Last summer, Ryan Coonerty, a county supervisor in Santa Cruz, got word that the neighboring county of San Mateo was about to take a bold step in adapting to climate change. Rising seas are already eroding San Mateo’s coast, and the county will need to spend billions of dollars on new sea walls and other infrastructure to protect itself in the years to come. So in July, San Mateo, along with Marin County and the city of Imperial Beach, sued 37 fossil fuel companies, arguing that they should help pay for the damage their products cause.

Santa Cruz had also been feeling the effects of climate change. Waves were taking chunks out of coastal roads, Coonerty says, destroying utility pipes beneath them. The sea wasn’t the only problem: there was a years-long drought, followed by historic wildfires, followed by unusually intense winter storms, which triggered landslides causing $140 million in road damage and cutting off entire neighborhoods. “We’ve never had storm damage like that before,” Coonerty says. “At the end of the day, this is going to be billions of dollars in damage to public infrastructure. And the question is, are the oil companies going to stick the public with the bill after they’ve reaped untold profits and lied to us?”

In December, Santa Cruz filed a suit of its own. Nine cities and counties have now brought similar lawsuits, including San Francisco, Oakland, and New York. In recent weeks, officials in Los Angeles and Florida have discussed joining the fray.

“Are the oil companies going to stick the public with the bill?”

There are several reasons why this wave of litigation is happening now. Frustration with the Trump administration’s opposition to climate action has led states and cities to take matters into their own hands. Recent floods, storms, and fires have also created a sense of urgency. Because of climate change, such events will only get more severe, and if cities are going to be prepared, they need to begin the expensive process of adapting their infrastructure now.

These lawsuits are also a sign that the science connecting climate change to damaging events has greatly improved. Santa Cruz, for instance, is suing not just for sea level rise, but for drought, wildfires, and other disasters, armed with recent research showing that climate change is already making them worse.

Not long ago, the phrase “no single event can be attributed to climate change” was repeated like a catechism. This is no longer true. Though scientists still warn that it’s inaccurate to speak of weather events being “caused” by climate change — weather always has multiple causes — better climate models, more powerful computers, and refined methodologies now allow researchers to quantify how climate change has increased the likelihood or severity of heat waves, droughts, deluges, and other extreme events.

The American Meteorological Society now publishes an annual compendium of studies examining the role of climate change in the previous year’s weather. This January’s issue marked an ominous milestone. For the first time, researchers found phenomena that couldn’t have happened in a world without industrial greenhouse gases. The record global heat of 2016, a strangely warm patch of water off Alaska known as “the blob,” and deadly heat waves in Asia weren’t just more likely because of climate change — they were only possible because of it.

For the first time, researchers found phenomena that couldn’t have happened in a world without industrial greenhouse gases

Meanwhile, new research is quantifying the amount of carbon dioxide that energy companies have added to the atmosphere over the course of their entire existence. Combined with attribution science, the two fields form a sort of climate forensics, enabling communities to point to an ostensibly natural disaster, find the fingerprints of climate change, and trace them back to an Exxon or BP.

If the current volley of lawsuits over adaptation costs are successful, they will likely be followed by others: Phoenix might sue over deadly heat, Boulder over its shrinking ski season, or Houston over torrential rain. The list of disasters exacerbated by climate change keeps getting longer. Recent attribution studies have found that climate change played a major role in everything from violent avalanches in Tibet to the bleaching of coral reefs in Australia.

Plaintiffs compare their cases to the pivotal tobacco litigation of the 1990s, hoping for a similar outcome but foreseeing similarly daunting obstacles. Like the states that brought the tobacco lawsuits, they face fantastically well-funded opponents and must convince courts of the causal link between major companies and widespread harm. No climate lawsuit has made it to trial in the US before.

Vic Sher, a partner at the firm Sher Edling LLP, which is leading several of the California lawsuits, says that one reason he believes the cities have a shot now is the science. “All of these earlier cases didn’t have the benefit of current attribution science, in terms of drawing the link between emissions and impacts, and emissions during a particular period, and attribution to particular corporations,” Sher says. “We have all that information now.”

Scientists have been able to quantify the effect of greenhouse gases on global average temperature for decades, but identifying their effect on specific weather events is far more challenging. Weather varies drastically by the day, season, and year before you even get to large-scale shifts like El Niño. It’s one thing to take all that data and find the trend of rising average temperature, and another to figure out how late June heat waves in the Mediterranean have changed and why.

There are two main steps to detecting climate change’s role in an event. The first is to look at historical data and determine the likelihood of the event in the current climate, and how that likelihood has changed over time. If there’s a trend, the next step is to see whether it’s due to greenhouse gases or one of the countless other variables that affect weather.

That’s done using computational models that simulate temperature, sea ice, moisture, and other elements of the climate system. Typically these models are used to predict how the future climate will respond to rising greenhouse gases, but attribution researchers use them to compare the current climate with a hypothetical one where greenhouse gas emissions never occurred. In a sense, we’re currently conducting a planet-wide experiment in what happens when you pump billions of tons of carbon dioxide into the atmosphere, but there’s no control group — an untouched planet against which we can measure the effects — so attribution researchers use models to simulate one.

We’re currently conducting a planet-wide experiment in what happens when you pump billions of tons of carbon dioxide into the atmosphere

Friederike Otto, a senior researcher at the University of Oxford and a lead scientist on the World Weather Attribution project, compares the process to figuring out whether dice are loaded. You roll a clean die and a loaded one over and over and compare the results. You won’t be able to point to a particular winning roll and say it happened because the die was loaded, but you can quantify how much more likely loading made it.

The first major attribution studies were done on heat waves, like the ones that killed tens of thousands of people in Europe in 2003 and Russia in 2010. The studies captured public attention and scientists began researching more events and delivering results more quickly. Since 2014, researchers at the World Weather Attribution project have been publishing assessments of heat waves, droughts, floods, and other events often weeks after they occur.

Attribution science is still new and there’s much room for improvement. In a 2016 report, the National Academies of Sciences declared that the field had advanced rapidly but needs better models and common standards for evaluating their quality. Even high-end models only simulate the climate system at a resolution of 25 square kilometers. That’s enough to capture large-scale phenomena like heat and rain, but not enough to handle weather with complex local dynamics, like thunderstorms or hurricanes.

These limitations will likely be raised by fossil fuel companies in court, as will general questions about the accuracy of the models. Models can be tested and fine-tuned by seeing how well they reproduce the recent past, but there isn’t always good historical data to test against and, of course, there’s none for the counterfactual, low greenhouse-gas version of the planet.

But when it comes to large-scale events, attribution research can be a useful tool for measuring how risk has changed. After Hurricane Harvey swamped Houston, two independent studies found that climate change had made the storm’s Earth-bending rainfall more likely and more intense. The year before, a study found that climate change had increased the odds of a catastrophic deluge in Louisiana. These studies show that torrential rain in the Gulf is no longer a freak event, and that flood maps need to be redrawn, building codes updated, and infrastructure made ready.

“Advances in the science of extreme weather event attribution have the potential to change the legal landscape in novel ways.”

The better attribution science gets, the easier it will be to argue that governments should have foreseen climate risks and prepared for them — and to hold them liable if they fail to. In the journal Nature this fall, a group of environmental lawyers listed a range of actors, from local governments to construction companies, that could face litigation for continuing to operate under a 20th century understanding of risk. “Advances in the science of extreme weather event attribution have the potential to change the legal landscape in novel ways,” they wrote.

The science can also bolster litigation against greenhouse gas emitters themselves. Robert Glicksman, a professor of environmental law at the George Washington University Law School, points out that when states sued tobacco companies, they relied on a form of causation similar to that provided by attribution studies. They couldn’t say definitively that a particular cancer was caused by smoking, but states could say smoking increased the probability of cancer among their residents, which was translating to higher health care costs that the tobacco companies were liable for. An analogous case is now being made by a growing number of cities and counties around the US: climate change has made certain disasters more likely, and local governments are bearing the costs.

The native Alaskan town of Kivalina sits on a narrow barrier island 80 miles above the Arctic circle. Rising temperatures have melted the sea ice that once protected it from fierce storms, resulting in rapid erosion, and it’s been evident for over a decade that the town will have to move. But relocating a town, even one of just 400 people, is expensive: between $100 and $400 million, and it’s unclear where the money will come from. So in 2008, the town decided to sue fossil fuel companies for the moving costs.

Kivalina’s case is the one that most closely resembles the current round of lawsuits, and its fate is inauspicious. It was ultimately dismissed on the grounds that greenhouse gas emissions are regulated on the federal level by the Clean Air Act and the Environmental Protection Agency.

The new lawsuits from Santa Cruz, New York, and elsewhere may fare better, according to Michael Burger, the executive director of Columbia University’s Sabin Center for Climate Change Law. They’re being brought under state law, not federal, which gives them a better chance of surviving similar motions to dismiss. “I think that this has been a theory that has been recognized for a while and it’s now having its moment,” Burger says.

Relocating a town, even one of just 400 people, is expensive: between $100 and $400 million

The new lawsuits will benefit from revelations about what fossil fuel companies knew about climate change and when they knew it. In the years since Kivalina, reporting by InsideClimate News and others has shown that fossil fuel companies, Exxon in particular, conducted their own research in the 1970s and ‘80s confirming the risks posed by greenhouse gasses. Internally, companies prepared their own infrastructure for climate change, raising the decks of drilling platforms and designing pipelines for predicted rising seas and melting permafrost. But externally, they denied the risk, marketing and selling fossil fuels while sowing doubt about climate change through front groups. The suits compare the disinformation campaign to that waged by tobacco companies, a comparison made easy by the fact that both industries paid some of the same people to downplay the danger.

The science connecting climate change to specific impacts has also improved since Kivalina. “The science that allows us to attribute climate-related effects to fossil fuel emissions, the body of science that allows us to tie greenhouse gas emissions to sea level rise, ocean acidification, changes in the hydrologic cycle, and so forth — that science is now robust in a way that it wasn’t previously,” says Sher, the lawyer leading several of the lawsuits.

Santa Cruz’s suit, for instance, cites attribution research on the role climate change played in the recent California drought and wildfires as part of its case that greenhouse gas emissions are making these disasters more likely. New York’s suit says the city is already experiencing rising temperatures and more extreme precipitation. All the current lawsuits focus on sea level rise, the impact of climate change that’s been best understood the longest. But that too has seen improvements in the ability to link climate change with specific local damage, like coastal flooding and erosion, key facts to prove in litigation.

“I think that, certainly, we will see attribution science front and center in all of these cases, whether it’s attribution of specific levels of sea level rise in specific places or attribution of extreme events,” says Burger, who is currently working on a paper on the role of attribution research in litigation.

But convincing courts that climate change is causing damage is only half the battle; plaintiffs also need to show that specific companies are responsible for climate change. For that, they have an expansive accounting project conducted by Richard Heede, co-founder of the Climate Accountability Institute. For years, Heede has compiled corporate records going back over a century — some found in dusty uncatalogued reports buried in libraries around the world — and calculated how much coal, oil, and gas the largest energy companies have extracted and sold over the course of their existence.

Two-thirds of the CO2 and methane pumped into the atmosphere since 1751 was sold or emitted by just 90 companies

Climate change can seem like an abstract problem in which everyone is complicit, but when it comes to extracting and selling fossil fuels, Heede’s research shows that that’s far from the case. In a 2014 study published in Climatic Change and cited in the lawsuits, he found that two-thirds of the CO2 and methane pumped into the atmosphere since 1751 could be traced back to just 90 companies — from investor-owned entities like Chevron and Exxon to state-owned concerns like Gazprom and Saudi Aramco — through their own emissions and the burning of the fuel they sold. Half of those emissions occurred after 1988 — well after companies knew climate change was a problem. Last year, Heede and other researchers used attribution science to show that those 90 companies are responsible for about half of the rise in global mean temperatures since 1880.

Heede’s research underpins the sweeping yet specific claims found in the lawsuits, like New York’s statement that the five companies it’s suing are responsible for over 11 percent of the greenhouse gases added to the atmosphere “since the dawn of the Industrial Revolution.” This research is also coming into play in the Philippines, where the country’s Commission on Human Rights is investigating fossil fuel company responsibility for climate change after Typhoon Haiyan, and in Germany, where a Peruvian farmer is suing the energy company RWE for contributing to the melting of a glacier that’s flooding his town.

If the new round of lawsuits go to trial, there will be a bitter fight over these two links in the causal chain: that these companies contributed to climate change, and that climate change is causing these particular harms.

“I think plaintiffs’ attorneys are playing a long game,” Glicksman says. “I think they realize they may lose some cases initially, but they’re hoping some trial court judge will endorse the causal link they’re trying to prove, and the likelihood of that happening will increase as the science gets more sophisticated.”

It will likely be years before any of these cases see a courtroom, if they ever do. First comes a long road full of procedural hurdles. The companies will likely argue that greenhouse gases are already regulated by federal statutes, that climate change is a problem for lawmakers not courts, and various other reasons the cases should be thrown out. As Sher puts it: “We will see whatever the highest paid lawyers in the world can come up with.”

Burger envisions the companies making a more general argument as well: why them? They may have taken the fossil fuels out of the ground and sold them, but so did many other companies. Furthermore, the government licensed their actions, utilities burned the fuel, car companies built cars for it, and individuals around the world used their products. You can already see this argument in Chevron’s complaint, filed in December. In it, Chevron says it only produced the fuel and isn’t responsible for the effects of burning it, but that if it is found liable, the Norwegian state oil company Statoil should pay up too.

Plaintiffs face opposition along other fronts as well. Days after Imperial Beach, Marin, and San Mateo filed their lawsuits, officials there received public records requests seeking correspondence between a long list of climate activists, scientists, and lawyers. The requests came from the Energy & Environment Legal Institute, a 501c3 funded by the fossil fuel industry that has previously gone to court seeking emails from climate researchers. The Union of Concerned Scientists has called their campaigns to get correspondence from climate scientists harassment.

Exxon itself has already struck back against the plaintiffs. In a petition filed last month in a Texas district court, the company accused the California communities of “abusive law enforcement tactics” designed to stifle the company’s “First Amendment right to participate in the national dialogue about climate change and climate policy.” Seeking depositions and documents, Exxon also accused the communities of failing to tell bondholders about the climate risks cited in their lawsuits. Weeks later, the Competitive Enterprise Institute made a similar argument in a letter urging the Securities and Exchange Commission to investigate the cities and counties for misleading bond investors about climate risk.

“This an outrageous abuse of the legal process.”

“This an outrageous abuse of the legal process that seeks to limit the ability of law enforcement and local government to protect their residents,” says John Cote, Communications Director at the San Francisco City Attorney’s Office. “It’s an attempted end-run around the California courts that have jurisdiction over this matter. In other words, it’s exactly what you would expect from a company like Exxon.”

The counterattacks don’t appear to have had a deterring effect. Days after Exxon’s petition, city councilors in Los Angeles introduced a motion to prepare a similar lawsuit, citing the recent devastating wildfires, heat waves, floods, and mudslides. In late January, Richmond, California, became the latest city to file a suit, listing damage from sea level rise, droughts, heat waves, and extreme rainfall. This month, Philip Levine, the former mayor of Miami Beach who is currently running for governor of Florida, said the state should look into suing fossil fuel companies, again comparing them to cigarette makers.

It’s standard practice for fossil fuel companies to push back fiercely against attempts to hold them accountable for climate change, but it’s easy to see why the current round of lawsuits would be worrisome. Research keeps finding a stronger climate signal in more and more disasters, from drought and wildfire to floods and avalanches. “If these cases survive, there’s a long list of climate change impacts that could become the subject of such litigation,” Burger says. The bill for adapting to them, whoever ends up paying it, will be large.