Stephen Hawking is wrong about runaway greenhouse on Earth and here's why

Stephen Hawking is a world-renowned theoretical physicist whose landmark contributions to cosmology, general relativity and quantum gravity changed the way we see the universe.

However, when it comes to climate change, the brilliant physicist is veering on the edge of total inaccuracy.

During his interview with the BBC for his 75th birthday celebrations, the Cambridge professor warned that U.S. president Donald Trump's decision to pull out of the Paris climate agreement may lead to runaway global warming, eventually turning Earth's atmosphere into something resembling Venus.

"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 two hundred and fifty degrees, and raining sulphuric acid," Hawking told BBC News.

While the prospect of the Earth morphing into Venus makes for a visually striking image, some climate change experts questioned the accuracy of his statement.

Among them, Zeke Hausfather, a climate scientist and energy systems analyst at Berkeley Earth and Gareth Jones, climate scientist at the Met Office:

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The problem with Hawking's answer is that there's no possibility of Venus' runaway greenhouse conditions occurring on Earth as of now (the situation will be different in a billion years with sun becoming 10% brighter).

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Hansen's argument

The physicist's response may be based on a controversial argument included in famed climate scientist James Hansen's book Storms of my Grandchildren, published in 2009.

In the book, Hansen argues that if we burn all reserves of oil, gas and coal "there is a substantial chance we will initiate the runaway greenhouse".

"If we also burn the tar sands and tar shale, I believe the Venus syndrome is a dead certainty," he continues.

Scientists think Venus experienced the runaway greenhouse early in its history. Just like the Earth, the planet once had an ocean but being closer to the sun, the atmosphere became so hot that hydrogen could escape from the upper atmosphere.

Today, Venus has a think atmosphere that is 96.5% CO2, which keeps its surface at nearly 900°F (482°C), and a surface pressure of 90 bars -- as opposed to 1 bar on Earth.

Hansen's argument was that the same could happen on Earth based not on the proximity to a brightening sun but from fossil fuel-burning humans.

This is because of a positive feedback loop happening now on Earth -- carbon dioxide warms the planet through the by trapping more heat in the atmosphere and oceans, causing more water to evaporate into the air, which subsequently amplifies the warming, as water vapor is also a greenhouse gas.

Not in a billion years

Earlier studies dismissed the possibility of a runaway state as impossible, but a paper published in Nature Geoscience in 2013 argued that “the runaway greenhouse may be much easier to initiate than previously thought.”

Much easier and theoretically possible, however, doesn't mean it will happen anytime now or for the next billion years, even if we burn all the fossil fuel available.

The paper's lead author, Colin Goldblatt of the University of Victoria in British Columbia, explained the conditions according to which the runaway greenhouse could actually happen on this planet:

What my results show is that if you put about ten times as much carbon dioxide in the atmosphere as you would get from burning all the coal, oil, and gas—about 30,000 parts per million—then you could cause a runaway greenhouse today.

So burning all the fossil fuels won't give us a runaway greenhouse. However, the consequences will still be dire. It won't sterilize the planet, but it might topple Western civilization. There are no theoretical obstacles to that.

In short, to cause Venus' runaway greenhouse today you'll need ten times the amount of CO2 we could release from burning all the coal, oil, and gas.

That will eventually happen as the sun becomes older and brighter, which is "in somewhere between half a billion and a billion years."

Hansen himself corrected his theory later on, writing that Venus-like conditions in the sense of 90 bar surface pressure and surface temperature of several hundred degrees "are only plausible on billion-year time scales".

It is also worth mentioning that other scientists, such as James Kasting, a geoscientist at The Pennsylvania State University, are still skeptical that an anthropogenic runaway could actually happen.