Our swiftly dim­ming planet

Popular Science - - CHARTED -

IF YOU’VE EVER MADE THE MIS­TAKE of wear­ing a black shirt on a swel­ter­ing day, you know that dark col­ors ab­sorb the sun’s heat, while light col­ors re­flect it. Our planet works the same way. White sur­faces, like the snow on the world’s glaciers, re­flect the sun’s rays back to­ward space. Darker ones, like forests and oceans, ab­sorb more heat. Over­all, about 30 per­cent of the en­ergy sent to Earth bounces back into outer space, sta­bi­liz­ing the global ther­mo­stat. But cli­mate change could up­end that bal­ance.

Sci­en­tists de­vel­oped a ra­tio, called albedo, to de­scribe just how much so­lar en­ergy any given sur­face re­flects. Albedo can range from zero to one: zero is a per­fect ab­sorber, an im­pos­si­bly dark sub­stance that takes in 100 per­cent of the sun’s en­er­gies, while one is a per­fect re­flec­tor, an in­cred­i­bly bright ma­te­rial bounc­ing all that en­ergy back. For ex­am­ple, a shirt with an albedo of 0.1 is 10 per­cent re­flec­tive and 90 per­cent ab­sorbent.

Now cli­mate sci­en­tists are look­ing into how Earth’s albedo fluc­tu­ates over time. They’re also in­ves­ti­gat­ing whether it will de­cline as our cli­mate changes. If our albedo takes a nose­dive, the same prin­ci­ple that cur­rently helps keep ev­ery­thing in bal­ance could make the world heat up even quicker in the fu­ture.

A warm­ing planet causes glaciers to shrink and sea ice to melt, re­veal­ing darker and more heat-ab­sorbent sur­faces that re­duce Earth’s albedo. At the same time, pol­lu­tion—in the form of black and gray soot—lands on ice sheets and makes them darker. This al­lows ice to ab­sorb more of the sun’s heat, melt­ing it more quickly.

Driven by this cy­cle, the planet could take in too much en­ergy from the sun and not cast enough back. As global warm­ing con­tin­ues, it might be even harder for Earth to keep its cool.

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