Already a wealth of modeling suggests that easily achievable amounts of global warming would fall far outside safety margins long before Earth began any runaway transition to Venus. In 2010 a study from Steven Sherwood at the University of New South Wales in Sydney and Matthew Huber at Purdue University calculated that warming slightly in excess of 10 degrees C—like that of the PETM and of pessimistic scenarios for future fossil-fuel burning—could render large portions of the planet uninhabitable for many creatures. Unprotected humans and other warm-blooded mammals can overheat and die in humid conditions hotter than about 35 degrees C, because their metabolisms produce more heat than can be easily dissipated into the surrounding air. The latest results from Kasting’s group, which are still under review, suggest that such conditions could prevail across much of the planet if human civilization burns enough fossil fuel to quadruple atmospheric levels of carbon dioxide.
Reaching such dangerous levels “is certainly doable,” Huber says. “It’s our decision whether or not to dedicate the next century to burning these reserves.... There used to be subtropical forests near the poles 50 million years ago, and that doesn’t sound so bad. But the fossil record closer to the equator is really poor, and that may be an indication that life was extremely stressed during these warm periods. If over half the surface area of the planet becomes inhospitable, it will not render Earth uninhabitable, but it will be unrecognizable and existentially challenging for the majority of the people, species and communities on Earth.”
As nightmarish as a runaway greenhouse seems, whether or not modern Earth is susceptible to it should perhaps be seen as essentially an academic point. Microbes could endure and even flourish on a planet at the brink of runaway, but people would still be steam-cooked whether or not such a hothouse world tipped over into a more Venusian climate. Leaving aside other effects of global warming like rising seas, stronger storms, longer droughts and plummeting biodiversity, Kasting says, “the problem of heat stress alone could become lethal to humans well before any runaway happens, and that danger may be much closer than previously realized. This is serious enough to warrant our full attention.”