Contributions of carbon majors to historical heatwaves
Authors: Quilcaille Yann , Gudmundsson Lukas , Gasser Thomas , Seneviratne Sonia I. 
Published: March 2024

While human-induced climate change shows no sign of slowing down, calls to steer to a more sustainable path grow louder. Countries are sued for their lack of ambitious climate action, and high-emitting companies for their responsibilities. However, climate litigation is often impeded by the lack of scientific evidence directly relevant to the legal cases. Available attribution research can provide support for claims, but some key elements are still missing. First, event attribution studies are limited to a few selected events, depending on available researchers&#8217; time and interests. Second, the contributions of high-emitting companies to recent extreme events has not yet been quantified. Here, we fill in both of these gaps. We present the first collective attribution of 149 historical heatwaves reported over the 2000-2021 period. We apply a well-established extreme weather attribution (Philip et al., 2020) to heatwaves reported in the EM-DAT database (EM-DAT, 2023). For each listed heatwave, we identify the event in observational data (ERA5, BEST) and CMIP6 data, then we estimate its occurrence probabilities for present and pre-industrial climate conditions. Subsequently, we calculate the contributions in global mean surface temperature of 110 fossil fuels and cement companies using their CO2 and CH4 emissions (Heede, 2014) and the reduced-complexity Earth system model OSCAR (Gasser et al., 2017). These contributions combined to the collective attribution allow for the calculation of the contributions of these carbon majors to all of the analyzed historical heatwaves. These carbon majors represent 76% of the CO2 emissions over 1850-2021, and half of this 76% is due to only six actors (nation-state of China for coal &amp; cement; nation-state of the Former Soviet Union for coal, oil and gas; Saudi Aramco; Chevron; ExxonMobil; Gazprom). In terms of global mean surface temperature, these six majors contribute to 0.30&#176;C, while the others contribute to an additional 0.34&#176;C. The majority of heatwaves are made substantially more probable and intense due to these six carbon majors. Though, other carbon majors cannot be neglected, as their sole contribution may be enough to make some heatwaves possible. This attribution of a large number of heatwaves and the link to the contributions of the carbon majors will provide useful resources for climate litigation, paving the way towards their legal responsibility.&#160;EM-DAT, CRED / UCLouvain: www.emdat.be, last access: 09.01.2024.Gasser, T., Ciais, P., Boucher, O., Quilcaille, Y., Tortora, M., Bopp, L., and Hauglustaine, D.: The compact Earth system model OSCAR v2.2: Description and first results, Geoscientific Model Development, 10, 271-319, 10.5194/gmd-10-271-2017, 2017.Heede, R.: Tracing anthropogenic carbon dioxide and methane emissions to fossil fuel and cement producers, 1854&#8211;2010, Climatic Change, 122, 229-241, 10.1007/s10584-013-0986-y, 2014.Philip, S., Kew, S., van Oldenborgh, G. J., Otto, F., Vautard, R., van der Wiel, K., King, A., Lott, F., Arrighi, J., Singh, R., and van Aalst, M.: A protocol for probabilistic extreme event attribution analyses, Adv. Stat. Clim. Meteorol. Oceanogr., 6, 177-203, 10.5194/ascmo-6-177-2020, 2020.