The Sabin Center maintains an open access database that attempts to post all of the climate change cases around the world. It has more than 3600 cases in 62 different jurisdictions and 28 international courts and regional tribunals. Of these, about 60% are in the United States. The largest numbers of these cases concern the adequacy of consideration of climate change in the environmental impact assessment processes, the adoption and implementation of climate legislation and regulations, or the impacts of climate on species. Only a small fraction have attempted to impose financial liability for climate-related harms.
In fact, to date, there has not been a single court decision anywhere in the world that imposes financial liability on any country or company because of its greenhouse gas emissions, though as I’ll discuss there are quite a few pending cases that try to do that. The reason that none of these cases has yet led to a ruling of financial responsibility is not because of any lack of science. With one exception discussed below, all of the decisions dismissing cases have been based entirely on legal grounds. Courts have found that certain cases are preempted or displaced by statutes, particularly the U.S. Clean Air Act, while others have been dismissed because they posed political questions, because the courts felt they lacked the power to issue certain kinds of rulings, because the plaintiffs lacked standing to sue; or on other legal grounds. Insufficient science has not at all been the problem, and so more science itself would not solve the problem.
Many courts in the U.S. and around the world have readily accepted climate science without serious dispute. So did four important decisions issued in the last two years by four international tribunals – the International Court of Justice, the European Court of Human Rights, the Inter-American Court of Human Rights, and the International Tribunal on the Law of the Sea. Every one of them relied on and accepted the findings of the IPCC.
There have been, at latest count, 49 decisions from U.S. courts that cited the IPCC. That includes the U.S. Supreme Court in its landmark 2007 decision in Massachusetts v. EPA. Overseas courts have similarly referenced the IPCC, including in the landmark cases of Urgenda v. Kingdom of the Netherlands, KlimaSeniorinnen v. Switzerland, Milieudefensie v. Royal Dutch Shell, Neubauer v. Germany, Notre Affaire a Tous v. France, and others.
Just about every court in the world that has looked at climate science has accepted the IPCC as the gold standard. Many other decisions in the U.S. and around the world have accepted the basic findings of climate science that the earth is getting warmer, that this is having or will have serious impacts, that the principal cause is greenhouse gas emissions, and that the chief source of greenhouse gas emissions is the combustion of fossil fuels. No court anywhere in the world has seriously questioned that.
In fact, there have been few attempts to question that. Even the oil companies, in defending the many lawsuits against them over climate damages, have accepted the IPCC. There was an attempt in 2010 in lawsuits challenging EPA’s 2009 endangerment finding under the Clean Air Act, but the U.S. Court of Appeals for the District of Columbia swiftly dismissed that argument, and the Supreme Court didn’t want to hear it.
The Trump administration has now revoked the endangerment finding, chiefly citing various legal arguments. The Department of Energy hand-picked a committee of five scientists, all of whom can be called contrarians, to issue a report saying that the impacts of climate change are not so bad after all. That report was roundly rebutted by a large number of climate scientists; a federal court ruled that the committee had been formed illegally; the committee was disbanded; and EPA did not rely on it in revoking the endangerment finding. The Trump EPA is saying, however, that U.S. motor vehicle emissions, the immediate target of their action, are such a small piece of the global climate change picture that it’s not worth the expense to regulate them. The legal challenges to the administration action are now back before the D.C. Circuit and may reach the Supreme Court, but probably not for another year or two.
Just about all the court decisions so far have been based purely on written submissions and on oral arguments by lawyers. In the U.S., there have only been three trials where climate scientists were on the witness stand and subject to cross examination by the lawyers on the other side. First, in 2007 a federal court heard a challenge to Vermont’s ability to regulate motor vehicle emissions. After hearing dueling claims, the judge accepted the conclusions of a team of climate scientists led by Jim Hansen. Second, in 2023 there was a trial in a case called Held v. Montana; the plaintiffs, represented by Our Children’s Trust, put several climate scientists on the witness stand. The state of Montana, as defendant, did not seriously challenge them. The trial court agreed with the scientist; and the state supreme court upheld the decision. Third, in 2025, also in Montana, a federal court heard testimony from climate scientists in a case called Lighthiser v. Trump. The court accepted the climate science but it dismissed the lawsuit based purely on legal grounds. That decision was just affirmed by the Ninth Circuit.
In several cases in other countries, climate scientists from both sides testified and were questioned by the judges. All these judges accepted the basic climate science.
One important case in the German courts did turn on a scientific issue. That case was Lliuya v. RWE. Briefly, the plaintiff was a resident of Huarez, Peru who claimed that his property was at risk of a glacial outburst flood caused by greenhouse gas emissions. After lengthy proceedings, the court heard from experts it had hired and from experts working for the parties. The court ruled for the plaintiff on numerous key legal grounds, but it dismissed the suit because it found that the risk of the glacial outburst was too far in the future and too low to be a basis for liability. To my knowledge, that is the only climate liability case that has been dismissed on scientific rather than legal grounds.
In the U.S., there are now pending about two dozen lawsuits brought mostly by U.S. states and cities against fossil fuel companies for money damages related to climate change. Various questions of climate science will be major issues if any of these cases ever get to trial. But that’s a real if.
The cases started back in 2004 and, for several years, were held up by disputes about whether they belonged in federal courts or state courts. They’ve landed in state courts. In two of these cases, the state supreme courts have held that the fundamental legal bases were sound, and the cases could proceed to discovery and maybe trial. Those were cases in Hawaii and Colorado. The Supreme Court of Maryland went the other way. In the other states, we have seen lower court decisions going both ways – some dismissing the suits on legal grounds; some saying they could go forward. Numerous appeals by both sides are pending in many of these cases.
The U.S. Supreme Court has agreed to hear one of these cases – Suncor v. County Commissioners of Boulder County, Colorado. The briefing is now taking place, with many amicus briefs on both sides. The Court will hear arguments in the fall; they haven’t set the date yet. We expect a ruling later this year or in the first half of 2027. Meanwhile, several but not all of the other pending cases have been put on hold, pending the decision of the Supreme Court.
There are at least four possible outcomes from the Supreme Court, listed here from best to worst from the perspective of the plaintiffs. First, the Supreme Court could rule for the plaintiffs and say the legal bases for the lawsuit are sound. Second, the Supreme Court could find that the Suncor case is premature; that it’s too early to decide before there has been a trial in that case and the verdict has gone through the state appellate process. The third possible outcome is that the Supreme Court rules for the defendant oil companies on the grounds that the claims based on public nuisance theories and the like are legally invalid, but allows the claims based on consumer deception to go forward. If any of the first three possibilities is what happens, the Suncor case and many of these other cases around the country will move forward to discovery and possibly trial.
The fourth possible outcome is that the Supreme Court knocks out even the consumer deception claims on legal grounds. For example, it might find that the cases are preempted by the Clean Air Act, or that the issues are of such national and global significance that they are not for state courts to decide. If that’s what happens, all these other cases will go away. Such a decision could also be bad news for the state climate superfund laws passed by Vermont and New York, which are already the subject of lawsuits.
If any of these cases do get to trial, there will be many issues of climate attribution. For example: to what extent are the specific injuries alleged by the plaintiffs, such as sea level rise and extreme rainfall, caused by climate change? How bad will they be, and when? It’s one thing to say that the seas will rise or the rain will get worse – it’s another to say what will happen in a particular place around a particular time, with enough precision to be a basis for awarding a specific amount of money damages to a particular plaintiff city.
Then there will be the issue of how much of the impacts are caused by the specific fossil fuel companies. Richard Heede has been studying and quantifying the contribution of different companies for several years, but if these cases move forward, the oil companies will work very hard to try to pick apart those studies, and also to claim that other sectors should also be liable, such as motor vehicle manufacturers and coal-burning electric utilities, and also that it’s governments that should be responsible for setting energy policy.
As noted above, most of these cases also allege that the oil companies engaged in deceptive conduct that affected consumer and governmental behavior. Proving these claims comes in three steps. First, did at least some of the companies engage in deceptive conduct, or at least did they make public statements that contradicted what they already knew about climate change? There is plenty of evidence of that, and more may come out in discovery. The second step is, did the deception affect consumer behavior or government policy? The third step is, did the changes in consumer behavior or government policy affect fossil fuel consumption? These latter two questions are attribution questions mostly for social scientists. They have been the subject of very little research, but if these cases go to trial, they will be key.
Even if the cases survive the Supreme Court’s Suncor decision, final resolution is still years away. First there would be discovery; then more motion practice; then trials; then appeals of the verdicts through the state court systems; and then probably another run at the Supreme Court.
But other cases that rely on climate science are moving much more quickly. Climate science is important in cases under the Clean Air Act, the National Environmental Policy Act, the Endangered Species Act, the securities disclosure laws, and others. Climate science is also important in the discussions about loss and damage, but those take place mostly in political forums rather than the courtroom, at least so far.
In sum, climate science is very important in many different kinds of lawsuits, but the way it is presented and used varies considerably with the kind of case. The emerging science of climate attribution will be very helpful in these cases, but in most cases, that’s not enough: many key decisions will hinge on purely legal issues that are mostly still open.
Michael Gerrard
Michael Gerrard is Andrew Sabin Professor of Professional Practice and founder and faculty director of the Sabin Center for Climate Change Law. He is former chair of the faculty of the Columbia Earth Institute and of the American Bar Association's Section of Environment, Energy and Resources. He practiced environmental law full-time in New York from 1979 to 2008, and has written or edited 14 books on environmental law.
