Though he sided with conservatives on most issues, Supreme Court Justice Anthony Kennedy sometimes broke ranks when it came to the environment. Kennedy, who announced Wednesday he will step down in July, was the key swing vote on one of the most important climate policies the United States has enacted: the regulation of greenhouse gases.

The 2007 Massachusetts v. Environmental Protection Agency decision forced the EPA to treat carbon dioxide as a pollutant. After a review of the science, the EPA issued an endangerment finding for greenhouse gases in 2009, which says carbon emissions are a threat to public health.

This was the impetus behind the Obama administration’s Clean Power Plan, which limited carbon dioxide emissions from power plants. It was also the basis for new Corporate Average Fuel Economy standards, which aimed to reduce greenhouse gas emissions by mandating higher efficiency from cars and trucks.

But replacing Kennedy with a more conservative judge could create an opportunity to narrow the scope of existing environmental laws, including those that regulate climate change.

EPA Administrator Scott Pruitt is in the process of rolling back Obama’s fuel economy standard. He’s also taken the initial steps to undo the Clean Power Plan. However, because of the endangerment finding, he is still required by law to come up with replacement regulations to deal with climate change (an extremely difficult task).

But would a Supreme Court with another conservative justice on bench take a whack at Massachusetts v. EPA?

It’s unlikely, according to Richard Lazarus, an environmental law professor at Harvard Law School who has argued cases before the Supreme Court. “I think on that question, the odds are very, very small,” he told me.

A new Supreme Court judge probably won’t overturn the legal basis for fighting climate change, but may weaken it

According to Lazarus, the court doesn’t typically revisit past cases unless there is a major constitutional question in play. The debate around the Massachusetts v. EPA decision centers on interpreting the language of the Clean Air Act (not a major constitutional question).

What a new Supreme Court justice could do is help limit how this applies, like denying private citizens standing to sue the government in federal court for injuries stemming from failing to adequately address climate change. Such rulings would narrow the real-world impacts of the court’s environmental judgments, which conservatives would celebrate.

Pruitt himself has little interest in genuinely challenging the endangerment finding, a process that would invite years of litigation. He doesn’t seem to want to mount a legal charge to overturn the Supreme Court decision behind it either. Instead, he’s opted to change how the EPA weighs costs and benefits in environmental regulations, tipping the scales in favor of weaker rules.

States and environmental groups would likely sue the EPA once these new regulations come out, arguing for stricter standards. That’s another place where a new justice on the bench could make a difference. “The question is whether the courts will defer to the EPA,” Lazarus said.

Kennedy’s environmental legacy extends beyond climate change, but his replacement will fill a different mold

Given the list of likely nominees, President Donald Trump will probably pick a replacement in the mold of Antonin Scalia, a solid conservative.

Amanda Reilly at E&E News reported that some of the potential nominees are skeptical of government efforts to protect the environment, and one already has climate change regulations in his crosshairs:

President Trump’s shortlist of potential Supreme Court nominees includes foes of expansive environmental regulation and critics of Obama-era attempts to regulate greenhouse gases. Notably, Trump last November added Judge Brett Kavanaugh of the U.S. Court of Appeals for the District of Columbia Circuit to the list. Appointed by President George W. Bush, Kavanaugh is among the D.C. Circuit’s most conservative judges and often hears environmental law cases. At the 2016 oral arguments over the Obama administration’s Clean Power Plan, Kavanaugh was blunt about where he stands: “Global warming is not a blank check,” he said.

But a conservative pedigree is no guarantee that a Supreme Court judge will rule in a predictable way. “You just never know,” said Myron Ebell, who leads the energy and environment division at the libertarian-leaning Competitive Enterprise Institute. “That’s the problem with giving someone a lifetime appointment.”

And the new justice will soon have an opportunity to align with or distinguish themselves from Kennedy. The Supreme Court is expected to hear the Weyerhaeuser Company v. US Fish and Wildlife Service case in October. One dimension of the case is the question of whether the government can regulate private land under the Endangered Species Act. Kennedy previously ruled that it could, irritating many conservatives.

Environmental groups may also become more reluctant to take suits all the way to the Supreme Court out of fear of establishing an unfavorable high court precedent. Instead, many cases will likely peter out in circuit courts.

Right now though, the most important decision falls to Trump: who he picks for the job.