Global dimming: Phenomenon of solar radiation declining

Researchers have shown that variations in the intensity of sunlight over decades are down to ultra-fine, human-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.

“In previous studies, we showed that the amount of sunlight that reaches the Earth’s surface is not constant over many decades but instead varies substantially — a phenomenon known as global dimming and brightening,” says ETH Professor Martin Wild of the Institute for Atmospheric and Climate Science.

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Why is solar radiation important?

Solar radiation provides heat, light, and energy necessary for all living organisms. Infrared radiation supplies heat to all habitats, on land and in the water. Without solar radiation, Earth’s surface would be about 32°C colder

Light is also provided by solar radiation. Predators would not be able to efficiently hunt prey without light from the sun and prey would not be able to take advantage of dark areas if predators were adapted to dark habitats. Human eyes are adapted to the visible spectrum, though some other species can see ultraviolet light in addition to colors

Reasons unknown

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.

A number of scientists suspected that cloud cover may have changed over the years, absorbing the sun’s rays more effectively during the dimming phase than during the brightening phase.

This is why researched analysed measurements collected between 1947 and 2017 in the Potsdam radiation time series. The series offers one of the longest, most homogeneous, continuous measurements of solar radiation on the Earth’s surface.

Dimming also occurred in cloud-free conditions

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 researchers identified aerosols entering the atmosphere due to air pollution as the major contributor to global dimming and brightening.

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.

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