The Intergovernmental Panel on Climate Change (IPCC) has stated that: Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations, mainly carbon dioxide. This conclusion is based on output from global climate computer models known as General Circulation Models (GCM).
David Douglass and John Christy, in a paper recently accepted for publication and already available on the internet, have come to a different conclusion. By considering observed, as opposed to modelled, temperature changes and at different latitude bands they conclude that:
1. El Nino and La Nina effects in the tropics have a more significant affect on global temperature anomalies than carbon dioxide, in particular it was an El Nino event that drove the 1998 global temperature maximum.
2. Variations in global temperatures since 1978 have mostly been due to climate effects in the northern hemisphere (northern extratropics) and these effects cannot be attributed to carbon dioxide.
3. Carbon dioxide has contributed a small amount to an increase in global temperatures but without what is commonly referred to as feed-back.
David Douglas and John Christy are practicing climate scientists from the Department of Physics and Astronomy, University of Rochester, and Department of Atmospheric Science and Earth System Science Center, University of Alabama, respectively. Their paper entitled ‘Limits on CO2 Climate Forcing from Recent Temperature Data of Earth’, was recently accepted for publication in Energy and Environment.
A regular at this blog, Cohenite, comments on the Douglass-Christy paper in a fairly technical note already posted at the community webpage of this blog, and entitled ‘Temperature Trends and Carbon Dioxide’, suggests that there is no evidence for a contribution from carbon dioxide to global temperatures and that the role of the sun has been underestimated.