Causes

Gases such as water vapor, methane, and carbon dioxide allow short-wave radiation from the sun to pass through to the surface of the earth, but do not allow long-wave radiation reflected from the earth to travel back out into space. This naturally occurring insulation process—dubbed the greenhouse

GLOBAL MEAN SURFACE AIR TEMPERATURE

(Departure from 1880 to 1920 base period)

GLOBAL MEAN SURFACE AIR TEMPERATURE

(Departure from 1880 to 1920 base period)

1850 1900 1950 2000 2050

Year

1850 1900 1950 2000 2050

Year

PERCENT REDUCTION IN JUNE-AUGUST SOIL MOISTURE

2xCO2

4xCO2

2xCO2

4xCO2

effect—keeps the earth warm: In its absence, the earth would be about 33°C cooler than it is now. However, as the concentration of greenhouse gases increases (due largely to human activities), most scientists agree that the effect is expected to intensify, raising average global temperatures.

The Antarctic Larsen B shelf is breaking up, as shown in these photographs from February and March 2002, causing fears of global warming. Seen in these photographs is the loss of 500 billion tons of ice. (© Reuters NewMedia Inc./Corbis. Reproduced by permission.)

However, the earth's climate is known to vary on long timescales. The existence of naturally occurring ice ages and warm periods in the distant past demonstrates that natural factors such as solar variability, volcanic activity, and fluctuations in greenhouse gases play important roles in regulating the earth's climate. A minority of scientists believe that purely natural variations in these factors can account for the observed global warming.

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