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Drought-fueled wildfires in Southern California, a devastating hurricane in the southern Appalachian Mountains, and catastrophic floods in New England are among the most recent disasters to bring the astronomical costs of climate change into focus.

As a growing number of local and national governments struggle to recover from—and protect against—more frequent and destructive climate disasters, some have sought compensation directly from fossil fuel companies through civil cases and “polluters pay” laws. But many of these actions are being challenged or slowed in court, partly due to the difficulty in showing that specific climate impacts occurred because of any one company’s greenhouse gas emissions.

A new study in the journal Nature, however, provides a tool for potentially recouping the costs of extreme weather amplified by climate change. The researchers lay out a scientific framework they say can be used to trace specific climate damages back to emissions from individual fossil fuel companies.

The framework combines climate modeling with publicly available emissions data to contrast the current climate and its impacts to what it would be like without the heat-trapping gases a company’s activities released into the atmosphere. This causal link is known as a “but for” standard—as in, a climate catastrophe likely would not have occurred but for an individual firm’s actions, the researchers report.

“We argue that the scientific case for climate liability is closed, even if the future of these cases remains an open question,” says , the study’s senior author and an associate professor in the . The study, he says, answers a question of whether science could ever link an individual firm’s emissions to climate change.

“Just over 20 years later, we find the answer to be ‘yes,’” says Mankin, who directs the . “Our framework can provide robust emissions-based attributions of climate damages at the corporate scale. This should help courts better evaluate liability claims for the losses and disruptions resulting from human-caused climate change.”

Mankin and the study’s first author, Stanford postdoctoral scholar Christopher Callahan, Guarini ’23, deploy the framework to provide the first causal estimates of regional economic losses due to extreme heat resulting from the emissions of individual fossil fuel companies.

Extreme heat linked to carbon dioxide and methane from 111 companies cost the world economy $28 trillion from 1991 to 2020, with $9 trillion of those losses attributable to the five top-emitting firms, according to the study. The highest-emitting investor-owned firm the researchers examined may be responsible for $791 billion to $3.6 trillion in heat-related losses over that period, according to the study.

“Our findings demonstrate that it is in fact possible to compare the world as it is to a world absent individual emitters,” says Callahan, who began working on the project as a PhD candidate in Mankin’s research group.

“The affluence of the Western economy has been based on fossil fuels,” Callahan says, “but just as a pharmaceutical company would not be absolved from the negative effects of a drug by the benefits of that drug, fossil fuel companies should not be excused for the damage they’ve caused by the prosperity their products have generated.”

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Geography professor Justin Mankin, left, and Christopher Callahan, then a PhD candidate at the Guarini School for Graduate and Advanced Studies, in 2022. The two have worked together on several climate-related studies. (Photo by Lars Blackmore)

The study, Callahan and Mankin say, benefits from 20 years of accumulating real-world climate impacts, the increased availability of climate and socioeconomic data, and methodological advances in “climate attribution science,” a form of modeling that allows scientists to track the effects of climate change almost in real time.

Climate attribution is the crux of Vermont’s 2024 Climate Superfund Act, which was partially informed by Mankin’s testimony and an early version of the Nature study. Passed in the wake of devastating statewide floods in 2023, the law empowers the Vermont attorney general to compel major fossil fuel companies to help cover the cost of disasters that can be scientifically linked to their emissions. A recent lawsuit challenges the state’s authority to collect such damages, as well as Vermont’s ability to accurately use climate attribution science to determine them.

The attribution framework reported in Nature incorporates established, peer-reviewed scientific methods for identifying the effect of specific emission levels on extreme weather. Callahan and Mankin also build on advances in the physical and social sciences that have drawn clearer connections between greenhouse gases, local climate change, and economic losses.

Critically, the model goes a step further than existing research by removing total emissions—measured in billions of tons—from the equation to identify a company’s specific greenhouse gas footprint. Previous attribution models have hinged on concentrations of greenhouse gases in the atmosphere, which are measured in parts-per-million that are harder to attribute to specific sources, Callahan says.

“Our approach simulates emissions directly, allowing us to trace warming and its repercussions back to specific emitters,” Callahan says. His and Mankin’s focus on extreme heat builds on their previous work calculating global financial losses due to heat waves and the economic damages individual countries have caused to others by contributing to climate warming.

“Extreme heat is indelibly linked to climate change itself, and the losses from it have been an instigator for legal claims. So, it’s an obvious place to illustrate the broad application of our approach,” Mankin says.

“We also live in a world that has warmed considerably over the past 20 years,” he says. “This analysis is not a predictive exercise where we ask what the future holds. Instead, it’s a documentary effort where we show what has already happened and provide the reason why.”