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Image of the Day: Lingering Night-Shining Clouds

Credit: P-M Hedén via NASA

These noctilucent, or night-shining, clouds are made of tiny ice particles. Because they're so high up — some 50 miles about Earth's surface — they're still illuminated by the Sun long after sunset.

One of the lingering uncertainties in climate science is whether the extra water vapor the atmosphere is expected to hold in a warmer world will form clouds like these, or low-altitude cumulus clouds instead. If it's the former, they'll help boost global warming by trapping extra heat beyond what's already being trapped by CO2 from fossil-fuel burning. If it's the latter, they could hold back warming by reflecting some sunlight back into space.



State Temperature Trends Since 1st Earth Day Every state in the U.S. has warmed since the first Earth Day, although the rate of warming varies.

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