Without action to stave off climate change, up to half of the plant and animal species in some of the world's most biologically diverse ecosystems could become locally extinct by the end of the century, according to a new report.
Imagine coastal East Africa missing seven out of 10 amphibians, six out of 10 birds and more than half of its mammals.
Or the Amazon missing two-thirds of every kind of species living there today.
If the world's countries can achieve the goals of the Paris climate agreement, that picture changes, according the study, released late Tuesday in the journal Climatic Change. The Paris Agreement calls for reducing global warming emissions enough to have a reasonable chance of keeping warming below 2 degrees Celsius.
"If you reduce the global temperature rise from 4.5 degrees to 2 degrees, instead of having almost half of the species being potentially lost from each grid cell we studied, that reduces to a quarter," said lead author Rachel Warren.
The study was conducted by a group of scientists from the University of East Anglia, the James Cook University and the conservation organization World Wildlife Fund. The researchers compared how various levels of warming would impact nearly 80,000 plant and animal species in 35 biodiverse regions around the world.
"One of the key messages is that if we're going to avoid the impacts of climate change on biodiversity in these priority places, the most effective way of doing that is climate change mitigation—reducing greenhouse gas emissions," said Warren, a climate change researcher at the University of East Anglia and a coordinating lead author of the Intergovernmental Panel of Climate Change's Fifth Assessment Report.
As part of the Paris Agreement, countries pledged to cut their emissions significantly in the next decade or so. But those pledges still fall short of the Paris goal, and even if they are achieved, the earth is headed toward roughly 3 degrees Celsius of warming. For this study, scientists looked at various scenarios and assessed what each would mean for species.
While middling reductions were better than nothing, they found that only a goal like keeping warming to 2 degrees made a big difference.
"In order for even terrestrial biodiversity to persist in places that we hold dear, we have to come in as close to or lower than 2 degrees as possible," said Jeff Price, a co-author on the study.
Price said the researchers also calculated what would happen at 6 degrees Celsius of warming, though that wasn't included in the published study. "Basically everything falls off the edge of the earth," he said. "Sort of the 'here there be dragons' scenario."
Even at 4.5 degrees Celsius, some of the scenarios explored in the study are pretty dire.
At that amount of warming, the study found that 96 percent of the breeding grounds of Sundarbans tigers in Bangladesh and India could be under water from sea level rise, and that there would be a sharp drop in male marine turtles from temperature-induced sex assignment of eggs.
Among the most affected areas: the Miombo Woodlands in south-central Africa, which are home to African wild dogs; South West Australia; and the Amazons-Guianas in French Guiana.
One key question is whether a given species is able to move along with the changing climate. For birds and certain mammals, there's at least a chance that they can follow their ideal habitat as it shifts toward the poles or into higher altitudes. For reptiles and plants, it's less likely. And for those species already at the poles—like the ones living in the Arctic—there's nowhere further north to go.
Nikhil Advani, the lead specialist on climate communities and wildlife for WWF who provided feedback but wasn't an author of the study, said the findings should be used in conjunction with other research.
Because the work is based on modeling at a high level to see how species are impacted in their range, it's hard to account for variables that will ultimately make a huge difference for species trying to survive climate change. The responses of humans to climate change—how humans use resources when they become more scarce, or migrate as sea levels rise—will have a major impact.