The 2025 Europe Heat Wave Was Hotter Because of Paris Emissions

A Europe heat wave in the summer of 2025 killed thousands of people across the continent. Emissions released in the decade since countries signed the Paris Agreement made it measurably hotter, according to a new study published on September 22 in the journal Geophysical Research Letters.

The researchers, led by doctoral student Jared Trok at Stanford's Doerr School of Sustainability, found a better-than-99-percent chance that emissions after 2015 intensified the 2025 Europe heat wave. In some regions, that decade of pollution lifted average daily temperatures by roughly a third of a degree Celsius, just over half a degree Fahrenheit, the study says.

The Europe heat wave struck a decade after governments pledged to curb warming, and the new research shows those pledges did not stop the added heat from arriving anyway.

A decade of emissions, measured against the weather

Attribution science has long tied extreme events like heat waves and floods to climate change in general. This study takes the method a step further by linking the Europe heat wave to a specific slice of pollution: the greenhouse gases released after the Paris Agreement was adopted.

The team used a diffusion model, a form of generative AI, trained on ten global climate models and checked against decades of real weather records. They then compared the actual 2025 heat wave against a counterfactual version of events in which emissions had stayed at 2015 levels. The gap between the two is what the added pollution accounts for.

The result was not limited to one summer. Running the same test backward, the researchers found that the hottest week of each Europe heat wave in every individual year since at least 2021 was very likely made more intense by post-2015 emissions. Near the Mediterranean, the decade's pollution added more than four hot days per year from 2022 onward.

Almost one-fifth of all carbon dioxide from burning fossil fuels has been emitted since the Paris Agreement was adopted, most of it from oil, natural gas, and coal, said the team behind the Europe heat wave analysis.

Small numbers carry outsized damage

During a Europe heat wave, that fraction of a degree sounds like a rounding error, and Trok said he was surprised that a single decade of emissions left such a clear fingerprint on recent heat waves. But he pointed to a large body of research showing that small temperature changes have disproportionate effects on heat deaths, electricity demand, crop yields, and labor productivity.

Previous work by Trok and co-author Noah Diffenbaugh had already found that small climbs in temperature can raise the risk of heat-related deaths in Europe faster than the temperature itself rises. The new result suggests that damage is already being added, year by year, by pollution released after the agreement meant to limit it.

AI is rewriting what attribution can prove

Diffenbaugh, a veteran of traditional climate modeling, said his own experience with global climate models would have made him call the influence of such a small slice of emissions highly uncertain. Natural variability usually drowns out the signal. That a diffusion model trained on those same climate models and then fed real meteorological data can pick the signal back out is a meaningful step forward, he said.

The authors describe the work as a step toward measuring how small amounts of emissions add to the damage from a single extreme event. That matters because knowing what continued pollution will cost as the world cuts back depends on being able to price each added fraction of a degree.

More of the same is already on the way

The researchers looked only at European temperatures, and they have not yet connected the added heat to deaths, lost crops, or power demand. But Diffenbaugh noted that even the most optimistic timelines for cutting emissions still involve more pollution in the future than the last decade produced. The study, he said, shows the extreme heat of recent years can be expected to intensify further, making every future Europe heat wave more dangerous.

The finding lands in a year when heat records kept falling on both sides of the Atlantic. The United States just endured its hottest summer on record, and the Planetary Health Check 2026 found record ocean heat and wildfires pushing Earth systems under mounting strain. Attribution studies like this one are starting to put a number on whose pollution helped make each record possible.

For anyone who grew up after 2015, the message is blunt. The emissions released during your lifetime have already made each Europe heat wave of the last few years worse, and the science can now say so with near certainty. What happens next depends on whether the pollution curve bends fast enough to keep the added fractions of a degree from piling up.