Aerosols alter solar radiation levels for decades

Based on the long-​term Potsdam radiation time series, ETH researchers have shown that variations in the intensity of sunlight over decades are down to ultra-​fine, man-​made dirt particles in the atmosphere.

In the late 1980s and 1990s, researchers at ETH Zurich discovered the first indications that the amount of sunlight reaching the Earth’s surface had been steadily declining since the 1950s. The phenomenon was known as “global dimming”. However, a reversal in this trend became discernible in the late 1980s. The atmosphere brightened again at many locations and surface solar radiation increased.

Natural variations or air pollution?

Yet little is known about the reasons for these fluctuations, which have been observed for decades. One particularly controversial point is whether the fluctuations are caused by air pollution, with aerosols blocking the sunlight, or whether they are a result of natural variations in the climate system.

This is why ETH Professor Martin Wild, of the Institute for Atmospheric and Climate Science, and colleagues from other research institutes analysed measurements collected between 1947 and 2017 in the Potsdam radiation time series, which is renowned among climate researchers. The series offers one of the longest, most homogeneous, continuous measurements of solar radiation on the Earth’s surface.

In this new study, they were able to show that rather than these fluctuations being due to natural changes in the cloud cover, they are instead generated by varying aerosols from human activity. The paper was published in the journal Geophysical Research Letters.

The researchers identified aerosols entering the atmosphere due to air pollution as the major contributor to global dimming and brightening. “Although we’d already assumed as much, we’d been unable to prove it directly until now,” he says.

Brightening after economic collapse

The fact that the transition from dimming to brightening coincided with the economic collapse of the former communist countries in the late 1980s supports the argument that these variations have a human cause. Moreover, around this time, many western industrialised nations introduced strict air pollution regulations, which improved air quality significantly and facilitated the transfer of the sunbeams through the atmosphere. Lastly, the atmosphere was recovering from the volcanic eruption of Mount Pinatubo, which had ejected vast amounts of aerosols into the air in 1991.

Dimming reduced evaporation and precipitation

Surface solar radiation is a key parameter for climate issues. Not only does it govern the temperature, it also has a fundamental impact on the water cycle by regulating evaporation, which, in turn, governs cloud formation and affects precipitation. During the global dimming, less water evaporated from the Earth’s surface, causing precipitation to decline worldwide.

Solar radiation also affects the cryosphere, i.e. glaciers, snow and ice. “Glacial retreat accelerated when the atmosphere began brightening again,” Wild says, adding: “It’s also becoming increasingly important for the solar industry to gain a better understanding of these fluctuations when it comes to planning new facilities.”

Photo credit: D€NNI$, flickr/Creative Commons

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