Extinction risk from climate change

A January 2004 paper published inNature estimated that between 15 and 37% of 1103 endemic or near-endemic known plant and animal species will be "committed to extinction" by 2050.[10] More properly, changes in habitat by 2050 will put them outside the survival range for the inhabitants, thus committing the species to extinction.

Other researchers, such as Thuiller et al.,[11] Araújo et al.,[12] Person et al.,[13] Buckley and Roughgarden,[14] and Harte et al.[15] have raised concern regarding uncertainty in Thomas et al.'s projections; some of these studies believe it is an overestimate, others believe the risk could be greater. Thomas et al. replied in Nature [16] addressing criticisms and concluding "Although further investigation is needed into each of these areas, it is unlikely to result in substantially reduced estimates of extinction. Anthropogenic climate change seems set to generate very large numbers of species-level extinctions." On the other hand, Daniel Botkin et al. state "... global estimates of extinctions due to climate change (Thomas et al. 2004) may have greatly overestimated the probability of extinction..."[17]

Mechanistic studies are documenting extinctions due to recent climate change: McLaughlin et al. documented two populations of Bay checkerspot butterfly being threatened by precipitation change.[18] Parmesan states, "Few studies have been conducted at a scale that encompasses an entire species"[19] and McLaughlin et al. agreed "few mechanistic studies have linked extinctions to recent climate change."[18]

In 2008, the white lemuroid possum was reported to be the first known mammal species to be driven extinct by climate change. However, these reports were based on a misunderstanding. One population of these possums in the mountain forests of North Queensland is severely threatened by climate change as the animals cannot survive extended temperatures over 30 °C. However, another population 100 kilometres south remains in good health.[20]

The risk of extinction does need to lead to a demonstrable extinction process to validate future extinctions attributable to climate change. In a study led by Barry Sinervo,[21] a mathematical-biologist at the University of California Santa Cruz, researchers analyzed observed contemporary extinctions (since dramatic modern climate warming began in 1975). Results of the study indicate that climate-forced extinctions of lizard families of the world have already started. The model is premised on the ecophysiological limits of an organism being exceeded. In the case of lizards, this occurs when their preferred body temperature is exceeded in their local environment. Lizards are ectotherms that regulate body temperature using heat sources of their local environment (the sun, warm air temperatures, or warm rocks). Surveys of 200 sites in Mexico showed 24 local extinctions (= extirpations), of Sceloporus lizards. Using a model developed from these observed extinctions the researchers surveyed other extinctions around the world and found that the model predicted those observed extirpations, thus attributing the extirpations around the world to climate warming. These models predict that extinctions of the lizard species around the world will reach 20% by 2080, but up to 40% extinctions in tropical ecosystems where the lizards are closer to their ecophysiological limits than lizards in the temperate zone.[22]

A July review suggested that most projections of extinction caused by climate change till then appear to have underestimated impacts, as they predicted average extinction rate of 7%-10% by 2100, while the extrapolation of observed responses resulted in 14%-15% extinction rate.[23]

January research published in Proceedings of the Royal Society B journal had also suggested that the previous projections of the effect of climate change on biodiversity overlooked two important factors: the differences in how quickly species relocate and competition among species. According to the researchers, led by Mark C. Urban, an ecologist at the University of Connecticut, diversity decreased when they took these factors into account, and that new communities of organisms, which do not exist today, emerged. As a result, the rate of extinctions may be higher than previously projected.[24] This research by Urban served as the starting point for the later projections issued by IPBES.[25]

Another January paper connected climate change with the observed decline in numbers and range reduction of the rusty blackbird, a formerly common yet currently vulnerable North American species.[26]

In February, a paper looked at 12 900 islands in the Pacific Ocean and Southeast Asia which host over 3000 vertebrates, and how they would be affected by sea level rise of 1, 3 and 6 meters (with the last two levels not anticipated until after this century). Depending on the extent of sea level rise, 15–62% of islands studied would be completely underwater, and 19–24% will lose 50–99% of their area. This was correlated with the total habitat loss for 37 species under 1 meter of sea level rise, and for 118 species under 3 meters.

A June study found that out of 16,857 bird, amphibian and coral species analyzed, 608–851 bird (6–9%), 670–933 amphibian (11–15%) and 47–73 coral species (6–9%) are vulnerable to climate change while already threatened with extinction according to the IUCN Red List. Meanwhile, 1,715–4,039 (17–41%) bird species, 698–1,807 (11–29%) amphibian species and 74–174 (9–22%) coral species were not vulnerable to extinction at the time of publication, but could be threatened under continued climate change, making them a future conservation priority.[27]

In May, a Science, journal paper found that most known species have small ranges, and the numbers of small-ranged species are increasing quickly. They are geographically concentrated and are disproportionately likely to be threatened or already extinct. According to the research, current rates of extinction are three orders of magnitude higher than the background extinction rate, and future rates, which depend on many factors, are poised to increase. Although there has been rapid progress in developing protected areas, such efforts are not ecologically representative, nor do they optimally protect biodiversity. In the researchers' view, human activity tends to destroy critical habitats where species live, warms the planet, and tends to move species around the planet to places where they do not belong and where they can come into conflict with human needs (e.g. causing species to become pests).[28][29]

Bumblebee collecting pollen.

A long-term study of more than 60 bee species published in the journal Science found that climate change causes drastic declines in the population and diversity of bumblebees across North America and Europe, at rates "consistent with a mass extinction." North America's bumblebee populations fell by 46% during the two time periods the study used, which were from 1901 to 1974 and from 2000 to 2014. North America's bumblebee populations fell by 46% because bee populations were hardest hit in warming southern regions such as Mexico. According to the study, there have been more frequent extreme warm years, which exceeded the species’ historical temperature ranges.[30]

In June, it was projected that native forest birds in Hawaii would be threatened with extinction due to the spread of avian malaria under the high-warming RCP 8.5 scenario or a similar scenario from earlier modelling, but would persist under the less extreme RCP 4.5[31]

In October, a study looked at the future persistence of the common lizard populations in Europe under future climate change. It found that under 2 °C (3.6 °F), 11% of the lizard population would be threatened with local extinction around 2050 and 14% by 2100. At 3 °C (5.4 °F) by 2100, 21% of the population are threatened, and at 4 °C (7.2 °F), 30% of the populations are.[32]

In 2016, the Bramble Cay melomys, which lived on a Great Barrier Reef island, was reported to probably be the first mammal to become extinct because of sea level rises due to human-made climate change.[1]

It was also reported that by 2060, one-third of the observed Adélie penguin colony along the West Antarctic Peninsula (WAP) will be in decline. The Adelie penguins are a circumpolar species, used to the ranges of Antarctic climate, and experiencing population decline. Climate model projections predict sanctuary for the species past 2099. The observed population is similarly proportional to the species-wide population (one-third of the observed population is equal to 20% of the species-wide population).[33]

Green sea turtle grazing grass.

Sex ratios for sea turtles in the Caribbean are being affected because of climate change. Environmental data were collected from the annual rainfall and tide temperatures over the course of 200 years and showed an increase in air temperature (mean of 31.0 degree Celsius). These data were used to relate the decline of the sex ratios of sea turtles in the North East Caribbean and climate change. The species of sea turtles include Dermochelys coriacea, Chelonia myads, and Eretmochelys imbricata. Extinction is a risk for these species as the sex ratio is being afflicted causing a higher female to male ratio. Projections estimate the declining rate of male Chelonia myads as 2.4% hatchlings being male by 2030 and 0.4% by 2090.[34]

A July paper found that under RCP 8.5, the scenario of continually increasing greenhouse gas emissions, numerous vulnerable and endangered vertebrate species living on the low-lying islands in the Pacific Ocean would be threatened by high waves at the end of the century, with the risk substantially reduced under the more moderate RCP 4.5 scenario.[35]

In October, it was found that the mountain goat populations of coastal Alaska would go extinct sometime between 2015 and 2085 in half of the considered scenarios of climate change.[36]

In March, a study from the University of East Anglia team analyzed the impacts of 2 °C (3.6 °F) and 4.5 °C (8.1 °F) of warming on 80,000 plant and animal species in 35 of the world's biodiversity hotspots. It found that these areas could lose up to 25% and 50% of their species, respectively: they may or may not be able to survive outside of them. It also estimated that under the higher warming level, Miombo Woodlands of South Africa would lose about 90% of its amphibians, 86% of birds, and 80% of mammals, The Amazon could lose 69% of its plant species, southwestern Australia 89% of its amphibians, the Fynbos in Western Cape egion of South Africa a third of its species and Madagascar 60%.[37][38]

In May, a Science Magazine paper estimated that under 1.5 °C (2.7 °F), 6% of insects, 8% of plants, and 4% of vertebrates would lose over half of their climatically determined geographic range (thus placing them at a greater risk of extinction due to other factors). At 2 °C (3.6 °F), such range losses would be seen in 18% of insects, 16% of plants, and 8% of vertebrates. At 3.2 °C (5.8 °F) (which correlated to the then-current global warming trajectory according to the authors), proportion of species with such range losses increase even further to ~49% of insects, 44% of plants, and 26% of vertebrates.[39] This estimate was later directly cited in the IPCC Sixth Assessment Report. According to the IUCN Red List criteria, such a range loss is sufficient to classify as species as "endangered", and it is considered equivalent to >20% likelihood of extinction over the 10–100 years.[9]

In July, it was found that two prominent species of seagrasses in the Mediterranean Sea would be substantially affected under the worst-case greenhouse gas emission scenario, with Posidonia oceanica losing 75% of its habitat by 2050 and potentially becoming functionally extinct by 2100, while Cymodocea nodosa would lose ~46% of its habitat and then stabilize due to expansion into previously unsuitable areas.[40]

In May, Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) released the summary of its Global Assessment Report on Biodiversity and Ecosystem Services. The report estimated that there are 8 million animal and plant species, including 5.5 million insect species. It found that one million species, including 40 percent of amphibians, almost a third of reef-building corals, more than a third of marine mammals, and 10 percent of all insects are threatened with extinction due to five main stressors. The land use change and sea use change was considered the most important stressor, followed by direct exploitation of organisms (i.e. overfishing). Climate change ranked third, followed by pollution and invasive species. The report concluded that global warming of 2 °C (3.6 °F) over the preindustrial levels would threaten an estimated 5% of the Earth's species with extinction even in the absence of the other four factors, while if the warming reached 4.3 °C (7.7 °F), 16% of the Earth's species would be threatened with extinction. In the oceans, they estimated that in the range between those "low" and "high" global warming scenarios, ocean net primary production would decline by 3% to 10% by the end of the century, while fish biomass would decline by 3% to 25%. Finally, even the lower warming levels of 1.5–2 °C (2.7–3.6 °F) would "profoundly" reduce geographical ranges of the majority of the world's species, thus making them more vulnerable then they would have been otherwise.[25]

In June, it was estimated that multiple bird species endemic to southern Africa's Kalahari Desert (Southern Pied Babblers, Southern Yellow-billed Hornbills and Southern Fiscals) would either be all-but-lost from it or reduced to its eastern fringes by the end of the century, depending on the emission scenario. While the temperatures are not projected to become so high as to kill the birds outright, they would still be high enough to prevent them from sustaining sufficient body mass and energy for breeding.[41]

Another June paper estimated that by 2050, climate change alone could reduce species richness of trees in the Amazon Rainforest by 31–37%, while deforestation alone could be responsible for 19–36%, and the combined effect might reach 58%. The paper's worst-case scenario for both stressors had only 53% of the original rainforest area surviving as a continuous ecosystem by 2050, with the rest reduced to a severely fragmented block.[42]

In the wake of 2019 Amazon rainforest wildfires, the World Wildlife Fund concluded that the jaguar is already "near threatened" and the loss of food supplies and habitat due to the fires make the situation more critical.[43] The fires affect water chemistry (such as decreasing the amount of dissolved oxygen in the water), temperature, and erosion rates, which in turn affects fish and mammals that depend on fish, such as the giant otter (Pteronura brasiliensis).[43]

The unprecedented fires of the 2019–20 Australian bushfire season that swept through 18 million acres (7 million hectares) claimed 29 human lives and stressed Australia's wildlife.[44] Before the fires, only 500 tiny Kangaroo Island dunnarts (Sminthopsis aitkeni) lived on one island; after half the island was burned, it is possible only one survived. Bramble Cay melomys (Melomys rubicola) became the first known casualty of human-caused climate change in 2015 due to rising sea levels and repeated storm surges; the greater stick-nest rat (Leporillus conditor) may be next.[45]

Emus (Dromaius novaehollandiae) are not in danger of total extinction, although they might suffer local extinctions as a result of bushfires; in northern New South Wales, coastal emus could be wiped out by fire .[45] The loss of 8,000 koalas (Phascolarctos cinereus) in NSW alone was significant, and the animals are endangered but not functionally extinct.[46][47]

A February paper studied 538 plant and animal species from around the world and how they responded to rising temperatures. From that sample, they estimated that 16% of all species could go extinct by 2070 under the "moderate" climate change scenario RCP 4.5, but it could be one-third under RCP 8.5, the scenario of continually increasing emissions.[48][49] This finding was later cited in the IPCC Sixth Assessment Report.[50]

Polar bear hopping from melting sea ice.

In July, a study in Nature Climate Change estimated the effects of Arctic sea ice decline on polar bear populations (which rely on the sea ice to hunt seals) under two climate change scenarios. Under high greenhouse gas emissions, at most a few high-Arctic populations will remain by 2100: under more moderate scenario, the species will survive this century, but several major subpopulations will still be wiped out.[51][52]

In September, a meta-analysis found that while 39% of vascular plant species were likely threatened, only 4.1% of this figure could be attributed to climate change, with land use change activities predominating. However, the researchers suggested that this may be more representative of the slower pace of research on effects of climate change on plants. For fungi, it estimated that 9.4% are threatened due to climate change, while 62% are threatened by other forms of habitat loss.[53]

A May study found that two Ethiopian bird species, White-tailed Swallow and Ethiopian Bush-crow, would lose 68-84% and >90% of their range by 2070. As their existing geographical range is already very limited, this means that it would likely end up too small to support a viable population even under the scenario of limited climate change, rendering these species extinct in the wild.[54]

In June, it was estimated that the current great ape range in Africa will decline massively under both the severe RCP 8.5 scenario and the more moderate RCP 4.5. The apes could potentially disperse to new habitats, but those would lie almost completely outside of their current protected areas, meaning that conservation planning needs to be "urgently" updated to account for this.[55]

An August paper published in Nature Communications found that the The "Big Five" mass extinctions were associated with a warming of around 5.2 °C (9.4 °F) and estimated that this level of warming over the preindustrial occurring today would also result in a mass extinction event of the same magnitude (~75% of marine animals wiped out).[56]

In February, the second part of the IPCC Sixth Assessment Report was published. It included median and maximum estimates of the percentage of species at high risk of extinction for every level of warming, with the maximum estimates increasing much more than the medians. For instance, for 1.5 °C (2.7 °F), the median was 9% and the maximum 14%, for 2 °C (3.6 °F) the median was 10% and the maximum 18%, for 3 °C (5.4 °F) the median was 12% and the maximum 29%, for 4 °C (7.2 °F) the median was 13% and the maximum 39%, and for 5 °C (9.0 °F) the median was 15% but the maximum 48%) at 5°C. The report also provided an additional extinction estimate for the most vulnerable groups of species at two levels of warming. At 3.2 °C (5.8 °F) 15% of invertebrates (including 12% of pollinators) were projected to be at a very high risk of extinction, as well as 11% of amphibians (with salamanders the most vulnerable, at 24%) and 10% of flowering plants. At 2 °C (3.6 °F), fewer than 3% of species from all groups would be at a very high risk of extinction, with the exception of salamanders at 7%.[9]

In May, the earlier predictions of iconic bird species getting extirpated from the Kalahari Desert by climate change were verified, as breeding success of the Southern Yellow-billed Hornbills was already observed to collapse in the hottest, southern parts of the desert. It was predicted that those particular subpopulations would disappear by 2027.[57][58]

In July, a survey of 3331 biodiversity experts from around the world was published in the journal Frontiers in Ecology and the Environment. The experts estimated that since the year 1500, around 30% (between 16% and 50%) of all species have been threatened with extinction - including the species which had already gone extinct. With regards to climate change, the experts estimated that 2 °C (3.6 °F) threatens or drives to extinction about 25% of the species, although their estimates ranged from 15% to 40%. When asked about 5 °C (9.0 °F) warming, they believed it would threaten or drive into extinction 50% of the species, with the range between 32 and 70%.[59]

Also in July, a Tohoku University Earth science scholar Kunio Kaiho disputed the 5.2 °C (9.4 °F) warming threshold for mass extinctions reported the previous year. After reanalysing sedimentary rock record, he estimated that the loss of over 60% of marine species and over 35% of marine genera was correlated to a >7 °C (13 °F) global cooling and a 7–9 °C (13–16 °F) global warming, while for the terrestrial tetrapods, the same losses would be seen under ~7 °C (13 °F) of global cooling or warming.[60] In November, his follow-up paper estimated that under what he considered the most likely scenario of climate change, with 3 °C (5.4 °F) of warming by 2100 and 3.8 °C (6.8 °F) by 2500 (based on the average of Representative Concentration Pathways 4.5 and 6.0), would result in 8% marine species extinctions, 16–20% terrestrial animal species extinctions, and a combined average of 12–14% animal species extinctions (The paper did not calculate estimates for plant extinctions). This was defined by the paper as a minor mass extinction, comparable to the end-Guadalupian and Jurassic–Cretaceous boundary events. It also cautioned that warming needed to be kept below 2.5 °C (4.5 °F) to prevent an extinction of >10% of animal species. Finally, it estimated that a minor nuclear war (defined as a nuclear exchange between India and Pakistan or an event of equivalent magnitude) would cause extinctions of 10–20% of species on its own, while a major nuclear war (defined as a nuclear exchange between United States and Russia) would cause the extinctions of 40-50% species.[61]

An August study estimated that while right now, 14.8% of the global range of all anurans is in a risk area, this will increase to 30.7% by 2100 under Shared Socioeconomic Pathway SSP1-2.6 (low emission pathway), 49.9% under SSP2-4.5, 59.4% under SSP3-7.0 and 64.4% under the highest-emitting SSP5-8.5. Extreme-sized anuran species are disproportionately affected: while currently only 0.3% of these species have >70% of their range in a risk area Of these species, this number will increase to 3.9% under SSP1-2.6, 14.2% under SSP2-4.5, 21.5% under SSP3-7 and 26% under SSP5-8.5[62]

In December, a Science Advances paper estimated that local extinctions of 6% of vertebrates alone would occur by 2050 under the "intermediate" SSP2-4.5, and 10.8% under the pathway of continually increasing emissions SSP5-8.5. By 2100, those would increase to ~13% and ~27%, respectively. These estimates included local extinctions from all causes, not just climate change: however, it was estimated to account for the majority (~62%) of extinctions, followed by secondary extinctions or coextinctions (~20%), with land use change and invasive species combined accounting for less than 20%.[63]

In January, a study estimated the proportion of vertebrates which would exposed to extreme heat beyond what they were known to have experienced historically in at least half their distribution by the end of the century. Under the highest-emission pathway SSP5–8.5 (a warming of 4.4 °C (7.9 °F) by 2100, according to the paper), this would include ~41% of all land vertebrates (31.1% mammals, 25.8% birds, 55.5% amphibians and 51% reptiles) . Under intermediate-high emission scenarios, estimates for all vertebrates are On the other hand, SSP1–2.6 (1.8 °C (3.2 °F) by 2100) would only see 6.1% of vertebrate species exposed to unprecendented heat in at least of their area, while SSP2–4.5 (2.7 °C (4.9 °F) by 2100) and SSP3–7.0 ((3.6 °C (6.5 °F) by 2100) would see 15.1% and 28.8%, respectively.[64]