Keywords: ocean health communication, ocean acidification, ocean deoxygenation, human physiology, metaphor and analogy, carbon cycle, microcosm-macrocosm analogy

What the future ocean has in common with an asthma attack

Introduction

Communicating the science of climate change to leaders, policymakers and the general public is an important aspect of addressing the climate crisis [START_REF] Bowman | Time to Take Action on Climate Communication[END_REF][START_REF] Hassol | Improving How Scientists Communicate About Climate Change[END_REF][START_REF] Moser | Communicating climate change: history, challenges, process and future directions[END_REF][START_REF] Moser | Reflections on climate change communication research and practice in the second decade of the 21st century: what more is there to say?[END_REF][START_REF] Somerville | Communicating the science of climate change[END_REF]. Effective climate change communication is necessary to raise public awareness, increase understanding in lay audiences, and advocate for policy change [START_REF] Moser | Communicating climate change: history, challenges, process and future directions[END_REF]. Most communication efforts have concentrated on explaining the link between rising atmospheric CO2 concentrations and global warming, rising sea-level and modified weather patterns [START_REF] Moser | Communicating climate change: history, challenges, process and future directions[END_REF][START_REF] Moser | Reflections on climate change communication research and practice in the second decade of the 21st century: what more is there to say?[END_REF][START_REF] Somerville | Communicating the science of climate change[END_REF]. A comparatively smaller effort has been directed to explaining the effect of anthropogenic CO2 emissions on interior ocean processes [START_REF] Schuldt | Communicating about ocean health: theoretical and practical considerations[END_REF]. Thus, ocean health issues such as ocean deoxygenation [START_REF] Oschlies | Drivers and mechanisms of ocean deoxygenation[END_REF] and ocean acidification [START_REF] Caldeira | Anthropogenic carbon and ocean pH[END_REF][START_REF] Gattuso | Ocean Acidification[END_REF] are not as widely known by the general public as climate change effects [START_REF] Capstick | Public understanding in Great Britain of ocean acidification[END_REF][START_REF] Chilvers | Public engagement with marine climate change issues: (Re)framings, understandings and responses[END_REF][START_REF] Cooke | Exploring the "Evil Twin of Global Warming": Public Understanding of Ocean Acidification in the United States[END_REF].

Ocean acidification [START_REF] Caldeira | Anthropogenic carbon and ocean pH[END_REF][START_REF] Gattuso | Ocean Acidification[END_REF] is the decrease in ocean pH that results from the absorption of anthropogenic CO2 from the atmosphere [START_REF] Khatiwala | Global ocean storage of anthropogenic carbon[END_REF][START_REF] Quéré | Global Carbon Budget 2018[END_REF]. Since the beginning of the industrial revolution, this process has decreased the pH of the surface ocean by 0.12 pH units [START_REF] Perez | Meridional overturning circulation conveys fast acidification to the deep Atlantic Ocean[END_REF], which corresponds to a 30% increase in acidity measured as the concentration of the H + ion. As the ocean acidifies, the saturation state with respect to calcium carbonate (CaCO3) minerals decreases [START_REF] Zeebe | CO2 in Seawater: Equilibrium, Kinetics, Isotopes[END_REF], and marine calcifying organisms such as calcifying algae, mollusks and corals experience reduced calcification [START_REF] Pörtner | Ocean systems[END_REF]. In conjunction with rising ocean temperature and changing nutrient concentrations, ocean acidification promotes toxic algal blooms, increases coral bio-erosion and alters sensory perception in fishes [START_REF] Doney | The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities[END_REF][START_REF] Gattuso | Ocean Acidification[END_REF][START_REF] Pörtner | Ocean systems[END_REF]. Ocean acidification has negative consequences for aquaculture and fisheries, and for the human communities that these economic activities sustain [START_REF] Doney | The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities[END_REF]. It is virtually certain (IPCC, 2021), based on the latest generation of Earth System models [START_REF] Kwiatkowski | Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections[END_REF], that acidification of the surface ocean will continue in response to the rise in atmospheric CO2.

Ocean deoxygenation is the decrease in dissolved oxygen that follows the warminginduced strengthening of upper water column stratification, and the enhancement of coastal nutrient inputs [START_REF] Oschlies | Drivers and mechanisms of ocean deoxygenation[END_REF]. As stratification of the upper ocean is strengthened, the ventilation of the ocean interior decreases, resulting in the decline of dissolved oxygen concentration. An additional cause of ocean deoxygenation is the decrease in oxygen solubility with increasing water temperature (Oschiles et al, 2018).

The effect is both pervasive -the oxygen content of the first 1000 m of the ocean has decreased by roughly 2% over the 1970-2020 period [START_REF] Schmidtko | Decline in global oceanic oxygen content during the past five decades[END_REF] -and local, with the expansion of oxygen minimum zones, where dissolved oxygen concentrations are vanishingly low [START_REF] Stramma | Expanding Oxygen-Minimum Zones in the Tropical Oceans[END_REF]. Earth System models predict that throughout the 21 st century ocean deoxygenation will continue under the effect of climate change [START_REF] Kwiatkowski | Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections[END_REF], with potentially negative consequences for ecosystems and the coastal economies they depend on [START_REF] Stramma | Expanding Oxygen-Minimum Zones in the Tropical Oceans[END_REF]. Communicating on ocean health implies familiarizing lay audiences with the cascade of cause-effect mechanisms that link increased atmospheric CO2 to evolving ocean chemistry. Faced with this challenge, communicators have to choose the most effective language to frame the issue [START_REF] Guenther | Framing as a Bridging Concept for Climate Change Communication: A Systematic Review Based on 25 Years of Literature[END_REF][START_REF] Lakoff | Why it Matters How We Frame the Environment[END_REF]. Framing, is setting an issue within an appropriate lexical context to achieve a desired interpretation or perspective -be it social, political or scientific [START_REF] Brugman | Political framing across disciplines: Evidence from 21st-century experiments[END_REF][START_REF] Entman | Framing: Toward clarification of a fractured paradigm[END_REF][START_REF] Fillmore | An Alternative to Checklist Theories of Meaning[END_REF]Thibodeau andBoroditsky, 2011, 2013). In climate change communication, metaphorical framing -i.e. using metaphors to provide context -is common practice [START_REF] Cred | The Psychology of Climate Change Comunication: A Guide for Scientists, Journalists, Educators, Political Aidies, and the Interested Public[END_REF][START_REF] Deignan | Metaphors of Climate Science in Three Genres: Research Articles, Educational Texts, and Secondary School Student Talk[END_REF][START_REF] Hassol | Improving How Scientists Communicate About Climate Change[END_REF][START_REF] Moser | Communicating climate change: history, challenges, process and future directions[END_REF][START_REF] Nerlich | Theory and language of climate change communication[END_REF][START_REF] Niebert | Understanding and communicating climate change in metaphors[END_REF][START_REF] Raimi | The Promise and Limitations of Using Analogies to Improve Decision-Relevant Understanding of Climate Change[END_REF][START_REF] Somerville | Communicating the science of climate change[END_REF][START_REF] Thibodeau | The earth is our home: systemic metaphors to redefine our relationship with nature[END_REF]. Alongside physical metaphors (e.g. "greenhouse effect" and "heat-trapping blanket"), a special place is occupied by medical metaphors [START_REF] Somerville | Medical Metaphors for Climate Issues[END_REF][START_REF] Stern | Simple Mental Models for Informing Climate Choices[END_REF] that compare climate change with human health (e.g. "Planet Earth is a sick patient due to climate change"; The Guardian, 2015), bridging the gap between the cognitive and affective domains of learning [START_REF] Chapman | Reassessing emotion in climate change communication[END_REF][START_REF] Duit | On the role of analogies and metaphors in learning science[END_REF].

Metaphorical framing has been extended to ocean health communication [START_REF] Bales | How to talk about climate change and the ocean: A FrameWorks Message Memo[END_REF][START_REF] Harrould-Kolieb | Framing ocean acidification to mobilise action under multilateral environmental agreements[END_REF][START_REF] Hawkins | How to talk about the ocean so that people will listen: A quick guide[END_REF][START_REF] Neil | Reframing the Ocean: A Frameworks Guide[END_REF][START_REF] Schuldt | Communicating about ocean health: theoretical and practical considerations[END_REF]. Thus, ocean acidification is described as "The evil twin of ocean warming" [START_REF] Pelejero | Paleo-perspectives on ocean acidification[END_REF], "Osteoporisis of the sea" [START_REF] Guinotte | Climate Change and Deep-Sea Corals[END_REF][START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF] and "the other CO2 problem" [START_REF] Doney | Ocean Acidification: The Other CO2 Problem[END_REF]; while ocean deoxygenation is referred to as "the ocean losing its breath" [START_REF] Gruber | Warming up, turning sour, losing breath: ocean biogeochemistry under global change[END_REF]. Similar to climate change metaphors, ocean health metaphors are provocative statements aimed at sparking interest for ocean health phenomena. But a gap remains between the metaphorical evocation of ocean health issues, and even a rudimentary understanding of their origin.

Making sense of ocean health phenomena requires that metaphors are followed by an explanation of the interconnectedness of natural systems, and of the processes that link anthropogenic causes to ocean change effects [START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF][START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF]. Explanatory metaphors, which are multi-phrase extensions of the metaphors introduced above, have proven to be effective when tested empirically [START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF][START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF]. Thus, seeing the Planet as a Body improves understanding of the interdependency of atmosphere, land and ocean, while introducing the ocean as Climate's heart, enhances understanding of the role played by the ocean within the planet [START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF]. Clearly, the human body frame is a powerful vehicle to foster awareness of ocean health issues. Yet, metaphors are not a substitute for content.

They do not fill the knowledge gap that prevents the public from arriving to a concrete understanding of ocean health issues [START_REF] Lindland | Getting Below the Surface: Mapping the Gaps between Expert and Public Understandings of the Ocean and Marine Conservation in the United Kingdom[END_REF][START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF]. Can we move beyond explanatory metaphors to provide explanatory scientific content without exiting the metaphoric Body frame? In other words, are there significant similarities between the ocean and the human body that can be exploited to adopt the human body as a model to deliver specific ocean-related knowledge? I found the clue to answering these questions in our historical past, in the philosophical quest of pre-modern thinkers to make sense of the unknown earth. From antiquity to the Renaissance, scientific thinkers by the likes of Hippocrates, Seneca and Leonardo da Vinci drew parallels between the Earth and the human body, in an attempt to conceptualize the natural world. They were drawn into this comparison by the intrinsic ability of metaphor to shape human thought. Their efforts over a period of two millennia are a window into how the human mind formulates interpretative models of the unknown Earth, including the ocean. In this paper, I build on this pre-modern tradition and, together with contemporary knowledge of human physiology and ocean biogeochemistry, put it at the service of ocean health communication.

The human Earth metaphor from ancient philosophy to early modernity

The idea that the Earth or the cosmos (intended as the physical universe) are in some way structurally similar to the human body is not new: it is present in ancient Iranian [START_REF] Götze | Persische Weisheit in griechischem Gewande: Ein Beitrag zur Geschichte der Mikrokosmos-Idee[END_REF], Mesopotamian [START_REF] Svärd | The Microcosm-Macrocosm Analogy in Mesopotamian and Medieval Arabic History of Science. In: Case Studies in Transmission[END_REF], Indian [START_REF] Wayman | The Human Body as Microcosm in India, Greek Cosmology, and Sixteenth-Century Europe[END_REF] and Chinese [START_REF] Raphals | Chinese Philosophy and Chinese Medicine[END_REF] philosophical traditions. In western philosophy, this view is embodied in the so-called microcosm-macrocosm analogy developed in ancient Greece, and transmitted to its medieval -Christian, Judaic and Islamic -and Renaissance descendants [START_REF] Allers | Microcosmus: From Anaximandros to Paracelsus[END_REF][START_REF] Barkan | Nature's Work of Art: The Human Body as Image of the World[END_REF][START_REF] Debus | The English Paracelsians[END_REF][START_REF] Kraemer | Microcosm[END_REF][START_REF] Malley | Andreas Vesalius of Brussels[END_REF]. This idea views man as a smaller representation of the cosmos, and the cosmos as an anthropomorphic entity: truths about the nature of the cosmos may be inferred from truths about human nature, and vice versa. The following examples illustrate how this idea was deeply rooted and persistent in pre-modern philosophical thinking.

In the words of Socrates (470-399 BC), as related in Plato's Philebus, the elements in man's body are derived from elements in the body of the cosmos: whatever man has, the cosmos must have [START_REF] Wayman | The Human Body as Microcosm in India, Greek Cosmology, and Sixteenth-Century Europe[END_REF]. Accepting these principles, the Greek physician Hippocrates (460-377 BC) developed a physiological and pathological system where the cosmos was a model to explain the human body, and imbalances in the body were a reflection of larger imbalances in the cosmos [START_REF] Jouanna | Greek Medicine from Hippocrates to Galen: Selected Papers[END_REF][START_REF] Schluderer | Imitating the Cosmos: The Role of Microcosm-Macrocosm Relationship in the Hippocratic Treatise On Regimen[END_REF][START_REF] West | The Cosmology of 'Hippocrates', De Hebdomadibus[END_REF]. Later, in the treatise De hebdomadibus, Hippocrates used knowledge of human anatomy and physiology to interpret the earth, giving rise to the makranthropic representation of the world (Le [START_REF] Blay | Microcosm and macrocosm: the dual direction of analogy in Hippocratic thought and the meteorological tradition[END_REF]. Thus, the rocky interior of the earth corresponded to the bones; the weaker parts surrounding it to the flesh; rivers corresponded to the veins; hot springs to the spinal marrow; ponds and marshes to the bowels; seas to the visceral fluids; the air to the breath in the body [START_REF] West | The Cosmology of 'Hippocrates', De Hebdomadibus[END_REF]. Taking this view, it was within man that the key to the cosmological riddle was to be found [START_REF] Allers | Microcosmus: From Anaximandros to Paracelsus[END_REF].

The makranthropic view of the earth was the base for interpretations of earthquakes by ancient Greek and Latin philosophers. In his treatise Meteorologica, Aristotle (384 -322 BC) claimed that: "We must suppose the action of the wind in the earth to be analogous to the tremors and throbbings caused in us by the force of the wind in our bodies. Thus, some earthquakes are a sort of tremor, other a sort of throbbing" (Le [START_REF] Blay | Microcosm and macrocosm: the dual direction of analogy in Hippocratic thought and the meteorological tradition[END_REF].

Hellenism, and the subsequent Roman conquest of Greece, were vehicles that spread Greek philosophy to the Latin world. Thus, three centuries after Aristotle, the Roman stoic philosopher and politician Seneca (4 BC -65 AD), in his treatise Naturales Quaestiones, saw earthquakes as a disease: "But in our bodies the movement of the veins also preserves its rhythm undisturbed while there is good health but when there is something wrong the movement pulses more rapidly and inhaling and exhaling give signs of effort and exhaustion. In the same way, the earth remains unshaken as long as its condition is normal. When something is wrong, then there is motion just like that of a sick body" (Le [START_REF] Blay | Microcosm and macrocosm: the dual direction of analogy in Hippocratic thought and the meteorological tradition[END_REF]. In summary, while earthquakes for Aristotle are a question of bad digestion, for Seneca they are a consequence of vascular and respiratory malfunction.

The ocean, and more in general the hydrological cycle, were also interpreted by ancient Greek and Latin thinkers using the human body as a model. Earth's water, in particular, was in turn related to bodily fluids like sweat, urine, or blood. Thus, in Meteorologica, Aristotle addresses the origin of ocean salinity: "the sea is a kind of sweat, exuded by the earth when the sun heats it, which explains its saltiness, all sweat being salty" (Le [START_REF] Blay | Microcosm and macrocosm: the dual direction of analogy in Hippocratic thought and the meteorological tradition[END_REF], and "…the salty sea as the final incarnation of riverine flow appropriately recalls the salinity of urine" [START_REF] Wee | Earthquake and Epilepsy: The Body Geologic in the Hippocratic Treatise On the Sacred Disease. In: The Comparable Body -Analogy and Metaphor in Ancient Mesopotamian, Egyptian, and Greco-Roman Medicine[END_REF]. Seneca, in his treatise Naturales Quaestiones, turns to blood as the model for earth's water: "The idea appeals to me that the earth is governed by nature and is much like the system of our own bodies in which there are both veins, receptacles for blood, and arteries, receptacles for air. In the earth, also there are some routes which waters runs, some though which air passes. And nature fashioned these routes so like human bodies that our ancestors even called them 'veins of water" (Le [START_REF] Blay | Microcosm and macrocosm: the dual direction of analogy in Hippocratic thought and the meteorological tradition[END_REF].

In medieval Europe, much of Christian philosophical thought was characterized by a subservience to the theological purpose [START_REF] Hart | The Physical Science of Leonardo da Vinci: A Survey[END_REF]. Christian theologians compared man and the cosmos starting from the notion, present in the Gospel, that man was created not only in the image of God, but also of the whole creation [START_REF] Kurdzialek | Mediaeval Doctrines on Man as Image of the World[END_REF]. We are confronted here by a microcosmic interpretation of man derived not from an urge to make sense of the natural physical world -like the ancient Greeks and Latins felt -but, rather, from the need to attribute a meaning to the theological mysteries that define the very heart of Christian faith [START_REF] Kurdzialek | Mediaeval Doctrines on Man as Image of the World[END_REF].

The comparison of the cosmos with the microcosmos with a physical scientific purpose was the realm of mediaeval alchemy. Alchemists believed that all things above and below are connected [START_REF] Gentner | The shift from metaphor to analogy in western science[END_REF], as codified in the medieval doctrine of signatures: "It is through similitudes that the otherwise occult parenthood between things is manifests and every sublunar body bears the traces of that parenthood impressed on it as a signature" [START_REF] Eco | Drift and Unlimited Semiosis[END_REF]. The egg, symbolizing the microcosm, was widely used in alchemical metaphors: "The shell of the egg is an element like earth, cold and dry; it has been called copper, iron, tin, lead. The white of the egg is the water divine, the yellow of the egg is couperose [sulfate], the oily portion is fire" (from the manuscript of St. Mark, 10 th -11 th century, cited in [START_REF] Gentner | The shift from metaphor to analogy in western science[END_REF].

The Renaissance revival of the thoughts, views, and ideals of Greek and Latin Antiquity led to the return of the ancient microcosm-macrocosm idea as a tool to conceptualize the human body and the cosmos. The direction taken -makranthropic or microcosmicdepended on the subject matter of study. For the Swiss physician, philosopher and alchemist Paracelsus (1493-1541), man resembled heaven and earth because he was composed of their matter -"hence he must have all their nature and all their parts, down to the last hair" [START_REF] Jevons | Paracelsus's Two-Way Astrology I. What Paracelsus Meant by 'Stars[END_REF]. Like Hippocrates, for Paracelsus there was a profound correspondence between the structure of the cosmos and the structure, working and arrangement of the human body; and health arose from the harmony between one and the 215 other. It was the work of the physician to understand these correspondences, which were the key to how the organs worked in health and disease [START_REF] Pagel | Vesalius and Paracelsus[END_REF]. It is within this microcosmic view of man, that Paracelsus interpreted epilepsy as "the earthquake of the microcosm" [START_REF] Storch | An Essay on the History of Epilepsy[END_REF]. With Leonardo da Vinci (1452-1519), the makranthropic view of the earth was again directed at interpreting large scale geological and hydrological processes. Thus, in his

The Motion and Measurement of Water (c. 1500), Leonardo revisits ancient Greek and Latin visions of the earth seen as a human body: Man has been called by the ancients a lesser world, and indeed the term is rightly applied; seeing that man is compounded of earth, water, air and fire, this body of the earth is the same. And as man has within himself bones as a stray and framework for the flesh, so the world has the rocks which are the supports of the earth, and man has within him a pool of blood wherein the lungs as he breathes expand and contract, so the body of the earth has its ocean which also rises and falls every six hours with the breathing of the world. As from the said pool of blood proceed the veins which spread throughout the earth -and is breathed out in baths and volcanoes, such as Mount Etna in Sicily.

Leonardo's mapping of the ocean onto the blood, and of "breathing" of the earth, onto a character conductible to blood flow "..its pulses" comes several decades before William Harvey's founding book The Circulation of the Blood (1628).

One of the best-known pictorial representations of the microcosm-macrocosm idea is Leonardo's drawing Vitruvian Man (1490) (Figure 2). In this drawing, the erect figure of a man fits both within a circle (arms spreadeagled) and a square (arms outspread), where the circle and the square, accepted by Pythagoreanism as the most beautiful shapes, symbolize the macrocosm [START_REF] Wayman | The Human Body as Microcosm in India, Greek Cosmology, and Sixteenth-Century Europe[END_REF]. But next to the makranthropic view of the earth, a new, more modern vision of geology, developed [START_REF] Laurenza | Leonardo's Theory of the Earth: Unexplored Issues in Geology from the Codex Leicester[END_REF]. In the Codex Leicester (1506-1510), Leonardo interprets the presence of fossil shells of marine organisms in continental outcrops as due to emersion of previously submerged sediments, and vertical movements of land as due to the effect of sediment erosion and unloading.

The times of the microcosm-macrocosm idea as an all-encompassing view of the earth as a material entity imbued with soul were nearing an end.

Starting with the scientific revolution and the Enlightenment, the use of the microcosmmacrocosm idea to conceptualize the physical universe was supplanted by modern scientific descriptions of the Earth [START_REF] Oldroyd | Thinking About the Earth: A History of Ideas in Geology[END_REF]. Amongst the last traces of its use can be found in the early work of James Hutton (1726-1797), the father of modern Geology.

Hutton, that is most famous for his opus Theory of the Earth (1785), had studied medicine in Leiden, graduating in 1749. His dissertation, On the circulation of blood in the Microcosm (1749), is a study of the chemistry of blood, its modification by physiological processes, its circulation, and purification. It is not surprising, then, that when Hutton went on to investigate the material cycles of the solid Earth, he concluded: "I consider the Earth to be a super organism and that its proper study should by physiology" [START_REF] Mcintyre | James Hutton and the Philosophy of Geology[END_REF]. The idea of the human body as a Microcosm of Nature could have led Hutton to realize that like the body, planet Earth is wasted and renewed by the flow of rivers, which keep it in equilibrium, just as the human body is kept in health by the addition of oxygen and nutrients and the removal of wastes by blood [START_REF] Norwick | Metaphors of Nature in James Hutton's Theory of the Earth With Proofs and Illustrations[END_REF]. By the time Hutton wrote The Theory of the Earth, however, he had become a mechanically and causally oriented investigator, such that he can be considered the transition between Renaissance science, which emphasized the microcosm-macrocosm idea, and modern science, which rejected interpreting inorganic objects in terms of organic objects [START_REF] Norwick | Metaphors of Nature in James Hutton's Theory of the Earth With Proofs and Illustrations[END_REF].

The fact that the microcosm-macrocosm idea has been so persistent in pre-modern philosophical thought indicates a deep-rooted tendency in the human mind for this kind of comparison [START_REF] Allers | Microcosmus: From Anaximandros to Paracelsus[END_REF]. This is all the more so if you consider that it probably developed independently in different cultures of antiquity [START_REF] Eliade | Patterns in Comparative Religion[END_REF]. This is not surprising, given the fundamental role played by metaphor in human thought (see next section). The microcosm-macrocosm idea is thus a kind of "symptom" of the thought process guiding man in his quest, during two millennia, to make sense of the unknown Earth [START_REF] Allers | Microcosmus: From Anaximandros to Paracelsus[END_REF]. We are faced today with the urgency of communicating effectively on Earth's changing environment to lay audiences. In this paper I reconsider the symbolic power of the human Earth metaphor, and use it to explain the challenges that human CO2 emissions impose on the ocean.

The role of metaphor and analogy in thought and communication

In this work I take advantage of the fundamental role that metaphor plays in human cognition and communication. I will distinguish between two types of similaritymetaphor and analogy -based on their ability to foster creative thinking and explain poorly known concepts [START_REF] Brown | Analogies in science and science teaching[END_REF][START_REF] Duit | On the role of analogies and metaphors in learning science[END_REF][START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF][START_REF] Gentner | The shift from metaphor to analogy in western science[END_REF][START_REF] Gentner | Structure Mapping in Analogy and Similarity[END_REF][START_REF] Little | Analogy in science: Where do we go from here?[END_REF]. In this section, I provide the basic theoretical principles that underpin the use I make of metaphor and analogy.

Metaphor as an essential component of human thought

Metaphors have been long considered only as figure of speech, a sophisticated device used to embellish discourse. The view of metaphor as a property of language alone, as opposed to cognition or action, follows from the objectivist western philosophical tradition [START_REF] Lakoff | Metaphors We Live By[END_REF]. In this tradition, metaphors are literally false statements that are intended to have a different meaning from the literal meaning (e.g. "anger is heat"). At the root of this view is the myth that the world is made up of distinct objects, with inherent properties and fixed relations among them; that these properties are independent of any people or other beings who experience them; that there is an objective reality, and that words have fixed meanings that can describe this reality correctly. In objectivist tradition, metaphors should always be avoided because their meaning is not clear and precise and does not fit reality in any obvious manner [START_REF] Lakoff | Metaphors We Live By[END_REF].

This view changed radically with the work of [START_REF] Lakoff | Metaphors We Live By[END_REF] in the field of cognitive linguistics, providing evidence that metaphor in language reflects underlying conceptual representations and thought processes. In other words, our ordinary conceptual system, in terms of which we both think and act, is fundamentally metaphorical in nature. Since the work of [START_REF] Lakoff | Metaphors We Live By[END_REF], evidence for a fundamental role of metaphors in human thought has been provided by psychological research [START_REF] Boroditsky | Metaphoric structuring: understanding time through spatial metaphors[END_REF]Thibodeau andBoroditsky, 2011, 2013), the study of gestures [START_REF] Mcneill | Hand and Mind: What Gestures Reveal about Thought[END_REF], the analysis of discourse [START_REF] Narayanan | KARMA: Knowledge-Based Action Representations for Metaphor and Aspect[END_REF] and of sign language [START_REF] Taub | Iconicity in American sign language: concrete and metaphorical applications[END_REF], amongst others. This research provides an explanation for the pervasive and universal tendency of pre-modern thinkers to conceptualize the Earth in terms of the human body.

Metaphor and analogy as communication tools

The fundamental role played by metaphor in human thought has important consequences for the concept of truth, and for how the general public understands -and forms its opinion on -issues so disparate as literature, politics and science [START_REF] Brugman | Recategorizing political frames: A systematic review of metaphorical framing in experiments on political communication[END_REF][START_REF] Semino | An Integrated Approach to Metaphor and Framing in Cognition, Discourse, and Practice, with an Application to Metaphors for Cancer[END_REF][START_REF] Thibodeau | The role of metaphor in communication and thought[END_REF]Thibodeau andBoroditsky, 2011, 2013).

Because human understanding is mostly metaphorical, and because the idea of truth is based on understanding, truth is always relative to a conceptual frame [START_REF] Lakoff | Metaphors We Live By[END_REF]. In other words, one cannot understand the meaning of a given word without access to all the essential knowledge -the frame -that relates to that word [START_REF] Fillmore | An Alternative to Checklist Theories of Meaning[END_REF][START_REF] Entman | Framing: Toward clarification of a fractured paradigm[END_REF]. In the case of ocean health, for example, the term ocean acidification is meaningless if it cannot be related to other elements of the ocean health frame (e.g. anthropogenic CO2 emissions, atmosphere-ocean CO2 exchange, surface ocean pH, biological calcification etc.).

When the purpose is communication, it is useful to distinguish metaphors from analogies, based on how information from the source domain is transferred to -mapped onto -the target domain [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF]). An analogy is a particular declaration of similarity asserting that a system of relations that holds between objects of the base domain (source of knowledge) also holds between objects of the target domain, regardless of whether or not the objects themselves are intrinsically similar [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF][START_REF] Gentner | Structure Mapping in Analogy and Similarity[END_REF]. A classic example is Rutherford's analogy: "The hydrogen atom is like our solar system" [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF]) (Fig. 3). In this analogy, the electron corresponds to the planets, while the atomic nucleus corresponds to the Sun. The strength of the analogy, however, lies in the fact that the electron revolves around (is attracted by) the atomic nucleus just as planets revolve around (are attracted by) the Sun: the mapping is on relations, rather than on object attributes. Analogy, as defined here, is a powerful tool to explain poorly known concepts. Figure 3 -An example of analogy: the electron revolves around (is attracted by) the atomic nucleus just as planets revolve around (are attracted by) the Sun, in Rutherford's analogy between the hydrogen atom and the solar system [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF].

What, then, is the place of metaphor? [START_REF] Gentner | Structure Mapping in Analogy and Similarity[END_REF] propose that metaphors occupy a continuum between analogies -that share many relational, but few attribute, similarities -and mere appearance matches -that share many attribute, but no relational, similarities (e.g. "a sunflower looks like the Sun"). It is clear that while analogies have explanatory power, mere appearances completely lack explanatory power.

Metaphors, that may imply both relational and attribute comparisons (e.g. "tires are like shoes"), lie along this continuum [START_REF] Gentner | Metaphor Is Like Analogy[END_REF][START_REF] Gentner | Structure Mapping in Analogy and Similarity[END_REF]. In addition, regardless of the degree to which they integrate relational similarities, metaphors very often have the additional property that the ground of the comparison is hidden [START_REF] Duit | On the role of analogies and metaphors in learning science[END_REF], resulting in literally absurd or false statements (e.g. "his eyes were burning coals"). The effect produced is that of surprise and provocation, leaving the addressee of the metaphor with the task of revealing -or even creating -the ground of the comparison. In other words, metaphors promote creative thinking. This is evident in art, where the provocation that arises from the coexistence of literal absurdity with figurative truth is used to deliver profoundly evocative messages (Fig. 4). Figure 4 -An example of metaphor: the physics is wrong, but there is something in the chemistry… (Marc Chagall, Over the Town, 1913) With these definitions in mind, it is evident that metaphors and analogies are complementary tools for framing ocean health issues [START_REF] Duit | On the role of analogies and metaphors in learning science[END_REF]. Metaphors, next to the role they play in conceptualization, are a provocative means to stimulate interest in ocean health issues, creating an expectation for further information; analogies, by providing basic information, give access to the understanding of how and why ocean health is deteriorating, making the underlying science accessible to the general public. Both should be used to frame ocean health issues effectively.

What the ocean has in common with the human body

Intrigued by the urge that pushed pre-modern thinkers to see the Earth as a human body, I explored the use of the human body as a model to frame ocean health issues. A consistent theme in the pre-modern comparison of the Earth to the human body is the parallel drawn between the water in the hydrological cycle and blood in the human body. I thus start from the idea that the global world ocean corresponds to the complete human circulatory system, and explore if the chemical composition of the ocean can be described with the same mathematical equations that calculate that of the human body. Should this be possible, it would provide a solid basis for using the human body as a model to explain the ocean.

At the base of this approach lies the realization that there are striking similarities in the structure and processes of the oceanic and human carbon cycles (Fig. 5), notwithstanding the 19 to 20 orders of magnitude difference in the volume and carbon mass of compartments between the two systems. Based on these similarities, I set up a carbon cycle model that simulates carbon exchange between two connected reservoirs and the atmosphere (see Appendix for a description of the numerical model). This model is simple enough to ignore the many differences in carbon compartmentalization between the human body and the ocean, but complex enough to consider the main carbon fluxes and processes that regulate acidity (measured as pH) in both systems.

Figure 5 -Similarities between the carbon cycle in the ocean and in the human body. The carbon cycle in the ocean is in many ways an analogue of the carbon cycle in the human body: both systems exchange CO2 between a relatively CO2-poor reservoir (lungs; surface ocean) and a relatively CO2-rich (body tissues; deep ocean) reservoir; in both systems net acidity is produced internally by metabolic processes in one of the reservoirs (tissue metabolism in the human body; organic matter mineralization in the deep ocean); both systems have a means to stabilize systemic pH via HCO3 -addition and H + removal (kidneys in the human body; carbonate compensation in the ocean). Export production in the ocean represents an additional carbon sink to the surface ocean which has no analog in the human body; this difference arises because the source of carbon fueling organic matter mineralization in the deep ocean is internal to the system (photosynthetic CO2 fixation), while in the human body the source of carbon for tissue metabolism is external (dietary input). Note: numbers refer to the analogies between the oceanic and the human carbon cycles listed in table 1.

A first indication that the similarities sketched in figure 5 have a sound physico-chemical basis is the observation that the model calculates pH, pCO2 and HCO3 -values characteristic of the human body [START_REF] Oldroyd | Thinking About the Earth: A History of Ideas in Geology[END_REF] -or of the ocean (Zeebe, 2012a) -when model parameters are set to their human physiological -or oceanographic -values, respectively (Fig. 2, Table 1, Figs. A2 and A3 in the appendix). In these two reference runs, the model captures the contrasts in pH and pCO2 between the relatively basic (pulmonary and surface ocean) and more acidic (body tissue and deep ocean) reservoirs, as they are driven by production of acidity in the body tissues and in the deep ocean (Fig. 5). I then used the model to simulate the response of the human body and the ocean to an abrupt increase in CO2. f value in equilibrium with pre-anthropogenic atmospheric pCO2; g ranges are taken from the results of pre-anthropogenic simulations with the LOSCAR model (Zeebe, 2012b); values in brackets are the results of the reference model runs (this work) for the human body and the preanthropogenic ocean; h in the human body model, total alkalinity corresponds largely to the concentration of bicarbonate (HCO3 -).

Acidification of the human body

In the human body, lung conditions that decrease the efficiency of pulmonary gaseous CO2 exchange result in the internal build-up of CO2 and body acidification, an acid-base disorder known as respiratory acidosis (Adrogué and Madias, 2005b). Typical examples are asthma, chronic obstructive lung disease (COPD), lung fibrosis and viral lung infections causing MERS, SARS and Covid-19, leading to acute respiratory distress syndrome (ARDS) (Adrogué and Madias, 2005b). The more affected the pulmonary exchange, the more severe the respiratory acidosis. In the worst cases, acute severe asthma, exacerbations of COPD and protracted, severe ARDS cause a sudden and strong acidification of the body called acute respiratory acidosis that, if protracted in time, can lead to respiratory failure [START_REF] Vasileiadis | Acid-Base Disturbances in Patients with Asthma: A Literature Review and Comments on Their Pathophysiology[END_REF]).

In the model, if impaired pulmonary gaseous CO2 exchange is simulated, the ensuing CO2 build-up results in a rapid pH drop, with pH falling below normal physiological values in a matter of minutes (Fig. 6a), consistent with acute respiratory acidosis (Adrogué and Madias, 2005b). Only on a time scale of several hours do enhanced renal H + excretion and HCO3 -production eventually restore pH to levels that approach the original, physiological pH (Fig. 6b-e). This phase, known as chronic respiratory acidosis, is typical of chronic lung diseases such as chronic obstructive pulmonary disease (COPD)

and pulmonary fibrosis (Adrogué and Madias, 2005b). The fundamental role played by the kidneys in the long-term regulation of body pH is evident if the model is run simulating impaired pulmonary gaseous CO2 exchange, but keeping renal H + excretion and HCO3 -production rates equal to their values in the reference model run. In these conditions (dashed red lie in figure 6a), no long-term pH recovery takes place. The robustness of the model as a tool to simulate the acid-base balance in the human body is confirmed by its ability to reproduce the transient pH, pCO2 and HCO3 -shifts observed in clinical studies with patients experiencing acute severe asthma [START_REF] Raimondi | Acid-base patterns in acute severe asthma[END_REF], as well as the steady-state pH, pCO2 and HCO3 -shifts observed in clinical studies with patients experiencing COPD with variable degrees of severity [START_REF] Gonzalez | Secondary Response to Chronic Respiratory Acidosis in Humans: A Prospective Study[END_REF] (Fig. 7a,b). that also appear in Fig. 3a,b; H + excretion and HCO3 -production rates (panels d and e, respectively) are those associated to renal processes that regulate pH of the human body in the time-scale of hours. In panels (f-j), orange areas represent pre-anthropogenic surface ocean composition calculated by the LOSCAR model (Zeebe, 2012b), while H + consumption and HCO3 -production rates (panels i and j, respectively) are those associated to carbonate compensation, that regulates oceanic pH in the time scale of millennia.

Ocean acidification

A surprisingly similar reaction of systemic pH to an increase in CO2 load is observed when the model is applied to simulate the pH shifts induced in the surface ocean by anthropogenic CO2 emissions (Fig. 6f-j). Starting from an idealized, pre-industrial oceanic pH, total alkalinity and pCO2 composition (Zeebe, 2012a), the injection in the atmosphere of 1000 PgC (Pg = Petagrams = 10 15 grams) equivalent CO2 in a time-frame of 500 years results in a decrease of surface ocean pH of 0.2 pH units and an increase in atmospheric CO2 concentration of 180 µatm in the same time-frame of the atmospheric CO2 injection. On the time scale of millennia, the model calculates a recovery of surface ocean pH and atmospheric pCO2, and a build-up of alkalinity in the ocean (Fig. 6f-h).

These shifts are analogous to those calculated by the LOSCAR model (Fig. A3 of the appendix), a more sophisticated 10-box atmosphere-ocean-sediment model designed specifically to investigate the reaction of the atmosphere-ocean system to carbon cycle perturbations (Zeebe, 2012a). In both the present model and in LOSCAR, the long-term recovery of oceanic pH is driven by the production of HCO3 -and consumption of H + in the deep ocean via carbonate compensation (Fig. 5). Carbonate compensation [START_REF] Broecker | The role of CaCO3 compensation in the glacial to interglacial atmospheric CO2 change[END_REF] is the buffering of oceanic acidity via the dissolution of seafloor and water column, suspended CaCO3. This process will eventually restore oceanic pH to its pre-industrial value [START_REF] Archer | Dynamics of fossil fuel CO2 neutralization by marine CaCO3[END_REF]. Carbonate compensation, however, takes place over time scales of millennia, while anthropogenic CO2 emissions and ocean acidification are taking place in the time scale of decades to centuries.

The similarity with the long-term control of pH by the kidneys in the human body is striking, and is confirmed if the ocean model is run with the same CO2 emission scenario, but keeping the rate of carbonate compensation equal to its value in the reference (anthropogenic CO2-emission free) model run: similar to what happens in the human body when an increased pulmonary CO2 load is not compensated by renal processes, oceanic pH decline is not followed by full pH recovery if the rate of carbonate compensation is held constant (Fig. 6f). When forced with standard International Panel for Climate Change anthropogenic CO2 emission scenarios [START_REF] Liddicoat | Compatible Fossil Fuel CO2 Emissions in the CMIP6 Earth System Models' Historical and Shared Socioeconomic Pathway Experiments of the Twenty-First Century[END_REF] (Fig. 3c), the model presented in this work calculates negative surface ocean pH shifts that are comparable to those predicted by the latest generation of Earth System models (Kwiatkowski et al., 2020) (Fig 7c,d).

This simple analysis shows that the human body and the ocean possess analogous mechanisms that stabilize systemic pH on long time scales, but also that these mechanisms are overwhelmed by short-term CO2 increases. Specifically, in the ocean, anthropogenic CO2 emissions are taking place on a time scale (decades to centuries) that is much shorter than that at which carbonate compensation acts to restore oceanic pH (millennia) [START_REF] Archer | Dynamics of fossil fuel CO2 neutralization by marine CaCO3[END_REF]. These results provide a solid basis for developing metaphors and analogies that frame ocean health issues in terms of human health.

Framing ocean health in terms of human health

The challenge in communicating about ocean health is to provide simple and psychologically effective statements that describe how ocean health is deteriorating, while making a mechanistic link to anthropogenic CO2 emissions. This implies providing the necessary cultural background -the frame -into which ocean heath issues develop.

In this section, I present an extended analogy between the human body and the oceanic carbon cycles that provides a network of easily accessible, interconnected knowledge linking declining ocean health to anthropogenic CO2 emissions.

First, the unknown "ocean" object is addressed with the introductory analogy: "The ocean is like the human circulatory system" (Table 2, analogy 0). Then, come five analogies that relate unknown elements of the oceanic carbon cycle to more familiar elements of of the human body (Table 2, analogies 1 to 5). These analogies convey in very simple terms the important facts that: (i) the surface ocean exchanges CO2 and O2 with the atmosphere;

(ii) ocean mixing allows the exchange of CO2 and O2 between the surface and deep ocean; (iii) CO2 is produced and O2 is consumed in the deep ocean by organic matter degradation; and (iv) carbon cycle processes control the chemical composition of the ocean, including its acidity and oxygenation. These basic carbon cycle facts are a prerequisite for understanding how anthropogenic CO2 emissions affect ocean health.

(0)
The ocean is like the human circulatory system: in the human body, the circulatory system circulates blood to all the tissues (arms, legs, organs…); in the earth, ocean mixing circulates water to all oceanic basins (Pacific, Atlantic, Indian…).

The ocean is the circulatory system of the earth

Structure and processes of the carbon cycle

(1) The surface layer is to the ocean what the lungs 2 are to the human body: in the human body, the lungs exchange O2 and CO2 with the atmosphere; in the ocean the same gaseous exchange occurs between the atmosphere and the surface ocean.

The surface ocean is the lungs of the ocean.

(2) Downwelling and mixing are to the ocean what arterial blood flow is to the human body: in the human body, arterial blood flow brings O2-rich and CO2-poor blood from the lungs to the body tissues; in the ocean, downwelling and mixing bring O2-rich and CO2-poor surface waters to the deep ocean.

Oceanic downwelling and mixing are the arteries of the ocean.

(3) The deep layer is to the ocean what the body tissues 2 are to the human body: in the human body, body tissue metabolism consumes O2 and produces CO2; in the deep ocean, organic matter degradation consumes O2 and produces CO2.

The deep ocean is the tissues of the ocean.

(4) Upwelling and mixing are to the ocean what the venous blood flow is to human body: in the human body, venous blood flow brings O2-poor and CO2-rich blood from the body tissues to the lungs; in the ocean, upwelling and mixing bring O2-poor and CO2-rich water from the deep ocean to the surface. Oceanic upwelling and mixing are the veins of the ocean.

(5) The carbonate compensation mechanism is to the ocean what the kidneys are to the human body: in the human body, the kidneys are responsible for the long-term (hours) stabilization of pH; in the ocean, the carbonate compensation mechanism stabilizes oceanic pH on the time scales of thousands of years.

Carbonate compensation is the kidneys of the ocean.

Ocean health (6) Anthropogenic CO2 emissions are to the ocean what an asthma attack is to the human body: in the human body, modified CO2 and O2 exchange between the lungs and the atmosphere leads to low body O2 levels (resulting in hypoxia and, in extreme cases, suffocation) and excess of acid in the blood (respiratory acidosis); in the ocean, modified CO2 and O2 exchange between the atmosphere and the ocean leads to ocean deoxygenation and ocean acidification.

Anthropogenic

CO2 emissions are the asthma attack of the ocean.

Note: 1 the numbering of analogies 1 to 5 corresponds to that of figure 1; 2 the lungs and tissues reservoirs include all CO2-poor/O2-rich and CO2-rich/O2-poor blood reservoirs in the human body, respectively, as explained in the Methods section.

565 I then map the consequences of an asthma attack in the human body to the consequences of anthropogenic CO2 emissions on the ocean (Fig. 8). During an asthma attack, impaired gaseous CO2 and O2 exchange in the lungs results in low body oxygen levels (hypoxia, and in the worst cases, suffocation), and in the build-up of body CO2 and excess acid in the blood. The net effect on oceanic CO2 and O2 levels caused by anthropogenic CO2 emissions is the same. Care must be taken, however, when carrying out this mapping operation, because the detailed processes by which CO2 and O2 shifts are generated in the human body by an asthma attack, and in the ocean by anthropogenic CO2 emissions, do not correspond directly. In the human body, it is the impaired transfer of gases across the internal surface of the lungs that is the cause, while in the ocean it is a more complex set of mechanisms that include the rise of atmospheric CO2 concentrations, ocean warming and the increased physical stratification of the ocean (Fig. 8). This implies keeping analogical mapping at a general level, by declaring that: anthropogenic CO2 emissions modify CO2 and O2 exchanges between the ocean and the atmosphere just as an asthma attack modifies lung-atmosphere CO2 and O2 exchanges. Detailed mapping attempts that involve intermediate ocean processes introduced above (dotted area and dotted arrows in figure 8) should be avoided.

Based on the above considerations, I propose that the following analogy can be used to raise awareness in the general public for the unfamiliar processes of ocean deoxygenation and ocean acidification, and link them to anthropogenic CO2 emissions:

Anthropogenic CO2 emissions are to the ocean what an asthma attack is to the human body: in the human body, modified CO2 and O2 exchange between the lungs and the atmosphere leads to low body O2 levels (resulting in hypoxia and, in extreme cases, suffocation) and excess of acid in the blood (respiratory acidosis); in the ocean, modified CO2 and O2 exchange between the atmosphere and the ocean leads to ocean deoxygenation and ocean acidification.

Declined metaphorically, the above analogy reads:

"Anthropogenic CO2 emissions are the asthma attack of the ocean."

In appendix A, I analyze the extended analogy "The ocean is like the human circulatory system" applying the principles of the structure-mapping theory (SMT) of Gentner (1983). SMT presents effective analogies as declarations of similarity that transfer a large number of relational structures from the base to the target domains (see section 3). The numerous relational mappings that arise from the analysis of the human body-ocean analogy support the conclusion that this analogy conveys, as [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF] put it, "a system of connected knowledge, rather than just an assortment of independent facts".

Communicating on ocean health using the human body analogy

There is a growing body of literature examining the most effective way to communicate on ocean health issues [START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF][START_REF] Lindland | Getting Below the Surface: Mapping the Gaps between Expert and Public Understandings of the Ocean and Marine Conservation in the United Kingdom[END_REF][START_REF] Neil | Reframing the Ocean: A Frameworks Guide[END_REF][START_REF] Schuldt | Communicating about ocean health: theoretical and practical considerations[END_REF][START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF]. The communication tools put forward are: metaphors, that correspond to the definition used in this work;

explanatory metaphors, that are multi-phrase extensions of metaphors; and explanatory chains, that are simplified scientific narratives, delivered in lay terms, that do not employ figurative language. Empirical research conducted in the field using surveys and interviews shows that two explanatory metaphors that frame the ocean in terms of the human body are particularly effective: seeing the Planet as a Body improves understanding of the interdependencies between the ocean, land and atmosphere, while seeing ocean as Climate's heart helps people understand the ocean's role in the climate system [START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF][START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF].

Other metaphors specifically address ocean acidification and ocean deoxygenation. Thus, ocean acidification is referred to as "the other CO2 problem" [START_REF] Doney | Ocean Acidification: The Other CO2 Problem[END_REF], "the evil twin of ocean warming" [START_REF] Pelejero | Paleo-perspectives on ocean acidification[END_REF] and "osteoporosis of the Sea" [START_REF] Guinotte | Climate Change and Deep-Sea Corals[END_REF][START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF], while ocean deoxygenation is communicated as "the ocean losing its breath" [START_REF] Gruber | Warming up, turning sour, losing breath: ocean biogeochemistry under global change[END_REF]. The "other CO2 problem" and "evil twin" metaphors lack mapping onto a specific ocean condition and could be applied to any ocean health issue aside ocean warming. The "osteoporosis" metaphor maps a wellknown human pathology onto ocean acidification, and hints at a link between the acidifying ocean and the dissolution of some ocean-related component. Finally, the "ocean breath" metaphor links the oceanic process to a well-known human physiological activity (breathing), and the commonly experienced physiological state of being "out of breath".

Together with providing important information on the effectiveness of communication strategies, field surveys expose important gaps in understanding that need to be addressed [START_REF] Lindland | Getting Below the Surface: Mapping the Gaps between Expert and Public Understandings of the Ocean and Marine Conservation in the United Kingdom[END_REF]. Three are considered here. First, lay audiences see the ocean from a "surface" perspective, and are not aware of the diversified nature of oceanic environments, especially the shallow-versus-deep differences in chemical properties.

Second, the public does not recognize the relationship between carbon emissions and forms of ocean change other than sea-level rise, like acidification and deoxygenation (see also [START_REF] Harrould-Kolieb | Framing ocean acidification to mobilise action under multilateral environmental agreements[END_REF]. Third, more in general, lay audiences have little knowledge of the role of the ocean in the carbon and oxygen cycles, and how these cycles play a fundamental role in determining ocean health.

The critical issue is that the public lacks the basic scientific background needed to understand how ocean health phenomena arise. One solution is to embed explanatory metaphors in explanatory chains that provide the necessary context. This has proven to be effective in explaining the succession of cause-effect mechanisms that links anthropogenic CO2 emissions to ocean acidification [START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF]. But explanatory chains are literal statements, that lie outside of the metaphorical body frame: the communicative power of relating the ocean to a familiar domain is inevitably lost. Can critical background information be delivered while remaining within the human body frame? And, can one single frame be found that both (i) provides basic information of the functioning of the oceanic CO2 and O2 cycles, and (ii) uses these notions to connect anthropogenic CO2 emissions to both acidification and deoxygenation phenomena? At the moment, the Planet as a Body, Climates heart and osteoporosis metaphors, while effective in their own right [START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF][START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF], provide only a loosely connected system of knowledge that achieves these goals only in part.

The extended analogy presented in this work provides a solution. By mapping ocean elements onto fundamental building blocks of a functioning human body (lungs, arteries, tissues, veins, kidneys), the message is delivered that the ocean is a complex system, with vertical chemical differentiation and internal interconnectedness and material flow, and that this dynamic system guarantees healthy oceanic levels of O2 and CO2. Ocean health issues -just like in the human body -arise when external forces disrupt the O2 and CO2

cycles. These notions can be easily packaged in the form of short narratives (see Text Box 1) supported by simple sketches such as the one in figure 5.

The human health frame proposed in this work will have to be tested empirically in the field with surveys and interviews to see if it offers an advantage over existing osteoporosis and ocean breath metaphors. Simple considerations suggest that some improvement is possible. Osteoporosis in an illness that develops over years to decades before appreciable effects on bone strength become apparent. Losing one's own breath, on the other hand, is not a serious condition and is reversible on very short time scales.

There is thus an association of ocean health with slow, or non-critical, physiological conditions, in these metaphors. Ocean deoxygenation and ocean acidification, however, are neither slow processes, nor readily reversible conditions on the human time scale. An asthma attack, on the other hand, is a very rapid, and possibly a serious, event, making it a good candidate base domain in the metaphorical framing of ocean health issues.

Text box 1 -Example of a narrative that introduces the processes of ocean acidification and ocean deoxygenation to the general public using the metaphors (titles) and the extended analogy (paragraphs) between the ocean and the human body presented in this work.

The ocean is the circulatory system of the Earth

Ocean currents move water between the surface and deep ocean basins (such as the Pacific, Atlantic and Indian deep oceans), much like veins and arteries circulate blood between the lungs and the rest of the body (such as arms, legs and organs). Just as each component of the human body serves a specific function that contributes to overall health, each domain of the ocean has a specific role that is essential for the ocean's health. For example, the surface of the ocean is where oxygen is produced through photosynthesis, much like the lungs are where oxygen is brought into the human body. The deep ocean, on the other hand, is where biological processes produce carbon dioxide acid, just as the human tissues is where carbon dioxide acid is added to the blood stream. Maintaining proper oxygen and carbon dioxide levels is essential for the ocean's health, just as it is for the human body. Without adequate oxygen, marine animals cannot thrive, and high levels of carbon dioxide acid make it challenging for marine organisms like corals, mussels and plankton to build their mineral skeletons.

Human CO2 emissions are the asthma attack of the ocean

Just as an asthma attack affects the functioning of the lungs and ultimately the entire body, human carbon dioxide emissions disrupt the way the ocean regulates oxygen and carbon dioxide levels. This has serious consequences for ocean health. During an asthma attack, the lungs struggle to transfer oxygen from the air to the bloodstream, which lowers oxygen levels in the body. At the same time, carbon dioxide acid builds up in the lungs and makes them more acidic. The body's circulation then carries this carbon dioxide-rich and oxygen-poor blood throughout the entire body, putting it at risk of respiratory failure and suffocation.

Similarly, when we emit carbon dioxide into the atmosphere, the surface ocean absorbs it and becomes more acidic. This process, called ocean acidification, results in weaker shells and skeletons of marine organism, toxic algal blooms and the destruction of coral reefs. At the same time, higher carbon dioxide levels in the ocean reduce the ocean's capability to carry oxygen. This process, called ocean deoxygenation, is transforming large portions of the ocean floor into deserts where animal life is absent. As ocean currents transport carbon dioxide-rich and oxygenpoor water from the surface to the deep ocean, the decline in ocean health expands geographically. Ocean acidification and ocean deoxygenation are already happening and are affecting the health of the ocean, and the economies that rely on the ocean as a resource, such as the fishing industry and tourism.

Caring for the health of the ocean

The ocean plays a crucial role in the functioning of the earth, but human carbon dioxide emissions have put it under tremendous stress. The ocean is showing clear signs of deteriorating health, with acid-rich and oxygen-poor waters becoming increasingly widespread. If we allow this planetary-scale asthma attack to continue unchecked, the ocean will become increasingly inhospitable for marine life, and we will lose a vital resource. To make matters worse, there is no medical treatment that can be administered to the ocean to stop deoxygenation and acidification from worsening in the coming decades and centuries. To slow these processes down, we must reduce our carbon dioxide emissions to ease the burden on the ocean. Only then can we hope to preserve the evolving health of our ocean for future generations.

Beyond the ocean: the Earth as a superorganism

In section 5, I have mapped the part of the human body under homeostatic control, including the lungs, tissues, veins, arteries and kidneys, onto elements of the ocean.

However, the Earth's surface also includes the continental domain, that contains significant amounts of carbon in soils and the biosphere. These components account for 10% of the total carbon exchange with the atmosphere [START_REF] Friedlingstein | Global Carbon Budget 2020[END_REF], and make up the thin layer of the Earth's outer surface, called the Critical Zone [START_REF] Richter | Monitoring Earth's Critical Zone[END_REF], where natural ecological, hydrological and geochemical processes interact with human activities such as agriculture, mining and urbanization, shaping the environment. Can the human body-ocean analogy be extended to include the Critical Zone? And is there a good reason to build this figurative superorganism?

Accepting the mapping of lungs, tissues, veins, arteries and kidneys onto the ocean domain, and dismissing other body organs (e.g. brain, liver, spleen, pancreas…), we are left with the gastro-intestinal (GI) tract, of which I give a very simplified description here [START_REF] Silverthorn | Human Physiology: An Integrated Approach[END_REF]. Food, after having been mechanically broken down in the mouth by mastication, is transferred to the GI tract (composed of esophagus, stomach and intestine)

where it is digested with the intervention of gut biota. Digestion decomposes food in an acidic environment, releasing nutrients. Nutrients are absorbed across the intestinal epithelium and pass into the internal, homeostatic part of the body. Only a portion of the ingested food is transformed into nutrients in the GI tract and then absorbed into the interior of the body; the rest is excreted in the form of feces. As opposed to the internal portion of the body that has a neutral and nearly constant pH (7.36 < pHblood < 7.42), the GI tract has a highly variable chemical composition and pH (1 < pHstomach < 2.5;6.4 < pHintestine < 7.5;Evans et al., 1988). This distinction gives rise in popular science writing to the analogy: "… the human body is just like a big donut. The GI tract is the donut hole."

I propose the following correspondence of the human GI tract with the continental domain: the un-weathered, pristine rock substrate, that is external to the superorganism and contains elements essential for life, is the food; physical erosion, that fragments rocks and increases the mineral surface area exposed to chemical weathering, is mastication; soils, and the biota they sustain (both vegetation and microbiota), are the GI tract of the superorganism; chemical weathering, that attacks rocks mostly in an acidic environment and mobilizes cation and anions in the dissolved form, is digestion; cations and anions, mobilized by continental weathering, are the nutrients; vegetation and soil microbiota, that actively participate in the weathering process, are the gut biota; rivers, that transport nutrients liberated by rock weathering to the ocean, are the intestinal epithelial transport;

clay minerals, which are the part of rocks that remain in the solid phase after the continental weathering process, are the feces.

A rapid examination of the above attribute matches, and consideration of how the human body and the Earths' surface carbon cycles operate, highlights numerous possible mappings between relations of the two domains [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF]. Some examples are: (i) These are only a few examples of mapping of relations between attributes of the human GI tract onto those of Earth's continental domain. Many more can be found. But the important point is that -together with the mapping of relations between the body 'interior' and the ocean domain (section 5) -it emerges that the Earth system, with its ocean and continental soils, and marine and continental life (the "Critical Zone"), can be seen as a superorganism.

physical
The superorganism discussed in this work is not the first example of a comparison between the Earth and a living organism that contains some elements of modern analogy.

Fifty years ago, independent scientist James Lovelock (1919[START_REF] Boyle | The evolution of biogeochemical recycling by persistence-based selection[END_REF]) and biologist Lynn Margulis (1938-2011) developed the Gaia Hypothesis (later the Gaia Theory) [START_REF] Lovelock | Gaia as Seen Through Atmosphere[END_REF][START_REF] Lovelock | Atmospheric Homeostasis by and for Biosphere -Gaia Hypothesis[END_REF], that sees the Earth's biosphere, atmosphere, oceans and lithosphere as a complex, self-regulating system, maintaining the planet's habitability -i.e. regulating temperature and surface composition (oxygen, pH, climate..) comfortable for life -in a process reminiscent of human homeostasis, but operating on a planetary scale [START_REF] Lenton | Gaia and natural selection[END_REF][START_REF] Lovelock | Gaia -the World as Living Organism[END_REF][START_REF] Lovelock | Geophysiology, the Science of Gaia[END_REF][START_REF] Lovelock | Atmospheric Homeostasis by and for Biosphere -Gaia Hypothesis[END_REF]. The controversial idea of a self-regulating Gaia sparked a heated debate among earth scientists and biologists that continues to this day [START_REF] Barlow | Open systems living in a closed biosphere: a new paradox for the Gaia debate[END_REF][START_REF] Boyle | The evolution of biogeochemical recycling by persistence-based selection[END_REF][START_REF] Doolittle | Making Evolutionary Sense of Gaia[END_REF][START_REF] Kirchner | The Gaia hypothesis: Can it be tested?[END_REF][START_REF] Kleidon | Testing the effect of life on Earth's functioning: How Gaian is the Earth system?[END_REF][START_REF] Kleidon | Life, hierarchy, and the thermodynamic machinery of planet Earth[END_REF][START_REF] Lenton | Gaia and natural selection[END_REF][START_REF] Lenton | Selection for Gaia across Multiple Scales[END_REF].

While there are advantages and shortcomings to seeing the Earth as a superorganism (some of which are addressed in [START_REF] Latour | Extending the Domain of Freedom, or why Gaia Is So Hard to Understand[END_REF]), it is worth discussing them briefly. James Hutton famously stated "I consider the Earth to be a super organism and that its proper study should by physiology" [START_REF] Mcintyre | James Hutton and the Philosophy of Geology[END_REF]. But if Earth is a superorganism, what is its metabolism? The Earth superorganism acquires carbon via the chemical weathering of continental carbon-bearing rocks and the riverine transport of dissolved HCO3 -to the ocean, while the energy for carbon fixation into biomass is provided by photosynthesis in the surface ocean. How should we call this metabolism?

Heterotrophs are organisms that cannot produce their own carbon compounds from CO2, but instead rely on other sources of organic carbon and nutrients, mainly plant or animal matter. However, the Earth superorganism couples a heterotrophic carbon acquisition mechanism (from the weathering of carbon-bearing rocks) to photosynthetic carbon fixation. Such a living strategy, called photoheterotrophy, is present in prokaryotes such as purple non-sulphur bacteria and green non-sulphur bacteria [START_REF] Da | Prokaryotic photosynthesis and phototrophy illuminated[END_REF], but also in some insects like the oriental hornet and some aphids [START_REF] Valmalette | Light-induced electron transfer and ATP synthesis in a carotene synthesizing insect[END_REF]. I thus propose to call Earth's metabolism geophotoheterotrophy, and accept that this is different from the human heterotrophic metabolism.

Another point is whether the Earth, like (supposedly) living organisms, is a materially open system with respect to carbon, rather than a closed system with minimal carbon exchanges with the exterior and a large degree of internal recycling. The degree of openness to carbon is deduced from the comparison of the rate of internal carbon mixing with the rate of carbon exchange with the exterior. Carbon input by rivers in the form of HCO3 -(which represents a carbon flow from the superorganism exterior to its interior) adds about 1.1 x 10 11 moles of carbon per day to the ocean [START_REF] Ridgwell | A Mid Mesozoic Revolution in the regulation of ocean chemistry[END_REF], and an equivalent carbon loss through sedimentation of carbonate rocks and organic matter is implied by steady state on long time scales. These fluxes are about 120 times smaller than the internal carbon mixing flux between the surface and deep ocean (1.3 x 10 13 moles of carbon per day; obtained by multiplying the mixing rate of ocean waters of 74 Sverdrups of [START_REF] Wallmann | Feedbacks between oceanic redox states and marine productivity: A model perspective focused on benthic phosphorus cycling[END_REF] by a concentration of dissolved inorganic carbon of 2 millimoles per litre), effectively implying a "materially nearly closed" system.

Surprisingly, this applies to a considerable degree also to the human body. Consider that the circulatory system in the human body results in a recycling flux of carbon, that permeates all tissues, equal to 180 moles of carbon per day (obtained by multiplying the cardiac output of 5 liters per minute by the concentration of dissolved inorganic carbon in blood equal to 25 millimoles per litre). This is 12 times larger than the daily carbon flux though the body (15 moles of carbon per day; assuming carbon uptake from the intestine is balanced by the daily exhaled CO2 flux; [START_REF] Atherton | Acid-base balance: maintenance of plasma pH[END_REF]): the human body is far from

being a completely open system with respect to carbon. Based on these estimates, we can state that the ocean internally recycles carbon at a much larger rate than the carbon exchange fluxes with the lithosphere just as the circulatory system of the human body recycles carbon at a rate much larger than the carbon flux from dietary uptake.

The outcome is equally surprising when the comparison is extended to the energy efficiency. Bomb calorimetric measurements show that nearly all of the chemical energy contained in food is taken up by the human body (e.g. 93% in [START_REF] Basolo | Procedures for measuring excreted and ingested calories to asses nutrient absorption using bomb calorimetry[END_REF][START_REF] Jumpertz | Energy-balance studies reveal associations between gut microbes, caloric load, and nutrient absorption in humans[END_REF]), while only a small portion is excreted with feces (about 7%). Is the Earth superorganism even remotely as efficient in harvesting energy as the human body? It would seem that this should be excluded on the basis that only about 0.3 % of the incident solar energy on the Earths' surface is converted to chemical energy by photosynthesis [START_REF] Kleidon | Life, hierarchy, and the thermodynamic machinery of planet Earth[END_REF]. However, bomb calorimetry accounts for all of the energy spent by the human body, while photosynthetic primary production is only one of multiple energy-demanding processes involved in the metabolism of the Earth superorganism. Take for example the hydrological cycle and atmospheric circulationprocesses that involve an energy flux equal to approximately 30 % of Earth's incident radiation [START_REF] Kleidon | Life, hierarchy, and the thermodynamic machinery of planet Earth[END_REF]. These processes are fundamental to the superorganism's metabolism because they drive continental weathering (Earth's "digestive system") and the transport of carbon from the continents to the ocean where it is fixed into organic carbon. As a result, the energy efficiency of the human body is only three times larger (rather than two orders of magnitude larger, if only photosynthesis is considered), than that of the Earth superorganism.

Finally, one cannot fail to notice that there is a major difference in the physical structure of the Earth superorganism, and in its relation to the external environment, compared to the human body. This is a consequence of the very different ratio of water to living carbon in the ocean vs the human body: while in the human body the skeleton and tissues dominate in volume over blood, conferring structure and strength, in the ocean, marine organisms (the "cells" of the superorganism "tissue"), are dispersed in ocean water. Thus, while the human body has internal structure and strength, which conveys motility with respect to its external environment, the Earth superorganism lacks internal physical strength, and is doomed by the laws of gravity to occupy a fixed space on the lithosphere (its food source), without the possibility of moving with respect to it.

The above considerations confirm that the Earth's lithosphere, ocean, atmosphere and life can indeed be seen as a single superorganism, displaying a form of weak homeostasis conducible to that of the human body. While recent attempts to frame the Earth system in terms of the human body are encouraging [START_REF] Volmert | Getting to the Heart of the Matter: Using Metaphorical and Causal Explanation to Increase Public Understanding of Climate and Ocean Change[END_REF][START_REF] Hendricks | Turning the Tide: Findings from Refraiming Research on Ocean Health and Marine Conservation in teh United Kingdom[END_REF], they lack the necessary level of detail in considering the multitude of earthly elements and capture the holistic interconnectedness of these elements only in part. The perspective presented in this work addresses these limitations, providing a frame that allows lay audiences to better conceptualize the Earth system, comprehend the impact of human activities on climate, and foster a healthier relationship between humanity and the Earth.

Conclusion

In this paper I show that, after the necessary simplifications, the oceanic carbon cycle can be conceptualized in terms of the human carbon cycle. This observation has led me to propose the human body as a frame to communicate on ocean health. This idea is not new, since the human body is already used as a base domain to frame ocean heath issues metaphorically. My work goes beyond existing human Earth metaphors because it highlights, using a mathematical model, a striking resemblance in the physical-chemical functioning of the Earth system and of the human body. This shows that the similarity between the Earth and the human body is analogical, rather than just metaphorical.

Because of this, it is possible to communicate on ocean health using language -lungs, veins, arteries, asthma, etc. -that is familiar to the general public. It is also possible to link the two most important ocean health issues -ocean acidification and ocean deoxygenation -to a single cause: anthropogenic CO2 emissions. Finally, using the human body as a model to explain the Earth is supported by contemporary research,

showing that metaphors play a key role in human cognition, and by the fact that comparisons between the Earth and the human body have been a constant preoccupation of mankind since Antiquity. Monod constant for carbon export production, 𝐾 456 (mol kg -1 ) -2 j ´ 10 -3

Ratio of inorganic to organic carbon in export production, PIC:POC (-) -0.24 k

Fraction of carbon export production buried in marine sediments, 𝐹 789 (-) -0.018 j Diffusion coefficient for CO2 exchange with the atmosphere, 𝐷 6: ! (mol atm -1 m -2 sec -1 ) 2.46 j ´ 10 -6 2 l ´ 10 -3

Rate of HCO3 -addition by an external source (diet or rivers), 𝑃 ;6: " # 12< (mol sec -1 ) 2.31 m ´ 10 -7 7.55 n ´ 10 5

Production rate of CO2 in the muscular tissues or the ocean, 𝑃 6: ! =1< (mol kg -1 sec -1 ) 3.67 o ´ 10 -6 -Production rate of H + in the muscular tissues, 𝑃 ; $ =1< (mol kg -1 sec -1 ) 1.80 o ´ 10 -6 0 Kinetic constant for HCO3 -production, 𝜑 6: ! (sec -1 ) 4.29 o ´ 10 -3 1.015 j ´ 10 -10

Renal HCO3 -filtration rate, 𝐷 ;6: " # (sec Next, we define attributes and relations in the base and the target domains (Table 3).

These are statements (collectively known as predicates) that describe given properties of object nodes. Importantly, attributes, A(bi), are predicates involving only one object node, while relations, R(bi, bj), are predicates involving a relation between two or more object nodes. Predicates from the base domain are carried across to the target domain using the object node substitutions defined in table 3.

An effective analogy is a comparison in which relational predicates, but few object attributes, can be mapped from base to target [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF]. This is the case for the human body-ocean analogy where, due to the very different nature of the domains being compared, there are few easily mappable object attributes. For example, one cannot say: "downwelling currents are separated from the rest of the ocean by a biological membrane, just as arteries are separated from the surrounding tissues by the arterial wall". In contrast, the numerous structural and functional similarities between the human body and oceanic carbon cycles (Fig. 5) give rise to a high number of relational predicates of the human body domain being mappable to the ocean domain, for example: "the tissues are CO2-rich and O2-poor compared to the lungs, just as the deep ocean is CO2-rich and O2-poor compared to the surface ocean" (Table 4).

In addition to the first-order relations, one can recognize second order relations that take at least one first-order relation as an argument (Table 4), enforcing connections among lower-order predicates. For example, one might map "The fact that CO2 is produced and O2 is consumed in the deep ocean causes the deep ocean to be CO2-rich and O2-poor compared to the surface ocean" from "The fact that CO2 is produced and O2 is consumed in the tissues causes the tissues to be CO2-rich and O2-poor compared to the lungs". The numerous mapping relations that arise from the analysis of the human body-ocean analogy support the conclusion that this analogy conveys, as [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF] put it, "a system of connected knowledge, rather than just an assortment of independent facts".

First-order relations

The tissues are CO2-rich and O2poor compared to the lungs R(b1,b3)] => [R(t1,t3)

The deep ocean is CO2-rich and O2-poor compared to the surface ocean

The lungs receive CO2-rich and O2-poor blood from veins R(b1,b4)] => [R(t1,t4)

The surface ocean receives CO2rich and O2-poor water from upwelling and mixing

The lungs loose CO2-poor and O2-rich blood to arteries R(b1,b2)] => [R(t1,t2)

The surface ocean loses CO2-poor and O2-rich water to downwelling and mixing

The tissues receive CO2-poor and O2-rich blood from arteries R(b2,b3)] => [R(t2,t3)

The deep ocean receives CO2poor and O2-rich water from downwelling and mixing

The tissues loose CO2-rich and O2-poor blood to veins R(b3,b4)] => [R(t3,t4)

The deep ocean loses CO2-rich and O2-poor water to upwelling and mixing

An asthma attack results in a modified O2 exchange between the lungs and atmosphere and to low body O2 levels R(b1,b6,b7)] => [R(t1,t6,t7) Anthropogenic CO2 emissions result in a modified O2 exchange between the ocean and the atmosphere and to ocean deoxygenation

Figure 1 -

 1 Figure 1 -Ocean acidification and ocean deoxygenation: the two main ocean health consequences of anthropogenic CO2 emissions. Ocean acidification occurs when anthropogenic CO2 is absorbed by the oceans and reacts with water, resulting in increased acidity. Ocean deoxygenation is driven by multiple factors including decreased solubility of oxygen in warmer waters and increased stratification of surface ocean waters due to warming, which leads to reduced mixing of oxygen-rich surface waters with deep waters where O2 is consumed by reaction with settling organic matter (see text).

  220

Figure 2 -

 2 Figure 2 -Leonardo da Vinci's Vitruvian Man (c. 1490) is probably the best-known pictorial representation of the microcosm-macrocosm idea (see text).

Figure 6 -

 6 Figure 6 -Analogous long-term pH regulation mechanisms in the human body and in the ocean.Reaction of pH, total alkalinity (TA in the ocean, HCO3 -in the human body) and pCO2 to an increase in the CO2 load in (a-e) the arteries of the human body and (f-j) the surface ocean. Dashed blue lines represent the results of the reference model runs for the human body and the pre-anthropogenic ocean; solid blue lines represent the result of model runs that simulate carbon cycle perturbations in the human body (respiratory acidosis) and the ocean (ocean acidification); red dashed lines represent the pH in model runs simulating the same carbon cycle perturbations but imposing renal and carbonate compensation H + and HCO3 -fluxes equal to their values in the reference model runs. In panels (a-e), the orange areas represent normal physiological values for arteries, taken from Androgué and Madias (2005b); red circles and dots represent the modelcalculated acute and chronic respiratory acidosis values of pH, HCO3 -concentration and pCO2

Figure 7 -

 7 Figure 7 -CO2 perturbations: respiratory acidosis (a,b) and ocean acidification(c,d). When applied to the human body, the clinical data[START_REF] Gonzalez | Secondary Response to Chronic Respiratory Acidosis in Humans: A Prospective Study[END_REF][START_REF] Raimondi | Acid-base patterns in acute severe asthma[END_REF] and model results (this work) refer to the acute-and chronic-phase arterial pCO2, pH and HCO3 - concentration in patients experiencing acute severe asthma (ASA) and chronic obstructive pulmonary disease (CODP); each blue dot represents a patient included in the COPD clinical trial[START_REF] Gonzalez | Secondary Response to Chronic Respiratory Acidosis in Humans: A Prospective Study[END_REF], the blue circle represents the average values for patients included in the ASA clinical trial[START_REF] Raimondi | Acid-base patterns in acute severe asthma[END_REF]; the red dot in panels a an b correspond to the modelcalculated (this work), steady state arterial pCO2, pH and HCO3 -composition shown in Fig.6a-c; the red circles in panels a an b correspond to the model-calculated (this work) acute state arterial pCO2, pH and HCO3 -composition shown in Fig.6a-c; the span of model calculated arterial pCO2, pH and HCO3 -compositions represented by the red line is obtained by simulating a variable degree of impediment of pulmonary gaseous exchange (see Appendix). When applied to the ocean, the model is forced with standard CO2 emission scenarios[START_REF] Liddicoat | Compatible Fossil Fuel CO2 Emissions in the CMIP6 Earth System Models' Historical and Shared Socioeconomic Pathway Experiments of the Twenty-First Century[END_REF] (panel c) used to force Earth System models that simulate future climate change trajectories[START_REF] Kwiatkowski | Twenty-first century ocean warming, acidification, deoxygenation, and upper-ocean nutrient and primary production decline from CMIP6 model projections[END_REF], including the decrease in surface ocean pH (panel d); in panel d, in order for the model presented in this work (solid lines) to fit the results of Earth System models (empty circles), I considered the presence of a net terrestrial CO2 sink corresponding to 17 to 25 % of anthropogenic CO2 emissions (panel c) (the present terrestrial CO2 sink is estimated at 18% of anthropogenic CO2 emissions[START_REF] Friedlingstein | Global Carbon Budget 2020[END_REF]).

Figure 8 -

 8 Figure 8 -Analogical mapping of the consequences of a severe asthma attack on the human body (base domain) onto the consequences of anthropogenic CO2 emissions on ocean health (target domain). Note that considering ocean processes (ocean warming and its effect on CO2 solubility and stratification) in detail should be avoided because they do not map clearly onto the human body domain.

  erosion increases the surface area of rocks involved in chemical weathering just as mastication increases the surface area of food being digested, (ii) chemical weathering on the continents is promoted by low soil pH just as the digestive processes in the GI tract of the human body is facilitated by low pH, (iii) soil biota play a fundamental role in the extraction of key cations and anions from rocks during chemical weathering just as gut microbiota play a key role in extracting nutrients from food, (iv) cations and anions released from rock weathering are transported by rivers to the ocean just as nutrients released form digestion of food are transported across the intestinal epithelial into the human body.

  Figure A2 -Full output of the numerical model of carbon exchange between two reservoirs and the atmosphere applied to the human body. The orange areas represent normal physiological values of pH, bicarbonate (HCO3 -) and pCO2 in arteries and veins taken from Adrogué and Madias (2005a). Dashed lines represent the results of the reference model run (this work); solid lines represent the result of the model run (this work) that simulates respiratory acidosis (see main text).

Figure A3 -

 A3 Figure A3 -Full output of the numerical model of carbon exchange between two reservoirs and the atmosphere applied to the pre-anthropogenic ocean. The orange areas represent the composition of the preanthropogenic ocean and the atmospheric pCO2 calculated by the LOSCAR model (Zeebe, 2012b). Dashed lines represent the results of the reference model run (this work); solid lines represent the result of the model run (this work) that simulates anthropogenic CO2 emissions (see main text). The red lines

Table 1 -

 1 Carbon cycle in the ocean and in the human body

		Human body	Pre-anthropogenic ocean
	Parameter
		Lungs	Tissues	Surface	Deep
	Reservoirs
	Volume (litres)	~1.3 a	~3.7 a	4.28 b ´ 10 19	1.25 b ´ 10 21
	Volume (km 3 )	-	-	4.28 b ´ 10 7	1.25 b ´ 10 9
	Dissolved inorganic carbon (DIC) (mol)	0.0318 c	0.096 c	7.5 e ´ 10 16	3.09 e ´ 10 18
	Dissolved inorganic carbon (DIC) (GtC d )	-	-	900 e	37100 e
	Composition
	pCO2 (mmHg)	36-44 e (40.0)	42-50 e (42.7)	-	-
	pCO2 (µatm)	-	-	~280 f (282)	(916)
	pH	7.36-7.44 e (7.40)	7.34-7.42 e (7.37)	8.16 -8.17 g (8.17)	7.77 -7.96 g (7.86)
	Total alkalinity h	21-27 e	23-29 e	2.27-2.30 g	~2.36 -2.42 g
	(mM)	(24.5)	(24.5)	(2.28)	(2.40)

Notes : a see Appendix for the explanation of how the total blood volume (5 litres) is subdivided between the lung and tissue reservoirs based on

[START_REF] Guyton | Textbook of Medical Physiology[END_REF]

; b Toggweiler (1999); c based on volumes reported in this table and HCO3 -concentration from Adrogué and Madias (2005a); d 1 GtC = 10 9 tonnes of carbon; e Friedlingstein (2020); e Adrogué and Madias (2005a);

Table 2 -

 2 Metaphors and analogies between the carbon cycle in the ocean and in the human body

	560
	Analogy 1	Metaphor
	Introduction

Table 4 -

 4 Mapping of predicates from the human body (base) domain to the ocean (target) domain in the human body/ocean analogy

	Predicates of the human body domain	Mapping function ("just as")	Predicates of the ocean domain
	Attributes
	The lungs exchange CO2 and O2 with the atmosphere	A(b1)] => [A(t1)	The upper ocean exchanges CO2 and O2 with the atmosphere
	The tissues produce CO2 and consume O2	A(b3)] => [A(t3)	The deep ocean produces CO2 and consumes O2
	The kidneys stabilize pH in the human body	A(b5)] => [A(t5)	Carbonate compensation stabilizes pH in the ocean

Acknowledgements

Two anonymous reviewers provided very useful suggestions that improved the manuscript.

Code availability

The Mathematica code to implement the numerical model is available from the author upon request.

Appendix A1 -Description of the numerical model

A.1.1 Differential equations

The model consists of five differential equations describing the evolution in time of the inventories of total dissolved inorganic carbon (TC = CO2 + HCO3 -+ CO3 2-) and total alkalinity (TA = 2 ´ CO3 2-+ HCO3 -+ B(OH)4 -+ OH --H + ) in two connected reservoirs (A and B), and of the inventory of gaseous CO2 in the atmosphere (Fig. A1). Reservoir A represents the surface ocean or the lungs, and exchanges CO2 with the atmosphere; reservoir B represents the deep ocean or the muscular tissues. In the following, inventories (denoted by m) are in moles, whereas concentrations (denoted by square brackets) are in moles kg -1 . pH and the concentrations of CO2, HCO3 -and CO3 2-are calculated from TC and TA interrogating interpolation functions built by solving DIC equilibria relevant to the human body and the ocean as described below. In differential equations A2 and A3 the terms in blue are specific to the model applied to the ocean; in differential equation A6, the term in red is specific to the model applied to the human body. The existence of these system-specific terms does not result in differences in the overall patterns of carbon flow and pH regulation in the ocean compared to those in the human body (see below).

,-# -𝑃𝑂𝐶 ,-. + 𝑃𝐼𝐶 ,-. + 𝐹 /$ 0→) -𝐹 /$ )→0

(A2)

!" '( ( !# = +𝐹 +$% ) * ,-# -2 • 𝑃𝐼𝐶 ,-. + 𝐹 /) 0→) -𝐹 /)

)→0

(A3)

)→0

(A5)

A.1.2 Carbon reservoirs

The volumes of the surface and deep ocean are taken from [START_REF] Toggweiler | Variation of atmospheric CO2 by ventilation of the ocean's deepest water[END_REF]. For the human body, volumes of the lungs and muscular tissues reservoirs are based on [START_REF] Guyton | Textbook of Medical Physiology[END_REF]. Since only 2 reservoirs are present in our model, I have included in the lung reservoir the blood that is actually present in the lungs (9%), in the arteries (8%), half of that in the heart (3.5%) and half of that present in capillaries (6%); while in the muscular tissue reservoir, we have included the blood that is present in the veins (64%), half of that present in the heart (3.5 %) and half of that present in the capillaries (6%).

This results in the lungs reservoir accounting for 26.5 % of the total blood inventory and the muscular tissue accounting for 73.5 % of the total blood inventory.

A.1.3 CO2 exchange with the atmosphere

In equations A1 and A2, the exchange of CO2 between reservoir A and the atmosphere is calculated as follows:

Where SA (in m 2 ) is the exchange surface between reservoir A and the atmosphere, equal to 70 m 2 for the lungs [START_REF] Bocci | The potential toxicity of ozone: side effects and contraindications of ozone therapy[END_REF] and 3.49 × 10 14 m 2 for the ocean [START_REF] Toggweiler | Variation of atmospheric CO2 by ventilation of the ocean's deepest water[END_REF],

) is a diffusion coefficient, and pCO2A and pCO2atm are the partial pressures (in atmospheres) of CO2 in reservoir A and in the atmosphere, respectively. Atmospheric pCO2 is calculated as follows:

where R (in lit*atm/(K*mol)) is the gas constant, TK (in Kelvins) is the temperature and

Vatm is the volume of the atmosphere (calculated inversing equation A7 and imposing the modern atmospheric CO2 inventory and pCO2). The pCO2 in reservoir A is calculated as:

where KH (mol kg -1 atm -1 ) is Henrys' constant for CO2 calculated after Austin et al.

(1963) at 37°C when modeling the human body and after [START_REF] Weiss | Carbon dioxide in water and seawater: the solubility of a non-ideal gas[END_REF] at 15°C and 35 g kg -1 salinity when modeling the ocean, and den (in kg lit -1 ) is the density of the fluid in reservoir A.

A.1.4 External HCO3 -source

Both the ocean and the human body receive a HCO3 -input flux (𝑃 +$% ) * ,-# , in mol sec -1 ) via continental runoff and dietary uptake, respectively. For the ocean, the bicarbonate weathering flux is set to 23.8 TmolHCO3-yr -1 (see below), which is intermediate between the value of Archer et al. (1998) (12 TmolHCO3-yr -1 ) and that of [START_REF] Berner | Global Environment: Water, Air, and Geochemical Cycles[END_REF] (32 TmolHCO3-yr -1 ). For the human body, I consider a HCO3 -uptake corresponding to drinking 2 liters of mineral water with a dissolved HCO3 -concentration equal to 10 mmol litre -1 .

A.1.5 Mixing between reservoirs

An exchange flux of TC and TA (in mol sec -1 ) takes place between reservoirs A and B associated to the mixing of ocean waters and the flow of blood through the circulatory system:

where MIX (in lit * sec -1 ) is the rate of oceanic mixing (upwelling, downwelling and vertical mixing) or the rate of blood flow (the cardiac output), and [i]X (in mol kg -1 ) is the concentration of species i in reservoir X.

A.1.6 Equilibrium DIC calculations and pH

In reservoirs A and B, equilibrium CO2, HCO3 -and CO3 2-concentrations and pH are calculated by interrogating interpolation functions built by interpolating the results of a high number (~180000) of equilibrium DIC calculations using the Interpolation function object of the software Wolfram Mathematica 12. The equilibrium calculations are carried out imposing temperature, salinity, and TC and TA ranges relevant for the human body and the ocean. In these calculations, the first dissociation constant for carbonic acid (CO2 + H2O <=> HCO3 -+ H + ), is set to the value (K1 = 7.9604 ´ 10 -7 ) used in the human physiology model of [START_REF] Cherif | A mathematical model of the four cardinal acid-base disorders[END_REF] when modeling the human body, while it is calculated as a function of salinity and temperature after [START_REF] Dickson | Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water[END_REF] when modeling the ocean. The second dissociation constant (K2) of carbonic acid (HCO3 -! CO3 2-+ H + ) is calculated as a function of salinity and temperature after [START_REF] Dickson | Handbook of Methods for the Analysis of the Various Parameters of the Carbon Dioxide System in Sea Water[END_REF]. When modeling the DIC system of the ocean, a total dissolved boron concentration (TB = B(OH)3 + B(OH)4 -) of 0.42 ´ 10 -3 is considered, while TB is set to 0 when calculating the DIC system in the human body.

A.1.7 Oceanic export production

In the ocean, carbon is exported from the surface layer in the form of particulate organic carbon (POC), according to the general process:

This process generates a sink of CO2 in the surface ocean layer that is equivalent to the POC export production flux (in molC sec -1 ), calculated as:

where Cexp (in TmolC yr -1 ) is the global ocean pre-anthropogenic export production, TC

) is the Monod constant for export production and Fexp is a constant that converts units of TmolC yr -1 into units of mol sec -1 . An export flux of particulate inorganic carbon (PIC) is associated to the POC export flux; this corresponds to the sinking of carbonate particles produced by planktic organism in the surface layer, according to the general process:

The PIC export flux (in molCaCO3 sec -1 ) is related to the POC export flux through the ratio of exported inorganic to organic particulate matter (PIC:POC):

Note that while the PIC export flux participates in the regulation of oceanic pH and has an analog process in the human body (renal HCO3 -filtration, see below), the POC export flux does not have an analogue in the human body.

A.1.8 Acidity production in reservoir B

In the deep ocean and in the muscular tissues, acidity is produced in the form of CO2, giving rise to the acidity flux 𝑃 $% # ",# (in mol sec -1 ); additionally, acidity is also produced in the form of H + in the muscular tissues, giving rise to acidity flux 𝑃 + , ",# (in mol sec -1 ). In the human body, the source of carbon that gives rise to the production of acidity is external (dietary carbon input); therefore, the acidity fluxes in the muscular tissues of the human body are imposed and taken from [START_REF] Davenport | The ABC of Acid-Base Chemistry[END_REF] and [START_REF] Hainsworth | Acid-Base Balance[END_REF]. In the deep ocean, since the source of carbon that gives rise to acidity production originates in the surface ocean layer, production of acidity (in mol sec -1 ) is calculated as the fraction of the POC export flux that is not accumulated in the sediments:

A.1.9 pH regulation

In both the ocean and the human body, the regulation of pH takes place via regulation of sources and sinks of H + and HCO3 -. In the human body, net excretion of HCO3 -is the result of the difference between the fluxes (in mol sec -1 ) of renal HCO3 -filtration (𝐹 +$% ) * 345# ) and renal HCO3 -reabsorption (𝐹 +$% ) * .+1,2 ):

where DHCO3 (in sec -1 ) is the renal filtration rate and fCO2 (in sec -1 ) is the acid secretion rate. In addition, the kidneys excrete H + with flux 𝐹 + , .+1,2 (in mol sec -1 ) that is proportional to the acidity of the muscular tissue reservoir:

where gH+ (sec -1 ) is a kinetic constant for removal of H + .

In the ocean, the production of HCO3 -and consumption of H + are associated to the process of carbonate compensation (the reverse of process 13). The source of HCO3 -via carbonate compensation is calculated with equation A17, while the H + sink, rather than independently calculated with equation A18, is stoichiometrically tied to HCO3 - production via carbonate compensation. Note that in the human body, the HCO3 -sink corresponding to equation A23 is not present, while HCO3 -is lost from the surface ocean by PIC export according to equation A14.

A.1.10 Differences between the oceanic and human body carbon cycles

In summary, the differences in the equations of the ocean vs human body model are minimal and can be summarized as follows:

(1) Carbon source for acidity production: while in the ocean the source of carbon that gives rise to acidity production in the deep layer originates within the surface layer (POC export production, process A11), in the human body the source of carbon is dietary input, which is external to the system.

(2) HCO3 -fluxes and pH regulation: in the ocean, the HCO3 -sink via PIC export production (process A14) is located in the upper ocean layer (reservoir A), while the HCO3 -source via carbonate compensation (equation A17) is located in the deep ocean layer (reservoir B). Instead, in the human body, both the HCO3 -sink (renal HCO3 -filtration, equation A16) and the HCO3 -source (renal HCO3 -reabsorption, equation A17) are located in the kidneys (considered here as connected to the tissue reservoir B).

A.1.11 Model parametrization

When modeling the carbon cycle in the ocean, reservoir volumes, the surface area of the ocean and mixing rates between the surface and deep ocean layers are taken from the literature as specified in Table 1, atmospheric pCO2 is set to the pre-industrial level of 280 µatm (Zeebe, 2012b), the ocean-atmosphere diffusion coefficient for CO2 is set to 2

´ 10 -3 mol atm -1 m -2 sec -1 following Broeker and Peng (1987), the ratio of inorganic to organic carbon export (PIC:POC) is set to 0.24 following [START_REF] Koeve | Upper ocean carbon fluxes in the Atlantic Ocean: The importance of the POC : PIC ratio[END_REF] and the flux of HCO3 -added by continental runoff is set to 7.55 ´ 10 5 mol sec -1 , which is intermediate between the fluxes reported in [START_REF] Archer | Dynamics of fossil fuel CO2 neutralization by marine CaCO3[END_REF] and [START_REF] Berner | Global Environment: Water, Air, and Geochemical Cycles[END_REF]. The remaining four parameters (𝐶 ,-. , 𝐾 FG$ , 𝐹 DE1 , 𝜑 $% # ) were set by running the model a very high number of times (up to 100 000 times) imposing combinations of values defined at regular intervals within rages encompassing a "first-guess" based on literature values.

The results were filtered to extract the parameter combination that most approached the target pCO2, pH and total alkalinity of the pre-anthropogenic ocean (Table 2). The best fit value for the fraction of carbon export production buried in marine sediments, 𝐹 DE1 (0.018), approaches the value (0.021) reported by [START_REF] Jahnke | The global ocean flux of particulate organic carbon: Areal distribution and magnitude[END_REF] for the deep sea. The best fit values of the maximum organic carbon export production, Cexp (1309 TmolC yr -1 ), and of the Monod constant for export production, KDIC (2 ´ 10 -3 mol kg -1 ), result in organic and inorganic carbon export productions of 671 and 160 TmolC yr -1 , respectively.

The modeled organic carbon export production approaches the value (500 TmolC yr -1 ) reported by [START_REF] Siegel | Global assessment of ocean carbon export by combining satellite observations and food-web models[END_REF] for the modern ocean; the modeled inorganic carbon export production approaches the upper limit of the range (33-150 TmolC yr -1 ) reported in [START_REF] Berelson | Relating estimates of CaCO3 production, export, and dissolution in the water column to measurements of CaCO3 rain into sediment traps and dissolution on the sea floor: A revised global carbonate budget[END_REF].

When modeling the carbon cycle in the human body, reservoir volumes, the surface area of the lungs and the cardiac output are taken from the literature as specified in Table A2, atmospheric pCO2 is set to the modern value of 410 µatm and the dietary flux of HCO3 - was set to 2.3 ´ 10 -7 mol sec -1 , which corresponds to a daily uptake of two liters of mineral water with a HCO3 -concentration of 10 mM. The remaining physiological parameters (𝐷 $% # , 𝑃 $% # ",# , 𝑃 + , ",# , 𝜑 $% # , 𝐷 +$% ) * , 𝛾 + , ) were set running the model multiple times (up to 100 000 model runs) imposing combinations of parameter values within ranges that are based on those presented in [START_REF] Cherif | A mathematical model of the four cardinal acid-base disorders[END_REF] (their table 1). The results were filtered to extract the parameter combination that most approached the target pCO2, pH and HCO3 -of the human body (Table 2). In this procedure, parameter values from [START_REF] Cherif | A mathematical model of the four cardinal acid-base disorders[END_REF] are converted, when applicable, from intensive quantities (e.g. mol lit - 1 sec-1), to extensive quantities (e.g. mol sec -1 ).

A.1.12 Simulating human and oceanic carbon cycle perturbations.

The reduced capacity of the lungs to exchange CO2 with the atmosphere is simulated by reducing the diffusion coefficient of gaseous exchange between lung and atmosphere (DCO2). When modeling anthropogenic CO2 emissions, an additional source of CO2 to the atmosphere (𝐹 $% # &'#( ) is considered. The magnitude and evolution in time of anthropogenic CO2 emissions correspond to scenarios SSP1-2.6, SSP1-4.5, SSP1-7.0 and SSP1-8.5 of [START_REF] Liddicoat | Compatible Fossil Fuel CO2 Emissions in the CMIP6 Earth System Models' Historical and Shared Socioeconomic Pathway Experiments of the Twenty-First Century[END_REF] that are the standard scenarios considered by the International Panel for Climate Change.

A.1.13 Code availability

The code used to generate the analysis presented in the paper and its Extended Data is available upon request to the author (aloisi@ipgp.fr). Table A1 - -metabolic production of H + in the muscular tissues (human body only); 𝐹 ;6: " # ECF< -renal HCO3 -filtration (only human body); 𝐹 ;6: " # 3;91G and 𝐹 ; $ 3;91G -production of HCO3 -and consumption of H + by pH-regulating processes (kidneys in the human body, carbonate compensation in the ocean).

Appendix A2 -Tables

correspond to the ranges of oceanic composition and atmospheric pCO2 calculated by the LOSCAR model when forced with the injection in the atmosphere of 1000 PgC equivalent CO2 in a time-frame of 500 years.

Appendix A4 -Enalysis of the extended anology "The ocean is like the human circulatory system" using the structure-mapping theory of Gentner (1983)

The structure-mapping theory of [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF] defines an analogy as an assertion that a relational structure normally applying in one domain can be applied in another domain [START_REF] Gentner | Structure-Mapping: A Theoretical Framework for Analogy[END_REF][START_REF] Gentner | Structure Mapping in Analogy and Similarity[END_REF]. In this appendix the structure-mapping theory is used to analyze the extended analogy "The ocean is like the human circulatory system". We start by representing the base and target domains in terms of object nodes (i.e. components of a given domain…), and define how the analogy maps object nodes of the base domain (b1, b2,…, bn) onto those of the target domain (t1, t2,…,tn) (table 3). An asthma attack results in a modified CO2 exchange between the lungs and atmosphere and to an excess of acid in the blood R(b1,b6,b8)] => [R(t1,t6,t8) Anthropogenic CO2 emissions result in a modified CO2 exchange between the ocean and the atmosphere and to ocean acidification

Second order relations (example)

The fact that CO2 is produced and O2 is consumed in the tissues causes the tissues to be CO2-rich and O2-poor compared to the lungs R'(A(b3), R(b1,b3)] =>[R'(A(t3), R(t1,t3)

The fact that CO2 is produced and O2 is consumed in the deep ocean causes the deep ocean to be CO2rich and O2-poor compared to the surface ocean