A handful of fossil fuel producers are responsible for more frequent heatwaves worldwide 

Across the globe, since the Indus­tri­al Revolu­tion, the vast major­ity of anthro­po­gen­ic green­house gases have been emit­ted by just a hand­ful of entit­ies – 178 accord­ing to the Car­bon Majors data­base. These ‘car­bon majors’ are private and pub­lic fossil fuel pro­du­cers as well as nation states. Whilst heat­waves have major health con­sequences, research pub­lished in Septem­ber 2025 shows that they are respons­ible for an increas­ingly sig­ni­fic­ant rise in the fre­quency and intens­ity of heat­waves¹.

The inter­na­tion­al team, led by sci­ent­ists from ETH Zurich, relied on an ‘attri­bu­tion study’, a meth­od developed in recent dec­ades in response to the urgent need to act to reduce GHG emis­sions. “Fol­low­ing the heat­wave that hit Europe in 2003, cer­tain NGOs approached us to ask wheth­er it was pos­sible to sci­en­tific­ally demon­strate the respons­ib­il­ity of the ‘car­bon majors’, with a view to tak­ing leg­al action,” says Pas­cal Yiou. “At the time, I didn’t believe it was pos­sible.” Since then, the increase in attri­bu­tion stud­ies has made it pos­sible to gauge the con­sequences of cli­mate change. Pas­cal Yiou con­tin­ues: “Today, we mon­it­or the influ­ence of cli­mate change on extreme events almost sys­tem­at­ic­ally, using tools such as the World Weath­er Attri­bu­tion and Clim­amet­er.”

What is new about the 2025 study? Pre­vi­ously, sci­ent­ists had focused on assess­ing the role of cli­mate change in spe­cif­ic weath­er events, but now the sci­ent­ists at ETH Zurich and their col­lab­or­at­ors have broadened their scope – in doing so the team assessed the respons­ib­il­ity of ‘car­bon majors’ for the occur­rence of all heat­waves over the past two decades.

In total, 180 car­bon majors are accoun­ted for… their emis­sions account for 57% of all CO2 emis­sions linked to human activ­it­ies over the same period. 

In total, 180 car­bon majors (com­pan­ies and states) are accoun­ted for between 1854 and 2023. Their emis­sions account for 57% of all CO2 emis­sions linked to human activ­it­ies over the same peri­od. 14 of them stand out, hav­ing emit­ted as much CO2 as the oth­er 166 com­bined: the former Soviet Uni­on, the People’s Repub­lic of China (coal), Saudi Ara­mco, Gazprom, Exxon­Mobil, Chev­ron, the Nation­al Ira­ni­an Oil Com­pany, BP, Shell, India (coal), Pemex, CHN Energy and the People’s Repub­lic of China (cement).

The team then com­piled heat­waves that had major human and/or eco­nom­ic con­sequences between 2000 and 2023, totalling 213 events. The res­ult shows that the ‘car­bon majors’ are respons­ible for an increase in the intens­ity of heat­waves, and this is grow­ing over the dec­ades. These extreme events are also more fre­quent due to the ‘car­bon majors’, as the authors write: “There are heat­waves that the car­bon majors have made at least 10,000 times more likely com­pared to the pre-indus­tri­al era, and which would have been vir­tu­ally impossible without anthro­po­gen­ic influ­ence.” The largest emit­ter, the former Soviet Uni­on, thus increased the prob­ab­il­ity of 53 heat­waves occur­ring by a factor of 10,000. In oth­er words, these 53 heat­waves would very likely not have occurred without the con­tri­bu­tion of the former Soviet Union.

“The meth­od used here is more com­plex than in pre­vi­ous stud­ies,” com­ments Pas­cal Yiou. “The authors con­sider numer­ous para­met­ers – glob­al tem­per­at­ure, emis­sions and GHG con­cen­tra­tions – that are not taken into account in con­ven­tion­al meth­ods, par­tic­u­larly those relat­ing to the phys­ic­al laws that describe met­eor­o­lo­gic­al extremes.” The art­icle notes that the assess­ment of the ‘car­bon majors’ con­tri­bu­tion to the rise in atmo­spher­ic CO₂ con­cen­tra­tions is con­sist­ent with oth­er stud­ies pub­lished in 2017².

“These attri­bu­tion stud­ies have a short­com­ing: they do not assess the cause of these extreme weath­er events,” notes Pas­cal Yiou. “Yet this is cru­cial to avoid mis­in­ter­pret­a­tions. For example, dif­fer­ent met­eor­o­lo­gic­al mech­an­isms are at play in the 2003 and 2021 heat­waves in Europe.” But incor­por­at­ing all the para­met­ers that describe atmo­spher­ic phys­ics makes the cal­cu­la­tion very com­plex, and there­fore costly in terms of com­put­ing time and resources. 

“This does not detract from the ser­i­ous­ness of this study, and two of our recent stud­ies focus­ing on Brit­ish Columbia and Par­is yield res­ults that are highly con­sist­ent with this one³,” con­tin­ues Pas­cal Yiou. Through this sys­tem­at­ic approach to major heat­waves, the authors demon­strate the import­ance of tak­ing action to lim­it cli­mate change. This con­clu­sion may carry weight in polit­ic­al dis­cus­sions.” The sci­entif­ic com­munity is now seek­ing to take a fur­ther step: that of “suc­cess­fully attrib­ut­ing the impact of extreme weath­er events to cli­mate change, a key factor for inter­na­tion­al com­pens­a­tion mechanisms.”

Accord­ing to the IPCC’s 2021 report⁴, it is “vir­tu­ally cer­tain that extreme heat events (includ­ing heat­waves) have become more fre­quent and intense in most regions since the 1950s […]. Cli­mate change caused by human activ­it­ies is the main driver of these changes. […] With every addi­tion­al degree of glob­al warm­ing, changes in [hot] extremes become more pronounced.”

Anaïs Marechal