Stephen Hawking fears Donald Trump’s decision to pull out of the Paris climate change agreement could be the “tipping point” which wipes out humanity and turns our planet into a living hell. …
“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,” he told the BBC.
That’s a severely ignorant thing to say by Hawking. Our atmosphere is not deep enough to possibly allow such a temperature increase, and our oceans soak up all the carbon dioxide after a while anyway. Yet another scientist dabbling outside his area and making an ass of himself.
The temperature at the top of the atmosphere is determined by basic physics — Earth must radiate as much heat out to space from its atmosphere as is being absorbed by the Earth from the Sun. Within the Earth’s atmosphere there is a temperature gradient — it gets colder as you get higher, about 6.5 degrees Celsius per kilometer, or 3.6 degrees Fahrenheit per 1,000 feet. So it is warmer at the surface of the Earth than up high in the atmosphere where the heat is being radiated by the atmosphere off into space. The temperature at the top of the atmosphere where the radiation is occurring from is a very chilly -18 degrees Celsius or zero degrees Fahrenheit. On average it’s about 5 kilometers or 16,000 feet from there to the surface, so the surface is comfortably warm. Our atmosphere simply isn’t deep/high enough for the temperature at the surface to reach anywhere near 250 degrees, and never will be unless we get an awful lot more oxygen and nitrogen gas from somewhere.
On Venus the temperature on the surface is like what Hawking is talking about, but that is only possible because the atmosphere on Venus is much deeper, about five times as high. There is a much greater distance from the top of the atmosphere where it radiates to space and the surface, so the surface can be much hotter than up in the atmosphere where it radiates.
Secondly, Earth has oceans that contain about fifty times as much carbon dioxide as the atmosphere. If we add more carbon dioxide to the atmosphere, it just gets soaked up by the oceans — after a delay of around 40 years. So the carbon dioxide we emit today goes into the ocean in about forty years on average. The carbon dioxide level is rising because we keeping emitting even more carbon dioxide, and it takes a while for it to be absorbed. It is generally reckoned that even if we blast ahead and burn all the fossil fuels humanity can get our hands on then we won’t be able to raise the carbon dioxide level more than about eight times the pre-industrial level. On the IPCC estimates from their models, this will cause about 10 degrees Celsius or 18 degrees Fahrenheit of global warming. Pretty serious, but nothing like Hawking’s 250 degrees. (By the way, the IPCC’s models are wrong, and they are overestimating the warming by about a factor of five to ten.)