Changes

[[Image:Artic 1979.jpg‎|left|thumb|275px|A computer projection comparing the Polar Ice Mass in 1979 (left) and 2000 (right).]]

[[Image:Artic 1979.jpg‎|left|thumb|275px|A computer projection comparing the Polar Ice Mass in 1979 (left) and 2000 (right).]]

Average [[Earth]] surface air temperature has risen about 1° F since 1970.  

Average [[Earth]] surface air temperature has risen about 1° F since 1970.  

<ref>Hansen's group at the Goddard Institute wrote, "Global warming is now 0.6 °C [1.0 °F] in the past three decades and 0.8 °C [1.4 °F] in the past century." http://data.giss.nasa.gov/gistemp/2005/</ref> How much of this is due to contamination of the temperature record from the [[urban heat island]] effect is hard to assess.  Efforts are currently underway to detect and remove this bias. Spurious warming trends that might be considered as global warming can occur almost anywhere. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>

<ref>Hansen's group at the Goddard Institute wrote, "Global warming is now 0.6&nbsp;°C [1.0&nbsp;°F] in the past three decades and 0.8&nbsp;°C [1.4&nbsp;°F] in the past century." http://data.giss.nasa.gov/gistemp/2005/</ref> How much of this is due to contamination of the temperature record from the [[urban heat island]] effect is hard to assess.  Efforts are currently underway to detect and remove this bias. Spurious warming trends that might be considered as global warming can occur almost anywhere. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>

Nonetheless, controversy has persisted over the influence of urban warming on reported large-scale surface-air temperature trends. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>

Nonetheless, controversy has persisted over the influence of urban warming on reported large-scale surface-air temperature trends. <ref>http://climatesci.colorado.edu/2006/01/29/do-urban-areas-have-larger-long-term-temperature-trends-than-other-locations/feed/</ref>

One of the main pieces of evidence for anthropogenic global warming presented during the 3rd IPCC report was a temperature reconstruction done by climatologist [[Michael Mann]]. The resulting graph of the Northern Hemisphere temperature is shaped like a hockey stick, and it is used as evidence for the unusualness of the earth's temperature in the past few decades.   

One of the main pieces of evidence for anthropogenic global warming presented during the 3rd IPCC report was a temperature reconstruction done by climatologist [[Michael Mann]]. The resulting graph of the Northern Hemisphere temperature is shaped like a hockey stick, and it is used as evidence for the unusualness of the earth's temperature in the past few decades.   

While dramatic, the graph encountered serious questions from the beginning.  It failed to reproduce two Northern Hemisphere climate episodes that were well documented in the literature, the Medieval Warm Period and The Little Ice Age (it should be noted that these were not global phenomenons, but were limited to Europe and Greenland; the Mann diagram concerned global average temperatures). Concerned about these discrepancies, [[Stephen McIntyre]] and [[Canada|Canadian]] economist Ross McKitrick attempted to reproduce the result and discovered a serious flaw in the statistical methods used to construct the hockey stick graph.  Because of this, the 4th Assessment Report of the IPCC included several different temperature reconstruction methods instead of favoring just one hypothesis.

While dramatic, the graph encountered serious questions from the beginning.  It failed to reproduce two Northern Hemisphere climate episodes that were well documented in the literature, the Medieval Warm Period and The Little Ice Age (it should be noted that these were not global phenomenons, but were limited to Europe and Greenland; the Mann diagram concerned global average temperatures).   Concerned about these discrepancies, [[Stephen McIntyre]] and [[Canada|Canadian]] economist Ross McKitrick attempted to reproduce the result and discovered a serious flaw in the statistical methods used to construct the hockey stick graph.  Because of this, the 4th Assessment Report of the IPCC included several different temperature reconstruction methods instead of favoring just one hypothesis.

==Natural Variability of the Climate System==

==Natural Variability of the Climate System==

... dealing with the threat of warming fits in with a great variety of preexisting agendas [like] dissatisfaction with industrial society (neopastoralism), ... governmental desires for enhanced revenues (carbon taxes), and bureaucratic desires for enhanced power. <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

... dealing with the threat of warming fits in with a great variety of preexisting agendas [like] dissatisfaction with industrial society (neopastoralism), ... governmental desires for enhanced revenues (carbon taxes), and bureaucratic desires for enhanced power. <ref>http://www.cato.org/pubs/regulation/regv15n2/reg15n2g.html</ref>

The global average surface temperature warmed about 0.9 °F over the second half of the 20th Century.

The global average surface temperature warmed about 0.9&nbsp;°F over the second half of the 20th Century.

Assessments of climate science by the [[United Nations]] (see [[IPCC]] - ''Intergovernmental Panel on Climate Change'')

Assessments of climate science by the [[United Nations]] (see [[IPCC]] - ''Intergovernmental Panel on Climate Change'')