The sun may rise every morning, but the amount of sunlight reaching Earth’s surface can substantially vary over decades, according to a perspective article led by an international research team.

The article, published in Advances in Atmospheric Science, suggests that stages of “dimming” and “brightening” correspond with increased air pollution and implementation of clean energy solutions, respectively.

“The amount of sunlight—which is solar radiative energy—we receive at Earth’s surface is not necessarily stable over the years but can undergo substantial decadal variations, as documented in the long-term historical solar radiation measurements we examined,” said corresponding author Martin Wild, professor with the Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology (ETH Zurich).

“These indicate declining solar radiative energy in large parts of the world from the 1950s to the 1980s—known as global dimming—and a partial recovery thereafter, known as brightening, at many observation sites.”

After reviewing the existing scientific literature on dimming and brightening research, which dates to the early 20th century and spans the globe, the researchers focused on regional-scale trends of solar radiative energy in China.

“China provides a conducive environment for understanding aerosol radiative effects—or how air pollutants impact the amount of solar radiative energy reaching Earth’s surface,” Wild said.

“China has well-organized, long-term and spatially uniform surface observations, making the country one of the most intensively studied world regions in terms of dimming and brightening, as reflected in an impressive number of related publications.”

In previous studies, the researchers found that the decadal variations in solar energy reaching Earth’s surface were particularly pronounced in China. Solar energy substantially declined from the 1960s to the 1990s, and the trend appeared to begin to reverse in the 2000s with a slight recovery in more recent years.

“The causes for these dramatic changes are thought to be primarily due to increasing air pollution in the ‘dimming’ phase, and due to the successful implementation of air pollution mitigation measures in the ‘brightening’ phase,'” Wild said.

Air pollution from the increasingly used fossil fuels and exacerbated carbon emissions over the years can literally intercept incoming solar radiation and prevent it from reaching the planet’s surface, Wild explained.

The more pollution in the atmosphere, the less sunlight reaches Earth’s surface. Now that China has started to reverse this trend by reducing air pollution, more sunlight is researching the surface, which could have compounding benefits.

“If China fully recovers from the dimming phase and reverts to the clean atmosphere levels of the 1960s, the country could make substantial gains in solar power production,” Wild said.

“These decadal changes of dimming and brightening in the available solar energy are not only crucial for various aspects of climate and environmental change, but are also of significant importance for resource assessments in the rapidly growing sector of solar power production.

“The availability of solar energy at Earth’s surface is such a critical factor for so many relevant ecological and societal aspects, as well as for the existence of life in the first place, that it deserves a sustained investment into its investigations as well as its long-term monitoring both from the surface and space.”

There are challenges, however, the researchers said, in fully understanding the dimming and brightening phenomenon’s magnitude, causes, predictability and implications in a wide range of applications.

The study authors recommend that researchers in the field work together and focus on optimally combining the diverse information contained in their different but complementary data sources, remedying remaining inconsistencies, to allow for the best possible understanding of variations in sunlight reaching Earth’s surface.

Other collaborators include Yawen Wang, Ocean University of China, Qingdao; Kaicun Wang, Peking University; and Su Yang, China Meteorological Administration.