On Tuesday, 375 top scientists signed an open letter warning against the consequences of backing out of the historic Paris Climate Agreement, which Republican presidential nominee Donald Trump has said he would do if elected.

The letter marks an unusual foray into presidential politics for most of these scientists, many of whom specialize in climate change-related fields. World-renowned theoretical physicist Stephen Hawking is among the signatories, as is former Obama energy secretary and Nobel Laureate Steven Chu.

The Paris Agreement, which is expected to go into force this year or in 2017, commits all nations to undertake steps to reduce greenhouse gas emissions and adapt to global warming impacts for the first time. At least 20 countries are expected to announce their ratification of the agreement in an event at the United Nations on Wednesday.

The list of scientists signing the letter includes 30 Nobel Laureates, and all signatories are members of the National Academy of Sciences (NAS).

The letter reads as both a warning of the seriousness of human-caused climate change, as well as a warning against electing a candidate who thinks climate change is a hoax and would withdraw from the Paris Agreement.

"Human-caused climate change is not a belief, a hoax, or a conspiracy. It is a physical reality," the letter states. "Our fingerprints on the climate system are visible everywhere."

Trump is on record saying he would pull out of the Paris Agreement if he were elected, and has expressed the view that climate change is a hoax, rather than a real and growing problem.

“We’re going to cancel the Paris Climate Agreement and stop all payment of U.S. tax dollars to U.N. global warming programs," Trump said in an energy policy speech on May 26.

Scientists assessing a melting glacier in Austria on Sept. 8, 2016. Image: sean gallup/Getty Images

Trump has stacked his campaign with advisers that have deep ties to the fossil fuel industry, including oil tycoon Harold Hamm and North Dakota Republican Rep. Kevin Kramer, who questions the existence of human-caused global warming.

"During the Presidential primary campaign, claims were made that the Earth is not warming, or that warming is due to purely natural causes outside of human control. Such claims are inconsistent with reality," the letter states.

In the letter, the researchers — who include MIT's Kerry Emanuel, who has been open about his past support for Republican candidates — warn against triggering climate "tipping points" as well as political points of no return.

"From studies of changes in temperature and sea level over the last million years, we know that the climate system has tipping points. Our proximity to these tipping points is uncertain," the letter states.

"The consequences of opting out of the global community would be severe and long-lasting"

"The political system also has tipping points," the letter states. "Thus it is of great concern that the Republican nominee for President has advocated U.S. withdrawal from the Paris Accord."

Astrophysicist Neil deGrasse Tyson, a recipient of the National Academy’s Public Welfare Medal and the director of the Hayden Planetarium in New York, endorsed the letter in a press release (though he did not sign onto it since he is not an elected member of the NAS).

“For lawmakers to not heed the advice of esteemed scientists on matters of science, in this the 21st century, signals the beginning of the end of an informed democracy,” Tyson said.

It's not anti-Trump, except...

The scientists, however, said they do not see the letter as a political effort against Trump, but rather as something that fulfills their obligation to speak up about the scientific evidence on climate change to correct the record based on candidates' statements.

August global average temperature anomaly. Image: NASA GISS

“This is fundamentally about the science. It’s not about Donald Trump," climate scientist Benjamin Santer of Lawrence-Livermore National Laboratory, said on a press call. He added that his expertise in finding the human fingerprints on modern climate change compelled him to act.

“It would be an epic failure to be silent and remain silent when that scientific understanding is dismissed as a hoax or a conspiracy,” he said.

"Ignorance is not a virtue, nor should it form the basis of public policy," Santer said.

Although Santer and other letter organizers see this as divorced from the presidential campaign in some way, it's clear that Trump was the impetus for the letter, at least to some degree.

"All of us reacted with some shock to language in the Republican platform," Emanuel said. "So we felt that we had to say something."

This isn't the first time that scientists have voiced concerns regarding public policy related to climate change. For example, in February, nearly 3,000 scientists from around the world condemned the Australian government's move to slash funding for climate research and lay off dozens of scientists.

However, the signing of this letter is a unique instance in which nearly 400 scientists are criticizing the political platform of the Republican Party, considering Democratic nominee Hillary Clinton's climate proposals track more closely with what scientists are saying regarding the severity of global warming and need to slash emissions of greenhouse gases.

Four scientists collaborated on the effort to write and solicit signatories for the letter: Santer, Emanuel, who specializes in studying tropical cyclones; and astrophysicists George Field and Ray Weymann.



