Variations in sunlight have more to do with pollution than clouds, says study

The amount of sunlight reaching Earth's surface has been fluctuating for decades now, and a new study supports the idea that human activity is to blame.

In the late 1980s, researchers first noticed a steady decline or 'dimming' in Earth's brightness in various parts of the world, including a near 30 percent drop in sunlight since the 1950s over a particular region in the Soviet Union.

Just a few decades later, after the most harmful aerosols were banned and the Soviet Union dissolved, the trend suddenly switched from a "global dimming" effect to a brightening one. 

Fine particulate matter, such as sulfate aerosols, have long been suspected of creating a haze in the atmosphere that blocks sunlight from entering. Evidence has shown this type of pollution reflects nearly all radiation it encounters in the atmosphere, while also reflecting or absorbing light.

Whether or not these particles are responsible for decades of global dimming remains contentious, and some argue natural variabilities, like cloud absorption of sunlight, are bigger factors than pollution in the amount of light that reaches Earth.

This new study sought to account for variations in sunlight under clear and cloudy conditions and found human-produced pollution is, in fact, a major culprit of dimming.

Wild and his colleagues used historical data collected between 1947 and 2017 by the Potsdam solar radiation record. The Potsdam record is regarded as one of the world's longest and best-maintained continuous measurements of solar radiation on the Earth's surface.

Even when skies are clear of clouds, the analysis reveals there can be strong dimming and brightening trends, similar to cloudy skies. 

"Our analysis shows, that strong decadal variations (dimming and brightening) not only appear when clouds are considered, but also remain evident under cloud-free conditions when cloud effects are eliminated," the authors write in their published paper.

With clouds ruled out, the authors argue variations in aerosols must be a substantial modifier of global dimming and brightening. 

"Although we'd already assumed as much, we'd been unable to prove it directly until now," says climate scientist and the study's lead author Martin Wild.

It would be nice to think global dimming is no longer a problem, but just because the world is brightening up, doesn't mean our future is.

Recently, some internationally banned aerosols have begun to mysteriously rise, and even if we get those under control scientists are worried historic dimming has already helped mask some of the effects of global warming.

More sunlight pouring into the planet isn't necessarily a good thing, they warn; it could make a future 'hothouse Earth' even hotter.

That's partly why some experts are researching ways to induce global dimming through solar geo-engineering, although others think that's far too risky. 

Recently, for instance, a study in the Arctic linked heavy mining practices to regional dimming, and this was found to reduce tree growth since the 1970s.

Forests are one of Earth's most important carbon sinks, and if tree growth slows with less sunlight to such a great extent, we could seriously be shooting ourselves in the foot.

There's clearly no easy solution, but this new research suggests we brought the problem on ourselves.

The study was published in the Geophysical Research Letters. 

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