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→Natural variability of the climate system
==Natural variability of the climate system==
==Natural variability of the climate system==
The Earth has experienced numerous [[Ice Age|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.
The Earth has experienced numerous [[Ice Age|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.
And it's not just the earth that has variations. Since the Viking spacecraft reached Mars in the 1970's until recent readings were taken, the average temperature on Mars has risen 1.1 degrees Fahrenheit just as the average temperature on the earth has risen. Since human industrialization is probably not to blame for the change on Mars, other causes are being considered. The scenario most postulated is dust storms.<ref>http://www.space.com/scienceastronomy/070404_gw_mars.html</ref>
==Politics of global warming==
==Politics of global warming==