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Global Dimming

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The ability of sulfate aerosols to create a global dimming effect has made them a possible candidate for use in climate engineering projects to limit the effect and impact of climate change due to rising levels of greenhouse gases. Delivery of sulfide gases such as sulfuric acid by artillery, aircraft and balloons in the stratosphere has been proposed. According to estimates, one kilogram of well placed sulfur in the stratosphere would roughly offset the warming effect of several hundred thousand kilograms of carbon dioxide. However, ozone depletion is a potential hazardous side effect of sulfur aerosols accumulated in the stratosphere.
Dozens of fires burning on the surface and a thick pall of smoke and haze filling the skies overhead in Eastern China. Smoke, pollution and other air particles are linked to global dimming. Photo taken by MODIS aboard NASA's Aqua satellite.

Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4% reduction over the three decades from 1960–1990.

Global dimming is thought to have been caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action.

It has interfered with the hydrological cycle by reducing evaporation and may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially masked the effect of greenhouse gases on global warming.

Aerosols and other particulates absorb solar energy and reflect sunlight back into space. The pollutants can also become nuclei for cloud droplets. Water droplets in clouds coalesce around the particles. Increased pollution causes more particulates and thereby creates clouds consisting of a greater number of smaller droplets (that is, the same amount of water is spread over more droplets). The smaller droplets make clouds more reflective, so that more incoming sunlight is reflected back into space and less reaches the Earth's surface. In models, these smaller droplets also decrease rainfall.

Clouds intercept both heat from the sun and heat radiated from the Earth. Their effects are complex and vary in time, location, and altitude. Usually during the daytime the interception of sunlight predominates, giving a cooling effect; however, at night the re-radiation of heat to the Earth slows the Earth's heat loss.

Research: In the mid-1980s, Atsumu Ohmura, a geography researcher at the Swiss Federal Institute of Technology, found that solar radiation striking the Earth's surface had declined by more than 10% over the three previous decades. His findings appeared to contradict global warming—the global temperature had been generally rising since the 70s. Less light reaching the earth seemed to mean that it should cool. Ohmura published his findings "Secular variation of global radiation in Europe" in 1989. This was soon followed by others: Viivi Russak in 1990 "Trends of solar radiation, cloudiness and atmospheric transparency during recent decades in Estonia", and Beate Liepert in 1994 "Solar radiation in Germany — Observed trends and an assessment of their causes". Dimming has also been observed in sites all over the former Soviet Union. Gerry Stanhill who studied these declines worldwide in many papers coined the term "global dimming".

Visible light and infrared radiation seem to be most affected rather than the ultraviolet part of the spectrum.

Over the last 50 years, pan evaporation has been carefully monitored. For decades, nobody took much notice of the pan evaporation measurements. But in the 1990s in Europe, Israel, and North America, scientists spotted something that at the time was considered very strange: the rate of evaporation was falling although they had expected it to increase due to global warming. The same trend has been observed in China over a similar period. A decrease in solar irradiance is cited as the driving force. Many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.

Causes: Besides sulfate aerosols particulates, the incomplete combustion of fossil fuels, such as diesel and wood release black carbon and soot into the air, volcanic ash and aircraft contrails.

Airborne volcanic ash can reflect the Sun's rays back into space and thereby contribute to cooling the planet. Dips in earth temperatures have been observed after large volcano eruptions such as Mount Agung in Bali that erupted in 1963, El Chichon (Mexico) 1983, Ruiz (Colombia) 1985, and Pinatubo (Philippines) 1991. But even for major eruptions, the ash clouds remain only for relatively short periods.

A 2007 NASA sponsored satellite-based study sheds light on the puzzling observations by other scientists that the amount of sunlight reaching Earth's surface had been steadily declining in recent decades, began to reverse around 1990. This switch from a "global dimming" trend to a "brightening" trend happened just as global aerosol levels started to decline.

It is likely that at least some of this change, particularly over Europe, is due to decreases in airborne pollution. Most governments of developed nations have taken steps to reduce aerosols released into the atmosphere, which helps reduce global dimming.

Effects on the hydrological cycle: Pollution produced by humans may be seriously weakening the Earth's water cycle — reducing rainfall and threatening fresh water supplies. Aome studies suggest that tiny particles of soot and other pollutants have a significant effect on the hydrological cycle. That is, the energy for the hydrological cycle comes from sunlight. As sunlight heats the ocean, water escapes into the atmosphere and falls out as rain. So as aerosols cut down sunlight by large amounts, they may be spinning down the hydrological cycle of the planet.

Effects on global warming: Some scientists now consider that the effects of global dimming have masked the effect of global warming to some extent and that resolving global dimming may therefore lead to increases in predictions of future temperature rise. Some researchers maintain, that till now we lived in a global warming plus a global dimming world and now we are taking out global dimming. So we end up with the global warming world, which will be much worse than we thought it will be, much hotter. The magnitude of this masking effect is one of the central problems in climate change with significant implications for future climate changes and policy responses to global warming.

Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency geoengineering measure. But others think that the idea that we should increase aerosol emissions to counteract global warming implies an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing the destruction of ozone layer which will result in monetary and health costs.

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Wikipedia article Global Dimming

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Last updated: January 2018
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