Donald Trump wasn’t exaggerating when he said during his election campaign that the U.S. could “cancel” the Paris Accord on climate change.

A decision due from the president this week on whether to pull the U.S. out of the deal involving almost 200 nations could have a domino effect on the participation of other countries in limiting fossil-fuel pollution, making it almost impossible and extremely expensive to stop catastrophic climate change.

That’s the conclusion of researchers and scientists evaluating the impact of Trump on the health of the climate. While forecasting the state of the environment more than 80 years into the future is a notoriously inexact exercise, academics gathered by the United Nations at the Intergovernmental Panel on Climate Change are concerned the world is headed for “extensive” species extinctions, serious crop damage and irreversible increases in sea levels even before Trump started to unpick the fight against global warming.

“Four years of the Trump administration may have only modest consequences, but eight years of bad policy would probably wreck the world’s chances of keeping warming below the international target of 2 degrees Celsius,” Michael Oppenheimer, professor of geosciences and international affairs at Princeton University, said by email. “The odds of our avoiding the climate-danger zone would fade to zero.” While a 2-degree shift wouldn’t be noticeable during the course of a day, it would represent a historic change for the Earth as a whole that’s faster than any change in the climate since the last ice age ended some 10,000 years ago. The scenarios that scientists are looking at depend on measurements of air and water temperatures taken at hundreds of sites around the world, as well as complex models about how trends will evolve in the coming decades.

Trump’s move would clearly make the outlook worse, according to Climate Interactive, a team of modelers backed by institutions such as MIT Sloan School of Management and the Rockefeller Brothers Fund.

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