The heat waves Europe has seen in recent years were made measurably more intense because of the greenhouse gasses emitted since the 2015 Paris Climate Agreement, according to a new study published today in Geophysical Research Letters.
“We found that the emissions released since the signing of this Paris Agreement in 2015 have substantially and significantly increased the intensity of Europe’s recent individual heat waves,” says lead author Jared Trok, a PhD student in Earth system science at the Stanford Doerr School of Sustainability. The study also determined that that held true for Europe’s hottest week in every individual year since at least 2021.
Their findings come on the heels of this summer’s back-to-back heat waves on the continent, which broke heat records and contributed to at least 35,000 excess deaths.
The Paris Agreement was a legally binding international treaty on climate change adopted by nearly 200 countries in December 2015, with the aim of limiting global warming to 1.5°C above pre-industrial temperatures. A decade later, the world is expected to blow past that 1.5°C target, with optimistic estimates finding that temperatures will peak at 1.8°C under current climate policies.
Beyond the 1.5°C threshold, every extra fraction of a degree matters. The world can expect to see intensified extreme weather, ecosystem loss, and flooding in coastal cities and island nations, along with far-reaching impacts to human health and food security.
Europe is particularly vulnerable in the face of a warming climate. The continent is heating up at more than twice the global average rate. The June and July 2025 heat waves broke records across the continent—only to be surpassed by 2026 heat waves. Surface air temperatures exceeded 40°C (104°F) in several countries, and up to 46°C (114.8°F) in Spain and Portugal.
The researchers created a generative AI model known as a diffusion model using simulated weather data from 10 global climate models to predict how extreme heat waves in Europe could have unfolded under different global emissions levels.
“This is a purpose built machine learning model that we trained on large amounts of climate model data…to make predictions about how individual heat waves might have unfolded differently if those same weather patterns had occurred under different levels of human emissions,” says Trok.
The team gave the model the observed weather conditions that caused Europe’s most intense annual heat waves from 2016 to 2025, which typically coincided with a persistent, high pressure system known as a heat dome. They had the model predict daily temperatures under emission levels at the time of each extreme heat event, and under the lower emissions levels of 2015.
They found that the emissions from the last decade increased the intensity of 2025 heat waves by roughly a third of a degree Celsius. “That third of a degree might seem very small, maybe imperceptible to us walking around, but there’s a large body of scientific literature that suggests that even small increments of temperature on a hot day can have these disproportionate impacts on the damages caused by those heat waves,” says Trok.
Scientists have long known that the burning of fossil fuels trap greenhouse gases in the atmosphere, raising global temperatures. In recent decades, researchers have developed methods to study how historical emissions have affected heat waves, hurricanes, droughts, wildfires, and other extreme weather events. The researchers say that the methodology represents a step forward for climate modeling.
“This research shows that we can systematically and objectively ask questions about subsets of historical emissions, including individual heat waves,” says co-author Noah Diffenbaugh, professor at the Stanford Doerr School of Sustainability. “A key part of what we’ve identified here is we’ve been able to say with high confidence that this subset of emissions has shifted the distribution of temperatures that could occur during this individual heat wave to such an extent that we can say we are very confident that this individual heat wave would have been cooler if the same weather patterns had occurred without this subset of emissions.”
It also serves as a warning, says Diffenbaugh, noting that the emissions that have already been released will impact our climate regardless of any mitigation efforts we take going forward. “Even in the context of the most optimistic, ambitious future for reducing emissions, there’s still more intensification of extremes that is essentially committed.”
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