Global dimming

Definition

The first widespread effort to measure solar radiation began during the 1957-1958 International Geophysical Year (IGY), when a global network of radiometer stations was established to measure the Earth’s radiation budget. In the following years, scientists discovered that the amount of solar radiation reaching the Earth’s surface was not constant. From 1960 to 1990, measurements of solar radiation decreased worldwide by 4 to 6 percent. The greatest rates of decline were in the heavily populated, industrialized regions of the Northern Hemisphere; smaller decreases were measured over the Arctic and Antarctic. The term “global dimming” was coined during the 1990s to describe this observed reduction in surface solar radiation.

Many scientists contend that results from the release of aerosols, or air pollution. Aerosols are solid particles or liquid droplets, typically 0.01 to 10 micrometers in size, that are produced by natural or anthropogenic sources and remain suspended in the atmosphere. Anthropogenic sources include the release of sulfur dioxide (SO₂) and black carbon from the combustion of fossil fuels. Some scientists contend that aerosols cause dimming by absorbing solar radiation or scattering it back into space and by modifying the optical characteristics and coverage of clouds. In clouds, anthropogenic aerosols act as cloud nuclei. Their presence significantly increases the number of sites on which clouds droplets can form, leading to the formation of clouds with many more small droplets than would occur naturally. This process leads to the formation of clouds that reflect greater amounts of solar radiation because of their higher albedos, expanded coverage, and longer lifetimes.

Significance for Climate Change

Global dimming is important to the broader discussion of climate change for many reasons. For one, it has fostered a more focused discussion of the global impacts of anthropogenic aerosols. While scientists have long observed the large-scale impacts of aerosols, such as the that followed the eruption of Mount Pinatubo in 1991, air pollution was often regarded as a small-scale problem confined to urban and industrialized regions. Observations of global dimming have fostered a broader consideration of how aerosols impact the global climate system.

Some scientists hypothesize that global dimming may have masked the effects of global warming from the 1960s to the present. They suggest that observations of increased global temperature, often attributed to GHG emissions, may be more significant than observed, because global dimming was simultaneously acting to cool surface temperatures. This theory provides a basis to explain why surface temperatures rose by 0.3° Celsius from 1960 to 1990, while the amount of solar radiation reaching the Earth’s surface declined by 4 to 6 percent.

Since 1990, the hypothesized impact of global dimming has raised concerns among global warming experts, because measurements of solar radiation suggest that dimming has waned in many parts of the world, a phenomenon that has earned the label “global brightening.” Some scientists hypothesize that focused efforts to reduce air pollution, such as the United States’ 1990 Clean Air Act Amendments, along with changes in the global economy, have fostered a worldwide reduction in aerosol emissions. Since aerosols have a much shorter residence time in the atmosphere than GHG emissions, there is a concern that continued efforts to reduce anthropogenic aerosols will cause global warming to proceed more rapidly over the next century than had been predicted.

In addition to its impact on global temperature, some scientists hypothesize that global dimming affects the hydrologic cycle by reducing the rate of evaporation and precipitation worldwide. Evidence for this hypothesis was originally uncovered from pan evaporation data collected over the latter half of the twentieth century, when scientists observed an annual decrease in the amount of water evaporated at many sites in the Northern Hemisphere. This result was unexpected, as it was generally thought that higher temperatures associated with global warming would increase rates of evaporation. Subsequent research, however, suggested that solar radiation may be a more important factor than originally believed in determining evaporation rates.

Provided this context, the reduction in solar radiation observed since the 1950s is viewed by many scientists as additional evidence for global dimming. It provides a basis to explain why evaporation rates decreased from 1960 to 1990 while global temperatures increased on average. In addition to the impacts outlined above, atmospheric scientists are investigating the broader impacts of global dimming on global circulation patterns, the distribution of rainfall worldwide, the vertical temperature profile, and the ability of plants to conduct under changing solar conditions.

Bibliography

Hansen, James, et al. “Earth’s Energy Imbalance: Confirmation and Implications.” Science 308, no. 5727 (June 3, 2005): 1431-1435.

Hunter, Chris. "Global Dimming. What Is It? What Are the Impacts?" Full Circle, 14 Apr. 2023, fullcircleci.co.uk/global-dimming-what-is-it-what-are-the-impacts/. Accessed 20 Dec. 2024.

Roderick, Michael L., and Graham D. Farquhar. “The Cause of Decreased Pan Evaporation over the Past Fifty Years.” Science 298, no. 5597 (November 15, 2002): 1410-1411.

Stanhill, Gerald. “Global Dimming: A New Aspect of Climate Change.” Weather 60, no. 1 (January, 2005): 11-14.