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Cloudy forecast

The biggest source of climate uncertainty is white and fluffy

2:30pm, March 7, 2014
Earth clouds

UP IN THE AIR  Clouds can both warm and cool the planet. How clouds will respond to global warming is the largest source of uncertainty in climate change predictions.

From space, clouds appear to perform an intricate and never-ending ballet. Thin streaks dance at the poles, vast storms plow across the jet streams, spinning cyclones get tossed up in the tropics and deep convecting monsters churn near the equator. Clouds whip and curl and billow, materializing seemingly out of nowhere and dissipating just as mysteriously.

The mystery deepens when scientists try to understand how clouds influence climate. Clouds lead a sort of double life, both trapping and deflecting planet-warming energy. Their molecules, like all water in the atmosphere, contribute to the greenhouse effect by lapping up infrared radiation emitted by Earth and redirecting some of that energy back toward the planet’s surface. But clouds’ white tops also reflect, collectively, almost a quarter of the solar radiation that reaches them, in effect shading the planet.

All told, clouds cool through reflection far more than they warm through the greenhouse effect.

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