Every Fraction of a Degree: Why 0.1°C of Warming Adds Millions at Risk

"You could think of it a bit like a tipping point for the human organism or body," Lenton says. "At some point, our physiology kind of reaches a tipping point and the worst tipping point is death."

What happens to coral reefs?

Corals are the architects of entire ecosystems. As they grow, they build the framework that supports vast numbers of other species, making reefs biodiversity hotspots. Hundreds of millions of people depend on them for food and livelihoods.

They're also highly vulnerable to heat. Prolonged marine heat waves cause corals to bleach — turning white — which can eventually kill them. Since the late 1990s, the world has seen four global bleaching events, and with a historically strong El Niño now raising water temperatures, scientists say a fifth is likely unfolding.

The numbers are already grim. Even before the previous bleaching event, which ran from 2023 to 2025, researchers found that 44% of coral species globally are at risk of extinction.

"These are incredibly worrying statistics, but they also emphasize that time matters," says David Obura, director of the coral research organization CORDIO East Africa, who worked on the study published last month in the journal Nature. "Every ten years, every five years counts."

The survival of most reefs hinges on the difference between 1.5 and 2°C. The Intergovernmental Panel on Climate Change found that beyond 2°C of warming, 99% of corals would be lost.

"At 2 degrees of warming, we anticipate most corals just won't survive, not in functional ways to build a reef ecosystem," Obura says. "You might still find some of the species, but many of them may be so rare or so compromised that they will have gone extinct by then."

Conservation groups and coastal communities aren't waiting. They're restoring corals, cutting water pollution, and even breeding more heat-tolerant strains to make reefs as resilient as possible.

"Most people around the world don't have the luxury of giving up on degraded ecosystems, because that's where they live," Obura says. "That's all they have. So we have to work out how to make them function again."

How does warming fuel storms and sea level rise?

Warmer air holds about 7% more water vapor for every degree of warming, which translates into heavier downpours. Rainstorms are already growing more intense across most of the U.S.

Researchers also warn of larger-scale tipping points, such as the collapse of Antarctic glaciers. If those glaciers destabilize, the melt would add vast volumes of water to the ocean and raise sea levels globally.

"We're never exactly sure where the tipping point is until we cross it, but we can be 100% confident that, for most of them, more warming is a bigger likelihood of crossing them," Lenton says. "Less warming is controlling that risk and that's why we should fight for every fraction of a degree that we can stave off."

What's on the table in Turkey?

Don't expect major new emissions-cutting commitments from COP31. Instead, discussions are focusing on a new goal for electrification — getting more cars and homes to run on electricity instead of fossil fuels. That would help cut overall emissions, because a growing share of the world's electricity now comes from cleaner sources like solar and wind.

The science, in short, hasn't changed the politics. But it has sharpened the arithmetic: smaller numbers of degrees, and even tenths of degrees, translate directly into lives, reefs, and coastlines spared.