Heathrow’s planned new runway just got canceled—thanks to the Paris Agreement

The project to expand London’s Heathrow airport was first proposed in 2009, and it has had a turbulent journey since. It was initially backed by a Labour government, canceled a year later, revived, then backed by a Conservative government in 2018. Proponents have touted the plan as a source of extra jobs, additional air routes, and fewer delays. The U.K. Department of Transport has estimated an economic benefit of £61 billion, or $79 billion, over the next 60 years.

But now, in one of the most concrete displays of global climate goals, a Court of Appeal ruling deemed the plan “legally fatal,” on the basis that it conflicts with the climate accords. It’s the first time that the Paris Agreement has been the rationale for a significant ruling anywhere in the world.

Since 2009, there have been a variety of public concerns about the construction, especially in local districts. Heathrow is the second busiest airport in the world by international passenger traffic, and the busiest in Europe; the runway would likely add increased noise and overcrowding, and require the demolition of homes to build.

But the basis for the ruling was environmental, specifically the increased carbon emissions likely to be caused by a potential 700 new planes flying in and out of Heathrow every day. That would prove inconsistent with the goals affirmed by Britain and almost 200 other nations in the Paris Agreement: reaching zero net emissions by 2050, and of keeping global temperatures from rising more than 2 degrees Celsius.

While the judgment stops short of saying new runway construction is totally illegal—rather declaring it “legally fatal in its present form”—the British government said Thursday it wouldn’t appeal the decision and escalate it to the Supreme Court. Prime Minister Boris Johnson personally opposed the project from the get-go when he was London mayor, then saying he’d “lie down in front of the bulldozers” to stop it going ahead. (That’s not the first time Johnson has used a tractor metaphor; he also quite literally bulldozed through a wall to symbolize Brexit breaking through the gridlock.)

London’s current mayor, Sadiq Khan, is also opposed. “Today’s landmark Heathrow judgment is a victory for Londoners,” he said in a statement Thursday. “We face a climate emergency and I’m delighted that the Court of Appeal has recognized that the government cannot ignore its climate change responsibilities.”

While the judgment doesn’t rule out new airport expansion plans, it does prove the potency of the internationally backed Paris Agreement, from which the United States announced its withdrawal in 2017.