Heatwaves, attributed to the carbon majors

The science of event attribution has become one of climate research’s most consequential branches: instead of asking only how the climate is changing on average, it asks how specific extremes, a particular heatwave, a particular flood, were made more likely or more intense by the warming caused by human emissions. A paper in the 11 September issue of Nature pushes the same question to the level of actors: rather than attributing heatwaves to humanity as a whole, it attributes them systematically to the emissions of the carbon majors, the companies that sit at the top of the global emissions ledger, and the numbers it reports are a ledger of responsibility.

The study, by an international team, connects two bodies of research that had largely run in parallel. The first is the accounting of corporate emissions, the long-running effort to quantify how much of the world’s fossil carbon has been extracted and sold by the largest producers. The second is event attribution, the method that uses climate models to estimate how a given extreme’s probability and intensity have been changed by warming. The paper’s contribution is to join them: to run the attribution not against all emissions but against the share attributable to the carbon majors, and to report how much of the heat in recent events belongs to that share.

The results give the attribution debate a sharper edge than it has usually had. A heatwave can be attributed to human influence in general, a statement that distributes responsibility across a century of emissions from every source; attributing it to a defined group of producers converts a diffuse statement into a specific one, and the specificity is exactly what the policy conversation needs. The paper is careful to present the method and the numbers, and the numbers have been read, in the discussion that followed, as a quantitative answer to a question that has usually been asked in moral and legal terms.

The significance of the work lies partly in its timing. The attribution literature has grown steadily more precise about the human role in extremes; this study adds precision about which humans, and the turn from the collective to the specific is a turn that many fields have made, from epidemiology to finance, as measurement has improved. The science does not decide who should pay for the heat; it measures whose carbon is in it, and the measurement is the foundation on which every subsequent decision, legal, regulatory or political, will have to be built.

There is a quiet power in the method’s logic. Every extreme event carries a signature of the carbon that helped shape it, and the signature can now be read at the level of the producers. The carbon majors have long been counted in tonnes and in dollars; the paper’s contribution is to count them in heat, in the intensity of the hottest days and the odds of their occurrence. The hottest summer on record, whatever it is, now has a fuller genealogy, and the biggest contributors to it are named in the peer-reviewed record.