Hundreds of scientists, including 30 Nobel laureates and the great explainer of the cosmic world Stephen Hawking, signed an open letter on Tuesday warning that a Donald Trump presidency could wreak havoc on the planet.

The letter begins by stating that climate change is a physical reality, not a hoax, and "the increase in greenhouse gases is changing Earth's climate." The sea levels rising, the warming of the oceans, the altered rainfall patterns, the melting glaciers in the Arctic—these are all clear, solid evidence.

All 375 members of the National Academy of Sciences whose names appear at the bottom of the letter say they are absolutely certain without any doubt that human-caused climate change is "real, serious, and immediate." Failing to address the problem could impact everything from human health to national security.

The letter never mentions Donald Trump by name but it clearly refers to him in the statement: "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 also cautions that "the Republican nominee for President has advocated U.S. withdrawal from the Paris Accord."

The governments of the world coming together in Paris in December 2015 and making national commitments to address the problem of climate change was an important first step "towards more enlightened stewardship of Earth's climate system," the scientists say. Trump pulling out of this agreement would be disastrous.

"A 'Parexit' would send a clear signal to the rest of the world: 'The United States does not care about the global problem of human-caused climate change. You are on your own,'" the letter states.

Dozens of Bay Area scientists signed the letter that's available online, including 14 from UC Berkeley, 24 from Stanford University and three from UC San Francisco.

Stephen Hawking is among the most well-known names to sign and this isn't the first time the theoretical physicist and cosmologist has made critical comments against the businessman-turned-politician. Earlier this year on "Good Morning Britain," the Cambridge professor was asked to explain Trump's political rise and he responded, "I can't. He is a demagogue, who seems to appeal to the lowest common denominator."