June 25, 2026 – by Santina Russo

Extracting and selling fossil fuels is a lucrative business for companies like ExxonMobil, Chevron, Saudi Aramco and Shell. But the environmental costs of the fossil business are borne by everyone. Through the CO₂ emissions they cause and the resulting global warming, humanity faces enormous challenges. In Switzerland, flood risks in cities are rising, and mudslides have killed people in mountain villages. Worldwide, rising sea levels threaten the homes of millions of people and freshwater shortages are fuelling conflicts.

For the first time now, climate research based on high-fidelity climate simulations has revealed how much the world’s 180 largest industrial polluters have contributed to one of the consequences of global warming driven by human-made CO₂ emissions: the extreme heatwaves of recent decades. More than that, the work of climate scientist Sonia Seneviratne from ETH Zürich and colleagues even quantifies the individual responsibility of these 180 emission giants for the observed heatwaves. Spoiler: it’s a lot.

Origins of the most extreme events

Specifically, Seneviratne’s team examined 213 extreme heatwaves that occurred between 2000 and 2023 across 63 countries. They determined their cause and asked whether these events would have happened without human-made CO₂ emissions. The data on the heatwaves come from the large international disaster database EM-DAT, which reports only the most extreme events: those that caused major economic damage or casualties, triggered a state of emergency, or led to calls for international assistance.

For their work, the scientists performed a systematic event attribution study, which for the first time not only analysed one event, but the entire history of the most severe heatwaves of 25 years. Such studies estimate how much a specific cause—most often human‑driven climate change—has altered the likelihood or intensity of a particular event.

The approach is based on the physical relationship between the change in mean global temperature and the probability of an extreme event. The understanding and quantification the processes underlying this relationship, in turn, is derived from multiple high-fidelity global climate simulations run and analysed by many climate scientists, including Seneviratne and her team, which are derived using supercomputing facilities around the world including CSCS’ Alps supercomputer.

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Our work provides the full causal chain from CO₂ emitters to climate extremes.

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Responsibility where it’s due

The results show that the probability of heatwaves has risen dramatically. Between 2000 and 2009, the likelihood for heatwaves increased 20-fold; between 2010 and 2019 it even rose by a factor of 200. A substantial share of the responsibility for these heatwaves can be traced back to the 180 biggest CO₂ emitters since pre-industrial times worldwide.

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The emissions of any single carbon major alone would have caused several heatwaves.

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These biggest polluters are commonly referred to as the carbon majors. The group includes large coal, oil and gas producers such as Chevron, BP, and Shell, but also mining and cement companies. Among them are the Swiss firms Glencore and Holcim. Glencore, a commodity trading and mining company, ranks 48th on the list of carbon majors, while the cement producer Holcim ranks 75th.

For each of the 180 carbon majors, the scientists accounted for the emissions across the full value chain of their products and linked those emissions to the heatwaves.

Individual carbon majors alone cause multiple heatwaves

The result is striking: every one of the carbon majors contributed substantially to all 213 heatwaves examined, making them both more likely and more intense. Many of these heatwaves became more than 10,000 times more probable due to the companies’ emissions, meaning they would not have occurred at all without them—even without any emissions from the other carbon majors. “In fact, the emissions of any single carbon major alone would have caused several heatwaves,” Seneviratne says. Depending on the emitter, individual contributions were enough to trigger between 16 and 53 heatwaves that would otherwise have been virtually impossible.

The companies’ emissions also made the heatwaves hotter, by an average of 1.7 degrees Celsius. Out of the millions of companies worldwide—around 25 million in the European Union alone—the 180 carbon majors are responsible for half of the temperature increase in the observed heatwaves, according to the team’s results.

Fuel for climate lawsuits

This systematic attribution of responsibility to individual large emitters is what’s new about the study, Seneviratne says. Earlier studies had already examined extreme weather events and often found a strong influence of climate change, but they did not identify who caused it. “Now, our work provides the full causal chain from CO₂ emitters to climate extremes, including a quantitative estimate of that relationship.”

This kind of individual attribution could prove decisive in future climate lawsuits. Such cases are already common and likely to become more frequent. One example is an ongoing lawsuit brought by a group of Indonesian citizens against the Swiss cement company Holcim, Switzerland’s first-ever civil climate lawsuit. The plaintiffs live on a small island and experience the impacts of global warming first-hand: rising sea levels regularly flood roads and houses, while saltwater increasingly contaminates freshwater wells. The residents are gradually losing their island and hold Holcim responsible for its share of the damage.

At the end of 2025, the court of the Canton of Zug formally admitted the case. It’s course and outcome will be worth watching. Seneviratne and her colleagues’ findings may well make it easier to hold individual companies accountable for their contribution to global warming and its severe consequences.

Cover Image: A major heatwave with record temperatures as they have happened across Asia and Europe. (Image credit: Adobe Stock, AI-generated)