image: Global mean downward surface shortwave radiation (DSSR) changes. (A) temporal evolution and (B) trends of DSSR during the historical period (1959-2014) simulated by the model; (C) contribution of greenhouse gases and anthropogenic aerosols to the changes in DSSR; and (D) trends of the two components of the DSSR (clear-sky component + cloud-induced component) in different experiments. Photo credit: Fengfei Song and Lixin Wu.
Credit: Photo credit: Fengfei Song and Lixin Wu.
This study is led by Dr. Fengfei Song and Dr. Lixin Wu (Frontier Science Center for Deep Ocean Multispheres and Earth System and Physical Oceanography Laboratory, Ocean University of China). Downward surface solar radiation (DSSR), a key component of Earth's surface energy budget, serves as the ultimate energy source for life on the planet and plays a critical role in determining the climatic conditions of various ecosystems. As a primary driver of energy production in photovoltaic systems, DSSR has become an increasingly important resource in the pursuit of meeting growing global energy demands while simultaneously reducing dependence on fossil fuels over the coming decades. This shift is pivotal in efforts to limit global warming to well below 2°C, or ideally 1.5°C, by the year 2100.
Analysis of Global DSSR Trends Using the Latest CMIP6 Climate Models
DSSR has exhibited significant decadal variations during the historical period, particularly over land. From the 1950s to the 1980s, a widespread "dimming" phenomenon was observed, followed by a "brightening" trend thereafter. This shift was especially evident in Europe and North America. A similar trend has been observed in China, where, after the dimming during the 1950s-1980s and a period of stability in the 1990s-2000s, DSSR has shown a recovery in certain regions since 2005. However, no recovery in DSSR has been observed in India. "Previous studies mainly focused on land areas, and due to the scarcity of oceanic observation data, our understanding of DSSR changes over oceans remains relatively limited," Dr. Song says.
The research team, utilizing the latest generation of CMIP6 climate models (considered the first generation of climate models that largely remediate the long-standing model biases in the DSSR), has conducted a comprehensive analysis of global average DSSR trends during the historical period (Figure 1). The results reveal a significant dimming of global average DSSR from 1959 to 2014, primarily driven by anthropogenic forcing (Figure 1A). This global dimming phenomenon during this historical period is evident in all climate models (Figure 1B).
New Insights on the Role of Water Vapor in DSSR Changes: A Key Factor Previously Overlooked
Previous studies have commonly attributed the decadal variations in DSSR to aerosol forcing. With global warming, atmospheric water vapor content has been steadily increasing, and water vapor itself can directly absorb solar radiation—an aspect that has previously been overlooked in prior researches. Based on results from single-forcing experiments (greenhouse gas forcing and anthropogenic aerosol forcing), the team found that both greenhouse gases and anthropogenic aerosols contribute equally to the weakening of global DSSR and the impact of greenhouse gases became more pronounced after 1979 (Figure 1C). "As previous studies mainly focused on land areas with higher aerosol forcing, they underestimated the role of water vapor in DSSR changes. But when we look at the global historical trends in DSSR, we find that the contributions of water vapor and anthropogenic aerosols are quite comparable," Dr. Song says.
Future DSSR Changes Depend Heavily on Emission Scenarios
The research team further analyzed the trends in global Downward Surface Solar Radiation (DSSR) under four different emission scenarios (SSP585: very high emissions, SSP370: high emissions, SSP245: moderate emissions, SSP126: low emissions). The various emission policies determine changes in global average temperature (and water vapor) as well as anthropogenic aerosol concentrations (Figures 2A, B). "In the SSP585 and SSP370 high-emission scenarios, global DSSR shows a long-term weakening trend. However, in the relatively cleaner SSP245 and SSP126 emission scenarios, global DSSR demonstrates a trend of long-term stability in SSP245 or even gradual recovery in SSP126." Dr. Wu says.
This study marks the first comprehensive analysis of the historical changes in DSSR from a global perspective (covering both land and oceans). It quantifies the relative contributions of greenhouse gases and anthropogenic aerosols and highlights the role of water vapor. Looking ahead, future DSSR changes will be highly dependent on emission policies. Implementing relatively cleaner and lower emission policies will help curb the weakening of DSSR, providing a crucial safeguard for a smooth transition from traditional fossil fuels to clean energy.
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See the article:
https://doi.org/10.1093/nsr/nwaf007
Journal
National Science Review