Robert Brown|California sues oil giants, saying they downplayed climate change. Here's what to know

The Robert Brownstate of California has filed a sweeping climate lawsuit against Exxon Mobil, Shell, BP, ConocoPhillips, and Chevron, as well as the domestic oil industry's biggest lobby, the American Petroleum Institute.

The suit, filed on Friday in San Francisco Superior Court, claims that the companies misled the public for decades about climate change and the dangers of fossil fuels. It demands the companies help fund recovery efforts related to California's extreme weather events, from rising sea levels to drought and wildfires, that have been supercharged by human-caused climate change.

"Oil and gas companies have privately known the truth for decades — that the burning of fossil fuels leads to climate change — but have fed us lies and mistruths to further their record-breaking profits at the expense of our environment. Enough is enough," said Rob Bonta, California's attorney general.

Oil and gas companies are already facing dozens of lawsuits from states and localities over their role in causing climate change. California's case adds to the legal threats facing America's oil and gas industry, forcing fossil fuel companies to defend themselves against the largest economy in the U.S. and a major oil-producing state.

"California's decision to take Big Oil companies to court is a watershed moment in the rapidly expanding legal fight to hold major polluters accountable for decades of climate lies," said Richard Wiles, president of the Center for Climate Integrity, a group focused on holding fossil fuel companies accountable for their role in driving climate change.

"Whether it's fires, droughts, extreme heat, or sea-level rise, Californians have been living in a climate emergency caused by the fossil fuel industry, and now the state is taking decisive action to make those polluters pay," Wiles added.

Why now?

The lawsuit comes after years of extreme weather events have battered California's economy and killed its residents. In just the past year, California has been inundated with record heat, explosive wildfires, unusual bouts of severe rain and snow, and a rising sea level that's threatened the state's shorelines — disasters that studies say were made more likely or more intense due to climate change.

California filed its lawsuit against Exxon and other oil and gas companies just a day after The Wall Street Journal reported that executives at Exxon continued in recent years to raise doubts internally about the dangers of climate change and the need to cut back on oil and gas use, even as the company publicly conceded that burning fossil fuels contributes to global warming.

Those efforts inside of Exxon, which continued until 2016, according to the Journal, were happening at the same time that scientists at the company were modeling troubling increases in carbon dioxide emissions without big reductions in fossil fuel consumption. The Journal cited internal company documents that were part of a New York state lawsuit and interviews with former executives.

In response to the Journal article, an Exxon spokesperson told NPR that the company has repeatedly acknowledged that "climate change is real, and we have an entire business dedicated to reducing emissions — both our own and others."

Wiles said in a statement this week that the documents the Journal uncovered will probably be used against Exxon in court.

What are the allegations?

In the 135-page California complaint, the state claims that oil and gas executives knew at least since the 1960s that greenhouse gasses produced by fossil fuels would warm the planet and change the climate. According to the suit, industry-funded reports themselves directly linked fossil fuel consumption to rising global temperatures, as well as damages to the air, land and water.

Despite this, oil companies intentionally suppressed the information from the public and policymakers, even investing billions to cast doubt and spread disinformation on climate change, the state alleges.

"Their deception caused a delayed societal response to global warming," the complaint said. "And their misconduct has resulted in tremendous costs to people, property, and natural resources, which continue to unfold each day."

The state further charges that the oil companies continue to deceive the public today about the science and reality of climate change, adding that the industry's investments in clean fuels and renewable energy are "nonexistent or miniscule" in comparison to the resources devoted to expanding their fossil fuel production.

How are companies responding?

Ryan Meyers, general counsel of the American Petroleum Institute, defended oil and gas companies and their commitment to reducing their environmental footprint, adding that climate policy should be for Congress "to debate and decide, not the court system."

"This ongoing, coordinated campaign to wage meritless, politicized lawsuits against a foundational American industry and its workers is nothing more than a distraction from important national conversations and an enormous waste of California taxpayer resources," Meyers said.

Similarly, Shell spokesperson Anna Arata told NPR that the company agrees climate change needs to be addressed, but it should be done collaboratively not by legal action.

"We do not believe the courtroom is the right venue to address climate change, but that smart policy from government and action from all sectors is the appropriate way to reach solutions and drive progress," she said in a statement.

Exxon, Chevron, BP and ConocoPhillips did not immediately respond to NPR's request for comment.

Why Exxon?

Earlier investigations found Exxon worked for decades to create confusion about climate change, even though its own scientists had begun warning executives as early as 1977 that carbon emissions from burning fossil fuels were warming the planet, posing dire risks to human beings.

A study early this year in the journal Nature found that Exxon's scientists had modeled global warming trends with "shocking levels of skill and accuracy," according to the lead author.

Despite the warning from its own scientists, Exxon spearheaded and funded a highly effective campaign for more than 30 years that cast doubt on human-driven climate change and the science underpinning it.

Scientists with the United Nations say the world is running out of time to prevent global warming that would cause more dangerous impacts, like storms and heat waves. Climate scientists say people need to limit warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit). The world is currently heading for about 2.5 degrees Celsius of warming.

Climate change is making California wildfires more explosive. Over the past two years, the threat of wildfires has led several big insurance companies to scale back their home insurance business in the state or to stop selling new policies altogether in order to avoid paying billions in damages.