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Nak Khid

What is Global Dimming? The cooling effect of Carbon soot in the atmophere

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Global dimming

 

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. However, after discounting an anomaly caused by the eruption of Mount Pinatubo in 1991, a very slight reversal in the overall trend has been observed.[1]

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 counteracted the effect of greenhouse gases on global warming.

The incomplete combustion of fossil fuels (such as diesel) and wood releases black carbon into the air. Though black carbon, most of which is soot, is an extremely small component of air pollution at land surface levels, the phenomenon has a significant heating effect on the atmosphere at altitudes above two kilometers (6,562 ft). Also, it dims the surface of the ocean by absorbing solar radiation

Recent reversal of the trend

Further information: Clean Air Act (United States)

Sun-blocking aerosols around the world steadily declined (red line) since the 1991 eruption of Mount Pinatubo, according to satellite estimates. Credit: Michael Mishchenko, NASA

Wild et al., using measurements over land, report brightening since 1990,[12][13][14] and Pinker et al.[15] found that slight dimming continued over land while brightening occurred over the ocean.[16] Hence, over the land surface, Wild et al. and Pinker et al. disagree. 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.[2][17]

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.[18]

Sulfate aerosols have declined significantly since 1970 with the Clean Air Act in the United States and similar policies in Europe. The Clean Air Act was strengthened in 1977 and 1990. According to the EPA, from 1970 to 2005, total emissions of the six principal air pollutants, including PM’s, dropped by 53% in the US. In 1975, the masked effects of trapped greenhouse gases finally started to emerge and have dominated ever since.[19]

The Baseline Surface Radiation Network (BSRN) has been collecting surface measurements. BSRN was started in the early 1990s and updated the archives in this time. Analysis of recent data reveals that the surface of the planet has brightened by about 4% in the past decade. The brightening trend is corroborated by other data, including satellite analyses.

Relationship to 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.[20] According to Beate Liepert, "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."[21] The magnitude of this masking effect is one of the central problems in current climate change with significant implications for future climate changes and policy responses to global warming.[20]

Interactions between the two theories for climate modification have also been studied, as global warming and global dimming are neither mutually exclusive nor contradictory. In a paper published on March 8, 2005 in the American Geophysical Union's Geophysical Research Letters, a research team led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics, New York, also showed that the apparently opposing forces of global warming and global dimming can occur at the same time.[22] Global dimming interacts with global warming by blocking sunlight that would otherwise cause evaporation and the particulates bind to water droplets. Water vapor is the major greenhouse gas. On the other hand, global dimming is affected by evaporation and rain. Rain has the effect of clearing out polluted skies.

Brown clouds have been found to amplify global warming according to Veerabhadran Ramanathan, an atmospheric chemist at the Scripps Institution of Oceanography in La Jolla, California. "The conventional thinking is that brown clouds have masked as much as 50 percent of global warming by greenhouse gases through so-called global dimming... While this is true globally, this study reveals that over southern and eastern Asia, the soot particles in the brown clouds are in fact amplifying the atmospheric warming trend caused by greenhouse gases by as much as 50 percent.

Possible use to mitigate global warming

Further information: Solar radiation management, Mitigation of global warming, and Albedo

Some scientists have suggested using aerosols to stave off the effects of global warming as an emergency geoengineering measure.[28] In 1974, Mikhail Budyko suggested that if global warming became a problem, the planet could be cooled by burning sulfur in the stratosphere, which would create a haze.[29][30] An increase in planetary albedo of just 0.5 percent is sufficient to halve the effect of a CO2 doubling.[31]

The simplest solution would be to simply emit more sulfates, which would end up in troposphere - the lowest part of the atmosphere. If this were done, Earth would still face many problems, such as:

Using sulfates causes environmental problems such as acid rain

Using carbon black causes human health problems

Dimming causes ecological problems such as changes in evaporation and rainfall patterns[

Droughts and/or increased rainfall cause problems for agriculture

Aerosol has a relatively short lifetime

The solution advocated is transporting sulfates into the next higher layer of the atmosphere - stratosphere. Aerosols in the stratosphere last years instead of weeks - so only a relatively smaller (though still large) amount of sulfate emissions would be necessary, and side effects would be less. This would require developing an efficient way to transport large amounts of gases into stratosphere, many of which have been proposed though none are known to be effective or economically viable.[33]

In a blog post, Gavin Schmidt stated that "Ideas that we should increase aerosol emissions to counteract global warming have been described as a 'Faustian bargain' because that would imply an ever increasing amount of emissions in order to match the accumulated greenhouse gas in the atmosphere, with ever increasing monetary and health costs."

 

 

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