Ultra-fine, human-made particles in the atmosphere are changing the amount of sunlight that reaches the ground, according to a new study.

From the 1950s through the 1980s, researchers saw steady declines in the amount of sunlight reaching the Earth's surface, in a phenomenon known as "global dimming." This trend mysteriously reversed in the late 1980s, when the atmosphere brightened again at many locations and surface solar radiation increased.

A new study in Geophysical Research Letters finds human-made aerosols, rather than natural changes in cloud cover, are responsible for the fluctuations. Surface solar radiation is a key parameter for climate, affecting temperature and impacting the water cycle by regulating evaporation, which, in turn, governs cloud formation and affects precipitation. Geophysical Research Letters is AGU's journal for high-impact, short-format reports with immediate implications spanning all Earth and space sciences.

"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," said atmospheric scientist Martin Wild of the ETH Zurich Institute for Atmospheric and Climate Science in Switzerland.

For decades, it was unclear if air pollution caused the fluctuations or natural variations in the climate system. Aerosols such as sulfates, nitrates and black carbon reflect or absorb sunlight in varying amounts depending on their physical properties. Some scientists suspected cloud cover may have changed over the years, absorbing the sun's rays more effectively during the dimming phase than during the brightening phase.

Dimming cloudless days
Wild and colleagues analysed measurements collected between 1947 and 2017 in the Potsdam radiation time series, which 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.

"In our analysis, we filtered out the effects of cloud cover to see whether these long-term fluctuations in solar radiation also occurred in cloud-free conditions," Wild said. the decadal fluctuations in the sunlight received at the Earth's surface were apparent even when skies were clear.

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," Wild said.

The transition from dimming to brightening coincided with the economic collapse of the former communist countries in the late 1980s. Around this time, many western industrialized nations introduced strict air pollution regulations, which improved air quality significantly.

During the 1990s, the atmosphere was recovering from the volcanic eruption of Mount Pinatubo, which had ejected vast amounts of aerosols into the air in 1991. All these factors could have contributed to the clearing of the air for transmission of sunlight, according to the authors.

Wild and his colleagues ruled out fluctuations in solar activity in an earlier study.

"The sun itself had only an infinitesimal, negligible effect, which in no way accounts for the magnitude of the intensity changes that had been observed over the years at the surface," Wild said.

During the global dimming, less water evaporated from the Earth's surface, causing precipitation to decline worldwide. Solar radiation also affects glaciers, snow and ice.

"Glacial retreat accelerated when the atmosphere began brightening again," Wild said. "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."

Research paper

Related Links
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