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Brown rat

The brown rat (Rattus norvegicus), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat and Norwegian rat, is a widespread species of common rat. One of the largest muroids, it is a brown or grey rodent with a body length of up to 28 cm long, and a tail slightly shorter than that. It weighs between 140 and 500 g. Thought to have originated in northern China and neighbouring areas, this rodent has now spread to all continents except Antarctica, and is the dominant rat in Europe and much of North America. With rare exceptions, the brown rat lives wherever humans live, particularly in urban areas.

Selective breeding of the brown rat has produced the fancy rat (rats kept as pets), as well as the laboratory rat (rats used as model organisms in biological research). Both fancy rats and laboratory rats are of the domesticated subspecies Rattus norvegicus domestica. Studies of wild rats in New York City have shown that populations living in different neighborhoods can evolve distinct genomic profiles over time, by slowly accruing different traits.

Naming and etymology
The brown rat was originally called the "Hanover rat" by people wishing to link problems in 18th-century England with the House of Hanover. It is not known for certain why the brown rat is named Rattus norvegicus (Norwegian rat), as it did not originate from Norway. However, the English naturalist John Berkenhout, author of the 1769 book Outlines of the Natural History of Great Britain, is most likely responsible for popularizing the misnomer. Berkenhout gave the brown rat the binomial name Rattus norvegicus, believing it had migrated to England from Norwegian ships in 1728.

By the early to the middle part of the 19th century, British academics believed that the brown rat was not native to Norway, hypothesizing (incorrectly) that it may have come from Ireland, Gibraltar or across the English Channel with William the Conqueror. As early as 1850, however, a new hypothesis of the rat's origins was beginning to develop. The British novelist Charles Dickens acknowledged this in his weekly journal, All the Year Round, writing: It is frequently called, in books and otherwise, the 'Norway rat', and it is said to have been imported into this country in a ship-load of timber from Norway. Against this hypothesis stands the fact that when the brown rat had become common in this country, it was unknown in Norway, although there was a small animal like a rat, but really a lemming, which made its home there.

Academics began to prefer this etymology of the brown rat towards the end of the 19th century, as seen in the 1895 text Natural History by American scholar Alfred Henry Miles: The brown rat is the species common in England, and best known throughout the world. It is said to have travelled from Persia to England less than two hundred years ago and to have spread from thence to other countries visited by English ships.

Though the assumptions surrounding this species' origins were not yet the same as modern ones, by the 20th century, it was believed among naturalists that the brown rat did not originate in Norway, rather the species came from central Asia and (likely) China.

Description
The fur is usually brown or dark grey, while the underparts are lighter grey or brown. The brown rat is a rather large murid and can weigh twice as much as a black rat (Rattus rattus) and many times more than a house mouse (Mus musculus). The head and body length ranges from 15 to 28 cm while the tail ranges in length from 10.5 to 24 cm, therefore being shorter than the head and body. Adult weight ranges from 140 to 500 g.  Exceptionally large individuals can reportedly reach 900 to 1000 g but are not expected outside of domestic specimens. Stories of rats attaining sizes as big as cats are exaggerations, or misidentifications of larger rodents, such as the coypu and muskrat. In fact, it is common for breeding wild brown rats to weigh (sometimes considerably) less than 300 g.

Brown rats have acute hearing, are sensitive to ultrasound, and possess a very highly developed olfactory sense. Their average heart rate is 300 to 400 beats per minute, with a respiratory rate of around 100 per minute. The vision of a pigmented rat is poor, around 20/600, while a non-pigmented (albino) with no melanin in its eyes has both around 20/1200 vision and a terrible scattering of light within its vision. Brown rats are dichromats which perceive colors rather like a human with red-green colorblindness, and their colour saturation may be quite faint. Their blue perception, however, also has UV receptors, allowing them to see ultraviolet lights that humans and some other species cannot.

Biology and behavior
The brown rat is nocturnal and is a good swimmer, both on the surface and underwater, and has been observed climbing slim round metal poles several feet in order to reach garden bird feeders. Brown rats dig well, and often excavate extensive burrow systems. A 2007 study found brown rats to possess metacognition, a mental ability previously only found in humans and some other primates, but further analysis suggested they may have been following simple operant conditioning principles.

Communication
Brown rats are capable of producing ultrasonic vocalizations. As pups, young rats use different types of ultrasonic cries to elicit and direct maternal search behavior, as well as to regulate their mother's movements in the nest. Although pups produce ultrasounds around any other rats at the age of 7 days, by 14 days old they significantly reduce ultrasound production around male rats as a defensive response. Adult rats will emit ultrasonic vocalizations in response to predators or perceived danger; the frequency and duration of such cries depends on the sex and reproductive status of the rat. The female rat also emit ultrasonic vocalizations during mating.

Rats may also emit short, high frequency, ultrasonic, socially induced vocalization during rough and tumble play, before receiving morphine, or mating, and when tickled. The vocalization, described as a distinct "chirping", has been likened to laughter, and is interpreted as an expectation of something rewarding. Like most rat vocalizations, the chirping is too high in pitch for humans to hear without special equipment. Bat detectors are often used by pet owners for this purpose.

In research studies, the chirping is associated with positive emotional feelings, and social bonding occurs with the tickler, resulting in the rats becoming conditioned to seek the tickling. However, as the rats age, the tendency to chirp appears to decline.

Brown rats also produce communicative noises capable of being heard by humans. The most commonly heard in domestic rats is bruxing, or teeth-grinding, which is most usually triggered by happiness, but can also be 'self-comforting' in stressful situations, such as a visit to the vet. The noise is best described as either a quick clicking or 'burring' sound, varying from animal to animal. Vigorous bruxing can be accompanied by boggling, where the eyes of the rat rapidly bulge and retract due to movement of the lower jaw muscles behind the eye socket.

In addition, they commonly squeak along a range of tones from high, abrupt pain squeaks to soft, persistent 'singing' sounds during confrontations.

Diet
The brown rat is a true omnivore and consumes almost anything, but cereals form a substantial part of its diet. The most-liked foods of brown rats include scrambled eggs, raw carrots, and cooked corn kernels. The least-liked foods were raw beets, peaches and raw celery. Foraging behavior is often population-specific, and varies by environment and food source. Brown rats living near a hatchery in West Virginia catch fingerling fish. Some colonies along the banks of the Po River in Italy dive for mollusks, a practice demonstrating social learning among members of this species. Rats on the island of Norderoog in the North Sea stalk and kill sparrows and ducks.

Also preyed upon by brown rats are chicks, mice and small lizards. Examination of a wild brown rat stomachs in Germany revealed 4,000 food items, most of which were plants, although studies have shown that brown rats prefer meat when given the option. In metropolitan areas, they survive mainly on discarded human food and anything else that can be eaten without negative consequences.

Reproduction and life cycle

The brown rat can breed throughout the year if conditions are suitable, with a female producing up to five litters a year. The gestation period is only 21 days, and litters can number up to 14, although seven is common. They reach sexual maturity in about five weeks. Under ideal conditions (for the rat), this means that the population of females could increase by a factor of three and a half (half a litter of 7) in 8 weeks (5 weeks for sexual maturity and 3 weeks of gestation), corresponding to a population growing by a factor of 10 in just 15 weeks. As a result, the population can grow from 2 to 15,000 in a year. The maximum life span is three years, although most barely manage one. A yearly mortality rate of 95% is estimated, with predators and interspecies conflict as major causes.

When lactating, female rats display a 24-hour rhythm of maternal behavior, and will usually spend more time attending to smaller litters than large ones.

Brown rats live in large, hierarchical groups, either in burrows or subsurface places, such as sewers and cellars. When food is in short supply, the rats lower in social order are the first to die. If a large fraction of a rat population is exterminated, the remaining rats will increase their reproductive rate, and quickly restore the old population level.

The female is capable of becoming pregnant immediately after giving birth, and can nurse one litter while pregnant with another. She is able to produce and raise two healthy litters of normal size and weight without significantly changing her own food intake. However, when food is restricted, she can extend pregnancy by over two weeks, and give birth to litters of normal number and weight.

Mating behaviors
Males can ejaculate multiple times in a row, and this increases the likelihood of pregnancy as well as decreases the number of stillborns. Multiple ejaculation also means that males can mate with multiple females, and they exhibit more ejaculatory series when there are several oestrous females present. Males also copulate at shorter intervals than females. In group mating, females often switch partners.

Dominant males have higher mating success and also provide females with more ejaculate, and females are more likely to use the sperm of dominant males for fertilization.

In mating, female rats show a clear mating preference for unknown males versus males that they have already mated with (also known as the Coolidge effect), and will often resume copulatory behavior when introduced to a novel sexual partner.

Females also prefer to mate with males who have not experienced social stress during adolescence, and can determine which males were stressed even without any observed difference in sexual performance of males experiencing stress during adolescence and not.

Social behavior
Rats commonly groom each other and sleep together. Rats are said to establish an order of hierarchy, so one rat will be dominant over another one. Groups of rats tend to "play fight", which can involve any combination of jumping, chasing, tumbling, and "boxing". Play fighting involves rats going for each other's necks, while serious fighting involves strikes at the others' back ends. If living space becomes limited, rats may turn to aggressive behavior, which may result in the death of some animals, reducing the burden over the living space.

Rats, like most mammals, also form family groups of a mother and her young. This applies to both groups of males and females. However, rats are territorial animals, meaning that they usually act aggressively towards or scared of strange rats. Rats will fluff up their hair, hiss, squeal, and move their tails around when defending their territory. Rats will chase each other, groom each other, sleep in group nests, wrestle with each other, have dominance squabbles, communicate, and play in various other ways with each other. Huddling is an additional important part of rat socialization. Huddling, an extreme form of herding and like chattering or "bruxing" is often used to commicate that they are feeling threatened and not to come near. The common rat has been more successful at inhabiting and building communities on 6 continents and are the only species to have occupied more land than humans.

During the wintry months, rats will huddle into piles – usually cheek-to-cheek – to control humidity and keep the air warm as a heat-conserving function. Just like elderly rats are commonly groomed and nursed by their companions, nestling rats especially depend on heat from their mother, since they cannot regulate their own temperature. Other forms of interaction include: crawling under, which is literally the act of crawling underneath one another (this is common when the rat is feeling ill and helps them breathe); walking over to find a space next to their closest friend, also explained in the name; allo-grooming, so-called to distinguish it from self-grooming; and nosing, where a rat gently pushes with its nose at another rat near the neck.

Burrowing
Rats are known to burrow extensively, both in the wild and in captivity, if given access to a suitable substrate. Rats generally begin a new burrow adjacent to an object or structure, as this provides a sturdy "roof" for the section of the burrow nearest to the ground's surface. Burrows usually develop to eventually include multiple levels of tunnels, as well as a secondary entrance. Older male rats will generally not burrow, while young males and females will burrow vigorously.

Burrows provide rats with shelter and food storage, as well as safe, thermo-regulated nest sites. Rats use their burrows to escape from perceived threats in the surrounding environment; for example, rats will retreat to their burrows following a sudden, loud noise or while fleeing an intruder. Burrowing can therefore be described as a "pre-encounter defensive behavior", as opposed to a "post-encounter defensive behavior", such as flight, freezing, or avoidance of a threatening stimulus.

Distribution and habitat

Possibly originating from the plains of northern China and Mongolia, the brown rat spread to other parts of the world sometime in the Middle Ages. The question of when brown rats became commensal with humans remains unsettled, but as a species, they have spread and established themselves along routes of human migration and now live almost everywhere humans are.

The brown rat may have been present in Europe as early as 1553, a conclusion drawn from an illustration and description by Swiss naturalist Conrad Gesner in his book Historiae animalium, published 1551–1558. Though Gesner's description could apply to the black rat, his mention of a large percentage of albino specimens—not uncommon among wild populations of brown rats—adds credibility to this conclusion. Reliable reports dating to the 18th century document the presence of the brown rat in Ireland in 1722, England in 1730, France in 1735, Germany in 1750, and Spain in 1800, becoming widespread during the Industrial Revolution. It did not reach North America until around 1750–1755.

As it spread from Asia, the brown rat generally displaced the black rat in areas where humans lived. In addition to being larger and more aggressive, the change from wooden structures and thatched roofs to bricked and tiled buildings favored the burrowing brown rats over the arboreal black rats. In addition, brown rats eat a wider variety of foods, and are more resistant to weather extremes.

In the absence of humans, brown rats prefer damp environments, such as river banks. However, the great majority are now linked to man-made environments, such as sewage systems.

It is often said that there are as many rats in cities as people, but this varies from area to area depending on climate, living conditions, etc. Brown rats in cities tend not to wander extensively, often staying within 20 m of their nest if a suitable concentrated food supply is available, but they will range more widely where food availability is lower. It is difficult to determine the extent of their home range because they do not utilize a whole area but rather use regular runways to get from one location to another. There is great debate over the size of the population of rats in New York City, with estimates from almost 100 million rats to as few as 250,000. Experts suggest that New York is a particularly attractive place for rats because of its aging infrastructure and high poverty rates. In 2023, the city appointed Kathleen Corradi as the first Rat Czar, a position created to address the city's rat population. The position focuses on instituting policies measures to curb the population such as garbage regulation and additional rat trapping. In addition to sewers, rats are very comfortable living in alleyways and residential buildings, as there is usually a large and continuous food source in those areas.

In the United Kingdom, some figures show that the rat population has been rising, with estimations that 81 million rats reside in the UK Those figures would mean that there are 1.3 rats per person in the country. High rat populations in the UK are often attributed to the mild climate, which allow them higher survival rates during the winter. With the increase in global temperature and glacier retreat, it is estimated that brown rat populations will see an increase.

In tropical and desert regions, brown rat occurrence tends to be limited to human-modified habitats. Contiguous rat-free areas in the world include the continent of Antarctica, the Arctic, some isolated islands, the Canadian province of Alberta, and certain conservation areas in New Zealand. Most of Australia apart from the eastern and south-eastern coastal areas does not have reports of substantial rat occurrences.

Antarctica is uninhabitable by rats. The Arctic has extremely cold winters that rats cannot survive outdoors, and the human population density is extremely low, making it difficult for rats to travel from one habitation to another, although they have arrived in many coastal areas by ship. When the occasional rat infestation is found and eliminated, the rats are unable to re-infest it from an adjacent one. Isolated islands are also able to eliminate rat populations because of low human population density and the geographic distance from other rat populations.

Rats as invasive species
Many parts of the world have been populated by rats secondarily, where rats are now important invasive species that compete with and threaten local fauna. For instance, Norway rats reached North America between 1750 and 1775 and even in the early 20th century, from 1925 to 1927, 50% of ships entering the port of New York were rat infested. The following sections provide some examples, including attempts of rat control.

Faroe Islands
The brown rat was first observed on the Faroe Islands in 1768. It is thought that the first individuals arrived on the southernmost island, Suðuroy, via the wreck of a Norwegian ship that had stranded on the Scottish Isle of Lewis on its way from Trondheim to Dublin. The drifting wreck, carrying brown rats, drifted northwards until it reached the village of Hvalba. Dispersion afterwards appears to have been fast, including all of Suðuroy within a year. In 1769, they were observed in Tórshavn on the southern part of Streymoy, and a decade later, in the villages in the northern part of this island. From here, they crossed the strait and occupied Eysturoy during the years 1776 to 1779. In 1779, they reached Vagar. Whether the rats dispersed from the already established population in Suðuroy, or they were brought to the Faroe Islands with other ships is unknown. The Northern islands were invaded by the brown rat more than 100 years later, after Norwegians built and operated a whaling station in the village of Hvannasund on Borðoy from 1898 to 1920. From there, the brown rat spread to the neighbouring islands of Viðoy and Kunoy. A recent genomic analysis reveals three independent introductions of the invasive brown rat to the Faroe Islands.

Today the brown rat is found on seven of the 18 Faroese islands, and is common in and around human habitations as well as in the wild. Although the brown rat is now common on all of the largest Faroese islands, only sparse information on the population is available in the literature. An investigation for infection with the spirochaete Leptospira interrogans did not find any infected animals, suggesting that Leptospira prevalence rates on the Faroe Islands may be among the lowest recorded worldwide.

Alaska
Hawadax Island (formerly known as Rat Island) in Alaska is thought to have been the first island in the Aleutians to be invaded by Norway rats (the Brown rat) when a Japanese ship went  aground in the 1780s. They had a devastating effect on the native bird life. An eradication program was started in 2007 and the island was declared rat-free in June 2009.

Alberta
Alberta is the largest rat-free populated area in the world. Rat invasions of Alberta were stopped and rats were eliminated by very aggressive government rat control measures, starting during the 1950s.

The only species of Rattus that is capable of surviving the climate of Alberta is the brown rat, which can only survive in the prairie region of the province, and even then must overwinter in buildings. Although it is a major agricultural area, Alberta is far from any seaport and only a portion of its eastern boundary with Saskatchewan provides a favorable entry route for rats. Brown rats cannot survive in the wild boreal forest to the north, the Rocky Mountains to the west, nor can they safely cross the semiarid High Plains of Montana to the south. The first brown rat did not reach Alberta until 1950, and in 1951, the province launched a rat-control program that included shooting, poisoning, and gassing rats, and bulldozing or burning down some rat-infested buildings. The effort was backed by legislation that required every person and every municipality to destroy and prevent the establishment of designated pests. If they failed, the provincial government could carry out the necessary measures and charge the costs to the landowner or municipality.

In the first year of the rat control program, 64 t of arsenic trioxide were spread throughout 8,000 buildings on farms along the Saskatchewan border. However, in 1953 the much safer and more effective rodenticide warfarin was introduced to replace arsenic. Warfarin is an anticoagulant that was approved as a drug for human use in 1954 and is much safer to use near humans and other large animals than arsenic. By 1960, the number of rat infestations in Alberta had dropped to below 200 per year. In 2002, the province finally recorded its first year with zero rat infestations, and from 2002 to 2007 there were only two infestations found. After an infestation of rats in the Medicine Hat landfill was found in 2012, the province's rat-free status was questioned, but provincial government rat control specialists brought in excavating machinery, dug out, shot, and poisoned 147 rats in the landfill, and no live rats were found thereafter. In 2013, the number of rat infestations in Alberta dropped to zero again. Alberta defines an infestation as two or more rats found at the same location, since a single rat cannot reproduce. About a dozen single rats enter Alberta in an average year and are killed by provincial rat control specialists before they can reproduce.

Only zoos, universities, and research institutes are allowed to keep caged rats in Alberta, and possession of unlicensed rats (including fancy rats) by anyone else is punishable by a penalty of up to C$5,000 or up to 60 days in jail. The adjacent and similarly landlocked province of Saskatchewan initiated a rat control program in 1972, and has managed to reduce the number of rats in the province substantially, although they have not been eliminated. The Saskatchewan rat control program has considerably reduced the number of rats trying to enter Alberta.

New Zealand
First arriving before 1800 (perhaps on James Cook's vessels), brown rats pose a serious threat to many of New Zealand's native wildlife. Rat eradication programmes within New Zealand have led to rat-free zones on offshore islands and even on fenced "ecological islands" on the mainland. Before an eradication effort was launched in 2001, the sub-Antarctic Campbell Island had the highest population density of brown rats in the world. Predator Free 2050 aims to eradicate rats from the entirety of New Zealand.

Diseases
Similar to other rodents, brown rats may carry a number of pathogens, which can result in disease, including Weil's disease, rat bite fever, cryptosporidiosis, viral hemorrhagic fever, Q fever and hantavirus pulmonary syndrome. In the United Kingdom, brown rats are an important reservoir for Coxiella burnetii, the bacterium that causes Q fever, with seroprevalence for the bacteria found to be as high as 53% in some wild populations.

This species can also serve as a reservoir for Toxoplasma gondii, the parasite that causes toxoplasmosis, though the disease usually spreads from rats to humans when domestic cats feed on infected brown rats. The parasite has a long history with the brown rat, and there are indications that the parasite has evolved to alter an infected rat's perception to cat predation, making it more susceptible to predation and increasing the likelihood of transmission.

Surveys and specimens of brown rat populations throughout the world have shown this species is often associated with outbreaks of trichinosis, but the extent to which the brown rat is responsible in transmitting Trichinella larvae to humans and other synanthropic animals is at least somewhat debatable. Trichinella pseudospiralis, a parasite previously not considered to be a potential pathogen in humans or domestic animals, has been found to be pathogenic in humans and carried by brown rats.

They can also be responsible for transmitting Angiostrongylus larvae to humans by eating raw or undercooked snails, slugs, molluscs, crustaceans, water and/or vegetables contaminated with them.

Brown rats are sometimes mistakenly thought to be a major reservoir of bubonic plague, a possible cause of the Black Death. However, the bacterium responsible, Yersinia pestis, is commonly endemic in only a few rodent species and is usually transmitted zoonotically by rat fleas—common carrier rodents today include ground squirrels and wood rats. However, brown rats may suffer from plague, as can many nonrodent species, including dogs, cats, and humans. During investigations of the plague epidemic in San Francisco in 1907, >1% of collected rats were infected with Y. pestis. The original carrier for the plague-infected fleas thought to be responsible for the Black Death was the black rat, and it has been hypothesized that the displacement of black rats by brown rats led to the decline of bubonic plague. This theory has, however, been deprecated, as the dates of these displacements do not match the increases and decreases in plague outbreaks.

During the COVID-19 pandemic, one study of New York City sewer rats showed that 17 percent of the city's brown rat population had become infected with SARS-CoV-2.

Uses in science

Selective breeding of white-marked rats rescued from being killed in a now-outlawed sport called rat baiting has produced the pink-eyed white laboratory rat. Like mice, these rats are frequently subjects of medical, psychological and other biological experiments, and constitute an important model organism. This is because they grow quickly to sexual maturity and are easy to keep and to breed in captivity. When modern biologists refer to "rats", they almost always mean Rattus norvegicus.

As pets
The brown rat is kept as a pet in many parts of the world. Australia, the United Kingdom, and the United States are just a few of the countries that have formed fancy rat associations similar in nature to the American Kennel Club, establishing standards, orchestrating events, and promoting responsible pet ownership.

The many different types of domesticated brown rats include variations in coat patterns, as well as the style of the coat, such as Hairless or Rex, and more recently developed variations in body size and structure, including dwarf and tailless fancy rats.

Working rats
A working rat is a rat trained for specific tasks as a working animal. In many cases, working rats are domesticated brown rats. However, other species, notably the Gambian pouched rat, have been trained to assist humans.

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Impossible object

An impossible object (also known as an impossible figure or an undecidable figure) is a type of optical illusion that consists of a two-dimensional figure which is instantly and naturally understood as representing a projection of a three-dimensional object but cannot exist as a solid object. Impossible objects are of interest to psychologists, mathematicians and artists without falling entirely into any one discipline.

Notable examples
Notable impossible objects include:

Explanations
Impossible objects can be unsettling because of our natural desire to interpret 2D drawings as three-dimensional objects. This is why a drawing of a Necker cube would most likely be seen as a cube, rather than "two squares connected with diagonal lines, a square surrounded by irregular planar figures, or any other planar figure". Looking at different parts of an impossible object makes one reassess the 3D nature of the object, which confuses the mind.

In most cases the impossibility becomes apparent after viewing the figure for a few seconds. However, the initial impression of a 3D object remains even after it has been contradicted. There are also more subtle examples of impossible objects where the impossibility does not become apparent spontaneously and it is necessary to consciously examine the geometry of the implied object to determine that it is impossible.

Roger Penrose wrote about describing and defining impossible objects mathematically using the algebraic topology concept of cohomology.

History
An early example of an impossible object comes from Apolinère Enameled, a 1916 advertisement painted by Marcel Duchamp. It depicts a girl painting a bed-frame with white enamelled paint, and deliberately includes conflicting perspective lines, to produce an impossible object. To emphasise the deliberate impossibility of the shape, a piece of the frame is missing. Swedish artist Oscar Reutersvärd was one of the first to deliberately design many impossible objects. He has been called "the father of impossible figures". In 1934, he drew the Penrose triangle, some years before the Penroses. In Reutersvärd's version, the sides of the triangle are broken up into cubes.

In 1956, British psychiatrist Lionel Penrose and his son, mathematician Roger Penrose, submitted a short article to the British Journal of Psychology titled "Impossible Objects: A Special Type of Visual Illusion". This was illustrated with the Penrose triangle and Penrose stairs. The article referred to Escher, whose work had sparked their interest in the subject, but not Reutersvärd, of whom they were unaware. The article was published in 1958.

From the 1930s onwards, Dutch artist M.C. Escher produced many drawings featuring paradoxes of perspective gradually working towards impossible objects. In 1957, he produced his first drawing containing a true impossible object: Cube with Magic Ribbons. He produced many further drawings featuring impossible objects, sometimes with the entire drawing being an impossible object. Waterfall and Belvedere are good examples of impossible constructions. His work did much to draw the attention of the public to impossible objects.

Some contemporary artists are also experimenting with impossible figures, for example, Jos de Mey, Shigeo Fukuda, Sandro del Prete, István Orosz (Utisz), Guido Moretti, Tamás F. Farkas, Mathieu Hamaekers, and Kokichi Sugihara.

Constructed impossible objects
Although possible to represent in two dimensions, it is not geometrically possible for such an object to exist in the physical world. However some models of impossible objects have been constructed, such that when they are viewed from a very specific point, the illusion is maintained. Rotating the object or changing the viewpoint breaks the illusion, and therefore many of these models rely on forced perspective or having parts of the model appearing to be further or closer than they actually are.

The notion of an "interactive impossible object" is an impossible object that can be viewed from any angle without breaking the illusion.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Impossible object


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David Stirling

Sir Archibald David Stirling, (15 November 1915 – 4 November 1990) was a Scottish officer in the British Army and the founder and creator of the Special Air Service (SAS). Under his leadership, the SAS carried out hit-and-run raids behind the Axis lines of the North African campaign. He saw active service during the Second World War until he was captured in January 1943. He spent the rest of the war in captivity, despite making several attempts to escape.

Stirling left the Regular Army in 1947. He founded the Capricorn Africa Society, which aimed to fight racial discrimination in Africa, but Stirling's preference to a limited, elitist voting franchise over universal suffrage limited the movement's appeal. He subsequently formed various private military companies and was linked with a failed attempt to overthrow the Libyan leader Muammar Gaddafi in the early 1970s. He also attempted to organise efforts to undermine trades unionism and to overthrow the British government, none of which made significant headway. He was made a Knight Bachelor in 1990, and died later the same year.

Early life
Stirling was born at his family's ancestral home, Keir House, in the parish of Lecropt, Perthshire on 15 November 1915. He was the son of Brigadier-General Archibald Stirling, of Keir, and Margaret Fraser, daughter of Simon Fraser, the Lord Lovat (a descendant of Charles II). Simon Fraser, 15th Lord Lovat was a first cousin. His paternal grandparents were Sir William Stirling-Maxwell, 9th Baronet and Lady Anna Maria Leslie-Melville.

Stirling was educated in England at the Catholic boarding school Ampleforth College. He was part of the Ampleforth Officer Training Corps. He attended Trinity College, Cambridge, for a year before going to Paris to become an artist.

Second World War and the founding of the SAS
Stirling was commissioned into the Scots Guards on 24 July 1937. At 6 ft with an athletic figure, Stirling was training to climb Mount Everest when the Second World War broke out in September 1939. In June 1940, he volunteered for the new No. 8 (Guards) Commando under Lieutenant-Colonel Robert Laycock, which became part of Force Z (later named "Layforce"). On 1 February 1941, Layforce sailed for the Middle East, in support of the capture of Rhodes, but were soon disbanded after suffering heavy casualties in the Battle of Crete and the Battle of the Litani River. Stirling remained convinced that due to the mechanised nature of war, a small team of highly trained soldiers with the advantage of surprise could attack several targets from the desert in a single night.

Believing that taking his idea up the chain of command was unlikely to work, Stirling decided to go straight to the top. On crutches following a parachuting accident, he stealthily entered Middle East headquarters in Cairo (under, through, or over a fence) in an effort to see Commander-in-Chief, Middle East Command General Sir Claude Auchinleck. Spotted by guards, Stirling abandoned his crutches and entered the building, only to come face-to-face with an officer with whom he had previously fallen out. Retreating rapidly, he entered the office of the deputy chief of staff, Major General Neil Ritchie. Stirling explained his plan to Ritchie, immediately after which Ritchie persuaded Auchinleck to allow Stirling to form a new special operations unit. The unit was given the deliberately misleading name "L Detachment, Special Air Service Brigade" to reinforce Dudley Clarke's deception of a parachute brigade existing in North Africa.

Stirling's new special operations unit was, at the outset, short of equipment (particularly tents and related gear) when the unit set up at Kibrit Air Base. The first operation of the new SAS was to steal from a nearby well-equipped New Zealand regiment various supplies including tents, bedding, tables, chairs and a piano. After at least four trips, they had a well-stocked camp.

After a brief period of training, an initial attempt at attacking a German airfield by parachute landing on 16 November 1941 in support of Operation Crusader proved to be disastrous for the unit. Of the original 55 men, some 34 were killed, wounded or captured far from the target, after being blown off course or landing in the wrong area, during one of the biggest storms to hit the region. Escaping only with the help of the Long Range Desert Group (LRDG) – who were designated to pick up the unit after the attack – Stirling agreed that approaching by land under the cover of night would be safer and more effective than parachuting. As quickly as possible he organised raids on ports using this simple method, bluffing through checkpoints at night using the language skills of some of his soldiers.

Under Stirling's leadership, the Lewes bomb, the first hand-held dual explosive and incendiary device, was invented by Jock Lewes. American Jeeps, which were able to deal with the harsh desert terrain better than other transport, were cut down, adapted and fitted with Vickers K machine guns fore and aft. Stirling also pioneered the use of small groups to escape detection. Finding it difficult to lead from the rear, Stirling often led from the front, his SAS units driving through enemy airfields in the Jeeps to shoot up aircraft and crew.

The first Jeep-borne airfield raid occurred soon after acquiring the first batch of Jeeps in June 1942, when Stirling's SAS group attacked the Italian-held Bagush airfield along with two other Axis airfields all in the same night. After returning to Cairo, Stirling collected a consignment of more Jeeps for further airfield raids. His biggest success was on the night of 26–27 July 1942 when his SAS squadron, armed with 18 jeeps, raided the Sidi Haneish landing strip and destroyed 37 Axis aircraft (mostly bombers and heavy transport) for the loss of two men killed. After a drive through the desert, evading enemy patrols and aircraft, Stirling and his men reached the safety of their advance camp at Qaret Tartura on the edge of the Qattara Depression. He was promoted to lieutenant-colonel in September 1942.

These hit-and-run operations eventually proved Stirling's undoing; he was captured by the Germans in January 1943 having been dubbed "The Phantom Major" by Field Marshal Erwin Rommel. Although Stirling escaped from the Germans, he was subsequently re-captured by the Italians, who took great delight in the embarrassment this caused to their German allies. He made four further escape attempts, before he was sent to Colditz Castle, where he remained as a prisoner for the rest of the war. He arrived on 20 August 1944 and was given the task of setting up the Colditz British Intelligence Unit by a Stay-Behind Order (SBO) which was in place in the area. Following Stirling's capture, Paddy Mayne took command of the SAS.

In North Africa, in the 15 months before Stirling's capture, the SAS had destroyed over 250 aircraft on the ground, dozens of supply dumps, wrecked railways and telecommunications, and had put hundreds of enemy vehicles out of action. Field Marshal Bernard Montgomery described Stirling as "mad, quite mad".

Post-war activities in Africa
Stirling transferred to the Regular Army Reserve of Officers in 1947. Stirling was granted the honorary rank of Lt. Col as a reservist, a rank that he retained on his retirement in 1965. Stirling was the founder of the Capricorn Africa Society, promoting freedom from racial discrimination in Africa. Founded in 1949, while much of Africa was still under colonial rule, it had its high point at the 1956 Salima Conference. However, because of his opposition to universal suffrage, preferring a qualified and very elitist voting franchise, educated Africans were divided on it and it attracted insufficient support. Consequently, the society's attempt to deal with the problem of different levels of social development in a non-racial way was ineffective, although it received surprising validation when the South African Communist Party used Stirling's multi-racial elitist model for its 1955 "Congress Alliance" with the African National Congress of South Africa. Stirling resigned as Chairman of the Society in 1959.

In September 1967 Len Deighton wrote an article in The Sunday Times Magazine about Operation Snowdrop, a raid led by Stirling. The following year Stirling was awarded "substantial damages" in a libel action about the article.

Mercenary and arms dealer
After the war, Stirling organised deals to provide British weapons and military personnel to other countries, such as Saudi Arabia, for various privatised foreign policy operations. Along with several associates, Stirling formed Watchguard International Ltd, initially with offices in Sloane Street (where the Chelsea Hotel later opened), latterly in South Audley Street in Mayfair.

Business was chiefly with the Gulf States. He was linked, along with Denys Rowley, to a failed attempt to overthrow the Libyan ruler Muammar Gaddafi in 1970 or 1971. Stirling was the founder of “private military company” KAS International, also known as KAS Enterprises.

Watchguard International Ltd was a private military company, registered in Jersey in 1965 by Stirling and John Woodhouse. Woodhouse's first assignment was to go to Yemen to report on the state of the royalist forces when a cease-fire was declared. At the same time Stirling was cultivating his contacts in the Iranian government and exploring the chances of obtaining work in Africa. The company operated in Zambia and in Sierra Leone, providing training teams and advising on security matters, but its founders' maverick ways of doing business caused its eventual downfall. Woodhouse resigned as Director of Operations after a series of disagreements and Stirling ceased to take an active part in 1972.

Great Britain 75
In mid-1970s, Stirling became increasingly worried that an "undemocratic event" would occur and decided to organise a private army to overthrow the government. He created an organisation called Great Britain 75 and recruited members from the aristocratic clubs in Mayfair; these were mainly ex-military men, and often former SAS members. The plan was that in the event of civil unrest resulting in the breakdown of normal Government operations, they would take over its running. He described this in detail in an interview from 1974, part of which is featured in Adam Curtis's documentary The Mayfair Set, episode 1: "Who Pays Wins".

In August 1974, before Stirling was ready to go public with GB75, the pacifist magazine Peace News obtained and published his plans. His biographer Alan Hoe disputed the newspaper's disparaging portrayal of Stirling as a right-wing 'Colonel Blimp'.

Undermining trades unionism
During the mid to late 1970s, Stirling created a secret organisation designed to undermine trades unionism from within. He recruited like-minded individuals from within the trade union movement, with the express intention that they should cause as much trouble during conferences as permissible. One such member was Kate Losinska, who was Head of the Civil and Public Services Association. Funding for this "operation" came primarily from his friend Sir James Goldsmith.

Honours
Stirling was awarded the Distinguished Service Order in recognition of gallant and distinguished service in the Middle East on 24 February 1942, appointed an Officer of the Order of the British Empire in recognition of gallant and distinguished service in the field on 14 November 1946 and appointed a Knight Bachelor in the 1990 New Year Honours for services to the military.

in 1984 the new base of the SAS was renamed Stirling Lines (from Bradbury Lines) in his honour.

In 2002 the SAS memorial, a statue of Stirling standing on a rock, was unveiled on the Hill of Row near his family's estate at Park of Keir. Two bronze plaques were stolen from the statue sometime around the end of May 2014. The current Laird of the Keir estate is his nephew Archie Stirling, a millionaire businessman and former Scots Guards officer.

In popular culture
Stirling was depicted by Connor Swindells in the 2022 television historical drama SAS: Rogue Heroes.

title: David Stirling


text:
Necker cube

The Necker cube is an optical illusion that was first published as a rhomboid in 1832 by Swiss crystallographer Louis Albert Necker. It is a simple wire-frame, two dimensional drawing of a cube with no visual cues as to its orientation, so it can be interpreted to have either the lower-left or the upper-right square as its front side.

Ambiguity
The Necker cube is an ambiguous drawing.

Each part of the picture is ambiguous by itself, yet the human visual system picks an interpretation of each part that makes the whole consistent. The Necker cube is sometimes used to test computer models of the human visual system to see whether they can arrive at consistent interpretations of the image the same way humans do.

Humans do not usually see an inconsistent interpretation of the cube. A cube whose edges cross in an inconsistent way is an example of an impossible object, specifically an impossible cube (compare Penrose triangle).

With the cube on the left, most people see the lower-left face as being in front most of the time. This is possibly because people view objects from above, with the top side visible, far more often than from below, with the bottom visible, so the brain "prefers" the interpretation that the cube is viewed from above.

There is evidence that by focusing on different parts of the figure, one can force a more stable perception of the cube. The intersection of the two faces that are parallel to the observer forms a rectangle, and the lines that converge on the square form a "y-junction" at the two diagonally opposite sides. If an observer focuses on the upper "y-junction" the lower left face will appear to be in front. The upper right face will appear to be in front if the eyes focus on the lower junction. Blinking while being on the second perception will probably cause you to switch to the first one.

It is possible to cause the switch to occur by focusing on different parts of the cube. If one sees the first interpretation on the right it is possible to cause a switch to the second by focusing on the base of the cube until the switch occurs to the second interpretation. Similarly, if one is viewing the second interpretation, focusing on the left side of the cube may cause a switch to the first.

The Necker cube has shed light on the human visual system. The phenomenon has served as evidence of the human brain being a neural network with two distinct equally possible interchangeable stable states. Sidney Bradford, blind from the age of ten months but regaining his sight following an operation at age 52, did not perceive the ambiguity that normal-sighted observers do, but rather perceived only a flat image.

During the 1970s, undergraduates in the Psychology Department of City University, London, were provided with assignments to measure their Introversion-Extroversion orientations by the time it took for them to switch between the Front and Back perceptions of the Necker Cube.

Apparent viewpoint
The orientation of the Necker cube can also be altered by shifting the observer's point of view. When seen from apparent above, one face tends to be seen closer; and in contrast, when seen from a subjective viewpoint that is below, a different face comes to the fore.

References in popular culture
The Necker cube is discussed to such extent in Robert J. Sawyer's 1998 science fiction novel Factoring Humanity that "Necker" becomes a verb, meaning to impel one's brain to switch from one perspective or perception to another.

The Necker cube is used to illustrate how vampires in Peter Watts' science fiction novels Blindsight (2006) and Echopraxia (2014) have superior pattern recognition skills. One of the pieces of evidence is that vampires can see both interpretations of the Necker Cube simultaneously, which sets them apart from baseline humanity.

title: Necker cube


text:
Urbanization

Urbanization (or urbanisation in British English) is the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It can also mean population growth in urban areas instead of rural ones. It is predominantly the process by which towns and cities are formed and become larger as more people begin living and working in central areas.

Although the two concepts are sometimes used interchangeably, urbanization should be distinguished from urban growth. Urbanization refers to the proportion of the total national population living in areas classified as urban, whereas urban growth strictly refers to the absolute number of people living in those areas. It is predicted that by 2050 about 64% of the developing world and 86% of the developed world will be urbanized. This is predicted to generate artificial scarcities of land, lack of drinking water, playgrounds and so on for most urban dwellers. The predicted urban population growth is equivalent to approximately 3 billion urbanites by 2050, much of which will occur in Africa and Asia. Notably, the United Nations has also recently projected that nearly all global population growth from 2017 to 2030 will be by cities, with about 1.1 billion new urbanites over the next 10 years. In the long term, urbanization is expected to significantly impact the quality of life in negative ways.

Urbanization is relevant to a range of disciplines, including urban planning, geography, sociology, architecture, economics, education, statistics and public health. The phenomenon has been closely linked to globalization, modernization, industrialization, marketization,  administrative/institutional power. and the sociological process of rationalization. Urbanization can be seen as a specific condition at a set time (e.g. the proportion of total population or area in cities or towns), or as an increase in that condition over time. Therefore, urbanization can be quantified either in terms of the level of urban development relative to the overall population, or as the rate at which the urban proportion of the population is increasing. Urbanization creates enormous social, economic and environmental challenges, which provide an opportunity for sustainability with the "potential to use resources much less or more efficiently, to create more sustainable land use and to protect the biodiversity of natural ecosystems." However, current urbanization trends have shown that massive urbanization has led to unsustainable ways of living. Developing urban resilience and urban sustainability in the face of increased urbanization is at the centre of international policy in Sustainable Development Goal 11 "Sustainable cities and communities."

Urbanization is not merely a modern phenomenon, but a rapid and historic transformation of human social roots on a global scale, whereby predominantly rural culture is being rapidly replaced by predominantly urban culture. The first major change in settlement patterns was the accumulation of hunter-gatherers into villages many thousands of years ago. Village culture is characterized by common bloodlines, intimate relationships, and communal behaviour, whereas urban culture is characterized by distant bloodlines, unfamiliar relations, and competitive behaviour. This unprecedented movement of people is forecast to continue and intensify during the next few decades, mushrooming cities to sizes unthinkable only a century ago. As a result, the world urban population growth curve has up till recently followed a quadratic-hyperbolic pattern.

History
From the development of the earliest cities in Indus valley civilization, Mesopotamia and Egypt until the 18th century, an equilibrium existed between the vast majority of the population who were engaged in subsistence agriculture in a rural context, and small centres of populations in the towns where economic activity consisted primarily of trade at markets and manufactures on a small scale. Due to the primitive and relatively stagnant state of agriculture throughout this period, the ratio of rural to urban population remained at a fixed equilibrium. However, a significant increase in the percentage of the global urban population can be traced in the 1st millennium BCE.

With the onset of the British agricultural and industrial revolution in the late 18th century, this relationship was finally broken and an unprecedented growth in urban population took place over the course of the 19th century, both through continued migration from the countryside and due to the tremendous demographic expansion that occurred at that time. In England and Wales, the proportion of the population living in cities with more than 20,000 people jumped from 17% in 1801 to 54% in 1891. Moreover, and adopting a broader definition of urbanization, while the urbanized population in England and Wales represented 72% of the total in 1891, for other countries the figure was 37% in France, 41% in Prussia and 28% in the United States.

As labourers were freed up from working the land due to higher agricultural productivity they converged on the new industrial cities like Manchester and Birmingham which were experiencing a boom in commerce, trade, and industry. Growing trade around the world also allowed cereals to be imported from North America and refrigerated meat from Australasia and South America. Spatially, cities also expanded due to the development of public transport systems, which facilitated commutes of longer distances to the city centre for the working class.

Urbanization rapidly spread across the Western world and, since the 1950s, it has begun to take hold in the developing world as well. At the turn of the 20th century, just 15% of the world population lived in cities. According to the UN, the year 2007 witnessed the turning point when more than 50% of the world population were living in cities, for the first time in human history.

Yale University in June 2016 published urbanization data from the time period 3700 BC to 2000 AD, the data was used to make a video showing the development of cities on the world during the time period. The origins and spread of urban centres around the world were also mapped by archaeologists.

Causes

Urbanization occurs either organically or planned as a result of individual, collective and state action. Living in a city can be culturally and economically beneficial since it can provide greater opportunities for access to the labour market, better education, housing, and safety conditions, and reduce the time and expense of commuting and transportation. Conditions like density, proximity, diversity, and marketplace competition are elements of an urban environment that deemed beneficial. However, there are also harmful social phenomena that arise: alienation, stress, increased cost of living, and mass marginalization that are connected to an urban way of living. Suburbanization, which is happening in the cities of the largest developing countries, may be regarded as an attempt to balance these harmful aspects of urban life while still allowing access to the large extent of shared resources.

In cities, money, services, wealth and opportunities are centralized. Many rural inhabitants come to the city to seek their fortune and alter their social position. Businesses, which provide jobs and exchange capital, are more concentrated in urban areas. Whether the source is trade or tourism, it is also through the ports or banking systems, commonly located in cities, that foreign money flows into a country.

Many people move into cities for economic opportunities, but this does not fully explain the very high recent urbanization rates in places like China and India. Rural flight is a contributing factor to urbanization. In rural areas, often on small family farms or collective farms in villages, it has historically been difficult to access manufactured goods, though the relative overall quality of life is very subjective, and may certainly surpass that of the city. Farm living has always been susceptible to unpredictable environmental conditions, and in times of drought, flood or pestilence, survival may become extremely problematic.

In a New York Times article concerning the acute migration away from farming in Thailand, life as a farmer was described as "hot and exhausting". "Everyone says the farmer works the hardest but gets the least amount of money". In an effort to counter this impression, the Agriculture Department of Thailand is seeking to promote the impression that farming is "honorable and secure".

However, in Thailand, urbanization has also resulted in massive increases in problems such as obesity. Shifting from a rural environment to an urbanized community also caused a transition to a diet that was mainly carbohydrate-based to a diet higher in fat and sugar, consequently causing a rise in obesity. City life, especially in modern urban slums of the developing world, is certainly hardly immune to pestilence or climatic disturbances such as floods, yet continues to strongly attract migrants. Examples of this were the 2011 Thailand floods and 2007 Jakarta flood. Urban areas are also far more prone to violence, drugs, and other urban social problems. In the United States, industrialization of agriculture has negatively affected the economy of small and middle-sized farms and strongly reduced the size of the rural labour market.

Particularly in the developing world, conflict over land rights due to the effects of globalization has led to less politically powerful groups, such as farmers, losing or forfeiting their land, resulting in obligatory migration into cities. In China, where land acquisition measures are forceful, there has been far more extensive and rapid urbanization (54%) than in India (36%), where peasants form militant groups (e.g. Naxalites) to oppose such efforts. Obligatory and unplanned migration often results in the rapid growth of slums. This is also similar to areas of violent conflict, where people are driven off their land due to violence.

Cities offer a larger variety of services, including specialist services not found in rural areas. These services require workers, resulting in more numerous and varied job opportunities. Elderly people may be forced to move to cities where there are doctors and hospitals that can cater to their health needs. Varied and high-quality educational opportunities are another factor in urban migration, as well as the opportunity to join, develop, and seek out social communities.

Urbanization also creates opportunities for women that are not available in rural areas. This creates a gender-related transformation where women are engaged in paid employment and have access to education. This may cause fertility to decline. However, women are sometimes still at a disadvantage due to their unequal position in the labour market, their inability to secure assets independently from male relatives and exposure to violence.

People in cities are more productive than in rural areas. An important question is whether this is due to agglomeration effects or whether cities simply attract those who are more productive. Urban geographers have shown that there exists a large productivity gain due to locating in dense agglomerations. It is thus possible that agents locate in cities in order to benefit from these agglomeration effects.

Dominant conurbation
The dominant conurbation(s) of a country can get more benefits from the same things cities offer, attracting the rural population and urban and suburban populations from other cities. Dominant conurbations are quite often disproportionately large cities, but do not have to be. For instance Greater Manila is a conurbation instead of a city. Its total population of 20 million (over 20% national population) make it a primate city, but Quezon City (2.7 million), the largest municipality in Greater Manila, and Manila (1.6 million), the capital, are normal cities instead. A conurbation's dominance can be measured by output, wealth, and especially population, each expressed as a percentage of the entire country's. Greater Seoul is one conurbation that dominates South Korea. It is home to 50% of the entire national population.

Though Greater Busan-Ulsan (15%, 8 million) and Greater Osaka (14%, 18 million) dominate their respective countries, their populations are moving to their even more dominant rivals, Seoul and Tokyo respectively.

Economic effects
As cities develop, costs will skyrocket. This often takes the working class out of the market, including officials and employees of the local districts. For example, Eric Hobsbawm's book The age of revolution: 1789–1848 (published 1962 and 2005) chapter 11, stated "Urban development in our period was a gigantic process of class segregation, which pushed the new labouring poor into great morasses of misery outside the centres of government, business, and the newly specialized residential areas of the bourgeoisie. The almost universal European division into a 'good' west end and a 'poor' east end of large cities developed in this period." This is probably caused by the south-west wind which carries coal smoke and other pollutants down, making the western edges of towns better than the eastern ones.

Similar problems now affect less developed countries, as rapid development of cities makes inequality worse. The drive to grow quickly and be efficient can lead to less fair urban development. Think tanks such as the Overseas Development Institute have proposed policies that encourage labour-intensive to make use of the migration of less skilled workers. One problem these migrant workers are involved with is the growth of slums. In many cases, the rural-urban unskilled migrant workers are attracted by economic opportunities in cities. Unfortunately, they cannot find a job and or pay for houses in urban areas and have to live in slums.

Urban problems, along with developments in their facilities, are also fuelling suburb development trends in less developed nations, though the trend for core cities in said nations tends to continue to become ever denser. Development of cities is often viewed negatively, but there are positives in cutting down on transport costs, creating new job opportunities, providing education and housing, and transportation. Living in cities permits individuals and families to make use of their closeness to workplaces and diversity. While cities have more varied markets and goods than rural areas, facility congestion, domination of one group, high overhead and rental costs, and the inconvenience of trips across them frequently combine to make marketplace competition harsher in cities than in rural areas.

In many developing countries where economies are growing, the growth is often random and based on a small number of industries. Youths in these nations lack access to financial services and business advisory services, cannot get credit to start a business, and have no entrepreneurial skills. Therefore, they cannot seize opportunities in these industries. Making sure adolescents have access to excellent schools and infrastructure to work in such industries and improve schools is compulsory to promote a fair society.

Environmental effects
Furthermore, urbanization improves environmental eminence through superior facilities and standards in urban areas as compared to rural areas. Lastly, urbanization curbs pollution emissions by increasing innovations. In his 2009 book Whole Earth Discipline, Stewart Brand argues that the effects of urbanization are primarily positive for the environment. First, the birth rate of new urban dwellers falls immediately to replacement rate and keeps falling, reducing environmental stresses caused by population growth. Secondly, emigration from rural areas reduces destructive subsistence farming techniques, such as improperly implemented slash and burn agriculture. Alex Steffen also speaks of the environmental benefits of increasing the urbanization level in "Carbon Zero: Imagining Cities that can save the planet",.

However, existing infrastructure and city planning practices are not sustainable. In July 2013 a report issued by the United Nations Department of Economic and Social Affairs warned that with 2.4 billion more people by 2050, the amount of food produced will have to increase by 70%, straining food resources, especially in countries already facing food insecurity due to changing environmental conditions. The mix of changing environmental conditions and the growing population of urban regions, according to UN experts, will strain basic sanitation systems and health care, and potentially cause a humanitarian and environmental disaster.

Urban heat island
Urban heat islands have become a growing concern over the years. An urban heat island is formed when industrial areas absorb and retain heat. Much of the solar energy reaching rural areas is used to evaporate water from plants and soil. In cities, there are less vegetation and exposed soil. Most of the sun's energy is instead absorbed by buildings and asphalt; leading to higher surface temperatures. Vehicles, factories, and heating and cooling units in factories and homes release even more heat. As a result, cities are often 1 to 3 C-change warmer than other areas near them. Urban heat islands also make the soil drier and absorb less carbon dioxide from emissions.

Water quality
Urban runoff, polluted water created by rainfall on impervious surfaces, is a common effect of urbanization. Precipitation from rooftops, roads, parking lots and sidewalks flows to storm drains, instead of percolating into groundwater. The contaminated stormwater in the drains is typically untreated and flows to nearby streams, rivers or coastal bays.

Eutrophication in water bodies is another effect large populations in cities have on the environment. When rain occurs in these large cities, it filters CO2 and other pollutants in the air onto the ground. These chemicals are washed directly into rivers, streams, and oceans, making water worse and damaging ecosystems in them.

Eutrophication is a process which causes low levels of oxygen in water and algal blooms that may harm aquatic life. Harmful algal blooms make dangerous toxins. They live best in nitrogen- and phosphorus-rich places which include the oceans contaminated by the aforementioned chemicals. In these ideal conditions, they choke surface water, blocking sunlight and nutrients from other life forms. Overgrowth of algal blooms makes water worse overall and disrupts the natural balance of aquatic ecosystems. Furthermore, as algal blooms die, CO2 is produced. This makes the ocean more acidic, a process called acidification.

The ocean's surface can absorb CO2 from the Earth's atmosphere as emissions increase with the rise in urban development. In fact, the ocean absorbs a quarter of the CO2 produced by humans. This helps to lessen the harmful effects of greenhouse gases. But it also makes the ocean more acidic. A drop in pH the prevents the proper formation of calcium carbonate, which sea creatures need to build or keep shells or skeletons. This is especially true for many species of molluscs and coral. However, some species have been able to thrive in a more acidic environment.

Food waste
Rapid growth of communities creates new challenges in the developed world and one such challenge is an increase in food waste also known as urban food waste. Food waste is the disposal of food products that can no longer be used due to unused products, expiration, or spoilage. The increase of food waste can raise environmental concerns such as increase production of methane gases and attraction of disease vectors. Landfills are the third leading cause of the release of methane, causing a concern on its impact to our ozone and on the health of individuals. Accumulation of food waste causes increased fermentation, which increases the risk of rodent and bug migration. An increase in migration of disease vectors creates greater potential of disease spreading to humans.

Waste management systems vary on all scales from global to local and can also be influenced by lifestyle. Waste management was not a primary concern until after the Industrial Revolution. As urban areas continued to grow along with the human population, proper management of solid waste became an apparent concern. To address these concerns, local governments sought solutions with the lowest economic impacts which meant implementing technical solutions at the very last stage of the process. Current waste management reflects these economically motivated solutions, such as incineration or unregulated landfills. Yet, a growing increase for addressing other areas of life cycle consumption has occurred from initial stage reduction to heat recovery and recycling of materials. For example, concerns for mass consumption and fast fashion have moved to the forefront of the urban consumers' priorities. Aside from environmental concerns (e.g. climate change effects), other urban concerns for waste management are public health and land access.

Habitat fragmentation
Urbanization can have a large effect on biodiversity by causing a division of habitats and thereby alienation of species, a process known as habitat fragmentation. Habitat fragmentation does not destroy the habitat, as seen in habitat loss, but rather breaks it apart with things like roads and railways This change may affect a species ability to sustain life by separating it from the environment in which it is able to easily access food, and find areas that they may hide from predation With proper planning and management, fragmentation can be avoided by adding corridors that aid in the connection of areas and allow for easier movement around urbanized regions.

Depending on the various factors, such as level of urbanization, both increases or decreases in "species richness" can be seen. This means that urbanization may be detrimental to one species but also help facilitate the growth of others. In instances of housing and building development, many times vegetation is completely removed immediately in order to make it easier and less expensive for construction to occur, thereby obliterating any native species in that area. Habitat fragmentation can filter species with limited dispersal capacity. For example, aquatic insects are found to have lower species richness in urban landscapes. The more urbanized the surrounding of habitat is, the fewer species can reach the habitat. Other times, such as with birds, urbanization may allow for an increase in richness when organisms are able to adapt to the new environment. This can be seen in species that may find food while scavenging developed areas or vegetation that has been added after urbanization has occurred i.e. planted trees in city areas

Health and social effects
In the developing world, urbanization does not translate into a significant increase in life expectancy. Rapid urbanization has led to increased mortality from non-communicable diseases associated with lifestyle, including cancer and heart disease. Differences in mortality from contagious diseases vary depending on the particular disease and location.

Urban health levels are on average better in comparison to rural areas. However, residents in poor urban areas such as slums and informal settlements suffer "disproportionately from disease, injury, premature death, and the combination of ill-health and poverty entrenches disadvantage over time." Many of the urban poor have difficulty accessing health services due to their inability to pay for them; so they resort to less qualified and unregulated providers.

While urbanization is associated with improvements in public hygiene, sanitation and access to health care, it also entails changes in occupational, dietary, and exercise patterns. It can have mixed effects on health patterns, alleviating some problems, and accentuating others.

Nutrition
One such effect is the formation of food deserts. Nearly 23.5 million people in the United States lack access to supermarkets within one mile of their home. Several studies suggest that long distances to a grocery store are associated with higher rates of obesity and other health disparities.

Food deserts in developed countries often correspond to areas with a high-density of fast food chains and convenience stores that offer little to no fresh food. Urbanization has been shown to be associated with the consumption of less fresh fruits, vegetables, and whole grains and a higher consumption of processed foods and sugar-sweetened beverages. Poor access to healthy food and high intakes of fat, sugar and salt are associated with a greater risk for obesity, diabetes and related chronic disease. Overall, body mass index and cholesterol levels increase sharply with national income and the degree of urbanization.[40]

Food deserts in the United States are most commonly found in low-income and predominately African American neighbourhoods. One study on food deserts in Denver, Colorado found that, in addition to minorities, the affected neighbourhoods also had a high proportion of children and new births. In children, urbanization is associated with a lower risk of under-nutrition but a higher risk of being overweight.

Infections
Urbanization has also been linked to the spread of communicable diseases, which can spread more rapidly in the favourable environment with more people living in a smaller area. Such diseases can be respiratory infections and gastrointestinal infections. Other infections could be infections, which need a vector to spread to humans. An example of this could be dengue fever.

Asthma
Urbanization has also been associated with an increased risk of asthma as well. Throughout the world, as communities transition from rural to more urban societies, the number of people affected by asthma increases. The odds of reduced rates of hospitalization and death from asthmas has decreased for children and young adults in urbanized municipalities in Brazil. This finding indicates that urbanization may have a negative impact on population health particularly affecting people's susceptibility to asthma.

In low and middle income countries many factors contribute to the high numbers of people with asthma. Similar to areas in the United States with increasing urbanization, people living in growing cities in low income countries experience high exposure to air pollution, which increases the prevalence and severity of asthma among these populations. Links have been found between exposure to traffic-related air pollution and allergic diseases. Children living in poor, urban areas in the United States now have an increased risk of morbidity due to asthma in comparison to other low-income children in the United States. In addition, children with croup living in urban areas have higher hazard ratios for asthma than similar children living in rural areas. Researchers suggest that this difference in hazard ratios is due to the higher levels of air pollution and exposure to environmental allergens found in urban areas.

Exposure to elevated levels of ambient air pollutants such as nitrogen dioxide (NO2), carbon monoxide (CO), and particulate matter with a diameter of less than 2.5 micrometres (PM2.5), can cause DNA methylation of CpG sites in immune cells, which increases children's risk of developing asthma. Studies have shown a positive correlation between Foxp3 methylation and children's exposure to NO2, CO, and PM2.5. Furthermore, any amount of exposure to high levels of air pollution have shown long term effects on the Foxp3 region.

Despite the increase in access to health services that usually accompanies urbanization, the rise in population density negatively affects air quality ultimately mitigating the positive value of health resources as more children and young adults develop asthma due to high pollution rates. However, urban planning, as well as emission control, can lessen the effects of traffic-related air pollution on allergic diseases such as asthma.

Crime
Historically, crime and urbanization have gone hand in hand. The simplest explanation is that areas with a higher population density are surrounded by greater availability of goods. Committing crimes in urbanized areas is also more feasible. Modernization has led to more crime as well, as the modern media has raised greater awareness of the income gap between the rich and the poor. This leads to feelings of deprivation, which in turn can lead to crime. In some regions where urbanization happens in wealthier areas, a rise in property crime and a decrease in violent crime is seen.

Data shows that there is an increase in crime in urbanized areas. Some factors include per capita income, income inequality, and overall population size. There is also a smaller association between unemployment rate, police expenditures and crime. The presence of crime also has the ability to produce more crime. These areas have less social cohesion and therefore less social control. This is evident in the geographical regions that crime occurs in. As most crime tends to cluster in city centres, the further the distance from the centre of the city, the lower the occurrence of crimes are.

Migration is also a factor that can increase crime in urbanized areas. People from one area are displaced and forced to move into an urbanized society. Here they are in a new environment with new norms and social values. This can lead to less social cohesion and more crime.

Physical activity
Although urbanization tends to produce more negative effects, one positive effect that urbanization has impacted is an increase in physical activity in comparison to rural areas. Residents of rural areas and communities in the United States have higher rates of obesity and engage in less physical activity than urban residents. Rural residents consume a higher percent of fat calories and are less likely to meet the guidelines for physical activity and more likely to be physically inactive. In comparison to regions within the United States, the west has the lowest prevalence of physical inactivity and the south has the highest prevalence of physical inactivity. Metropolitan and large urban areas across all regions have the highest prevalence of physical activity among residents.

Barriers such as geographic isolation, busy and unsafe roads, and social stigmas lead to decreased physical activity in rural environments. Faster speed limits on rural roads prohibits the ability to have bike lanes, sidewalks, footpaths, and shoulders along the side of the roads. Less developed open spaces in rural areas, like parks and trails, suggest that there is lower walkability in these areas in comparison to urban areas. Many residents in rural settings have to travel long distances to utilize exercise facilities, taking up too much time in the day and deterring residents from using recreational facilities to obtain physical activity. Additionally, residents of rural communities are traveling further for work, decreasing the amount of time that can be spent on leisure physical activity and significantly decreases the opportunity to partake in active transportation to work.

Neighbourhoods and communities with nearby fitness venues, a common feature of urbanization, have residents that partake in increased amounts of physical activity. Communities with sidewalks, street lights, and traffic signals have residents participating in more physical activity than communities without those features. Having a variety of destinations close to where people live, increases the use of active transportation, such as walking and biking. Active transportation is also enhanced in urban communities where there is easy access to public transportation due to residents walking or biking to transportation stops.

In a study comparing different regions in the United States, opinions across all areas were shared that environmental characteristics like access to sidewalks, safe roads, recreational facilities, and enjoyable scenery are positively associated with participation in leisure physical activity. Perceiving that resources are nearby for physical activity increases the likelihood that residents of all communities will meet the guidelines and recommendations for appropriate physical activity. Specific to rural residents, the safety of outdoor developed spaces and convenient availability to recreational facilities matters most when making decisions on increasing physical activity. In order to combat the levels of inactivity in rural residents, more convenient recreational features, such as the ones discussed in this paragraph, need to be implemented into rural communities and societies.

Mental health
Urbanization factors that contribute to mental health can be thought of as factors that affect the individual and factors that affect the larger social group. At the macro, social group level, changes related to urbanization are thought to contribute to social disintegration and disorganization. These macro factors contribute to social disparities which affect individuals by creating perceived insecurity. Perceived insecurity can be due problems with the physical environment, such as issues with personal safety, or problems with the social environment, such as a loss of positive self-concepts from negative events. Increased stress is a common individual psychological stressor that accompanies urbanization and is thought to be due to perceived insecurity. Changes in social organization, a consequence of urbanization, are thought to lead to reduced social support, increased violence, and overcrowding. It is these factors that are thought to contribute to increased stress.

A 2004 study of 4.4 million Swedish residents found that.

Changing forms
Different forms of urbanization can be classified depending on the style of architecture and planning methods as well as the historic growth of areas.

In cities of the developed world urbanization traditionally exhibited a concentration of human activities and settlements around the downtown area, the so-called in-migration. In-migration refers to migration from former colonies and similar places. The fact that many immigrants settle in impoverished city centres led to the notion of the "peripheralization of the core", which simply describes that people who used to be at the periphery of the former empires now live right in the centre.

Recent developments, such as inner-city redevelopment schemes, mean that new arrivals in cities no longer necessarily settle in the centre. In some developed regions, the reverse effect, originally called counter urbanization has occurred, with cities losing population to rural areas, and is particularly common for richer families. This has been possible because of improved communications and has been caused by factors such as the fear of crime and poor urban environments. It has contributed to the phenomenon of shrinking cities experienced by some parts of the industrialized world.

Rural migrants are attracted by the possibilities that cities can offer, but often settle in shanty towns and experience extreme poverty. The inability of countries to provide adequate housing for these rural migrants is related to overurbanization, a phenomenon in which the rate of urbanization grows more rapidly than the rate of economic development, leading to high unemployment and high demand for resources. In the 1980s, this was attempted to be tackled with the urban bias theory which was promoted by Michael Lipton.

Most of the urban poor in developing countries unable to find work can spend their lives in insecure, poorly paid jobs. According to research by the Overseas Development Institute pro-poor urbanization will require labour-intensive growth, supported by labour protection, flexible land use regulation and investments in basic services.'

Suburbanization
When the residential area shifts outward, this is called suburbanization. A number of researchers and writers suggest that suburbanization has gone so far to form new points of concentration outside the downtown both in developed and developing countries such as India. This networked, poly-centric form of concentration is considered by some emerging pattern of urbanization. It is called variously edge city (Garreau, 1991), network city (Batten, 1995), postmodern city (Dear, 2000), or exurb, though the latter term now refers to a less dense area beyond the suburbs. Los Angeles is the best-known example of this type of urbanization. In the United States, this process has reversed as of 2011, with "re-urbanization" occurring as suburban flight due to chronically high transport costs.

Planned urbanization
Urbanization can be planned urbanization or organic. Planned urbanization, i.e.: planned community or the garden city movement, is based on an advance plan, which can be prepared for military, aesthetic, economic or urban design reasons. Examples can be seen in many ancient cities; although with exploration came the collision of nations, which meant that many invaded cities took on the desired planned characteristics of their occupiers. Many ancient organic cities experienced redevelopment for military and economic purposes, new roads carved through the cities, and new parcels of land were cordoned off serving various planned purposes giving cities distinctive geometric designs. UN agencies prefer to see urban infrastructure installed before urbanization occurs. Landscape planners are responsible for landscape infrastructure (public parks, sustainable urban drainage systems, greenways etc.) which can be planned before urbanization takes place, or afterwards to revitalize an area and create greater livability within a region. Concepts of control of the urban expansion are considered in the American Institute of Planners.

As population continues to grow and urbanize at unprecedented rates, new urbanism and smart growth techniques are implemented to create a transition into developing environmentally, economically, and socially sustainable cities. Additionally, a more well-rounded approach articulates the importance to promote participation of non-state actors, which could include businesses, research and non-profit organizations and, most importantly, local citizens. Smart Growth and New Urbanism's principles include walkability, mixed-use development, comfortable high-density design, land conservation, social equity, and economic diversity. Mixed-use communities work to fight gentrification with affordable housing to promote social equity, decrease automobile dependency to lower use of fossil fuels, and promote a localized economy. Walkable communities have a 38% higher average GDP per capita than less walkable urban metros (Leinberger, Lynch). By combining economic, environmental, and social sustainability, cities will become equitable, resilient, and more appealing than urban sprawl that overuses land, promotes automobile use, and segregates the population economically.

Urbanization throughout the world
Presently, most countries in the world are urbanized, with the global urbanization average numbering 56.2% in 2020. However, there are great differences between some regions; the nations of Europe, the Middle East, the Americas and East Asia are predominantly urbanized. Meanwhile, two large belts (from central to eastern Africa, and from central to southeast Asia) of very lowly urbanized countries exist, as seen on the map here. These labeled countries are among the least urbanized.

Historical

 * Neolithic Revolution
 * Oppidum
 * Polis
 * Urban revolution

Regional

 * Urbanization in Africa
 * Urbanization in China
 * Urbanization in India
 * Urbanization in Pakistan
 * Urbanization in the United States

title: Urbanization


text:
Salting the earth

Salting the earth, or sowing with salt, is the ritual of spreading salt on the sites of cities razed by conquerors. It originated as a curse on re-inhabitation in the ancient Near East and became a well-established folkloric motif in the Middle Ages. The best-known example is the salting of Shechem as narrated in the Biblical Book of Judges, 9:45. The supposed salting of Carthage is not supported by historical evidence.

Cities
The custom of purifying or consecrating a destroyed city with salt and cursing anyone who dared to rebuild it was widespread in the ancient Near East, but historical accounts are unclear as to what the sowing of salt meant in that process. In the case of Shechem, various commentaries explain it as:
 * ...a covenantal curse, a means of ensuring desolation, a ritual to avert the vengeance of the shades of the slaughtered, a purification of the site preparatory to rebuilding, or a preparation for final destruction under the herem ritual.

Ancient Near East
Various Hittite and Assyrian texts speak of ceremonially strewing salt, minerals, or plants (weeds, "cress", or kudimmu, which are associated with salt and desolation ) over destroyed cities, including Hattusa, Taidu, Arinna, Hunusa, Irridu, and Susa. The Book of Judges (9:45) says that Abimelech, the judge of the Israelites, sowed his own capital, Shechem, with salt, c. 1050 BC, after quelling a revolt against him. This may have been part of a ḥērem ritual (see Salt in the Bible).

Carthage
At least as early as 1863, various texts claimed that the Roman general Scipio Aemilianus plowed over and sowed the city of Carthage with salt after defeating it in the Third Punic War (146 BC), sacking it, and enslaving the survivors. The salting was probably modeled on the story of Shechem. Though ancient sources do mention symbolically drawing a plow over various cities and salting them, none mention Carthage in particular. The salting story entered the academic literature in Bertrand Hallward's article in the first edition of the Cambridge Ancient History (1930), and was widely accepted as factual. However, there are no ancient sources for it and it is now considered apocryphal.

Palestrina
When Pope Boniface VIII destroyed Palestrina in 1299, he ordered that it be plowed "following the old example of Carthage in Africa", and also salted. "I have run the plough over it, like the ancient Carthage of Africa, and I have had salt sown upon it ..." The text is not clear as to whether he thought Carthage was salted. Later accounts of other saltings in the destructions of medieval Italian cities are now rejected as unhistorical: Padua by Attila (452), perhaps in a parallel between Attila and the ancient Assyrians; Milan by Frederick Barbarossa (1162); and Semifonte by the Florentines (1202).

Jerusalem
The English epic poem Siege of Jerusalem (c. 1370) recounts that Titus commanded the sowing of salt on the Temple, but this episode is not found in Josephus's account.

Spanish Empire
In Spain and the Spanish Empire, salt was poured onto the land owned by a convicted traitor (often one who was executed and his head placed on a picota, or pike, afterwards) after his house was demolished.

Portugal
This was done in Portugal as well. The last known event of this sort was the destruction of the Duke of Aveiro's palace in Lisbon in 1759, due to his participation in the Távora affair (a conspiracy against King Joseph I of Portugal). His palace was demolished and his land was salted. A stone memorial now perpetuates the memory of the shame of the Duke, where it is written: "In this place were put to the ground and salted the houses of José Mascarenhas, stripped of the honours of Duque de Aveiro and others ... Put to Justice as one of the leaders of the most barbarous and execrable upheaval that ... was committed against the most royal and sacred person of the Lord Joseph I. In this infamous land nothing may be built for all time."

Brazil
In the Portuguese colony of Brazil, the leader of the Inconfidência Mineira, Tiradentes, was sentenced to death and his house was "razed and salted, so that never again be built up on the floor, ... and even the floor will rise up a standard by which the memory is preserved (preserving) the infamy of this heinous offender ..." He suffered further indignities, being hanged and quartered, his body parts carried to various parts of the country where his fellow revolutionaries had met, and his children deprived of their property and honor.

Legends
An ancient legend recounts that Odysseus feigned madness by yoking a horse and an ox to his plow and sowing salt.

title: Salting the earth