"We are close to the tipping point, where global warming becomes irreversible. Trump's action could push the Earth over the brink, to become like Venus, with a temperature of 250 degrees [Celsius], and raining sulfuric acid," he told BBC News, referring to the president's decision to pull the US out of the Paris climate deal.
But most climate experts say that scenario is a dramatic and implausible exaggeration: Relative to Venus, planet Earth is much farther from the sun and given its chemical makeup will never have such a thick carbon dioxide atmosphere, so it could not likely reach temperatures of 482°Fahrenheit (250°C) that Hawking described in the interview, they say.
However, the general trend of runaway and catastrophic climate change is a real concern, experts said.
"Hawking is taking some rhetorical license here," Michael Mann, a climate scientist at the Pennsylvania State University, told Live Science in an email. "Earth is further away from the sun than Venus and likely cannot experience a runaway greenhouse effect in the same sense as Venus — i.e. a literal boiling away of the oceans. However Hawking's larger point — that we could render the planet largely uninhabitable for human civilization if we do not act to avert dangerous climate change — is certainly valid."
Venus is the second planet from the sun and the brightest planet in the solar system; though the planet is named after the Roman goddess of love and desire, don't expect to take a trip to the balmy planet with your sweetheart anytime soon. Despite being the same size as Earth and having roughly the same gravity as our home planet, it's a far cry from our water-drenched planet. Venus is the hottest planet in the solar system, with temperatures reaching 870°F (466°C). The reason for these sweltering temperatures is Venus' thick carbon-dioxide atmosphere that is dotted with sulfuric acid clouds; the atmosphere traps much more heat than our own does. It is also much closer to the sun, meaning it absorbs much more solar radiation than Earth. Churning volcanoes add to Venus' reputation as an inferno.
The leading theory about how Venus came to be such a hellscape is that the planet got caught in a feedback loop, wherein the planet absorbed more solar radiation than it released, causing more water vapor to get trapped in its atmosphere. That, in turn, led to greater heat absorption, and runaway warming (also called a runaway greenhouse effect).
"Basically, Venus was in a state of heat stroke — the planet was in a warming state and it couldn't cool down," said Tyler Robinson, an astrobiologist at the University of Washington.
Unlikely on Earth
Though most humans take for granted the relative constancy of an Earth-like climate, our planet has undergone dramatic changes in its 4.5-billion-year history. During the Great Oxygenation Event, around 2.5 billion years ago, photosynthetic cyanobacteria fueled a huge rise in oxygen in the atmosphere.
Around 650 million years ago, the entire planet froze, in a phenomenon known as "snowball Earth." And during the dinosaur age, the planet was, on average 18°F (10°C) hotter than it is now, with a carbon dioxide-rich atmosphere. And huge "carbon excursions" have led to massive extinctions in the past — such as the end-Permian extinction around 252 million years ago, when roughly 95 percent of sea life died out due to ocean acidification.
So it's not unreasonable to contemplate the possibility of a runaway climate scenario, Robinson said. Still, most experts, including Robinson, see that possibility as incredibly unlikely.
While in theory, a process similar to the one experienced on Venus could take place on Earth, the process would most likely occur over hundreds of millions of years, most experts believe, Robinson said. There are also very low odds that Earth's oceans could literally boil away like Venus' primeval oceans did, Robinson said.
Earth, meanwhile, is protected from solar radiation by an atmosphere that is dramatically different from that of Venus.
"Venus' atmosphere is about 100 times thicker than Earth's atmosphere, and composed almost entirely of CO2 [carbon dioxide]," Robinson said. By contrast, Earth's atmosphere is mostly molecular nitrogen and oxygen, with less than 0.04 percent coming from carbon dioxide, Robinson told Live Science in an email.
Without a thick carbon dioxide atmosphere and the extra dose of solar radiation from the sun, only willful malice is likely to cause a runaway greenhouse scenario, said Kevin Zahnle, a space scientist at NASA Ames Research Center, who has analyzed runaway greenhouse projections for the planet.
"There is no rational expectation of a runaway [greenhouse effect] in the facts as we know them," Zahnle told Live Science in an email.
For one, there were much warmer climates on Earth in the relatively recent past, such as during the Eocene epoch (between 56 million and 34 million years ago), and no signs of a runaway greenhouse effect, Zahnle said. At that time, CO2 levels were likely three times higher than they are now. Even imagining a future with cars, planes, and air conditioning on full blast, no climate projections predict such high levels of CO2 in our atmosphere, he said.
"A runaway greenhouse effect is not in the cards," added Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado.
Of course, there is always the possibility of deliberate sabotage, Zahnle said.
"Dr. Evil might consider an engineering solution akin to the engineering solutions proposed to terraform Mars, but the scale of the effort would be stupendous," Zahnle said. "You'd need fluorocarbons — so Dr. Evil would need to create a worldwide religion dedicated to the sacred use of hairspray and underarm deodorants," Zahnle said. (In the past, some consumer aerosol products contained fluorocarbons, though the US banned the ingredients in the late 1970s.)
Climate catastrophe possible
Nontheless, Earth doesn't have to become like Venus for life on Earth to become hellish.
The Paris Agreement aimed to keep warming below 3.6°F (2°C) compared with preindustrial temperatures, but even reaching that level for sustained periods could cause changes that are already underway to completely disrupt ecosystems and farming, Trenberth told Live Science.
Read more at Discovery News