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

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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.

The study serves as a useful reminder that scientists can't determine habitability only from estimates of how much radiation reaches a planet, says Larry Esposito, a researcher who studies planetary atmospheres at the University of Colorado at Boulder. A planet's current climate and the history of that climate play key roles, too.

The atmospheric model used in looking at the greenhouse effect on Earth represents "a first pass at doing the problem again," says Dr. Goldblatt. It doesn't account for clouds, which would be crucial to determining the mount of sunlight reaching Earth's surface. Instead, the model operates assuming clear skies.

"You start off with simple models. You try to understand the answers. Then you go on to more complex models," he says.

Over the past 25 years, researchers have developed more-detailed measurements of water vapor and how it interacts with the infrared radiation the Earth's surface sends skyward. These improvements prompted the team to try to take another crack at measuring the energy needed to trigger a runaway greenhouse effect.

Water vapor and other greenhouse gases absorb most of that radiation and re-radiate it in all directions, including back toward Earth's surface. But radiation in a narrow band of wavelengths can escape, allowing some of that heat to head back toward space.

As the atmosphere warms, more water evaporates, and the atmosphere's ability to hold moisture increases. Runaway heating can occur when warming temperatures push enough water vapor into the air to in effect slam the infrared window shut, Goldblatt explains.

Nor is sunlight alone in determining the surface temperature. A study published earlier this year in the journal Astrobiology described how tidal heating – the friction created within a planet as it is tugged by a star's gravity – could produce enough heat at the planet's surface to push an otherwise stable climate into runaway greenhouse warming.

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