On Tuesday, US Environmental Protection Agency administrator Lee Zeldin announced that the agency is taking aim at the endangerment finding, a 2009 rule that’s essentially the tentpole supporting federal greenhouse-gas regulations.

This might sound like an obscure legal situation, but it’s a really big deal for climate policy in the US. So buckle up, and let’s look at what this rule says now, what the proposed change looks like, and what it all means.

To set the stage, we have to go back to the Clean Air Act of 1970, the law that essentially gave the EPA the power to regulate air pollution. (Stick with me—I promise I’ll keep this short and not get too into the legal weeds.)

There were some pollutants explicitly called out in this law and its amendments, including lead and sulfur dioxide. But it also required the EPA to regulate new pollutants that were found to be harmful. In the late 1990s and early 2000s, environmental groups and states started asking for the agency to include greenhouse-gas pollution.

In 2007, the Supreme Court ruled that greenhouse gases qualify as air pollutants under the Clean Air Act, and that the EPA should study whether they’re a danger to public health. In 2009, the incoming Obama administration looked at the science and ruled that greenhouse gases pose a threat to public health because they cause climate change. That’s the endangerment finding, and it’s what allows the agency to pass rules to regulate greenhouse gases.  

The original case and argument were specifically about vehicles and the emissions from tailpipes, but this finding was eventually used to allow the agency to set rules around power plants and factories, too. It essentially underpins climate regulations in the US.

Fast-forward to today, and the Trump administration wants to reverse the endangerment finding. In a proposed rule released on Tuesday, the EPA argues that the Clean Air Act does not, in fact, authorize the agency to set emissions standards to address global climate change. Zeldin, in an appearance on the conservative politics and humor podcast Ruthless that preceded the official announcement, called the proposal the “largest deregulatory action in the history of America.”

The administration was already moving to undermine the climate regulations that rely on this rule. But this move directly targets a “fundamental building block of EPA’s climate policy,” says Deborah Sivas, an environmental-law professor at Stanford University.

The proposed rule will go up for public comment, and the agency will then take that feedback and come up with a final version. It’ll almost certainly get hit with legal challenges and will likely wind up in front of the Supreme Court.

One note here is that the EPA makes a mostly legal argument in the proposed rule reversal rather than focusing on going after the science of climate change, says Madison Condon, an associate law professor at Boston University. That could make it easier for the Supreme Court to eventually uphold it, she says, though this whole process is going to take a while. 

If the endangerment finding goes down, it would have wide-reaching ripple effects. “We could find ourselves in a couple years with no legal tools to try and address climate change,” Sivas says.

To take a step back for a moment, it’s wild that we’ve ended up in this place where a single rule is so central to regulating emissions. US climate policy is held up by duct tape and a dream. Congress could have, at some point, passed a law that more directly allows the EPA to regulate greenhouse-gas emissions (the last time we got close was a 2009 bill that passed the House but never made it to the Senate). But here we are.

This move isn’t a surprise, exactly. The Trump administration has made it very clear that it is going after climate policy in every way that it can. But what’s most striking to me is that we’re not operating in a shared reality anymore when it comes to this subject. 

While top officials tend to acknowledge that climate change is real, there’s often a “but” followed by talking points from climate denial’s list of greatest hits. (One of the more ridiculous examples is the statement that carbon dioxide is good, actually, because it helps plants.) 

Climate change is real, and it’s a threat. And the US has emitted more greenhouse gases into the atmosphere than any other country in the world. It shouldn’t be controversial to expect the government to be doing something about it. 

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