In 2018, local officials in Colorado sued oil companies in state court, charging that pollution from fossil fuels contributed to global warming and harmed Coloradans through extreme summer temperatures, more frequent wildfires and higher risk of some insect-borne diseases. Their complaint did not cite potentially key scientific evidence. That’s because it didn’t exist yet.

Just then gaining public recognition was a field of research known as climate attribution, looking at how global warming affects the atmosphere in specific places or during specific weather events.

The field is now advanced, and climate advocates hope to harness it as legal evidence against polluters. But in the US, that may rest on the outcome of the 2018 case, which is now before the Supreme Court. It will hear arguments Monday in .

The key issue before the justices is whether climate lawsuits can be brought under state tort law, or if federal law preempts that. The high court will either leave open a path for dozens of suits around the US seeking climate damages, or close it.

Or the outcome could be somewhere in between. “It is possible that the court will rule in a way that preserves at least some, if not all, of state tort claims related to climate harms,” said Rachel Rothschild, an associate professor at the University of Michigan Law School, who on Saturday published an article on the topic in the .

Scientists in the last decade honed their ability to conduct rapid analyses of extreme weather events, to estimate within days whether greenhouse gases changed the likelihood or intensity of a given heat wave, flood, cyclone, drought or wildfire. In 2021, the most authoritative global climate science body, the United Nations Intergovernmental Panel on Climate Change, confirmed extreme-event attribution science as a central topic of study.

This July, the US National Academies published a 250-page assessment of the field that validated the approach, finding human-induced changes to the global climate have “a direct and well-understood impact” on extreme heat and rainfall. (It also noted some “significant challenges” the method faces, including limited observations to draw on and an imperfect understanding of how atmospheric dynamics themselves may be changing.) As attribution science has become more mainstream, efforts to discredit it have also gained momentum.

“You’ve seen more concern expressed about the science by the fossil fuel companies,” said Alice Hill, an energy and environment fellow at the Council on Foreign Relations who is also a former prosecutor and judge. “And that’s because that would show liability.”Interest-group campaigns have targeted climate scientists whose research could support litigation against fossil-fuel producers. Under pressure from Republican officials, the Federal Judicial Center removed a chapter from its latest science manual for judges that discussed climate research, including on attribution.

Republican lawmakers have moved to preempt more suits like Boulder’s. GOP members of Congress introduced a bill that would protect fossil-fuel companies from lawsuits. Some states have already passed shield laws for oil and gas producers, and others are pursuing similar legislation.

Newer attribution techniques entered the case only in May, after the Supreme Court had agreed to hear it. Defendants Suncor Energy Inc. and ExxonMobil Holding Corp. filed a brief arguing that atmospheric greenhouse gases “cannot be unmixed and traced to their sources in particular States or countries.”

That led the Natural Resources Defense Council, an environmental nonprofit, to file a friend-of-the-court brief supporting Boulder in early August, arguing that the companies “overlook an extensive body of established climate science that draws these causal lines.”However robustly scientific conclusions may stand up in peer-reviewed journals and global assessments, that doesn’t ensure a smooth entry into the court system. Several other groups filed amicus briefs addressing how the new science may fall short of legal standards for establishing causality.

The conservative Pacific Legal Foundation wrote that historical decisions to produce oil predate ideas about climate torts, and unfairly penalize companies by applying modern judgments to long-ago actions. Boulder’s arguments are “exactly the type of tenuous chain theory that fails the directness requirement of due process,” its brief says.

The Breakthrough Institute, a research nonprofit focused on technological solutions to environmental problems, also pushed back at the field’s findings. It disputed that attribution analysis can link any emitter’s carbon dioxide — which mixes quickly into the atmosphere — to particular harms. And if that were true, the brief says, it would imply that anyone suffering a weather-related injury could sue any emitter anywhere.

Justice Samuel Alito has recused himself in , and the rest of the justices will issue a ruling next year. Even if the five conservatives and three liberals allow for climate tort suits in some capacity, there’s still a long road to a future where judges in other courtrooms decide individual cases on the merits of scientific evidence presented in trial.

“The judges will have to make a determination whether the attribution science is developed enough to be admitted and used,” Hill said.

And for Boulder, if it continues to press its claims, “There’ll be many steps before [it] can get to a finding of liability, if it ever does.”