UN study puts best-case warming at 1.7 to 1.8 degrees

Every tenth of a degree exposes 140 million more people to heat

A recent United Nations study has narrowed the planet’s best-case outcome to 1.7 or 1.8 degrees Celsius of warming — roughly 3 degrees Fahrenheit — while fossil fuel emissions held to their current path would put the world around 2.8 degrees Celsius, or 5 degrees Fahrenheit, hotter by the end of the century. Those numbers are the backdrop against which delegations are working at the COP31 climate summit in Turkey, where the question is what the negotiations can still deliver.

The summit arrives a decade after countries agreed to limit warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, and that target is now out of reach. Global average temperatures already sit close to 1.5 degrees Celsius above the level recorded before the Industrial Revolution. Heat-trapping emissions from burning fossil fuels continue to rise in the atmosphere, and most countries are not on track to meet their climate pledges to cut emissions by transitioning to cleaner energy.

The gap between 1.5 and 2 degrees Celsius can look trivial on a thermometer, but the research on those increments describes sharply different outcomes: more warming raises the risk of extreme weather worldwide and could push some of the planet’s most sensitive ecosystems toward collapse. Tim Lenton, a climate professor at the University of Exeter, said the decimals are where the damage accumulates. “The crucial point is that every fraction of a degree matters,” Lenton said. “Every fraction of a degree that we avoid is really able to do a huge amount of limiting harm.”

Much of that damage is heat the planetary average conceals. The 1.5-degree target is a number measured across the whole planet, and what it masks is the increasingly extreme heat events pushing the average upward. 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 producing heavy death tolls — in Europe, an estimated 50,000 people died of heat-related causes this past summer. Heat is particularly dangerous for people with chronic health conditions, as well as those who are elderly or pregnant, and the real toll stays hidden because many heat-related deaths go uncounted. Lenton compared the hazard to a threshold inside the body. “You could think of it a bit like a tipping point for the human organism or body,” Lenton said. “At some point, our physiology kind of reaches a tipping point and the worst tipping point is death.”

Coral reefs sit near the same divide. Corals are architects of an entire ecosystem: as they grow they build the framework that supports vast numbers of other species, which makes reefs biodiversity hotspots, and they are 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 them to bleach, turning white, which can eventually kill them. Four global bleaching events have occurred since the late 1990s, and with a historically strong El Niño now raising water temperatures, scientists say the fifth is likely unfolding. 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. David Obura, director of the coral research organization CORDIO East Africa, worked on the study published last month in the journal Nature. “These are incredibly worrying statistics, but they also emphasize that time matters,” Obura said. “Every ten years, every five years counts.”

Given that susceptibility to heat, the survival of most coral reefs turns on the difference between 1.5 and 2 degrees of warming. The Intergovernmental Panel on Climate Change found that beyond 2 degrees Celsius, 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 said. “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.”

Faced with projections that most coral reefs cannot survive 2 degrees of warming, conservation groups and coastal communities are working in many areas to make reefs as healthy as possible so they can better withstand climate change — 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 said. “That’s all they have. So we have to work out how to make them function again.”

Storm intensity scales with the same increments. Every degree of climate change affects how hard storms can hit, because warmer air holds 7% more water vapor per degree, which produces heavier downpours; rainstorms are already getting more intense in most of the U.S. Higher levels of warming could also open other risky tipping points, research shows, including the collapse of Antarctic glaciers — if they destabilize, the melt would add vast volumes of water to the ocean and raise sea levels globally. Lenton cautioned that these thresholds cannot be read in advance. “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 said. “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.