Changes

<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>

<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>

Assessments by the [[United Nations]] promote the world view that most of the warming since around 1850 is mostly human-caused. In February, 2007, the [[Intergovernmental Panel on Climate Change]], a body of scientists<ref>http://www.chicagotribune.com/news/nationworld/chi-0704280477apr29,1,4275902.story?coll=chi-newsnationworld-hed</ref> established by the [[United Nations]], 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.

Assessments by the [[United Nations]] promote the world view that most of the warming since around 1850 is likely human-caused. In February, 2007, the [[Intergovernmental Panel on Climate Change]], a body of scientists<ref>http://www.chicagotribune.com/news/nationworld/chi-0704280477apr29,1,4275902.story?coll=chi-newsnationworld-hed</ref> established by the [[United Nations]], 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.

There have been many cycles of naturally-caused global warming and cooling over many millions of years (see [[climate cycles]]). Some scientists, including [[Richard Lindzen]] of [[MIT]], [[Sallie Baliunas]] of [[Harvard]] and [[Fred Singer]] (independent), say that the recent warming could be part of another natural cycle.  

There have been many cycles of naturally-caused global warming and cooling over many millions of years (see [[climate cycles]]). Some scientists, including [[Richard Lindzen]] of [[MIT]], [[Sallie Baliunas]] of [[Harvard]] and [[Fred Singer]] (independent), say that the recent warming could be part of another natural cycle.  

==The Hockey Stick Reconstruction==

==The Hockey Stick Reconstruction==

[[Image:Hockey stick chart ipcc.jpg|right|thumb|300px|The 2001 IPCC report shows blue proxy values along the "shaft" with red instrumental values superimposed as the "blade"]]

[[Image:Hockey stick chart ipcc.jpg|right|thumb|300px|The Hockey Stick graph in 2001 IPCC report shows blue proxy values along the "shaft" with red instrumental values superimposed as the "blade"]]

One of the main arguments for anthropogenic global warming 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.  However, Stephen McIntyre and economist Ross McKitrick later 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.

One of the main arguments for anthropogenic global warming 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.  However, Stephen McIntyre and economist Ross McKitrick later 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.