Steve McIntyre from Climate Audit has uncovered another error in An Inconvenient Truth. Al Gore argues that temperatures during the Medieval Warm Period were less than modern times, refering to the work of Dr Thompson (the graph is given the folky title of Dr Thompson's thermometer). However, where did this graph actually come from?
Dr Thompson's Thermometer was taken from Tropical Glacier and Ice Core Evidence of Climate Change on Annual to Millennial Time Scales (Thompson 2003). Specifically, from Figure 7d (rotated to match Al Gore's graph courtesy of Climate Audit):
While the graph was published in Thompson's paper, the actual data is a combination of Mann's hockey stick (Mann 1998) and CRU's surface measurements (Jones 1999). So Al Gore did err in attributing this graph to Thompson. What is the significance? Gore was trying to make the following point:
"As Dr Thompson’s thermometer shows, the vaunted Medieval Warm Period (the third little red blip from the left below) was tiny in comparison to the enormous increases in temperature in the last half-century - the red peaks at the far right of the graph."
Since Mann's study in 1999, there have been a number of proxy studies reconstructing past temperature, using a variety of proxy methods such as tree rings, bristle cones, stalagmites, coral, etc. The results are published in Surface Temperature Reconstructions for the Last 2,000 Years (freely available online). In summating the various studies, the NOAA came to the following conclusion (there's a lot more so it's definitely worth a read):
The similarity of characteristics among the different paleoclimatic reconstructions provides confidence in the following important conclusions:
- Dramatic warming has occurred since the 19th century.
- The recent record warm temperatures in the last 15 years are indeed the warmest temperatures the Earth has seen in at least the last 1000 years, and possibly in the last 2000 years.
So while Al Gore was in error attributing the Mann/Jones graph to Dr Thompson, the main conclusion that temperatures during the Medieval Warm Period were less than modern times is correct. This is an important point and seems to be overlooked in the eagerness to debunk Gore.
Posted by John Cook on Thursday, 15 November, 2007
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