California, Massachusetts, Connecticut and New York are leading a group of 20 other states in suing the US Environmental Protection Agency for renouncing its ability to regulate greenhouse gas emissions, The New York Times reports. The lawsuit specifically argues that the EPA’s decision to rescind a 2009 study that determined greenhouse gases are dangerous to public health was illegal. The study, which is the source of what’s called the “Endangerment Finding,” was one of several justifications — along with things like the Clean Air Act — for the agency’s ability to regulate emissions.

Rescinding the finding nullified the EPA’s evidence for things like emissions standards and a variety of other regulations that attempted to reduce the amount of greenhouse gases produced by the automotive, coal and oil industries. The Trump administration framed the rollback as a cost-saving measure, but it was also a major blow to the government’s ability to fight climate change. Greenhouse gases, which include things like carbon dioxide, methane and nitrous oxide, collect in the atmosphere and warm the planet, upsetting weather patterns and negatively impacting the environment. Determining the changes caused by greenhouse gases posed a risk to public health gave the EPA the authority to regulate them under its existing mandate to address air pollution. An authority it could have again, depending on the result of this litigation.

Of course, winning a lawsuit isn’t necessary to restore the EPA’s role in fighting climate change. Congress could do that now by passing a new law. The legal route is just faster, and potentially riskier. The New York Times writes that this new lawsuit was filed in the US Court of Appeals for the District of Columbia, and could ultimately be combined with an existing lawsuit from environmental groups. Depending on how the case fairs in the lower court, it may eventually be appealed to the US Supreme Court, who could decide on an even more restrictive interpretation of the EPA’s role.

Under President Donald Trump, the EPA has already rolled back clean water rules and attempted to stifle research. The Trump administration has separately tried to undermine the authority of independent agencies like the EPA and FTC, something the Supreme Court has yet to determine to be illegal.