Medieval warming study is blow to climate change deniers

CLIMATE change deniers have been deprived of one of their favourite arguments against human-induced global warming.

During the Medieval Climate Anomaly (MCA), Europe basked in balmy weather, and some claim that whatever natural mechanism caused it is warming the world today. To find out, Valerie Trouet at the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf and colleagues studied the growth rate of trees in Morocco and a stalagmite in Scotland, both dating back 1000 years, to determine rainfall levels during the MCA.

They found a big difference in rainfall, and hence pressure systems, in each region at this time. This suggests that from 1050 to 1400 the North Atlantic experienced a strongly positive North Atlantic Oscillation (NAO) - the regional climate system that drives winds from the Atlantic over Europe. The more positive the NAO, the more warm air is blown towards the continent.

To investigate why the warm winds were so persistent, lasting 350 years, the team combined their data with information from other regions of the world. It turns out that the El Niño system was in the negative La Niña mode, which, as the two systems are connected by ocean currents, could have reinforced the NAO (Science, vol 324, p 78). A persistently positive NAO is not operating today.

External forces like abrupt changes in solar output or volcanism could have started and stopped the cycle, says Trouet, who hopes to pinpoint the trigger at a climatology workshop in May.