Global Warming's Link to Clearer Skies on Earth

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Christopher Joyce reports a recent scientific study that shows the Earth is getting brighter, meaning more of the sun's rays are getting through the atmosphere and warming the planet's surface. While researchers believe that the brightness is a sign of diminishing pollution, the change also reveals the extent of the planet's global warming problems.

ALEX CHADWICK, host:

This is DAY TO DAY. I'm Alex Chadwick.

In a moment, what to think about The New Yorker magazine's long series of reports on global warming.

First, the Earth may be getting sunnier, at least in some places. Scientists who measure the amount of sunlight that hits the ground say a long period of dimming seems to be coming to an end. NPR's Christopher Joyce has more.

CHRISTOPHER JOYCE reporting:

We may be 93 million miles away from the sun, but it can still get hot out there. On a summer day in, say, Utah or Pennsylvania, the sun can blast a square meter of the ground with some 600 watts of energy. Professor Robert Charlson of the University of Washington puts that in perspective.

Professor ROBERT CHARLSON (University of Washington): Ten 60-watt light bulbs warming the top of your desk, and that would get hot.

JOYCE: Not hot enough to fry an egg.

Prof. CHARLSON: It might.

JOYCE: OK. Maybe hot enough to poach an egg. The point is the Earth takes a lot of heat, but the coverage is patchy. If the atmosphere over one place is polluted or sooty or there's been a big volcano recently, a lot of the incoming solar energy gets blocked. Clouds also reflect some light away from the ground; so does snow. So calculating Earth's total solar energy budget is harder than balancing Donald Trump's checkbook. Nonetheless, some scientists found that parts of the planet started dimming in the 1960s. It was a controversial finding, but it got atmospheric scientists to pay more attention. Now two groups of researchers say they've seen a reversal in that dimming. Martin Wild with the Swiss Federal Institute of Technology says instruments on the ground noticed the change about 15 years ago.

Mr. MARTIN WILD (Swiss Federal Institute of Technology): And what they found there is that this dimming did not last, but rather what they find at many sites are signs that the sunlight is actually increasing, so we actually find a brightening.

JOYCE: Another scientific team found the same trend when they examined the data from Earth-observing satellites. That group was led by Rachel Pinker at the University of Maryland. She says no one's sure why Earth seems to be getting brighter, but they have some idea.

Ms. RACHEL PINKER (University of Maryland): Perhaps we have less pollution in the atmosphere, so if we have a combination of cleaner, clear sky and less clouds, this is a recipe of increased surface radiation.

JOYCE: Some of that cleaner sky was most obvious over Eastern Europe. When the Soviet Union collapsed, industrial activity shrank and there was less air pollution blocking solar energy coming in. And other scientists have observed fewer clouds in some parts of the sky. So goodbye, gray skies; hello, blue. But scientists aren't singing. Martin Wild notes that the buildup of carbon-based gases in the atmosphere has been warming the Earth already; that's the so-called greenhouse effect. The days of dimmer skies might have hidden the greenhouse effect, so what happens when it gets sunnier?

Mr. WILD: Now that this dimming does no longer counterbalance the greenhouse effect, we see much stronger evidence for the greenhouse warming at the surface.

JOYCE: Robert Charlson, the atmospheric scientist at the University of Washington, says something else might be at work here, even measurement error. Moreover, the measurements ignore big sections of the planet. But, he says, the results could be significant, and scientists should try to improve their methods.

Prof. CHARLSON: That these trends, uncertain though they may be, that they are of this magnitude says we should pay attention to them. We should try to understand them.

JOYCE: The research appears in the latest issue of the journal Science.

Christopher Joyce, NPR News.

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