Changes

==Natural variability of the climate system==

==Natural variability of the climate system==

The Earth has experienced numerous [[ice age]]s over the past two million years, during which global temperatures were approximately 6 degrees Celsius cooler than the present. The frigid temperatures allowed ice sheets to expand southward, covering much of [[Asia]], [[Europe]], and [[North America]]. The cooling associated with ice ages is gradual, while the terminations are relatively rapid. However, even the rapid terminations of ice ages take centuries to millennia, which contrasts with the effects predicted for anthropogenic global warming. Projections of future climate predict global surface temperature to rise by 2 degrees Celsius (give or take about 1.5 C) between about the year 2000 and 2100. This predicted rapid warming is attributed to increases in atmospheric carbon dioxide, which through a complex set of climatic feedbacks (e.g., water vapor, lapse rate, albedo, vegetative, and cloud feedbacks) leads to the warming. Ultimately this is the same physical mechanism as the [[greenhouse effect]], in which outgoing infrared radiation is retained by the Earth system, raising the global temperature.

The Earth has experienced numerous [[ice age]]s over the past two million years, during which global temperatures dropped approximately 6 degrees Celsius and then returned to normal. The frigid temperatures allowed ice sheets to expand southward, covering much of [[Asia]], [[Europe]], and [[North America]]. The cooling associated with ice ages is gradual, while the terminations are relatively rapid. However, even the rapid terminations of ice ages take centuries to millennia, which contrasts with the effects predicted for anthropogenic global warming. Projections of future climate predict global surface temperature to rise by 2 degrees Celsius (give or take about 1.5 C) between about the year 2000 and 2100. This predicted rapid warming is attributed to increases in atmospheric carbon dioxide, which through a complex set of climatic feedbacks (e.g., water vapor, lapse rate, albedo, vegetative, and cloud feedbacks) leads to the warming. Ultimately this is the same physical mechanism as the [[greenhouse effect]], in which outgoing infrared radiation is retained by the Earth system, raising the global temperature.

There is a close correlation between the average temperature of the Earth's surface and the amount of [[carbon dioxide]] in the atmosphere. During warm periods, there are high concentrations of atmospheric carbon dioxide, and lower levels during cool periods.

There is a close correlation between the average temperature of the Earth's surface and the amount of [[carbon dioxide]] in the atmosphere. During warm periods, there are high concentrations of atmospheric carbon dioxide, and lower levels during cool periods.