Richard Heede spent a decade digging through “disheveled, dusty” tomes in libraries around the world searching for the answers he thought could help save humanity.

The Norway-born academic’s task was direct, but far from simple: Find out how many greenhouse gases the world’s fossil fuel companies, cement-makers and other industrial giants had pumped into the atmosphere since the Industrial Revolution. A geographer by training, he tagged library visits onto work trips to pore over annual company and shareholder reports. (“Nobody had seen them for decades,” he recalled.) He painstakingly traced mergers and acquisitions as companies morphed and amalgamated. He enlisted volunteers across the globe.

Like most analysts, Heede started his work on climate change focused on what individual consumers could do to reduce their emissions. After all, it was the consumer who was “consuming” the product and actually releasing the emissions from the oil, gas or coal. But over time, he recognized there was a flaw in that approach: Individual consumers can make choices only among what’s already on the market — but who decided what was on the market? Other, larger forces had shaped an economy dependent on fossil fuels, he realized — companies who developed the markets for fossil fuels and influenced decisions to build the infrastructure that supported them.

He asked himself: Shouldn’t the companies who profited from those decisions play a role in mitigating them? With world governments making little progress toward reducing emissions, perhaps pressuring the companies whose products were causing the harm might have more effect?

“With federal policy being unsupportive and still emphasizing continued energy development, I just thought it would be a new lever to look at the companies that have their hand on the tiller,” said Heede, who now lives in Colorado. “And pressure can be exerted in a number of ways.”

By 2013, roughly a decade after Heede began his search, he had his answer: Just 90 companies had contributed nearly two-thirds of the world’s industrial emissions. He could even pinpoint the share of those emissions for which companies existing today are responsible.

In effect, Heede had established a pillar of a new field of research, now known as attribution science. But it wasn’t just an academic exercise: It’s a weapon that climate campaigners are starting to wield to put fossil fuel companies on the hook for billions of dollars in damages. It’s a kind of end run around a political system they see as forced into gridlock by fossil fuel industry influence.

Heede and his collaborators are part of a paradigm shift in how to assign blame for climate change. For decades, as signs have grown that the planet is warming, the public and defenders of industry have laid the blame on end users, the ordinary people who drove their cars too much or blasted air conditioning in their homes. Those add up. But attribution science has the effect of moving the blame back one step, away from consumers and onto the companies that extracted the oil, coal and gas that have powered our planet for decades.

If blame can be attributed to corporations or governments, they believe, it can have two powerful effects: create a strong incentive for those companies to once and for all move away from fossil fuels, and unleash — through lawsuits — financial resources that could be used to seed new technologies and better prepare communities for the calamities climate change is expected to bring.

Attribution science is now about to receive a very real test in the courts, as cities, states and ordinary citizens across the world are using it to try to send fossil fuel companies the bill for climate change damage.

The first lawsuit over what fossil fuel companies revealed about the costs of climate change comes from the New York attorney general, who has accused Exxon Mobil of securities fraud. The suit alleges that Exxon used a different internal estimate for the cost that carbon-reduction policies would have on its business than it told investors; oral arguments for that case begin this week.

“I didn’t know that the legal interest would pick up so fast,” Heede said in an interview. “I didn’t know how this data was going to be used. But I knew that for any legal action — or, for that matter, shareholder pressure or regulatory pressure — we had to know who the companies were and what they contributed.”

ATTRIBUTION SCIENCE originated with the very legal question it might now be used to address.

In 2003, the science journal Nature published an article by Oxford University scientist Myles Allen titled “Liability for climate change.” Allen wondered aloud how to solve “the attribution problem” to demonstrate precisely how much burning fossil fuels were responsible for worsening climate-linked developments. If one could do that, he surmised, it would be possible to sue fossil fuel companies for damages.

A year later, Allen and colleagues Dáithí Stone, Peter Stott and Mark Hawkins published the first “extreme event attribution” study. Using computer models, they compared human-caused, post-Industrial Revolution emissions with scenarios lacking such emissions, known as natural variability. They concluded that man-made greenhouse gas emissions had more than doubled the likelihood of the deadly European heatwave of 2003, which killed 27,000 people. The analysis was straightforward and remains the bedrock of extreme event attribution science.

In the years that followed, more severe weather patterns followed, including Superstorm Sandy on America’s Eastern Seaboard in 2012. Weather forecasters cited in media reports tended to be uncomfortable linking these events to climate change, often saying it was hard to connect individual weather events to an overall pattern of global warming.

But the emerging attribution scientists didn’t agree. In 2015, Fredi Otto, acting director at the Environmental Change Institute at Oxford University, and Heidi Cullen, a climate scientist who is now at the Monterey Bay Aquarium Research Institute, formed the World Weather Attribution group with the goal of providing “rapid response” analysis of climate-fueled disasters. They wanted to show just how much human emissions had worsened those events or made them likelier to occur.

“We got pushback from the scientific community saying, ‘Well, actually this is way too fast. You can’t do science so fast, and the models are not good enough,’” said Otto, who did her postdoctorate work under Allen.

Skeptics have since mostly dissipated, though some remain. The Bulletin of the American Metereological Society began publishing annual event attribution analyses in 2011. Reruns of the World Weather Attribution analyses confirmed the group’s initial efforts. It helped that the National Academy of Sciences wrote a 2016 report on climate attribution science and identified World Weather Attribution’s methods as the gold standard. Last year, the European Union’s Copernicus Climate Change Service asked World Weather Attribution to write a road map to operationalize the science so it can better predict and respond to disasters; a prototype could launch this fall.

On top of that, computer modeling vastly improved, and quickly, permitting the volume and sophistication required for World Weather Attribution group’s mission. Michael Wehner, a scientist at Lawrence Berkeley National Laboratory, said he today could complete his study of the devastating 2013 Colorado floods in a matter of days, whereas the original study took his team three years. He has since expanded his work to hurricanes, finding that human-driven climate change boosted Hurricane Harvey’s August 2017 rainfall by as much as 38 percent.

“Read the papers. If you don’t believe the papers, then do your own,” Wehner said of attribution science critics. “This body of literature, which is now fairly large, clearly tells us that dangerous climate change is upon us, and people are suffering and dying — and it’s real, and it’s going to get worse.”

Some scientists aren’t yet ready to fully embrace extreme event attribution science. The reinsurance industry, which covers megalosses in the event primary insurers can’t foot the bill, contends other factors like building codes and zoning decisions contribute a great deal to the overall financial toll from major disasters.

“We follow the science very closely. We try to understand what this actually means to the risk which we are taking. But when it really comes down to attribution of certain events to climate change, our view is that this piece of science is at an early stage,” said Ernst Rauch, chief climate scientist and global head of climate and public sector business development with Munich Re, a reinsurance company.

Even the National Academy of Sciences in its 2016 report sounded a cautionary note. Finding the precise climate fingerprint for events with nonmeteorological factors, such as drought and wildfire, can be challenging. Events backed up by robust observational records with well-understood physical effects, like long-term temperature increases, are most scientifically sound.

“The ability to attribute the causes of some extreme event types has advanced rapidly since the emergence of event attribution science a little more than a decade ago, while attribution of other event types remains challenging,” the report said.

For any potential uncertainty about climate attribution, Heede said there’s at least one definite truth about fossil fuel companies that should override the rest.

“They were aware decades ago what trouble climate change would be,” he said.

WHILE THE CLIMATE science community was initially cautious about attribution science, lawyers were immediately intrigued.

“I think they recognized that it was a bit of a game-changer,” said Cullen, the climate scientist.

The two scientific breakthroughs – the ability to link global warming to the intensity of storms, rising sea levels and worsening heatwaves combined with the ability to trace historical emissions to individual companies dating back to the Industrial Revolution – have laid the legal groundwork for a string of lawsuits getting underway in coming months.

“We can actually close this causal chain now,” said the Environmental Change Institute’s Otto . “If the first judge has the guts to actually accept such a claim and give a verdict against a big polluter, then that will force these companies to change their business models.”

So far, 13 state and local governments have filed lawsuits against oil and gas companies like Exxon Mobil, BP and ConocoPhillips, including the cities of New York, Baltimore, Oakland, San Francisco and Richmond; Imperial Beach and Santa Cruz, Calif.; the counties of Marin, San Mateo and Santa Cruz, Calif.; King (Wash.) and Boulder (Colo.); and the state of Rhode Island. Another lawsuit by Pacific Coast fishermen against Chevron also seeks climate damages.

The cases face long odds. The legal arguments and the science are largely untested in the context of suing individual companies for the complicated effects of and myriad sources driving global climate change.

“It is very difficult to see how there can be a specific nexus between the conduct of these defendants and any of the adaptations and other costs that these municipalities are incurring,” said Brendan Collins, an environmental attorney at Ballard Spahr LLP who is not involved in the cases.

It’s not the first time plaintiffs have sought climate change damages from energy companies.

Fossil fuel companies have successfully swatted away past cases, all of which have been federal, in part because the courts say Congress has responsibility for regulating greenhouse gas emissions, but Congress hasn’t yet amended the 1990 Clean Air Act to incorporate concerns about climate change. So, climate cases get stuck.

The novel legal innovation this time is that plaintiffs are aiming to sue in state courts. States have common law provisions that allow for claims under two legal theories: public nuisance, that a party is interfering with the rights of citizens, or product liability, that the dangers of using a product must be communicated to the public. Plaintiffs are trying to make the case that energy companies that extract, transport or market fuels are a public nuisance because they are destroying their residents’ enjoyment of a stable climate and forcing costs on them. Under product liability, plaintiffs are arguing that the companies pushed those products into the market knowing these probable, damaging outcomes.

The lawsuits have caught the attention of business groups like the U.S. Chamber of Commerce, which penned two white papers outlining the threats to corporations presented by public nuisance lawsuits in nonfederal courts. But the courts have so far rebuffed fossil fuel companies’ attempts to punt the cases to federal jurisdiction.

The lawsuits aren’t all the same. Some cities are suing over past and future costs to defend against sea level rise; fishermen want compensation for harming a West Coast crab fishery; suits filed on behalf of children contend fossil fuel companies deprived future generations’ quality of life by driving climate change. Most of the lawsuits argue that fossil fuel companies marketed a product they knew to be harmful without acknowledging — and, in fact, sowing doubt and confusion about — those harms.

Plaintiffs see a parallel to the tobacco industry. That industry fell victim to product liability lawsuits because companies knew their product caused harm but misled the public. More contemporary examples of successful lawsuits include a case against Monsanto for cancer linked to its weed-killer, Roundup, and action against pharmaceutical companies for distributing opioids despite known risks and abuse.

Fossil fuel companies have argued that they had no control over how their product was used after selling it — it’s people who put gasoline in their cars, not Exxon Mobil. They are also expected to argue that all levels of government have cemented fossil fuels’ place in American society through tax incentives, infrastructure spending and other policies. That is the crux of what the companies argued in a federal case in the U.S. District Court for the Northern District of California brought in 2017 by the cities of Oakland and San Francisco.

“Plaintiffs do not assert that the mere extraction or sale of fossil fuels created the alleged nuisance (nor could they), but rather that the combustion of fossil fuels by third-party users — such as Plaintiffs themselves — causes global warming and rising seas,” attorneys for BP, Chevron, ConocoPhillips, Exxon Mobil and Shell wrote in a March 2018 motion to dismiss.

The federal case ultimately was dismissed, but Oakland and San Francisco are appealing.

“They will try to claim the harm from their product is not at all their responsibility,” said Peter Frumhoff, science director with the Union of Concerned Scientists, who helped develop the framework for using climate attribution science in the courts. “It’s always somebody else, isn’t it?”

The American Petroleum Institute spokesperson Reid Porter said the oil and gas trade association would not comment on pending litigation.

“The natural gas and oil industry is actively addressing the complex global challenge of climate change, investing in cutting edge technologies, efficiency improvements, and cleaner fuels,” he said in an email.

While the novel legal arguments might face an uphill battle, they’re already affecting the court of public opinion — and in that venue, the plaintiffs seem to be winning. The idea of prosecuting companies has gained currency among Democratic presidential candidates. Much like the fight against pharmaceutical companies that knowingly pushed opioids on American consumers, the push against fossil fuel companies resonates with anti-corporate zeal sweeping voters in both parties.

The political atmosphere is also far more partisan than the major environmental courtroom victories of decades ago, and climate change is firmly ensconced in the U.S. culture wars, noted Durwood Zaelke, founder and president of the Institute for Governance & Sustainable Development. Zaelke’s organization started the Center for Climate Integrity, which is aiding the civil climate lawsuits.

But shifts are occurring beneath the surface, Zaelke said. Investors are prodding companies into action on climate, with companies like BP agreeing to develop a strategy to survive as a business under policies designed to keep global temperatures from rising 2 degrees Celsius. Republicans are no longer debating whether humans are baking the planet, even going as far as to offer policy solutions. Young activists have put climate change front and center on the global agenda.

Events and public opinion are trending in the right direction, Zaelke said. But it will still take an enormous leap for a breakthrough. He thinks climate attribution and the courts could provide the needed boost.

“I’m in a pretty optimistic mood right now believing these strands can come together,” Zaelke said. “We all have a chance to be a little more heroic when circumstances demand, and that’s where we are — circumstances demand more heroism from the lawyers, the youth, the judges.”