Long-Term Decline in Downward Surface Solar Radiation Highlights Emission Impacts

Long-Term Decline in Downward Surface Solar Radiation Highlights Emission Impacts
by Simon Mansfield
Sydney, Australia (SPX) Feb 20, 2025

A new study led by Dr. Fengfei Song and Dr. Lixin Wu from the Frontier Science Center for Deep Ocean Multispheres and Earth System and the Physical Oceanography Laboratory at Ocean University of China has examined long-term trends in downward surface solar radiation (DSSR).

As a crucial component of Earth's energy balance, DSSR influences global climate patterns and is essential for renewable energy production. Given its importance in photovoltaic energy generation, understanding DSSR trends is vital in the effort to reduce reliance on fossil fuels and mitigate global warming, aiming to keep temperature increases well below 2C by 2100.

Historical DSSR Variations and CMIP6 Climate Model Insights

DSSR has shown significant decadal changes, particularly over land. A general "dimming" phase from the 1950s to the 1980s was followed by a "brightening" trend, notably across Europe and North America.

In China, DSSR declined during the earlier period, stabilized in the 1990s-2000s, and began to recover in some regions after 2005. However, India has not experienced a similar recovery. "Previous research has primarily focused on land-based observations, while DSSR trends over oceans remain less understood due to limited data availability," Dr. Song explains.

Utilizing the latest CMIP6 climate models, which address previous biases in DSSR simulations, the research team conducted a comprehensive analysis of historical global DSSR trends. Their findings indicate a significant decline in global DSSR between 1959 and 2014, primarily driven by anthropogenic factors. This trend was consistently observed across multiple climate models, confirming the widespread nature of global dimming during this period.

Water Vapor's Overlooked Role in DSSR Reduction

Historically, DSSR variations have been attributed largely to aerosol concentrations. However, rising atmospheric water vapor levels, a consequence of global warming, have also been identified as a key factor.

Water vapor can directly absorb solar radiation, a mechanism often overlooked in previous studies. By analyzing single-forcing experiments, the researchers determined that both greenhouse gases and anthropogenic aerosols have contributed equally to DSSR reduction, with the greenhouse gas effect becoming more pronounced after 1979. "Earlier studies, focusing on high-aerosol land areas, underestimated the role of water vapor. However, when examining global DSSR trends, it is evident that water vapor and aerosols have had comparable impacts," Dr. Song states.

Future DSSR Trends Depend on Emission Scenarios

The study further explores future DSSR projections under four distinct emission scenarios: SSP585 (very high emissions), SSP370 (high emissions), SSP245 (moderate emissions), and SSP126 (low emissions).

These scenarios dictate changes in global temperatures, water vapor levels, and aerosol concentrations. "In high-emission scenarios (SSP585 and SSP370), DSSR continues to decline over the long term. However, under cleaner emission pathways such as SSP245, DSSR stabilizes, and in SSP126, it even shows signs of gradual recovery," Dr. Wu explains.

This research represents the first global-scale assessment of historical DSSR trends, encompassing both land and ocean regions. It quantifies the relative contributions of greenhouse gases and aerosols while emphasizing the significant influence of water vapor. The study underscores the importance of emission reduction policies, as cleaner energy strategies can help curb DSSR declines, supporting a stable transition to renewable energy sources.

Research Report:A long-term decline in downward surface solar radiation