Stephen Hawking is at it again. Two months ago the wheelchair-bound British scientist warned we were in danger of going extinct within a hundred years and must, therefore, start colonizing Mars. Now he's warning that President Trump will singlehandedly cause Earth to become as hot and lifeless as Venus.

"By denying the evidence for climate change, and pulling out of the Paris Climate Agreement," Hawking declares, "Donald Trump will cause avoidable environmental damage to our beautiful planet, endangering the natural world, for us and our children."

Hawking is a fellow theoretical physicist who has made significant contributions to our speculations about the behavior of black holes. I met him at Harvard some thirty years ago and subsequently interviewed him for ABC News, shortly after the publication of his first book, "A Brief History of Time."

Stephen was a far-out thinker back then, but credible; it's what made him exciting. But now that he has allowed the media to turn him into the Amazing Kreskin of science, I find it hard to take him seriously.

The saddest thing about his latest wild-eyed pronouncement is that it contributes to the politicization of a timely and legitimate topic, climate change. Hawking has committed the unforgivable sin of allowing hyperbole and his progressive political views to grossly distort his cold-eyed scientific judgment.

There is presently a strong, well-publicized scientific consensus that humanity's environmental slovenliness is largely to blame for the observed warming of Earth's atmosphere — roughly 1.4 degrees Fahrenheit since 1880, according to NASA's Goddard Institute for Space Studies. The consensus is based on observations of temperatures around the world, our understanding of the greenhouse effect — the proven warming caused by certain gases, such as methane, water vapor, and carbon dioxide — and computer modeling of the ultra-complex interactions among land, air, and water.

Beyond that, there isn't much truth to what Hawking said. Earth and Venus are about the same size, but otherwise quite different. Their atmospheres contain 0.04 percent and 96.5 percent carbon dioxide, respectively; and their average temperatures are 59 and 867 degrees Fahrenheit, respectively. It would take something far more fantastical than even Hawking's lively imagination to trigger a runaway greenhouse effect that remakes our planet into Venus.

In fact, in 2012 scientists in Canada and England explicitly studied and discounted the likelihood of such a thing happening, given our current behavior and understanding of atmospheric science. "The good news," they report, "is that almost all lines of evidence lead us to believe that it is unlikely to be possible, even in principle, to trigger a full runaway greenhouse by addition of non-condensible greenhouse gases such as carbon dioxide to the atmosphere."

Another widely promulgated fiction is that a scientific consensus is somehow infallible. Tell that to Galileo, who overturned millennia of scientific thinking when he observed the phases of Venus through his telescope and recognized from them that it had to be orbiting the Sun, not the Earth.

Caution should be even more in order when it comes to predictions about the long-term behavior of something as complex as the climate. Not only are there a dizzying number of positive and negative feedback mechanisms that even supercomputers have a hard time juggling, many of them are fundamentally unpredictable. In science-speak, the climate system is chaotic, as the Intergovernmental Panel on Climate Change acknowledges.

"The climate system is particularly challenging since it is known that components in the system are inherently chaotic," the IPCC says. "There are feedbacks that could potentially switch sign, and there are central processes that affect the system in a complicated, non-linear manner."

If long-term predictions about the climate are unreliable, how much more so are the effects of a complex, non-binding political agreement based on those unreliable long-term climate predictions? And so Hawking is speaking not as an impartial scientist, but a partisan politician when he warns that President Trump's decision will lead to our planet's ruin.

Hawking, of all people, should know about the limits of science's ability to predict the future. When he was 21, doctors diagnosed him with Lou Gehrig's Disease and predicted he wouldn't live past 25. Last January he turned 75.

Michael Guillen, a former Emmy-winning ABC News Science Editor, taught physics at Harvard and is now president of Spectacular Science Productions. His novel, The Null Prophecy, debuts today.

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