Climate Change 2001 Assessment

1. Has the climate changed during the 20th century?

• 1.1 Has the world warmed?
• 1.2 What other climate changes have been observed?
• 1.3 What aspects of our climate have NOT changed?

1.1 Has the world warmed?

An increasing number of observations indicates that the world has warmed:

Variations of the Earth’s surface temperature over the last 140 years.

Variations of the Earth’s surface temperature over the last 1000 years.

1.1.1 The average surface temperature has increased over the 20th century by about 0.6C (± 0.2°C). This increase occurred mainly from 1910 to 1945 and 1976 to 2000. The increase is larger at night time and over land area.

It is likely⁷ that in the Northern Hemisphere, over the past 1000 years:

• the temperature increase in 20th century was the largest,
• the 1990s was the warmest decade and 1998 the warmest year.

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1.1.2 Temperatures have risen during the past four decades in the lowest 8 kilometers of the atmosphere. More...

1.1.3 Snow cover and ice extent have decreased. More...

1.1.4 The sea level has risen by 10 to 20 cm (4 to 8") during the 20th century. Ocean temperature has increased since the late 1950s. More...

See also summary of changes in IPCC TS figure 7a

1.2 What other climate changes have been observed?

• Precipitation is likely⁷ to have increased in the 20th century in some land areas and decreased in others.
• It is likely⁷ that there has been some increase in cloud cover.
• It is very likely⁷ that there has been less extreme low temperatures and slightly more extreme high temperatures.
• There have been more warm episodes of the El Niño since the mid-1970s.
• There were relatively small global increases in severe drought or severe wetness over the century but an increase in droughts in some regions in recent decades.

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See also summary of changes in IPCC TS figure 7b

1.3 What aspects of our climate have NOT changed?

• No warming is apparent in some parts of the Southern Hemisphere oceans and parts of Antarctica.
• No systematic rainfall change over the Southern Hemisphere.
• No significant trends of Antarctic sea-ice extent.
• No clear change in tropical and extra-tropical storm intensity and frequency or in the frequency of tornadoes, thunder days, or hail events.

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See also summary of changes in IPCC TS figure 7b