When President Trump announced the U.S. was leaving the Paris Climate Agreement, he promised to save trillions of dollars in climate-related spending.

But a year after his historic decision, Trump’s promise could become a costs nightmare.

Instead of savings, data published in the journal Nature shows that Trump’s decision to pull out of the deal could actually end up costing the U.S. economy trillions of dollars before the end of the century.

A group of researchers at Stanford University used 50 years of historical data to calculate the relationship between heat and productivity, and found that meeting the Paris Agreement’s most ambitious target of 1.5 degrees warming could save the economy trillions of dollars in lost productivity.

But if we miss the target — an outcome made all the more likely by Trump’s exit — things start looking grim. In the U.S. alone, the cost of failing to meet the most ambitious Paris target of 1.5 degrees, and warming by 2 degrees, could be around $6 trillion.

The costs spiral into the tens of trillions if you take into account effects on the entire global economy, and the fact that we’re currently nowhere close to limiting warming to 2 degrees.

If, for example, all countries follow America’s lead and fail to slash their emissions, the world is likely to warm 4 degrees, putting a 20 percent dent in global GDP, the researchers estimated.

VICE News spoke to researcher Marshall Burke about why a warming world is such a huge financial liability.