Spur to Research: Global warming

This blog is part of a research project on global warming
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Is global warming human-caused?

Smog in Cairo

To get a good understanding of the possible effects of global warming, 'global dimming' needs to be considered. Global dimming is the gradual decline in the amount of sunlight reaching the Earth. The phenomenon is thought to be caused by air pollution and it seems to counter the effects of global warming . Some scientists believe that global dimming already has led to droughts and famine.

What is 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.

It is thought to have been caused by an increase in particulates such as sulphate aerosols in the atmosphere due to human action. The switch from a "global dimming" trend to a "brightening" trend in 1990 happened just as global aerosol levels started to decline.

Global dimming 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.

Deliberate manipulation of this dimming effect is now being considered as a geoengineering technique to reduce the impact of climate change."

From Wikipedia - http://en.wikipedia.org/wiki/Global_dimming

Image via Wikipedia

"What is global dimming?

Fossil fuel use, as well as producing greenhouse gases, creates other by-products. These by-products are also pollutants, such as sulphur dioxide, soot, and ash. These pollutants however, also change the properties of clouds.

Clouds are formed when water droplets are seeded by air-borne particles, such as pollen. Polluted air results in clouds with larger number of droplets than unpolluted clouds. This then makes those clouds more reflective. More of the sun’s heat and energy is therefore reflected back into space.

This reduction of heat reaching the earth is known as Global Dimming."

From Global Issues - http://www.globalissues.org/article/529/global-dimming#Whatisglobaldimming

BBC Horizon video