Fair-ish and Balanced-ish
Tuesday, March 11, 2003
Medieval Warm Period
One theme in global warming skeptics litany is the medieval warm period. This is commonly presented as evidence that the earth's climate can change without human influence (something which isn't debated by scientists, rather this is a strawman put forward by the skeptics). However one common meme within this theme is that the temperatures were higher than modern temperatures. For example;
[B]etween 900 and 1100 the climate dramatically warmed. Known as the Medieval Warm Period, the temperature rose by more than 1° F to an average of 60° or 61° F, as much as 2° F warmer than today. Again, the temperature during this period is similar to Greenhouse predictions for 2100, a prospect global warming theory proponents insist should be viewed with alarm. But judging by how Europe prospered during this era, there is little to be alarmed about. The warming that occurred between 1000 and 1350 caused the ice in the North Atlantic to retreat and permitted Norsemen to colonize Iceland and Greenland. Back then, Greenland was actually green. Europe emerged from the Dark Ages in a period that was characterized by bountiful harvests and great economic prosperity. So mild was the climate that wine grapes were grown in England and Nova Scotia. (Link)
Or as Australia's own, Aaron Oakley writes:
During the medieval optimum the world was warmer than it is today (I understand that they were even growing wine grapes in England). Europe was healthier than it was during the subsequent little ice age. (Can be found by going here then clicking the comments link).
These English grapes strike me as a pretty weak piece of evidence for higher temperatures, given that England has plenty of vineyards today.
Likewise, Greenland is still green (remember that Eric the Red wouldn't have be describing the interior or arial photos of Greenland, but rather potential colonisation sites).
So how does science see the medieval warm period? This section is from the the IPCC's 2001 report into the scientific basis for global warming:
As with the "Little Ice Age", the posited "Medieval Warm Period" appears to have been less distinct, more moderate in amplitude, and somewhat different in timing at the hemispheric scale than is typically inferred for the conventionally-defined European epoch. The Northern Hemisphere mean temperature estimates of Jones et al. (1998), Mann et al. (1999), and Crowley and Lowery (2000) show temperatures from the 11th to 14th centuries to be about 0.2°C warmer than those from the 15th to 19th centuries, but rather below mid-20th century temperatures. The long-term hemispheric trend is best described as a modest and irregular cooling from AD 1000 to around 1850 to 1900, followed by an abrupt 20th century warming. Regional evidence is, however, quite variable. Crowley and Lowery (2000) show that western Greenland exhibited anomalous warmth locally only around AD 1000 (and to a lesser extent, around AD 1400), with quite cold conditions during the latter part of the 11th century, while Scandinavian summer temperatures appeared relatively warm only during the 11th and early 12th centuries. Crowley and Lowery (2000) find no evidence for warmth in the tropics. Regional evidence for medieval warmth elsewhere in the Northern Hemisphere is so variable that eastern, yet not western, China appears to have been warm by 20th century standards from the 9th to 13th centuries. The 12th and 14th centuries appear to have been mainly cold in China (Wang et al., 1998a,b; Wang and Gong, 2000). The restricted evidence from the Southern Hemisphere, e.g., the Tasmanian tree-ring temperature reconstruction of Cook et al. (1999), shows no evidence for a distinct Medieval Warm Period. (Link)
A graph showing tree ring derived temperatures can be found here.
A more recent paper (Jan Esper, Edward R. Cook and Fritz H. Schweingruber, Science 2002, 295, 2250 - 2253) claims that the temperatures reconstructions cited above underestimate the magnitude of the warming. It suggests that the MWP was (at it's peak) about as hot as 1990 (ie. significantly cooler than today).
Update: The Greenland link doesn't work (that will teach me to deep link). To see the picture, go here and click on the picture of the ruins of the Hvalsey Church (this was one of the Church's in the Vikings Eastern Settlement. The important thing to note is the colour of the background.