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When would global warming destroy life on Earth? Study hazards a guess.

Two new studies look at when a runaway greenhouse effect makes a planet uninhabitable. For Earth, the data suggest that time is still distant, even with current levels of global warming.

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This is a picture of the surface of Venus, which is hot enough to melt lead, thanks to a runaway greenhouse effect at some point in the planet's past. Two recent studies look at how such processes might occur.

JPL/AP/File

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A runaway greenhouse effect – where a planet's atmosphere traps so much heat that temperatures rise to life-snuffing levels – may be easier to achieve than previously believed. And there may be more than one way to drive the increase.

Those are the implications of two recent studies looking at what planetary scientists describe as one of the fundamental processes that can render a planet uninhabitable.

In the sun's neighborhood, Venus is the textbook example. It is thought to have had oceans on its surface early in its history, but the planet's proximity to the sun and the relatively high concentration of heat-trapping carbon dioxide in its atmosphere combined to evaporate the oceans, triggering runaway warming that drove surface temperatures to levels that can melt lead.

The most recent of the two studies, published Monday in the journal Nature Geoscience, found that the amount of energy needed to shift a planet's climate into thermal overdrive at Earth's distance from the sun was about 10 percent less than estimates many scientists have been using for more than two decades.

The research suggests that from a standpoint of Earth's climate, it would likely take another 1.5 billion years, even accounting for the pace at which human activities are pumping greenhouse gases into the air, for a runaway greenhouse effect to take over, says Colin Goldblatt, an assistant professor at the University of Victoria in British Columbia who studies the evolution of Earth's climate.

The results also imply that a star's habitable zone – where a planet could capture enough warmth from its sun to allow liquid water to remain stable on the surface – may be smaller than previously estimated. If the results hold up, this would reduce the number of extrasolar planets deemed potentially habitable.

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