Temperature Reconstructions for the Last Millennium

Gerald R. North
Department of Atmospheric Sciences, Texas A&M University, College Station, TX

The time dependence of large-scale surface temperatures (hemispheric or global average) over the last millennium provides perspective for understanding present and future states of global climate. To estimate these past temperatures proxies must be used since the instrumental record only extends back about 150 years. The main technique used in reconstructions by proxies involves utilization of the instrumental record as a means of calibration. As an example consider tree rings. First the specialist investigates the available sites and decides what factors are controlling the local response to environmental conditions. Once these are identified and if the species and specimens at the site are sensitive to temperature change, one can use the tree ring widths (or other parameters) to design a ‘thermometer’ that can be used for extrapolation. The steps along the way are essentially the methods of statistical regression.

The findings of the 2006 National Academy of Sciences Committee can be summarized as follows:
1) There was a warm period in the middle ages and a cool one centered in the 1600s, with an upswing in temperatures after that to the present.
2) It can be said with a high level of confidence that the last three decades were warmer than any such interval in the last 400 years.
3) It is plausible that the last three decades were the warmest in a thousand years.

While this methodology does not make use of climate models, they are essential to understand the record and to project climate into the next century.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas