Why even a fraction of a degree can escalate climate change impacts

Coral reefs are highly vulnerable to heat, like these bleached corals in Thailand in 2024. If the planet continues to warm at the current rate, the majority of the world's reefs aren't likely to survive. Sirachai Arunrugstichai/Getty Images hide caption

Countries have gathered at the annual climate change negotiations at the COP31 summit in Turkey to discuss how to achieve a goal that is increasingly unachievable. A decade ago, countries agreed to limit climate warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit). Now, that goal is out of reach.

Globally, average temperatures are already close to 1.5 degrees Celsius warmer than they were before the Industrial Revolution. Heat-trapping emissions from burning fossil fuels are still rising in the atmosphere, and most countries are not on track to meet their climate pledges to cut emissions by transitioning to cleaner energy.

The best-case scenario for the planet is now 1.7 or 1.8 degrees Celsius of warming (roughly 3 degrees Fahrenheit), according to a recent United Nations study. If fossil fuel emissions stay on their current path, the world would be around 2.8 degrees Celsius (5 degrees Fahrenheit) hotter by the end of the century.

While the difference between 1.5 and 2 degrees Celsius may seem tiny, scientific research shows those increments can produce dramatically different outcomes for the planet. More warming raises the risk of extreme weather globally and could lead to the collapse of some of the planet's most sensitive ecosystems.

"The crucial point is that every fraction of a degree matters," says Tim Lenton, climate professor at the University of Exeter. "Every fraction of a degree that we avoid is really able to do a huge amount of limiting harm."

Coral reefs already close to the edge

Corals are architects of an entire ecosystem. As they grow, they create the framework that supports vast numbers of other species, making reefs biodiversity hotspots. They're also vital to hundreds of millions of people who depend on them for food and livelihoods.

Corals are also highly vulnerable to heat. Prolonged marine heat waves cause corals to bleach, turning them white, which can eventually kill corals. Since the late 1990s, there have been four global bleaching events. With a historically strong El Niño now raising water temperatures, scientists say the fifth global bleaching event is likely unfolding.

Even before the previous bleaching event 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."

Given their susceptibility to heat, the survival of most coral reefs depends on the difference between 1.5 and 2 degrees of warming. The Intergovernmental Panel on Climate Change found that beyond 2 degrees Celsius 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. "So 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."

With the dire forecasts for the future of coral, conservation groups and coastal communities are working to make reefs as healthy as possible in many areas so they can better withstand climate change. That includes restoring corals, reducing water pollution and even breeding more heat-tolerant corals.

"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."

Millions more exposed to extreme heat

The goal of 1.5 degrees Celsius is an average temperature, measured across the planet. What it masks is the increasingly extreme heat events that are raising the average.

Researchers have found that for every tenth of a degree of warming, an additional 140 million people are exposed to dangerous levels of heat.

Heat waves are already causing heavy death tolls, like in Europe, where an estimated 50,000 people died of heat-related causes this past summer. Heat is particularly dangerous for those with chronic health conditions, as well as for people who are elderly or pregnant. The real toll of heat waves is often hidden, since many heat-related deaths are undercounted.

"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."

Powerful storms

Every degree of climate change also affects the intensity of storms. Warmer air can produce heavier downpours because it holds 7% more water vapor for every degree. Rainstorms are already getting more intense in most of the U.S.

Scientific research also shows higher levels of warming could lead to other risky tipping points, like the collapse of Antarctic glaciers. If they destabilize, the melt would add vast volumes of water to the ocean, raising 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."

At the ongoing climate negotiations in Turkey, countries are not expected to make major new commitments to cut climate emissions. Discussions are focusing on a new goal for electrification, meaning more cars and homes would run on electricity instead of fossil fuels. That would help reduce overall emissions, since a growing share of the world's electricity comes from cleaner sources, like solar and wind.

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