The amount of sunlight reaching Earth’s surface has fluctuated for decades, and a new study supports the idea that human activity is to blame.
In the late 1980s, researchers first noticed a steady decrease or ‘eclipse’ in Earth’s brightness in various parts of the world, including a nearly 30 percent decline in sunlight since the 1950s over a particular region of the Soviet Union .
Just a few decades later, after the most harmful aerosols were banned and the Soviet Union dissolved, the trend suddenly shifted from a “global dimming” effect to an illuminating one.
It has long been suspected that fine particles, such as sulfate aerosols, create a haze in the atmosphere that blocks sunlight. There is some evidence that this type of pollution reflects almost all the 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 disputed, and some argue that natural variables, such as the absorption of sunlight by clouds, are greater factors than pollution in the amount of light reaching Earth.
This new study tried to explain variations in sunlight under bright and cloudy conditions and found that human-caused pollution is in fact a major culprit of dimming.
Wild and his colleagues used historical data collected between 1947 and 2017 by the solar radiation record in Potsdam. The Potsdam record is considered one of the world’s longest and best-preserved continuous measurements of solar radiation on the Earth’s surface.
Even if the sky is clear of clouds, the analysis shows that there can be strong dimming and lighter trends, similar to cloudy skies.
“Our analysis shows that strong decadal variations (dimming and brightening) not only occur when cloud is taken into account, but also remain apparent under cloud-free conditions when cloud effects are eliminated,” the authors write in their published paper.
Since clouds are excluded, the authors argue that variations in aerosols must be a substantial modifier of global dimming and brightening.
“Although we had already assumed that, we had not been able to directly prove it so far,” said climate scientist and lead author Martin Wild.
It would be nice to think that global dimming is no longer an issue, just because the world is lighting up doesn’t mean our future is.
Recently, some internationally banned aerosol cans have begun to mysteriously rise, and even if we get those under control, scientists are concerned that historical dimming has already helped mask some of the effects of global warming.
More sunlight streaming into the planet isn’t necessarily a good thing, they warn; it could make a future ‘greenhouse earth’ even hotter.
That’s partly why some experts are exploring ways to induce global dimming through solar geoengineering, although others think that’s far too risky.
For example, a recent study in the Arctic has linked heavy mining practices with regional dimming, and this was found to reduce boom growth since the 1970s.
Forests are one of the Earth’s most important carbon sinks, and if tree growth slows down so much with less sunlight, we could seriously shoot ourselves in the foot.
Obviously there is no easy fix, but this new research suggests that we caused the problem ourselves.
The study is published in the Geophysical Research Letters