American politics, just like climate change, have reached a “tipping point” with Donald Trump being named the Republican Party’s nominee, said acclaimed physicist Stephen Hawking and nearly 400 other scientists in an open letter Tuesday.

The letter, signed by 375 members of the National Academy of Sciences, is meant to urge American policymakers and leaders to make climate change their most pressing concern, not only because of record warming in the last year, but because of the possibility of Trump winning the presidency.