In 1998, 46 states and the District of Columbia signed on to the largest civil litigation settlement in US history, the tobacco Master Settlement Agreement. Stunning in its scope and scale, the agreement forced the four largest tobacco companies to stop advertising to youth, limit lobbying, restrict product placement in media, and fund anti-smoking campaigns. It also required them to pay out more than $206 billion over 25 years.

Tobacco companies had in previous decades successfully swatted down hundreds of private lawsuits. But states found an opening by suing companies for the harm they caused to public health. “This lawsuit is premised on a simple notion: You caused the health crisis, you pay for it,” said then-Mississippi Attorney General Michael Moore in 1994.

Now another wave of lawsuits is trying to hold powerful institutions accountable for an even bigger crisis, by making them pay and change their ways. At least eight US cities, five counties, and one state are suing some of the world’s largest fossil fuel companies for selling products that contribute to global warming while misleading the public about their harms. In parallel, 21 young people are trying to suspend fossil fuel development as part of their high-profile climate rights case, Juliana v. United States, against the government. (The case has a hearing Tuesday at the Ninth Circuit Court of Appeals in Portland, Oregon.)

And with several new Democratic attorneys general elected in the 2018 midterm elections, even more litigation may be on the horizon.

“It feels like there is a lot of climate change litigation right now,” said Paul Sabin, a professor of environmental history at Yale. “But this is only the beginning.”

The momentum is building in other countries too. In Canada, the Netherlands, and Ireland, citizens are taking their governments to court to demand more ambitious policies to fight climate change.

In the European Union, a member of the European parliament in March formally requested that Exxon Mobil, the largest investor-owned oil company in the world, have its lobbying privileges revoked. The call for revocation came after a public hearing that outlined the company’s role playing down climate science and spreading misinformation.

At stake in these cases are billions of dollars in liability and legal precedents that will last generations. For the plaintiffs — children, farmers, fishermen, and cities vulnerable to drought and sea level rise — weak federal climate policy and dire warnings from scientists about the future are driving a sense of urgency. And litigation offers something missing from every other climate change mitigation strategy, whether it’s the Green New Deal or a carbon tax: a villain.

“Big oil knew for decades that greenhouse gas pollution from their operations and their products were having a significant and detrimental impact on the earth’s climate,” Rhode Island Attorney General Peter F. Kilmartin said last year from atop a seawall, announcing his state’s suit against companies like Exxon Mobil, Chevron, BP, and Royal Dutch Shell. “Instead of working to reduce that harm, these companies chose to conceal the dangers, undermine public support for greenhouse gas regulation, and engage in massive campaigns to promote the ever-increasing use of their products and ever-increasing revenues in their pockets.”

With so many lawsuits filed across so many jurisdictions, the likelihood of a climate case getting to a trial is growing. Outside the courtroom, public opinion is starting to shift, with a majority of Americans wanting the government to address climate change, according to several recent polls. The hope among the plaintiffs is that one of the suits could lead to comprehensive action on climate change that the political process has failed to provide.

However, the plaintiffs are in uncharted legal territory, and opponents say these cases hinge on radical, unprecedented expansions of existing laws. The litigation also circumvents the legislative process, which is arguably where climate change policies should be implemented in the first place.

As federal appeals courts weigh whether some of these lawsuits should be allowed to go to trial, it’s helpful to understand the background of environmental litigation, the arguments being made, and what they could mean for the fight against climate change. Here’s what you need to know.

Environmental lawsuits have a long history, but climate liability and climate rights are a new frontier

“Litigation has been a crucial strategy for environmental activism since the ’60s and the ’70s,” said Sabin, the Yale professor. In fact, we’ve already seen a successful lawsuit centering on climate change. The Environmental Protection Agency was forced to regulate carbon dioxide to fight climate change as the result of a 2007 Supreme Court decision in a lawsuit, Massachusetts v. EPA. It’s the most significant example of a climate change mitigation policy established through the courts.

One of the earliest cases to invoke harm caused by greenhouse gases dates back to 1986. In the City of Los Angeles v. National Highway Traffic Safety Administration lawsuit, the city and environmental organizations challenged NHTSA’s rollback of a vehicle emissions law. An appeals court ruled in favor of the federal government.

But the current wave of litigation is bringing up new legal questions in the context of climate change for the first time. For local governments suing fossil fuel companies, the fight is over who’s on the hook for paying for the damages stemming from rising average temperatures. In the youth lawsuits, the key issue is whether a stable climate is a civil right.

Another interesting factor in these cases is that climate science itself isn’t up for debate. The lawsuits center on some fundamental interpretations of law, but in nearly all cases, the parties agree on these facts: Greenhouse gases from burning fossil fuels are heating up the planet, which in turn is fueling sea level rise, more extreme weather, and changes in the overall climate.

In San Francisco and Oakland’s lawsuits against oil companies, for example, the presiding federal judge even asked for a climate change tutorial from the plaintiffs and the defendants. Both sides largely agreed on the fundamentals.

“Chevron accepts the consensus in the scientific communities on climate change,” said Theodore Boutrous, an attorney who presented a climate tutorial on behalf of Chevron last year and agreed with the Intergovernmental Panel on Climate Change’s conclusions that human activity is warming the planet. “There’s no debate about climate science.”

The dispute, then, is over how to apply existing laws. Since the legal principles these lawsuits invoke have never been applied to climate change, the outcomes stand to set huge legal precedents. Depending on how they’re decided, these lawsuits could open the floodgates to new litigation. At the same time, judges are uncertain about how to proceed and have reached widely differing conclusions on similar lawsuits. Which is why the tension and drama around these suits is so high, compared to a fight over, say, a carbon tax.

The plaintiffs in the suits against the government and against fossil fuel producers all say that their objective is to see the cases through to the end, but they stand to accomplish a lot well before the cases come to fruition.

“I think that litigation serves multiple goals,” Sabin said. Beyond winning the case, forcing powerful businesses and institutions to publicly grapple with their impact on the planet is an end unto itself. The discovery process where parties have to reveal some of their inner workings could also be enlightening. “Those [goals] include framing an issue in a public setting, drawing attention an issue. They include uncovering documents and revealing what’s been going on.”

Local governments are suing fossil fuel companies for posing a public nuisance with their products

So far, 14 US cities, counties, and one state have sued fossil fuel companies. Most of these cases are still undergoing pretrial legal motions. The lawsuits brought by the cities of San Francisco and Oakland were dismissed but are being appealed.

In these cases, the local governments are claiming fossil fuel producers have created a public nuisance. This refers to an activity that impairs the use of a public good through damage, creating hazards, and reducing comfort. It’s a principle that’s long been used to litigate environmental issues from protecting drinking water to controlling air pollution.

The argument in these cases is that fossil fuel companies have known for years that their products release greenhouse gases that warm the planet, which in turn harms the public interest: Rising seas are encroaching on shoreline properties, and drier weather is increasing wildfire risks for homes.

At the same time, the plaintiffs say, coal miners and oil drillers obfuscated their products’ impact on the environment despite their own internal research showing that carbon dioxide from burning fossil fuels is warming the planet. “The bottom line story is: ‘You made this product. You knew while making this product that it was going to cause these horrific problems. And you did not tell anyone,’” said David Bookbinder, chief counsel at the Niskanen Center.

The state of Rhode Island, for instance, filed suit in Providence/Bristol County Superior Court against 14 oil and gas companies last year. The complaint notes that the state has more than 400 miles of coastline threatened by sea level rise as warmer temperatures melt polar ice. That in turn is fueling larger storm surges, saltwater intrusion, erosion, and nuisance flooding. More carbon dioxide in the atmosphere is also making the ocean more acidic, threatening shellfish in the Narragansett Bay.

The complaint alleges that many of these fossil fuel companies knew about how their products caused climate change decades ago but hid that information, even as they started protecting their own facilities from consequences like rising oceans and melting polar ice. More recently, oil companies have even asked for government funding to build sea walls to protect coastal refineries from these climate change impacts.

“By 1988, Defendants had amassed a compelling body of knowledge, unavailable to the general public and the broader scientific community, about the role of anthropogenic greenhouse gases and specifically those emitted from the normal use of Defendants’ fossil fuel product, in causing global warming,” according to Rhode Island’s complaint. “Defendants took affirmative steps to conceal, from the State and the general public, the foreseeable impacts of the use of their fossil fuel products on the Earth’s climate and associated harms to people and communities.”

As recourse, Rhode Island wants oil companies to pay for sea walls and other infrastructure to protect human safety and property as well as punitive damages.

Fossil fuel companies say that climate change is too big a problem for the courtroom

The obvious counterargument is that humanity has also benefited immensely from fossil fuels. Coal, oil, and natural gas have provided lifesaving, economy-boosting heat and electricity to billions. The companies extracting these fuels say they would be out of business if people weren’t buying what they’re selling.

And it’s not just the SUVs that run on gasoline or the power plants that burn coal that are driving climate change; it’s the way we build our roads to accommodate cars rather than public transit, design our cities for sprawl rather than density, and orient our diets around meat and dairy instead of vegetables, fruits, and grains.

According to fossil fuel producers, putting companies on trial before a judge and jury doesn’t come anywhere near close enough to solving the problem, nor does it achieve justice.

Joshua Lipshutz, an attorney at the Gibson Dunn law firm who served as legal counsel for Chevron, has argued that climate change is a fundamentally different animal compared to past applications of public nuisance torts. Usually, such rules are applied to a specific instance of pollution, like a leaking gas pipe. This problem has a defined scope (the amount of gas released), it was something that wasn’t supposed to happen, and there’s a specific solution (fixing the pipe, and if negligence is found, making the polluter pay a fine).

However, with climate change, the plaintiffs are seeking damages for future harms, things that have not occurred yet. They’re also blaming a form of pollution, greenhouse gases, that can’t be attributed to any one entity. Cars, airplanes, furnaces, and power plants all emit carbon dioxide. The US isn’t the only country spewing carbon dioxide, and none of the plaintiffs are seeking an injunction against producing even more fossil fuels.

Another complicating factor is that every tier of government has pursued policies that have encouraged the use of fossil fuels, from building highways to subsidizing airports to constructing greenhouse gas-emitting power plants. So if an oil and gas company can be held liable for carbon dioxide emissions, so too perhaps can city governments, power companies, and automakers.

“The plaintiff’s theory, if you take it to its natural conclusion, you can bring this lawsuit against anyone,” Lipshutz said. “This really is sort of an unprecedented type of lawsuit where you’re seeking to [punish] an important part of our economy that’s perfectly lawful ... and for companies to pay for future damage that has not yet occurred.”

But as specious as oil companies may think these suits are, they are taking them extremely seriously. Exxon Mobil, the world’s largest investor-owned oil company, launched a million-dollar push last year for carbon tax legislation that includes immunity from climate change-related lawsuits. Exxon is also facing lawsuits for allegedly misleading investors about the risks of climate change to its business as well as the risks of future climate regulations. (Exxon did not respond to requests for comment.)

Meanwhile, a federal judge in Boston in March allowed a suit filed by the Conservation Law Foundation against Exxon to go forward. The suit alleges the company failed to protect an oil storage facility against the impacts of climate change and is notable because Exxon’s defense may be that it knew climate change was a threat and was prepared for it, an argument that would undermine its defense in other lawsuits.

Fighting these cases also costs these companies, in time, money, and unwanted attention in the spotlight, so there’s pressure to end these lawsuits quickly. “The longer the cases against oil companies drag on, the less happy investors will be,” said Daniel Farber of the Center for Law, Energy, and the Environment at the University of California Berkeley.

As for the courts, judges can’t even agree on who has jurisdiction over these kinds of lawsuits. At the US District Court for the Northern District of California, one judge, William Alsup, decided to move lawsuit from San Francisco and the City of Oakland against oil companies to federal court. He later dismissed the claim on the merits.

Another federal judge at the same court, Vince Chhabria, sent climate lawsuits filed by Marin and San Mateo counties as well as the city of Imperial Beach against 37 fossil fuel companies to California state court.

Both sets of decisions are being appealed at the Ninth Circuit Court of Appeals.

In these public nuisance lawsuits, the plaintiffs believe they have a better shot of winning their cases in state courts. The defendants think their case is stronger in federal courts and have pushed for federal courts to hear these cases when they’ve been filed in lower courts. “The reality is, we think that ultimately it shouldn’t matter whether the cases should be heard in state court or federal court,” Lipshutz said. “They are not viable legal claims.”

Fossil fuel companies also say that long-shot lawsuits, especially when they come from states, counties, and cities, are a waste of the public’s money and time that would be better devoted to adapting to climate change and directly mitigating emissions.

“Focusing resources on a novel, never-accepted theory just isn’t the way to have a productive chance at really addressing global warming,” Chevron’s Boutrous told Vox last year. “No tort theory has ever been developed that comes close to covering these issues.”

But even if these climate lawsuits aren’t decided in favor of cities or states, the discovery process, where the defendants may be required to turn over internal documents to the court, could create new lines of attack. Already, we’ve seen leaked internal documents from oil companies spur lawsuits, so further revelations could lead to even more litigation. That’s why the fossil fuel companies named in these suits are rushing to have these suits thrown out before they actually begin.

The children’s climate change lawsuit against the federal government is facing a make-or-break ruling this year

In 2015, 21 plaintiffs filed a lawsuit against the federal government in the United States District Court in Oregon. The plaintiffs, now between the ages of 11 and 22, include Sophie Kivlehan, 20, the granddaughter of the famed climate scientist James Hansen, and Kelsey Cascadia Rose Juliana, 22, the namesake of the case Juliana v. US. The case is backed by the nonprofit Our Children’s Trust, which has also backed similar suits in eight other states.

The suit argues that the US government undertook policies that contributed to climate change. This includes leasing public lands for mining, drilling, and fracking to extract fossil fuel. The complaint notes that the federal government has long known about the consequences of burning fossil fuels, namely climate change.

By pursuing these policies, the federal government is denying young people the constitutional right to a public resource, a safe climate.

“That’s the brilliance of having children as the plaintiffs,” said Ann Carlson, a professor of environmental law at the University of California Los Angeles. “They’re arguing about the future of the planet.”

For the plaintiffs, the goalposts are clear. “The issue here is climate change and the only measure of success is when this federal government starts recognizing its lengthy responsibility in action in causing climate change and a plan is implemented to cease emissions,” said Philip Gregory, one of the lead attorneys representing the children in the suit. That would mean a suite of aggressive policies to limit global warming.

In a surprising move, the Supreme Court stepped in to pause Juliana v. US last year just days before the trial was set to begin. Then suddenly the high court allowed it to go ahead. Then in November, the Ninth Circuit paused the case to hear an appeal from the federal government. On Wednesday, the court will hear an appeal of a lower court decision.

The key argument from the children is that the federal government violated the civil rights of the plaintiffs. Gregory drew an analogy to racial discrimination:

Let’s say the federal government develops a parking lot, and it leases a restaurant in the parking lot, and that restaurant even though it’s privately owned, engages in segregation. Let’s just say that. Well that’s a constitutional violation that the government is leasing property that is causing harm. That harm being segregation. So now let’s change the words. The federal government has federal lands that it’s leasing to companies to remove coal that the federal government knows will be burned and cause fossil fuel emissions which will harm children and future generations. The federal government knows that.

It’s this knowledge of resulting harm — harm that the government has studied for decades — that makes the federal government liable for the fossil fuel emissions resulting from its policies, Gregory argues. And not only did the government know about the harms of fossil fuels, it has had a growing suite of alternatives at its disposal. “What we’re saying here is the evidence is uncontradicted that we have viable alternatives to a fossil fuel energy system,” Gregory said.

This is a new, untested argument, and it could set a precedent. “It’s clear that federal courts in the United States have not previously recognized a constitutional right to a clean environment or to a stable climate system,” said Michael Burger, executive director of the Sabin Center for Climate Change Law at Columbia University. “In my view there is a compelling legal argument for it.”

Asked for comment, the Justice Department referred Vox to an 82-page brief filed in its appeal of the suit before the Ninth Circuit. The brief, presented by acting Assistant Attorney General Jeffrey Bossert Clark, outlines a number of counterarguments.

The first is that the plaintiffs don’t have standing and cannot demonstrate a particular injury since climate change is something that affects the whole world in complicated ways. The Justice Department argues that the plaintiffs didn’t go through the proper regulatory channels outlined under the Administrative Procedure Act and that the Constitution doesn’t hold any right to a stable climate system.

“Plaintiffs’ alleged fundamental right to a ‘livable climate’ finds no basis in this Nation’s history or tradition and is not even close to any other fundamental right recognized by the Supreme Court,” Clark’s filing states.

The Justice Department is also arguing that existing laws such as the Clean Air Act already cover climate change.

The question now is how the federal appeals court will weigh these arguments Wednesday.

Fishermen and farmers are also suing for damages caused by climate change

There are numerous other climate change liability lawsuits pending in the US and around the world, but the circumstances around them are more unique.

A group of Oregon and California fishermen, represented by the largest commercial fishing industry trade group on the West Coast, filed a lawsuit in California Superior Court against 30 fossil fuel companies. Rising carbon dioxide levels in the atmosphere are causing the ocean to warm and to acidify, and fishermen’s yields of valuable catches like Dungeness crab are declining, the group noted. The Pacific Coast has already seen blooms of toxic algae spurred by warmer oceans. That algae has in turn made it unsafe to eat many of the fish and other animals in the water.

Unlike the lawsuits filed by the cities, the fishermen can point to direct monetary harm that has already occurred from warming. Several heat waves have struck the Pacific Ocean since 2014. The crab fishing season in 2015 was delayed due to the presence of an algae neurotoxin in shellfish. The delay forced some fishing operations ashore for good while harming the finances of others.

These damages may give the case stronger legal footing, but it’s still too early to tell how the lawsuit will proceed.

Meanwhile, citizens in the Netherlands, Ireland, and Pakistan have sued their governments for failing to address climate change. The government of the Philippines is currently conducting a human rights inquiry into fossil fuel producers and weighing whether to enter litigation against these companies.

Saul Luciano Lliuya, a farmer in Peru, is suing the German energy giant RWE. The glaciers in the Andes mountains have lost half of their ice in the past 40 years, and he was worried that this would brings risks of landslides and flooding to his hometown of Huaraz, home to 120,000 people.

In 2015, he filed suit against RWE, a company with about $50 billion in annual revenue, for $20,000, the estimated cost to build a dam to control flooding around his city. RWE produces about 73 percent of its electricity from fossil fuels. It’s also headquartered in Essen, 6,500 miles away from Huaraz.

Yet surprisingly, a German court ruled in 2017 that the case has merit and is now collecting evidence for the proceedings.

For activists, climate lawsuits are a high-stakes, high-reward gambit

“These cases are sort of on the cutting edge,” said Farber, of the Berkeley law center. “I think it’s kind of a long shot that they’re actually going to succeed in the end, although maybe a long shot worth taking just because the payoff would be so great.”

Climate change lawsuits could lead to multibillion-dollar payouts, and force an unwilling government to make cutting greenhouse gases a central priority. Both types of cases could set precedents that would last for decades. But litigation takes years of effort and can cost millions. If a court or a jury rules against the plaintiffs, they could end up worse off than when they started.

Given recent history, many of the nuisance lawsuits against fossil fuel companies will likely be tossed out. But it only takes one successful case to set an industry-rocking precedent.

And a successful lawsuit still might not address the underlying problem of greenhouse gas emissions, which are still rising in the US and still need a comprehensive policy solution.

“These cases are one moment in time in something that’s going to be going on for 100 years,” Yale’s Sabin said. “So whether they win or not, they’re part of that longer process and if they win, they’ll reshape the conversation.”