"The amount of sunlight - which is solar radiative energy - we receive at the 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 Martin Wild, lead author and professor at the Swiss Federal Institute of Technology (ETH Zurich).

These variations include a marked decline in solar radiation across much of the world from the 1950s to 1980s, a period now referred to as "global dimming." This was followed by a gradual increase known as "brightening" in subsequent decades. Wild and colleagues attribute these changes to human-driven factors, notably air pollution and its mitigation.

In analyzing regional patterns, the researchers focused on China due to its extensive and consistent surface radiation records. Wild explained that "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. 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."

They found that solar radiation in China declined significantly from the 1960s to the 1990s. However, in the early 2000s, this downward trend began to reverse, corresponding with national efforts to reduce emissions and improve air quality.

"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," said Wild.

Pollution from fossil fuels and other emissions can block or scatter incoming sunlight, directly affecting how much energy reaches the ground. As China curbs emissions, a clearer atmosphere is allowing more solar energy to reach the surface.

"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. He emphasized the broader significance of this trend: "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 the 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."

Despite the clear trends, challenges remain. The researchers note ongoing difficulties in quantifying the full magnitude, causes, and implications of sunlight variability. They advocate for better integration of complementary data sources and interdisciplinary collaboration to enhance predictive capability and reduce uncertainties.

Additional contributors to the article include Yawen Wang of Ocean University of China, Kaicun Wang of Peking University, and Su Yang of the China Meteorological Administration.

Research Report:A Perspective on Global Dimming and Brightening Worldwide and in China

Related Links
Institute of Atmospheric Physics, Chinese Academy of Sciences
Solar Science News at SpaceDaily