Changes

In February, 2007, the [[IPCC|Intergovernmental Panel on Climate Change]], whose members were appointed by their governments, released their fourth assessment report on climate change. That report concluded that most of recent global warming was "very likely" caused by human activity<ref>http://news.bbc.co.uk/2/hi/science/nature/6321351.stm</ref>. It says that increases in atmospheric concentrations of [[greenhouse gas]]es cause warming. Several members of this panel later complained their contributions were misrepresented in the final report.

In February, 2007, the [[IPCC|Intergovernmental Panel on Climate Change]], whose members were appointed by their governments, released their fourth assessment report on climate change. That report concluded that most of recent global warming was "very likely" caused by human activity<ref>http://news.bbc.co.uk/2/hi/science/nature/6321351.stm</ref>. It says that increases in atmospheric concentrations of [[greenhouse gas]]es cause warming. Several members of this panel later complained their contributions were misrepresented in the final report.

In 2008, a study revealed that the worlds glaciers had shrunk an average of 4.9 feet in 2006, compared to an average of one foot a year from 1980 to 1999.<ref>[http://www.cnn.com/2008/WORLD/weather/03/16/un.climate.ap/index.html?iref=mpstoryview Glaciers shrinking at record rate]</ref>  On 25 March, 2008, the antarctic Wilkens ice shelf began to break up, losing 220 square miles, with an area the size of [[Connecticut]] at risk.<ref>[http://www.cnn.com/2008/TECH/science/03/25/antarctic.ice/index.html Massive ice shelf on verge of breakup]]</ref>.

In 2008, a study revealed that the worlds glaciers had shrunk an average of 4.9 feet in 2006, compared to an average of one foot a year from 1980 to 1999.<ref>[http://www.cnn.com/2008/WORLD/weather/03/16/un.climate.ap/index.html?iref=mpstoryview Glaciers shrinking at record rate]</ref>  On 25 March, 2008, the antarctic Wilkens ice shelf began to break up, losing 220 square miles, with an area the size of [[Connecticut]] at risk.<ref>[http://www.cnn.com/2008/TECH/science/03/25/antarctic.ice/index.html Massive ice shelf on verge of breakup]</ref>.

===IPCC controversy===

===IPCC controversy===

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 [[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==