The Callahan climate attribution study, published in peer-reviewed journal Earth’s Future, has done something courts have long said was impossible: it draws a direct line from a single company’s historical emissions to a specific extreme weather event, and puts a percentage on it. That matters enormously for the more than 40 climate lawsuits filed against major oil companies in the United States since 2017.
What the Callahan climate attribution study actually found
Christopher Callahan, an Earth systems scientist and assistant professor at Indiana University‘s O’Neill School of Public and Environmental Affairs, ran more than 150 simulations across eight different climate models. The goal was to build a statistical map linking atmospheric carbon dioxide concentrations to the probability of extreme heat and extreme rainfall. He then plugged in historical emissions data from individual companies and countries.
The results are concrete. Fossil fuel firms collectively increased the likelihood of the 2021 Pacific Northwest heat wave by 31 per cent. They raised the probability of Pakistan’s catastrophic 2022 extreme rainfall by seven per cent. ‘There is no level of emissions that does not contribute to changing climate impacts,’ Callahan said. ‘Every ton matters.’
What makes his method different from earlier work in source attribution is speed and accessibility. Previously, linking a specific emitter to a specific event meant chaining multiple climate models together in a process that could take months and required substantial computing resources. Because the relationship between cumulative emissions and warming follows predictable proportions, Callahan can calculate each emitter’s share of a given event using historical data alone. ‘What this science tells us,’ he explained, ‘is that there is a direct relationship between how much an entity emits and the extreme events that alter people’s lives.’
Kim Cobb, an observational climate expert at Brown University, called the work ‘extremely valuable’ and said it ‘would have been a pipe dream 10 years ago; it simply wouldn’t be possible.’ Jim Hurrell, a professor of atmospheric science at Colorado State University, pointed to uses beyond the courtroom: ‘This science could be really useful to help communities plan for the future.’
The legal obstacle this research is designed to meet
Causation has always been the sticking point in climate liability cases. At a 2025 Federalist Society panel on climate litigation, Michael Buschbacher, a partner at Boyden Gray PLLC who has backed industry opposition to climate liability laws, framed the difficulty directly: ‘How do you show that emissions from company X cause injury Y from a hurricane or whatever?’
The Callahan climate attribution study is a direct answer to that question. Even Kyle Kohli, writing in Energy In Depth, a publication of the Independent Petroleum Association of America, acknowledged what the science means for plaintiffs: ‘Climate attribution research seeks to connect greenhouse-gas emissions to specific climate impacts, and increasingly, to particular industries or companies. That makes it especially valuable to climate plaintiffs attempting to overcome one of their biggest legal obstacles: proving causation.’
At least five of the 40-plus US cases have entered the discovery phase, where executives could be compelled to hand over internal documents and testify under oath about what their companies knew and when. The American Petroleum Institute listed stopping ‘extreme climate liability policy’ as one of its top 2026 priorities, a signal of how seriously the industry takes the legal threat.
Industry efforts to limit the science’s reach
The fossil fuel industry has known about its role in warming the planet for more than 50 years. Its response, documented across decades, has been to fund doubt wherever the science gained traction. A Politico investigation found a secret campaign by oil industry allies to discredit a National Academies of Sciences, Engineering, and Medicine report on extreme weather attribution. The campaign came just months after Republican attorneys general successfully forced the removal of a NAS climate sciences chapter from a reference manual for federal judges. A NASA scientist who asked not to be named called that decision ‘pathetic. If they cannot stand up for science, then who is going to?’
Researchers are not divided on the underlying findings. Michael Mann, a climate scientist at the University of Pennsylvania, called the NAS report ‘great’ and said it ‘didn’t pull any punches.’ Kristina Dahl, a climate scientist and vice president for science at Climate Central, described it as ‘a really excellent roundup of the state of the science.’
Callahan is clear-eyed about the political context his research inhabits. ‘Our responsibility as scientists is not to never do science that is relevant to the political conversation,’ he said. ‘We should follow the results of our questions wherever they lead and report them regardless of what they are.’ Climate attribution science has been developing for over two decades; what the Callahan climate attribution study adds is the ability to name names, and name percentages, in a courtroom.
