Tuesday, March 17, 2009

Global Dimming

As you probably know by now, carbon dioxide and other greenhouse gases present in the atmosphere in relatively small amounts can cause global warming because these "trace" gases trap solar energy absorbed at the earth's surface and prevent it from being radiated as heat back into space. However, these gases are transparent and have no effect on atmospheric visibility. Sunlight passes right through them, just as it does through the oxygen and nitrogen that are the main constituents of our atmosphere.

While the climate warming impacts of increased greenhouse gases are clear, the effects of increased aerosols are not. Aerosols are solid particles or liquid droplets suspended in air. They include soot, dust and sulfur dioxide particles, and are what we commonly think of when we talk about air pollution. Aerosols come, for example, from the combustion of fossil fuels, industrial processes, and burning of tropical rainforests.

Aerosols can be hazardous to both human health and the environment. They can also affect global temperature by either reflecting light back into space, thus reducing solar radiation at the Earth's surface, or absorbing solar radiation, thus heating the atmosphere. The variable cooling and heating effects of aerosols also modify properties of cloud cover and rainfall. However, studies of the long-term effects of aerosols on climate change have been largely inconclusive up to now due to limited aerosol measurements.

In the March 13 issue of Science, it was announced that scientists have recently compiled the first database of aerosol measurements over the past 35 years, making possible new research into how air pollution affects climate change. Using this new database, researchers now can compare long-term temperature, rainfall and cloud cover data with aerosol measurements.

The database includes visibility measurements taken from 1973 - 2007 at 3,250 meteorological stations all over the world and released by the National Climatic Data Center. Visibility is the distance an observer can see clearly from the measurement source. The more aerosols present in the air, the shorter the visibility distance. A preliminary analysis of the database measurements shows a steady increase in aerosols over the period from 1973 to 2007.

Increased aerosols in the atmosphere block solar radiation from the earth's surface, and have thus caused a net "global dimming," particularly in south and east Asia, South America, Australia, and Africa. The only region that does not show an increase in aerosols is Europe, which has actually experienced a "global brightening." Trends in North American stations were relatively flat.

It's worth noting that previous studies of "global dimming" have been based on "all-sky" observations, conditions that include clouds, which are obviously a big factor in how much sunlight gets to the surface. However, the new study considered only clear sky conditions (i.e., only when there are no clouds) to better attribute the effects of aerosols independent of the effects of cloud cover.

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