Stéphanie Jenouvrier
email: sjenouvrier@whoi.edu

Marika Holland

David Iles

Sara Labrousse

Laura Landrum

Jimmy Garnier

Hal Caswell

Henri Weimerskirch

Michelle Larue

| Rubao

Christophe Barbraud

The Paris Agreement objectives will likely halt future declines of emperor penguins

Keywords: Antarctica, climate change mitigation, dispersion, emission reduction pledges, seabirds

The Paris Agreement is a multinational initiative to combat climate change by keeping a global temperature increase in this century to 2°C above preindustrial levels while pursuing efforts to limit the increase to 1.5°C. Until recently, ensembles of coupled climate simulations producing temporal dynamics of climate en route to stable global mean temperature at 1.5 and 2°C above preindustrial levels were not available. Hence, the few studies that have assessed the ecological impact of the Paris Agreement used ad-hoc approaches. The development of new specific mitigation climate simulations now provides an unprecedented opportunity to inform ecological impact assessments. Here we project the dynamics of all known emperor penguin (Aptenodytes forsteri) colonies under new climate change scenarios meeting the Paris Agreement objectives using a climate-dependent-metapopulation model. Our model includes various dispersal behaviors so that penguins could modulate climate effects through movement and habitat selection. Under business-as-usual greenhouse gas emissions, we show that 80% of the colonies are projected to be quasiextinct by 2100, thus the total abundance of emperor penguins is projected to decline by at least 81% relative to its initial size, regardless of dispersal abilities. In contrast, if the Paris Agreement objectives are met, viable emperor penguin refuges will exist in Antarctica, and only 19% and 31% colonies are projected to be quasiextinct by 2100 under the Paris 1.5 and 2 climate scenarios respectively. As a result, the global population is projected to decline by at least by 31% under Paris 1.5 and 44% under Paris 2. However, population growth rates stabilize in 2060 such that the global population will be only declining at 0.07% under Paris 1.5 and 0.34% under Paris 2, thereby halting the global population decline. Hence, global climate policy has a larger capacity to safeguard the future of emperor penguins than their intrinsic dispersal abilities.

| INTRODUC TI ON

The Paris Agreement is a multinational initiative to curb future atmospheric warming, with an explicit goal of limiting the global temperature increase to 'well below 2°C', and ideally to 1.5°C relative to the preindustrial mean (UNFCCC, 2015). Pathways to reach the Paris Agreement objectives require globally unified climate policy in the short-term, and transformation of global energy supply, including large-scale shifts away from fossil fuel use and increasing investment in renewable energies, combined with carbon dioxide removal through afforestation (Rogelj et al., 2018). The Paris Agreement therefore represents global recognition of the importance of pursuing climate change solutions in the near-term.

Global-scale vulnerability assessments based on species traits and expert opinion indicate that over one-quarter of birds, amphibians, and corals are highly vulnerable to 2°C degrees of warming [START_REF] Foden | Identifying the world's most climate change vulnerable species: A systematic trait-based assessment of all birds, amphibians and corals[END_REF]. Recently, [START_REF] Warren | The projected effect on insects, vertebrates, and plants of limiting global warming to 1.5°C rather than 2°C[END_REF] reported widespread projected range loss in insects, plants, and vertebrates under current emissions pledges, but meeting the Paris Agreement 1.5°C objective would dramatically curb these range contractions. However, projected range contractions do not necessarily correspond directly to changes in population abundance [START_REF] Ehrlén | Predicting changes in the distribution and abundance of species under environmental change[END_REF]Schurr et al., 2012). Instead, mechanistic models that link climate to demographic processes are required to account for the complex population responses that can arise under scenarios of future change [START_REF] Jenouvrier | Impacts of climate change on avian populations[END_REF]. Unfortunately, climate-coupled demographic projections are rare because they require detailed, long-term, longitudinal data. Indeed, only 18 studies have used this approach to project the future abundance of bird populations under specific climate change scenarios (reviewed in [START_REF] Iles | Effects of climate change on birds, chap. 12: Projected population consequences of climate change[END_REF]. No studies have yet explicitly examined the consequences of meeting the Paris Agreement objectives on population dynamics for any species.

Climate change occurs differently among regions (Stocker et al., 2013), resulting in a shifting mosaic of habitat quality across a species' range (McRae et al., 2008;[START_REF] Travis | Dispersal and species responses to climate change[END_REF]. On ecological timescales, population viability of sensitive species will therefore depend on the capacity for individuals to disperse to suitable habitats. Yet, dispersal generates complex population-level responses to environmental change, including 'ecological traps' or 'attractive sinks' [START_REF] Kristan | The role of habitat selection behavior in population dynamics: Source-sink systems and ecological traps[END_REF], rescue effects, emergent metapopulation synchrony [START_REF] Lande | Spatial scale of population synchrony: Environmental correlation versus dispersal and density regulation[END_REF], and habitat-structured transient dynamics [START_REF] Iles | Source-sink dynamics of bumblebees in rapidly changing landscapes[END_REF]. These effects can either dampen or amplify species' sensitivity to climate change [START_REF] Bowler | Causes and consequences of animal dispersal strategies: Relating individual behaviour to spatial dynamics[END_REF][START_REF] Travis | Dispersal and species responses to climate change[END_REF]. Predicting population responses to climate change therefore requires a full consideration of species' dispersal capabilities [START_REF] Travis | Modelling dispersal: An eco-evolutionary framework incorporating emigration, movement, settlement behaviour and the multiple costs involved[END_REF]. However, most quantitative projections of species abundance under future climate ignore this important mechanism [START_REF] Travis | Dispersal and species responses to climate change[END_REF] and the relative extent to which dispersal and global climate policy can reduce the projected species extinction of climate change is an open question [START_REF] Warren | The implications of the United Nations Paris Agreement on climate change for globally significant biodiversity areas[END_REF].

Finally, accurately assessing future climate impacts on ecosystems requires the use of fully coupled climate models that track the nonlinear temporal evolution of the ocean-atmosphere system [START_REF] Jenouvrier | Impacts of climate change on avian populations[END_REF]. Until recently, ensembles of internally consistent coupled climate simulations that produce stable equilibrium global mean temperature at 1.5 and 2°C above preindustrial levels were not available. The development of these specific mitigation models (Sanderson et al., 2017) now provides an unprecedented opportunity to examine the temporal climate dynamics that would result from meeting the Paris Agreement objectives, and their concomitant effects on natural populations. Correctly representing the temporal evolution of climate under the Paris Agreement is particularly important for understanding wildlife responses because short-term rates of population change and historical 'legacy' effects can strongly influence population viability [START_REF] Lande | Risks of population extinction from demographic and environmental stochasticity and random catastrophes[END_REF]. To date, no studies have used these mitigation climate models to inform ecological impact assessments.

Emperor penguins (Aptenodytes forsteri) are iconic examples of a species threatened by future climate change [START_REF] Barbraud | Emperor penguins and climate change[END_REF][START_REF] Forcada | Penguin responses to climate change in the southern ocean[END_REF]Jenouvrier, Caswell, et al., 2009;[START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF][START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF]Ropert-Coudert et al., 2019). The emperor penguin is classified as 'near threatened' by the International Union for the Conservation of Nature and is currently under consideration for inclusion under the United States Endangered Species Act.

Beyond these focused conservation measures, can near-term global action on climate policy curb the projected declines of emperor penguin populations?

Projections indicate that most breeding colonies will be endangered by 2100 under 'business as usual' emissions scenarios [START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF], resulting in dramatic declines in the global population size even under optimistic dispersal scenarios [START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF]. These declines occur through projected loss of Antarctic sea ice, to which the emperor penguin life cycle is closely tied. Emperor penguins directly rely on seasonally stable sea ice as a breeding platform during the Antarctic winter and throughout the spring chick-rearing period [START_REF] Ainley | Impacts of cetaceans on the structure of southern ocean food webs[END_REF].

During the nonbreeding season, sea ice serves as a platform from which they feed, molt, and seek refuge from predators. Sea ice also influences critical components of the food chain (e.g., krill:

Euphausia superba and silverfish: Pleuragramma Antarctica; La [START_REF] Mesa | Influence of environmental conditions on spatial distribution and abundance of early life stages of Antarctic silverfish, Pleuragramma antarcticum (Nototheniidae), in the Ross Sea[END_REF]Meyer et al., 2017). Therefore, sea ice conditions affect the survival and reproduction of emperor penguins both directly (e.g., early sea ice breakup can jeopardize chick survival) and indirectly through the food web (Abadi, [START_REF] Fe R E N C E S Abadi | Integrated population modeling reveals the impact of climate on the survival of juvenile emperor penguins[END_REF]Jenouvrier, Barbraud, Weimerskirch, & Caswell, 2009;[START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]. Specifically, the breeding success is reduced in years with higher sea ice cover because foraging trips are longer, energetic costs for adults are higher, and offspring provisioning is lower [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]Massom et al., 2009;[START_REF] Zimmer | Foraging movements of emperor penguins at Pointe Géologie, Antarctica[END_REF]. In addition, adult survival is maximized at intermediate levels of sea ice because neither the complete absence of sea ice (low food resources and/or high predation), nor heavy and persistent sea ice (longer foraging trip), provide satisfactory conditions [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF].

Here we provide a global assessment of the potential impacts of limiting global warming to 1.5 or 2°C on emperor penguins using a set of new climate scenarios, and compare the importance of biological mechanisms for a species to cope with climate change (i.e., dispersal) with the ability of meeting the Paris Agreements. We utilize a unique ensemble of transient climate simulations which are subject to emissions scenarios that have been specifically designed to meet the Paris Agreement targets (Sanderson et al., 2017) and provide the first analysis of Antarctic sea ice conditions in these simulations.

Our study integrates these new climate projections with a mechanistic metapopulation model previously developed by [START_REF] Jenouvrier | Mating behavior, population growth, and the operational sex ratio: A periodic two-sex model approach[END_REF] and [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF][START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF][START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF], providing fundamental insight into the capacity for near-term global action on climate policy to alter the future of an iconic marine predator.

| MATERIAL S AND ME THODS

Our analysis focuses on estimating the demographic responses of emperor penguin populations to three climate scenarios, each representing a distinct future that is contingent on human policy decisions in the near-term. There are 54 known emperor penguin colonies around the coast of Antarctica [START_REF] Fretwell | An emperor penguin population estimate: The first global, synoptic survey of a species from space[END_REF][START_REF] Fretwell | Penguins from space: Faecal stains reveal the location of emperor penguin colonies[END_REF]LaRue, Kooyman, Lynch, & Fretwell, 2015; Figure S1).

Emperor penguins breed in large colonies (>100 individuals) on sea ice, forming a set of discrete, yet potentially connected local populations over the entire species range along the Antarctic coast.

Hence, we link the new simulations of future sea ice conditions to a metapopulation model developed by [START_REF] Jenouvrier | Mating behavior, population growth, and the operational sex ratio: A periodic two-sex model approach[END_REF][START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF][START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF][START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF]. The metapopulation model describes the demography and dispersal behavior of emperor penguins across their Antarctic range. It includes vital rates (survival and reproduction) that depend on the mean sea ice conditions during four seasons of the emperor penguin life cycle (nonbreeding, laying, incubating, and rearing), and accounts for differences in the impact of sea ice conditions on adult survival between males and females. In the sections below, we more fully describe climate scenario and sea ice projections, as well as dispersal scenario and the sea ice-dependent metapopulation model.

In addition, in order to assess the consequence of potential biases in sea ice simulations, we performed a sensitivity analysis of the sea ice-dependent metapopulation model to quantify which of the sea ice seasonal means affect the most the population growth rate projected at each colony throughout this century. Finally, we analyze the uncertainties of the sea ice-dependent metapopulation model.

| Climate scenarios

Our new analysis incorporates two unique mitigation scenarios developed by the National Center for Atmospheric Research using a General Circulation Model (GCM) to explicitly evaluate future climate trajectories under the Paris Agreement temperature targets (described in Sanderson et al., 2017). These mitigation scenarios are unique in that they were explicitly designed to meet the global temperature change targets set in the Paris Agreement (Sanderson & Knutti, 2016). This is in contrast to other emissions scenarios, for example, the SRES (Special Report on Emissions Scenarios; Nakicenovic et al., 2000) or the representative concentration pathways (RCP; [START_REF] Van Vuuren | The representative concentration pathways: An overview[END_REF] scenarios used in the fourth and fifth IPCC assessment reports, respectively. While these older scenarios consider a range of possible human activity and resulting greenhouse gas emissions, they had no explicit consideration of the Paris Agreement targets and hence do not lead to a 1.5 or 2°C global average warming by 2100. Previous ecological studies have instead used ad hoc approaches to obtain climate projections meeting the Paris Agreement temperature targets using RCP scenario simulations (e.g., [START_REF] Warren | The projected effect on insects, vertebrates, and plants of limiting global warming to 1.5°C rather than 2°C[END_REF]. Furthermore, we compare the resulting emperor penguin population projections obtained with these two new Paris scenario to those obtained from a 'business-as-usual' climate scenario (RCP 8.5;Meinshausen et al., 2011) that represents a future in which greenhouse gas emissions continue unabated. By using climate simulations with these new Paris Agreement scenarios, we are able to directly consider the avoided impacts for emperor penguins in 1.5 or 2°C climate futures relative to a 'business-as-usual' scenario.

The RCP 8.5 scenario uses the same Community Earth System Model (CESM) model and includes a large ensemble of simulations [START_REF] Kay | The community earth system model (CESM) large ensemble project: A community resource for studying climate change in the presence of internal climate variability[END_REF], allowing us to assess the importance of internal variability. This model was contributed to the Coupled Model Intercomparison Project 5 (CMIP5) and included in the IPCC fifth assessment report. Previous studies have projected emperor penguin populations under 'business-as-usual' climate scenarios (SRES A1.B) using the climate model ensemble from the IPCC's fourth assessment report, that is, CMIP3 multimodel dataset [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF][START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF]. The CMIP3 includes older generation models than that available in CMIP5 and a direct comparison between the climate projections from CMIP3 and CMIP5 is not possible because they use different scenarios describing the amount of greenhouse gas in the atmosphere in the future (SRESs: Nakicenovic et al., 2000vs. RCPs: Van Vuuren et al., 2011, respectively). However, for both large-scale climate patterns and the magnitudes of climate change, there is overall consistency between the projections based on CMIP3 and CMIP5 (Stocker et al., 2013).

| Sea ice projections

The climate model simulations used in this study are produced from the CESM using the Community Atmosphere Model, version 5 (CESM1-CAM5; [START_REF] Hurrell | The community earth system model: A framework for collaborative research[END_REF]. This is a fully coupled earth system model which incorporates atmosphere, ocean, sea ice and terrestrial components. It has a nominal 1° resolution in both the atmosphere and the ocean. This model has a very good overall simulation of climate as compared to other models (e.g., [START_REF] Knutti | Robustness and uncertainties in the new CMIP5 climate model projections[END_REF] and numerous aspects of the Antarctic climate, such as wind variability and the sea ice response to that variability (e.g., [START_REF] Landrum | Stratospheric ozone depletion: An unlikely driver of the regional trends in Antarctic sea ice in austral fall in the late twentieth century[END_REF], are well simulated. Our use of a large ensemble of simulations allows us to quantify the uncertainties related to internal climate variability. Specifically, we use simulations from the CESM Large Ensemble project (CESM-LENS; [START_REF] Kay | The community earth system model (CESM) large ensemble project: A community resource for studying climate change in the presence of internal climate variability[END_REF] peror penguin, at each colony, following the approach described by [START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF]. Here we present for the first time, the projection of sea ice in Antarctica from these unique climate ensembles.

| Dispersal scenarios

Individual dispersal behaviors for emperor penguins are poorly understood because emperor penguin have been marked at only one site (Pointe Géologie; [START_REF] Wienecke | Review of historical population information of emperor penguins[END_REF], and no recapture occurred at other colonies. Until recently, emperor penguins were considered to be highly philopatric (Prevost, 1961). Recent studies have now shown some degree of genetic homogenization for emperor penguin colonies, suggesting high connectivity in these populations via individual dispersal among colonies [START_REF] Clucas | Comparative population genomics reveals key barriers to dispersal in southern ocean penguins[END_REF][START_REF] Younger | The challenges of detecting subtle population structure and its importance for the conservation of emperor penguins[END_REF]. In addition, recent work suggests that emperor penguin colonies can move onto ice shelves and perhaps found new colonies [START_REF] Fretwell | An emperor penguin population estimate: The first global, synoptic survey of a species from space[END_REF][START_REF] Fretwell | Emperor penguins breeding on iceshelves[END_REF].

Hence, we explore various scenarios combining different dispersal rates, dispersal behaviors, and dispersal distances (see [START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF] for more details). Our sea ice-dependent metapopulation model assumes that individuals only emigrate from poor quality breeding sites when environmental conditions lead to negative fitness. With an informed search, individuals select habitats that maximize fitness within their dispersal range; this behavior occurs among some colonial seabirds that prospect for breeding sites using the presence and reproductive success of residents [START_REF] Doligez | Public information and breeding habitat selection in a wild bird population[END_REF]. In contrast, random search behavior results in undirected movements with respect to habitat quality. The shortdistance dispersal scenario allows for regional movements among colonies, while long-distance dispersal creates a more connected metapopulation across the entire continent.

| Sea ice-dependent metapopulation model

The sea ice-dependent metapopulation model was developed by [START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF] using MATLAB R2018, The MathWorks, Inc., Natick, Massachusetts, United States. The model projects the population vector n-comprising the population size n i in each colony i-from time t to t + 1 using: to indicate that the projection interval is divided into two main phases of possibly different duration: the reproduction phase (F) followed by the dispersal phase (D). 1 The reproduction matrix F is constructed using the density-dependent Ricker model. The dispersal phase (D) combines various dispersal behaviors and dispersal events. The projection matrices D and F depend on both the current population density n(t) and the habitat characteristics (including sea ice concentrations anomalies), x(t), that vary among colonies and over time, t. The global population size at time t is

given by N t = ∑ i n i (t).

| Reproduction phase

The reproduction matrix, F, is constructed using the Ricker model including the intrinsic growth rate of each colony r i (t) and the carrying capacity of each colony K i . Negative density-dependence effects occur within crowded favorable habitats (r i > 0 and

n i > K i )
while populations tend to go extinct within poor habitat colonies (r i ≤ 0).

The intrinsic growth rate

For each projection interval t, the intrinsic growth rate of each colony r i (t) is projected by a nonlinear, stochastic, sea ice-dependent, two-sex, stage-classified matrix A[x(t), n(t)]. It is described in more detail in [START_REF] Jenouvrier | Mating behavior, population growth, and the operational sex ratio: A periodic two-sex model approach[END_REF][START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]. A[x(t), n(t)] includes a sequence of seasonal behaviors (arrival to the colony, mating, breeding) and accounts for differences in adult survival between males and females as function of sea ice concentration anomalies x(t).

A[x(t), n(t)] depends on n(t) because the reproduction is function of

the proportion of males and females within the population through mating processes [START_REF] Jenouvrier | Mating behavior, population growth, and the operational sex ratio: A periodic two-sex model approach[END_REF].

The matrix A[x(t), n(t)] includes five stages: male and female prebreeders (birds that have yet to breed for the first time), breeding pairs, and male and female nonbreeders (birds that have bred before but do not do so in the current year). The vital rates describing the transitions between these stages from year t to t + 1 includes the probability that an individual of a given stage returns to the breeding site, the probability of mating as a function of the availability of potential mates, the probability of breeding success (raising an offspring given that the female lays an egg), the primary sex ratio (fixed at 0.5), the survival of offspring during the first year at sea, and the annual survival of prebreeders, nonbreeders and male and female breeders.

These vital rates are functions of sea ice concentration anomalies relative to the average from 1979 to 2007 during four seasons: (a) the nonbreeding season from January to March; (b) ( 1)

n(t + 1) = D[x(t), n(t)]F[x(t), n(t)]n(t), 1
In this paper, matrices are denoted by upper case bold symbols (e.g., F) and vectors by lower case bold symbols (n); f ij is the (i, j) entry of the matrix F, n i is the ith entry of the vector n.

the arrival, copulation, and laying period (April-May), hereafter called the laying period; (c) the incubation period (June-July);

(d) the rearing period (August-December). These relationships and their estimations are described in detail in [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]. [START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF] estimated the carrying capacity of each colony as K i = 2N 0 , with N 0 the initial size of the population observed in 2009 [START_REF] Fretwell | Penguins from space: Faecal stains reveal the location of emperor penguin colonies[END_REF][START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF].

| The dispersal phase

The model includes intercolony movements. A dispersal event in-

The first dispersal event

The emigration rate for each colony i depends on the overall quality of the habitat, which is measured by the median of the realized population growth r * i . The emigration rate increases linearly from m 1 = 0 at r ≥ 0 to m 1 = 1 at critical value r * c < 0. The emigration matrix thus only depends on the ratio r * (t)∕r * c ,

A critical threshold r * c close to 0, corresponds to high dispersion scenario while a larger negative threshold reflects low dispersion.

Once individuals have left their colonies, we assume that they search for a new colony using two different behaviors: an informed searching behavior (S I ) and a random searching behavior (S R ).

The random search assumes that dispersers randomly seek a colony within the limits of the maximum dispersal distance. Thus the probability of selecting a colony depends on the mean dispersal distance of the emperor penguin, d, and the matrix of distance between colonies (dist(i, j)) included in the vector of habitat descriptors x.

The matrix dist(i, j) corresponds to the coastal distance between colonies i and j derived from the location of know emperor penguin colonies.

Conversely, the informed search assumes that dispersers search for the most favorable habitat they can reach; we use r * as a descriptor of the quality of the habitat. Thus the informed search matrix is also a function of r * :

If the selected colony is not at carrying capacity, individuals settle in this new habitat. However, individuals are not able to settle in colonies that have reached their carrying capacities after the first dispersal event, and will conduct a novel search during the second dispersal event.

The second dispersal event

The surplus individuals leave and randomly settle in another colony regardless of their dispersal strategy in their first event. Thus the emigration matrices depend on the carrying capacity K, the population vector n at the end of the first dispersal event, and a random search matrix:

| Sensitivity analysis

We conduct a sensitivity analysis to quantify in which seasons, sea ice conditions (SIC) affect the most the intrinsic population growth rate projected at each colony. Using simulations, we calculate the sensitivity of the population growth rate to a perturbation in SIC over a specific season for each colony, at each time step t from 2009 to 2100, for each demographic projection (including uncertainties in demographic processes). We used a local sensitivity approach by adding a small perturbation (p = 1e -5) to the large ensemble mean of RCP 8.5 at each specific year, ran the demographic model without dispersion with the perturbed SIC, extracted the population growth rate at each colony, and compared it to the population growth rate without perturbation. We summarized results by showing the absolute value of these sensitivities across demographic simulations and time, as our main question is how robust our demographic model is to potential biases in seasonal SIC projections. We used the large ensemble mean of RCP 8.5 because it shows a larger range of sea ice changes throughout the century, but results are consistent across climate scenarios. More details are shown in Figure S2.

(2) (4)

D: = D 2 D 1 , (3)
M 1 : = M 1 r * (t) r * c . (

| Uncertainties

The model includes multiple sources of stochasticity and uncertainties related to climate and demography [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF].

Climate uncertainty reflects the chaotic temporal evolution of the coupled ocean-atmosphere system (often called 'internal variabil- In step 3, for each year of each projection, we randomly draw demographic parameters from distributions describing residual process variance. This incorporates additional unexplained temporal variance in demographic rates that is driven by environmental factors other than sea ice (e.g., variation in predator and prey populations). The black line is the mean of Antarctic sea ice extent from the large ensemble of CESM, while the red line is the mean of the observations. Sea ice extent is defined as the total area covered by ice of greater than 15% concentration

| RE SULTS

| Comparison of Antarctic sea ice projections to observations

Our sensitivity analysis reveals that population growth rates are largely influenced by the Antarctic sea ice conditions during the laying season, that are very well resolved in CESM model (Figures 1 and2). While there are biases in the climatology in the Eastern Weddell during the nonbreeding period, the sensitivity of the population growth rate to sea ice conditions during the nonbreeding is small (Figure 2). While the sensitivity of the growth rate to sea ice conditions during the rearing season is overall relatively large, it varies considerably among colony and time (Figure 2b;

Figure S2). These sensitivities are very small for colonies 30-35, locations where the largest differences between the climatology simulated by CESM and observations are observed in the west Pacific Ocean.

| Antarctic sea ice projections

The largest decline in sea ice conditions relative to historical levels are projected during the nonbreeding and laying seasons of the emperor penguin life cycle, regardless of the climate scenario (Figures 345).

Large sea ice declines are projected under the 'business-as-usual' climate scenario RCP 8.5, and colonies are likely to experience complete loss of sea ice during the nonbreeding, incubation, F I G U R E 4 Antarctic sea ice projections using Paris Agreement's 2°C goal. legends as Figure 3 F I G U R E 5 Antarctic sea ice projections using Paris Agreement's 1.5°C goal. Same legends as Figure 3 Under Paris Agreement's 1.5°C goal, most colonies experience sea ice declines smaller than 10% to their historical mean during the breeding seasons (except colonies in Dronning, Enderby, and Kemp lands).

F I G U R E 2

| Emperor penguin population projections

Under the 'baseline' RCP 8.5 scenario in which greenhouse emissions remain unmitigated throughout the 21st century, by 2100 all emperor penguin colonies are projected to decline, and 43 of the 54 (80%)

colonies are projected to decline by more than 90% and thus be quasiextinct (Figures 6a and7a; Figures S3-S6). In this scenario, annual mean Antarctic sea ice extent declines by 48%, and the most endangered colonies in Queen Maud, Enderby, and Kemp Land will likely experience complete loss of sea ice during the critical laying season (Figures 2 and3). The colonies that are projected to be quasiextinct by 2100 all experience sea ice decline larger than 50% relative to historical mean during the laying season. Globally, the total abundance of emperor penguins is projected to decline by 86% (median of projections; Figure 8) relative to its initial size if individuals do not disperse among colonies. Simultaneously, global population growth rate is projected to decrease dramatically (Figure 5; S1). Dot colors show the conservation status. Following [START_REF] Jenouvrier | Projected continent-wide declines of the emperor penguin under climate change[END_REF], 'vulnerable' (green) is a likely population decline by more than 30%; 'endangered' (yellow) is a likely population decline by more than 50%; 'quasi-extinct' (red) is a likely population decline by more than 90%. Blue color refers to populations that are not likely to decline by more than 30%. A likely outcome is defined by IPCC as a probability >66% (Tables S2-S4). AS, Amundsen Sea; BS, Bellingshausen Sea; IO, Indian Ocean; RS, Ross Sea; WPO, Western Pacific Ocean; WS, Weddell Sea In contrast, under the Paris Agreement 1.5 and 2°C climate scenarios, only 10 (19%) and 17 (31%) colonies are projected to be quasiextinct by 2100, respectively (Figure 6). Under Paris 2, the annual mean sea ice extent loss by 2100 is 13%, whereas under Paris 1.5 the loss is only 5%. The colonies that are not endangered (blue colonies on Figure 6) are more likely to experience sea ice decline smaller than 10% relative to the historical mean during the four seasons of their life cycle (Figures 4 and5). The most threatened colonies are located in eastern Antarctica where projected declines in sea ice extent are largest (Figure 6b). Colonies in the Ross Sea experience less sea ice loss and thus are projected to increase from their current size by 2100 (Figures S2-S4).

Yet, in total, 56% and 65% of emperor penguin colonies (under Paris 1.5 and 2, respectively) are likely to experience declines of 50% by 2100. As a result, the median of the global population is projected to decline by 44% under Paris 2, and by 31% under Paris 1.5 without dispersal. Despite these declines in abundance that occur in the first half of the century, population growth rates stabilize by 2100 such that the global population will be only declining at 0.07% (a half-life of 952 years) under

| Uncertainties

Our projections account for uncertainty in estimates of demographic parameters and their relationship to climate, as well as temporal variance in population growth that is not related to sea ice conditions. In addition, under each climate scenario, we conduct a set of population projections using dispersal models that include no dispersal and various combinations of dispersal behaviors, including (a) high or low movement between breeding colonies; (b) two types of search strategies for new colonies (random or informed); and (c) either short-(up to 1,000 km) or long-distance movements (up to 6,000 km). In aggregate, these simulations place bounds on the degree to which emperor penguins could modulate climate effects through movement and habitat selection.

F I G U R E 8 Global number of breeding pairs of emperor penguins from 2009 to 2100 projected for various climate scenarios without dispersal. This global population size is calculated using sea ice concentration anomaly projections from the Community Climate System Model using RCP 8.5 (red), Paris Agreement's 2°C goal (blue), and Paris Agreement's 1.5°C (green). The thick lines are the median and the colored areas are the 90% envelopes from stochastic simulations of population trajectories

F I G U R E 9
Global number of breeding pairs of emperor penguins from 2009 to 2100 projected for various dispersal and climate scenarios. These median global population sizes are calculated using sea ice concentration anomaly projections from the Community Earth System Model using (a) RCP 8.5, (b) Paris Agreement's 2°C goal, and (c) Paris Agreement's 1.5°C. Thick light gray line is the population trajectory without dispersal. Colored solid lines are population trajectories under high emigration rates, while dashed lines show low emigration rates. Blue lines are trajectories under informed search, while red lines show the random search. The distance of dispersal is large (6,000 km). Figure S7 shows the confidence envelope associated with these projections processes and mating processes at each colony in response to sea ice (Figure 11). Structural uncertainties due to dispersal processes are relatively small (Figure 7). [START_REF] Iles | Effects of climate change on birds, chap. 12: Projected population consequences of climate change[END_REF]. Forecasts that fail to include these key sources of uncertainty will be falsely overconfident, eroding trust in ecological science and hindering ecological understanding [START_REF] Clark | Ecological forecasts: An emerging imperative[END_REF][START_REF] Dietze | Ecological forecasting[END_REF]. Forecasting therefore requires a quantification of uncertainty in each model component, and importantly, full propagation of these uncertainties to forecasts.

Forecast uncertainty is dominated by different processes across various spatial and temporal horizons [START_REF] Dietze | Ecological forecasting[END_REF][START_REF] Hawkins | The potential to narrow uncertainty in regional climate predictions[END_REF]). Here we found that the uncertainties related to climate internal variability and unexplained temporal variance in vital rates (process variance) are relatively large (Figure 10). If these uncertainties are ignored, the global number of breeding pairs is projected to increase from 2009 to ~2033, while no such increase is projected when all uncertainties were accounted for (Figure 10). In addition, if these uncertainties are ignored, the global population size by 2100 is twice as large than that when all uncertainties are accounted for (medians: 71,080 vs. 35,150). Indeed, an increase in sea ice variability is likely to reduce the stochastic population growth rate of emperor penguin, at least when the average sea ice conditions are not far from historical sea ice mean (see figure 6a in [START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]. In addition, an increase in unexplained temporal variance in vital rate is likely to reduce the stochastic population growth rate [START_REF] Koons | Is lifehistory buffering or lability adaptive in stochastic environments?[END_REF][START_REF] Tuljapurkar | Population dynamics in variable environments I. Long-run growth rates and extinction[END_REF]. Hence, including both climate internal variability and process variance reduce the projected number of breeding pairs, and affect strongly the projected trajectory throughout the century (see also figure 4c In addition, we account for the uncertainty due to internal variability using a large ensemble of simulations with the CESM1 [START_REF] Hurrell | The community earth system model: A framework for collaborative research[END_REF][START_REF] Kay | The community earth system model (CESM) large ensemble project: A community resource for studying climate change in the presence of internal climate variability[END_REF]. 1), our use of the CESM1 simulations to characterize evolving sea ice conditions under the different scenarios provides a reasonable approach to diagnose the role of meeting the Paris Agreement targets for emperor penguins. In addition, the largest differences between CESM1 sea ice simulations and observations over the historical period, occur during seasons or regions for which sea ice conditions have little effect on the emperor penguin population growth rates (Figure 2), hence our results are robust to these climate biases. Finally, while they are large uncertainties in projected sea ice loss in Antarctica [START_REF] Collins | Long-term climate change: Projections, commitments and irreversibility[END_REF], we have a high confidence on the avoided impacts for emperor penguins in 1.5 or 2°C climate futures relative to a 'business-as-usual' scenario.

| Emperor penguin responses to climate change

Adaptive capacity and dispersal ability are two key attributes that affect the resilience of species to climate change [START_REF] Williams | Towards an integrated framework for assessing the vulnerability of species to climate change[END_REF]. In our study, incorporating dispersal led to steeper population declines by 2100 than models in which individuals were completely site-faithful under RCP 8.5 (Figure 7). This counterintuitive result occurs because dispersal allows poor quality habitats that would otherwise be sequestered to function as connected demographic sinks that rapidly deplete the entire metapopulation [START_REF] Jenouvrier | Influence of dispersal processes on the global dynamics of Emperor penguin, a species threatened by climate change[END_REF]. The adaptive capacity of emperor penguins is unknown, but is likely limited because they have a long life spans, delayed maturity, and low reproductive rates, coupled with low genetic diversity [START_REF] Younger | The challenges of detecting subtle population structure and its importance for the conservation of emperor penguins[END_REF].

The biological capacity for emperor penguins to 'cope' with climate change through adaptation or dispersal to suitable habitats is therefore likely to be minimal (but see [START_REF] Younger | Too much of a good thing: Sea ice extent may have forced emperor penguins into refugia during the last glacial maximum[END_REF]. Nevertheless, here we have demonstrated that global climate policy has the capacity to safeguard the future of emperor penguins in ways that their intrinsic biological properties do not as Paris Agreement climate scenarios resulted in dramatically higher population viability than a baseline scenario.

| Climate change mitigation

Globally, the projected abundance of emperor penguins in 2100 was higher under Paris 1.5 than Paris 2. Additionally, the temporal dynamics and end-of-century abundance of individual colonies often differed markedly under these two climate thresholds. For This 'half degree difference' has also recently been shown to affect projected range sizes for a large proportion of insects, vertebrates, and plants [START_REF] Warren | The projected effect on insects, vertebrates, and plants of limiting global warming to 1.5°C rather than 2°C[END_REF]. While this half-degree difference is likely to affect many climate properties, in polar systems it will have especially large impacts on sea ice conditions. For example, the chance that the Arctic experiences an ice-free year by 2100 is 30% under 1.5° of warming, but rises to 100% under 2° of warming [START_REF] Jahn | Reduced probability of ice-free summers for 1.5°C compared to 2°C warming[END_REF]. Here we showed that in Antarctica (Figures 4 and5), the projected response of sea ice to this half-degree difference varies across regions, resulting in extremely strong effects on particular emperor penguin colonies (Figures S3-S5).

Emperor penguins are considered 'indicators' of climate change in the Southern Ocean because they are highly sensitive to environmental conditions in multiple stages of their annual cycle and across large spatial extents [START_REF] Barbraud | Emperor penguins and climate change[END_REF]). Yet, many other polar species across a diversity of life histories are also tied to sea ice conditions (Thomas & Dieckmann, 2008). Icedependent species occupy all levels of the food web in Antarctica, including primary producers (e.g., ice algae and phytoplankton), zooplankton (e.g., krill), and secondary and tertiary consumers (e.g., Antarctic silverfish, Weddell seals: Leptonychotes weddellii, Snow petrels: Pagodroma nivea). Similarly strong linkages to sea ice exist for species in the Arctic (e.g., for Polar bears: Ursus maritimus; [START_REF] Hunter | Climate change threatens polar bear populations: A stochastic demographic analysis[END_REF] where climate change is progressing even more rapidly.

It is therefore likely that meeting the Paris Agreement objectives, or failing to do so, will have wide-ranging consequences that extend far beyond the effects we demonstrated for emperor penguins.

Climate change may combine with and potentially exacerbate other pressures that influence the viability of polar species (Rintoul et al., 2018). For example, melting sea ice may increase human access to new fishing areas, increasing competition with Antarctic predators such as emperor penguins. Similarly, range shifts of other species will likely alter the composition of marine communities, with potentially important impacts across the food web. These interactive and multispecies effects are difficult to anticipate, and represent an important avenue of future research.

Nevertheless, our study emphasizes that near-term global policy decisions over the next decade will have dramatic impacts on the viability of an iconic Antarctic predator, and will likely shape the future of earth's biota more generally.

  which include 40 ensemble members run from 1920-2100 with historical forcings over the 20th century and the RCP 8.5 21st-century emissions scenario. These are compared to Paris target agreement simulations (Sanderson et al., 2017) which employ emissions scenarios that result in a 1.5°C global average warming (Paris 1.5) or a 2°C global average warming (Paris 2) by 2100. All simulations use the identical climate model and the Paris target simulations are branched from the CESM-LENS simulations in 2006 and run from 2006-2100. We calculate the seasonal sea ice conditions means for the four seasons of the life cycle of the em-

  cludes the three stages: (a) emigration from the resident colony; (b) search for new colony among other colonies with an average dispersal distance d (transfer); and (c) settlement in a new colony. The duration of the transfer phase can vary, as the final settlement in a new colony may occur after several events (e.g., an individual may not settle in its first choice habitat if that habitat has reached its carrying capacity n i ≥ K i ). In our model, movements of individuals among colonies are divided into two successive dispersal events to account for a timelimited search. Indeed for emperor penguins the breeding season lasts 9 months, and thus the timing for prospecting other colonies during the nonbreeding season is limited. During the first dispersal event (D 1 ) individuals may select the habitat with highest quality (informed search) or settle in a random habitat. During the second dispersal event (D 2 ) individuals that reached a saturated colony leave and settle randomly in a new colony (see figure 1 in Jenouvrier et al., 2017). The latter is a way to account for a dispersal cost of gathering information for the informed search (see discussion in Jenouvrier et al., 2017). The dispersal projection matrix D is thus and each dispersal matrix D e is written to indicate that matrices for searching behavior, S e , and emigration, M e , depend on the population size at the start of the event (n e ) as well as the environmental conditions x(t).

  5) S R : = S R [x, d]. (6) S I : = S I [r * (t), x, d].

  ity'). We used multiple ensemble runs of the same climate model and climate scenario, each with vanishingly small differences in initial climate conditions to account for such uncertainty[START_REF] Hawkins | The potential to narrow uncertainty in regional climate predictions[END_REF]. Parameter uncertainty describes statistical uncertainty in the estimates of demographic parameters (e.g., survival and reproduction, and their responses to sea ice concentration anomalies, see[START_REF] Jenouvrier | Effects of climate change on an emperor penguin population: Analysis of coupled demographic and climate models[END_REF]. Process variance (i.e., environmental stochasticity) reflects true 'unexplained' temporal variance in demographic rates that is not accounted for by sea ice.To decompose these various sources of uncertainties in our global projections of population growth rate and size, we used a stepwise, nested approach to estimate sources of uncertainty in projections of future population growth rates[START_REF] Gauthier | Partitioning prediction uncertainty in climate-dependent population models[END_REF]. In step 1, we only include parameter uncertainty in projections. This entails generating repeated demographic projections, where random draws from the sampling distributions for each demographic rate are used to parameterize the metapopulation model. In this step, only a single climate run is used, and unexplained temporal process variance in demographic rates is ignored.In step 2, for each of the repeated projections in step 1, we conduct an additional series of projections using the range of available climate model ensemble members. Uncertainty in future climate causes projections of sea ice from each ensemble member to diverge, relative to their nearly identical initial conditions. Population projections at this stage thus include uncertainty in climate (estimated from multiple climate ensemble members), and for each ensemble member, parameter uncertainty arising from a series of demographic projections based on random draws from sampling distributions.

Figure 1 compares

 1 Figure 1 compares Antarctic sea ice conditions simulated by the GCM CESM model to satellite observations for the specific seasons of the emperor penguin life cycle across the entire Antarctic coast. The range of Antarctic sea ice conditions simulated by the climate model overlaps very well with the range of observations over the historical period, except in few regions and seasons. Specifically, the

F

  I G U R E 1 Antarctic sea ice extent along the coast of Antarctica from the Community Earth System Model (CESM) and observation for the various seasons of the emperor penguin life cycle (panels). The lower panel shows the Antarctic coast and dots are the location of colonies (FigureS1; TableS1). Inside numbers refer to the emperor colonies, while x-labels show longitudes. On the top four panels across seasons, the shading areas show the entire range of Antarctic sea ice extent. In the observations this represents all possible time values 1979-2005 at each longitude for specific season (panel). For the Large Ensemble (CESM-LE), the range represents several runs and all possible time values 1979-2005.

N

  Sensitivity of the population growth rate to Antarctic sea ice conditions for the four seasons of the life cycle of emperor penguin. (a) Median of the absolute value of the sensitivity of the population growth rate calculated at each colony, at each time step, and each simulation for the RCP 8.5 scenario. (b) Median of the absolute value of the sensitivity of the population growth rate across colony. More details are shown in Figure S2 U R E 3 Antarctic sea ice projections using RCP 8.5. These are projections from the Community Earth System Model for each emperor penguin colony (y-axis) from 2009 to 2100 (x-axis), for each season of the emperor penguin life cycle (panels). The y-axis refers to the colony number used in Figure S1. Medians of the large ensemble of Antarctic sea ice concentration anomalies at each penguin colony are shown (color bar) as function of time (x-axis). Dotted black contour shows a 90% decline in sea ice relative to the historical mean (1979-2007), while the thick black contour shows a 10% decline and laying seasons at the end of the century. By 2060, most colonies (except in the Eastern Weddell Sea and Ross Sea) experience declines larger than 50% relative to historical levels during the nonbreeding and laying seasons. In contrast, for the climate scenario meeting the Paris Agreement's goals, such large sea ice declines are limited to fewer colonies in Dronning, Enderby, and Kemp lands (Figure S1).

Figure

  FigureS6), resulting in annual declines of 4.06% per year by the end of the century (a half-life of 17 years). Under these conditions, the species will go extinct rapidly. Furthermore, even under a dispersal scenario that leads to the most optimistic population outcome (short

F

  I G U R E 7 Projected intrinsic population growth rate of emperor penguin colonies through to 2100 for each climate scenario without dispersal. The figure shows the median of the year-to-year population growth rates from 2009 to 2100 for each colony. Growth rates are based on sea ice concentration anomaly projections from the Community Earth System Model using (a) RCP 8.5, (b) Paris Agreement's 2°C goal, and (c) Paris Agreement's 1.5°C. The y-axis refers to the colony number used in Figure S1. Blue (red) colors show a positive (negative) population growth rate. The white contour represents a null growth rate, indicating stable populations , low emigration rate, and informed search),the median of the global population is projected to decline by 81% (Figure9a; FigureS7). Larger global declines are projected with other dispersal scenarios, up to a decline of 99% relative to its current size with long-distance dispersal and high emigration rate regardless of dispersal behavior (random or informed search). By including all uncertainties (FigureS8), the 90% confidence envelope of the global population projections by 2100 ranges from a decline of 99.2% to 67% relative to the 2009 initial size.

Paris 1 .

 1 5, and 0.34% (half-life of 201 years) under Paris 2. Thus, emperor penguins will persist if the Paris Agreement objectives are met, with two main refuges in the Ross and Weddell Seas. Other dispersal scenario project slightly larger global declines. For example, if individuals have high emigration rates, shortrange dispersal, and select habitats nonrandomly (Figure S5), the global population will decline by 34% by 2100 under Paris 1.5. By including all uncertainties (Figure S8), the 90% confidence envelope of the global population projections by 2100 ranges from a decline of 38.6% and 49. 3% under Paris 1.5 and 2, respectively, to an increase of 161.2% and 89% under Paris 1.5 and 2, relative to the 2009 initial size.

  climate and environmental stochasticity are large for the population growth rate (Figure 10). Complex patterns of uncertainties occur at the level of the global population size because the model is highly nonlinear by accounting for density dependence

Figures

  Figures S3-S5 show the uncertainties of the population projections at each colony for the three climate scenarios (RCP 8.5, Paris Agreement's 2°C goal, and Paris Agreement's 1.5°C) and various dispersal scenarios contrasting short/long distance of dispersal, low/ high emigration rate, and informed versus random search. Figure S7 shows the uncertainties of the global population projections according to the climate and dispersal scenarios. Figures S8-S10 summarize all the structural uncertainties in the metapopulation model for each climate scenario, and show larger confidence envelope. The violin plots illustrate the distribution of the global population sizes by 2100. For RCP 8.5 all population projections are projected to

F I G U R E 1 0 1 |

 11 Uncertainty decomposition for the global population size of emperor penguins. The population projections at each colony are based on sea ice concentration anomaly projections from the Community Earth System Model under RCP 8.5 without dispersion. They are three main sources of uncertainty: parameter uncertainty, climate uncertainty, and process variance (colored areas). The white thick line shows the median of these projections F I G U R E 11 Uncertainty decomposition for annual global population growth rate of emperor penguins. Same legends as Figure 10 for colored areas. The white dashed line shows a stable population, that is, a null population growth rate 4.Climate and demographic uncertainties Uncertainty is central to population forecasting and enters in every step, from climate modeling (e.g., uncertainty in current climate conditions and future climate change) to demographic modeling (e.g., uncertainty in the responses of vital rates to climate parameters, residual vital rate covariation, etc.) to population modeling (e.g., uncertainty in population abundance and stage structure; Iles &

  [START_REF] Jenouvrier | Impacts of climate change on avian populations[END_REF]. These results emphasize the importance for incorporating the natural climate variability and nonstationary climate dynamics predicted by an ensemble of climate models[START_REF] Jenouvrier | Impacts of climate change on avian populations[END_REF].There are three sources of uncertainty in climate model projections[START_REF] Hawkins | The potential to narrow uncertainty in regional climate predictions[END_REF] which include (a) scenario uncertainty associated with the future greenhouse gas emissions; (b) internal variability associated with the chaotic nature of the climate system; and (c) model structural uncertainty due to errors in the models themselves. In this study, we assess the role of the greenhouse gas scenario in driving differences in projected emperor penguin responses to climate change, hence account for scenario uncertainty. In doing so, we consider model simulations with the RCP 8.5 emissions scenario (Van Vuuren et al., 2011) which has increasing greenhouse gas emissions over the 21st century and reaches a global radiative forcing of 8.5 W/m 2 and compare these to model simulations which use unique Paris target forcing scenarios (Sanderson et al., 2017). These Paris target scenarios were explicitly designed to reach 1.5 or 2°C of global average warming by 2100 as outlined in the Paris Agreement.

  example, the well-studied breeding colony at Pointe Géologie experiences continued declines throughout the 21st century under 2°C warming, but remains largely stable, especially in the second half of the century under 1.5°C (FiguresS3-S5). Additionally, the Cape Darnley breeding colony declines precipitously under Paris 2 and is rapidly trending toward extinction at 2100, but has largely stabilized by 2100 (albeit at a lower abundance) under Paris 1.5.

  The ensemble simulation sets use the same model and forcing but differ very slightly in their initial state. This results in a different time evolution of climate conditions due to chaotic dynamics and allows us to isolate the uncertainty in sea ice projections that is associated with internal variability. Finally, here we do not consider the uncertainty associated with model structure.

	This model structural uncertainty can be large for projections of
	Antarctic sea ice (e.g., IPCC AR5, Chapter 12; Turner, Bracegirdle,
	Phillips, Marshall, & Hosking, 2013), with important consequences
	for emperor penguin population projections (Jenouvrier et al.,
	2012). However, the Paris target forcing scenario has only been
	applied in a single model (the CESM1) and so it is not possible to as-
	sess model structural uncertainty with this scenario. Additionally,
	most of the available climate model projections do not include a
	large ensemble of members and thus we are not able to assess the
	role of internal variability in projection uncertainty for those mod-
	els. Given this, and the very good quality of the CESM1 Antarctic
	sea ice simulation for present day conditions (Figure

ACK N OWLED G EM ENTS

We acknowledge Institute Paul Emile Victor (Programme IPEV 109), and by Terres Australes et Antarctiques Franҫaises for TA penguin data. We acknowledge D. Besson and K. Delord for TA penguin data management. We acknowledge the support of NSF OPP 1643901 (PICA) to SJ and RJ, and NSF OPP 1744794 to SJ, SL and ML.

Funding information

NSF, Grant/Award Number: 1643901 and 1744794

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DATA AVA I L A B I L I T Y S TAT E M E N T

All data are available in the main text or the Supporting Information.

Detailed code and materials used in the analysis will be avail-

CO N FLI C T O F I NTE R E S T

The authors declare no competing interests.