President Trump’s decision to pull out of the Paris climate agreement is a moral outrage and an insult to future generations.

For the scientific community, it also stokes another fear: that America now appears to be an anti-evidence, backward-thinking place, undeserving of top scientific talent.

That reaction was best captured by Richard Horton, editor of the Lancet medical journal. In a series of tweets during Trump’s Paris announcement Thursday, Horton urged fellow researchers, clinicians, and scientists to hold the president to account for a “historic error” and a “turn to the medieval”:

For all of us who admire the idea of American democracy, we must mourn the death of reason imposed by President Trump. An historic error. — richard horton (@richardhorton1) June 1, 2017

If we didn't know it before, we know it now. President Trump leads a rogue anti-science US government that must be defeated. — richard horton (@richardhorton1) June 1, 2017

That the planet is warming is a well-established scientific fact — a fact that Trump has chosen to alternately ignore, deny, or call “a Chinese hoax.”

The decision to back out of the historic agreement moves against “200 years of scientific inquiry,” Bill McKibben, the environmentalist and author, wrote at the New York Times. “It’s a stupid and reckless decision — our nation’s dumbest act since launching the war in Iraq. But it’s not stupid and reckless in the normal way. Instead, it amounts to a thorough repudiation of two of the civilizing forces on our planet: diplomacy and science.”

Trump also made his decision about Paris with virtually no science advisers on staff. But long before today, he has been chipping away at environmental protections and the funding for the Environmental Protection Agency. His proposed immigration policies have made the US less desirable for recruiting global talent into America’s research labs and hospitals, and his proposed budgets have aimed at gutting the US research enterprise. (Though whether Congress will let him is another question.)

Horton urges the health and science communities to “unite in resistance” and argues that the Trump age spells the end of “America as a beacon of science-led government”:

The idea of America as a beacon of science-led government is temporarily over. The health and science communities must unite in resistance. — richard horton (@richardhorton1) June 1, 2017

No scientist, no health professional can be either neutral or silent about President Trump's turn to the medieval. This is a bad, sad day. — richard horton (@richardhorton1) June 1, 2017

Scientists, for their part, have already been marching by the thousands in streets across the globe. But it’s going to take a lot more pressure and resolve — on cities, states, and the most powerful companies in the world — to scale back emissions on their own, without the leadership of the US.