The planet is about 1.3 degrees Celsius warmer today than it was in the late 1800s. That seemingly small increase has impacted the natural world in some pretty profound ways. Birds have become smaller. Lizards, insects, and snails have changed color. Some goats have become more nocturnal. These are adaptations that help animals survive climate change.
Many species, though, haven’t been able to adapt fast enough. Rising temperatures have not only eroded animal populations, such as by stoking wildlife-killing wildfires in Australia and the Amazon, but they have also driven entire species to extinction. Several years ago, an Australian rodent called Bramble Cay melomys went extinct, largely due to sea level rise. Warming temperatures spread disease-ridden mosquitos into higher elevations in Hawaii, killing every last individual of certain bird species.
That’s the situation today. It’s already bleak. So what happens to wildlife populations if — or more like when — Earth gets even hotter?
That urgent question is at the center of a new study, published in the journal Science. The research analyzes how different degrees of warming, relative to the pre-industrial-era average, affect the portion of species globally that are at risk of going extinct.
The numbers reported in the study are alarming, but they also underscore an important message: If countries can rein in their greenhouse gas emissions, they stand to save thousands of species from meeting their permanent end.
Current climate policies put half a million species at risk of extinction
This new study is a meta-analysis, meaning it synthesized the results of other existing studies — 485 of them, to be precise. It estimates the percentage of known plant and animal species globally that are projected to become extinct under different climate scenarios in the future. Those scenarios include the current level of warming and targets under the Paris Agreement, a global UN agreement to curb climate change, as well as more extreme emissions scenarios.
- 1.3 degrees (current warming): 1.6 percent of species
- 1.5 degrees (aspirational Paris Agreement target): 1.8 percent of species
- 2 degrees (official Paris agreement target): 2.7 percent of species
- 2.7 degrees (where current policies and pledges get us): 5 percent of species
- 4.3 degrees (higher emissions scenario): 14.9 percent of species
- 5.4 degrees (a worst-case warming scenario): 29.7 percent of species
Scientists still don’t have a firm grasp of how many species there are on Earth, so it’s hard to translate these percentages into actual numbers of extinct animals. Scientists now think there are 10 million species or more, according to ecologist Mark Urban, the study’s sole author. That would put projected losses under current warming at around 160,000 species, rising to half a million should the world fail to enact additional policies to curb carbon emissions. That is a lot. We are talking about potentially losing half a million species to extinction from climate change.
“There’s this acceleration of extinction risk with each increment of temperature rise,” said Urban, director of the Center of Biological Risk at the University of Connecticut, told Vox.
To be clear, Urban is reporting extinction risk. This is different from extinction certainty. He compares a species at risk of extinction to a jug of water with a crack in it: “We know the water is pouring out,” he said. “What we don’t know, however, is how big the crack is. So we don’t know how long it’ll take [for the water to run out].” Meaning, for the species to go extinct.
The study also has some important limitations. Models like this are only as good as the data that feeds into them, said Chrystal Mantyka-Pringle, a researcher at Wildlife Conservation Society Canada who was not a part of the study. And research tends to be biased toward certain types of animals and areas that are easier (and cheaper) to study, such as forests in the US or Europe and not, say, the Arctic. Yet the study is sound, according to Mantyka-Pringle and Ilya Maclean, a conservation ecologist at the University of Exeter, who was also unaffiliated with this research. “It’s a pulse check on where we are,” Mantyka-Pringle told Vox.
Frogs and animals on mountains and islands are most at risk
The paper also explored which animals and habitats are most and least at risk from warming. Birds, for example, face a lower risk of extinction, Urban found, likely because they can easily move around. As the ideal habitat for avian species shifts toward the poles or up mountains, birds will try to shift with it. That said, warming still poses a serious problem to many avian species, and other research suggests birds face a high extinction risk in North America.
On the flip side, organisms that can’t easily move, such as plants or those that are highly dependent on water, will be more at risk, Urban says. Indeed, Urban’s analysis found that amphibians, including frogs, are most sensitive to a warming climate. Climate change can intensify droughts, and frogs need water. Hotter temperatures may also contribute to the toll of chytrid fungus, a deadly pathogen that has wiped out dozens of frog species. What’s more is that amphibians don’t typically travel very far, so it’s harder for them to simply shift their habitat.
As for different habitats, mountains, islands, and freshwater ecosystems appear most sensitive to climate change, according to the analysis. Again, this isn’t shocking. Animals that live on mountains can move to higher elevations as temperatures warm, but they’ll eventually reach the top and literally just run out of room. That’s led some scientists to describe climate change as an “escalator to extinction” for mountain species.
Islands face similar space issues, and they tend to be home to species with smaller populations to begin with. Plus, they already contend with other threats such as invasive species, which warming can make worse. This point is key: Climate change is just one threat, but it often intensifies other ones, such as droughts tied to deforestation or the spread of wildlife disease.
Are these numbers really that bad?
Barring the worst-case scenarios for greenhouse emissions, the study projects that a relatively small portion of species will go extinct from climate change — a single-digit percentage. Is that really a problem?
That is the widely held answer from ecologists, at least. Even a small percentage translates to tens of thousands of species, each of which has value for human society, whether or not we have figured it out.
A single animal species may be crucial to a community’s culture, such as the Chinook salmon, which is considered sacred by some Indigenous tribes in the American West. And we already know that the natural world holds the cure for many life-threatening diseases. Researchers estimate that an astonishing 70 percent of antibiotics and cancer treatments in use today are rooted in natural organisms.
“Most humans love nature, and that’s one aspect of it, but biodiversity is also the foundation of our health and our wealth and our cultures,” Urban said.
Allowing even one species to go extinct is like playing Russian roulette, he said.
“We don’t know what we’re losing,” Urban said. “But when you really lose a species, that is truly irreversible. That’s the end.”