Medieval Warm Period, Busted

Climate Change Denialists, or Sceptics (Skeptics) as they like to call themselves, will not be liking this piece of research one single teensy-eensy-weensy bit.

http://www.newscientist.com/article/dn16892-natural-mechanism-for-medieval-warming-discovered.html

“Natural mechanism for medieval warming discovered : 19:00 02 April 2009 by Nora Schultz : Europe basked in unusually warm weather in medieval times, but why has been open to debate. Now the natural climate mechanism that caused the mild spell seems to have been pinpointed. The finding is significant today because, according to Valerie Trouet at the Swiss Federal Institute for Forest, Snow, and Landscape Research in Birmensdorf, the mechanism that caused the warm spell in Europe – and meant wine could be produced in England as it is now – cannot explain current warming. It means the medieval warm period was mainly a regional phenomenon caused by altered heat distribution rather than a global phenomenon.”

There’s a sting in this story, and it has to do with the Pacific Ocean. It seems that natural factors such as volcanic eruptions could have created a lasting, self-reinforcing La Niña effect in the ENSO (El Niño Southern Oscillation) and that this caused the Medieval Warm Period in Europe.

“The idea to use growth rings to work out past climate change is not new, but Trouet’s team is the first to look back beyond 1400 in the European record. They found that the strongly positive NAO [ North Atlantic Oscillation ] lasted for about 350 years from 1050 to 1400. By combining their data with information from other regions of the world during medieval times and plugging it into different models, the researchers have also come up with a hypothesis of what made the warm winds so persistent. “It turns out that in the tropical Pacific, the El Niño system was in a negative La Niña mode, meaning it was colder than normal,” says Trouet.”

If this permament La Niña were triggered by natural regional Warming, then it could also happen as a result of man-made Global Warming.

This would exacerbate the megadrought conditions currently being experienced in parts of the United States, China, Australia and the European Mediterranean region and elsewhere.

“Mann is also concerned that the dominance of medieval La Niña conditions now indicated by Trouet’s work might make it more likely that the current man-made warming could also put the El Niño system back into a La Niña mode, although most climate models so far had predicted the opposite. “If this happens, then the implications are profound, because regions that are already suffering from increased droughts as a result of climate warming, like western North America, will become even drier if La Niña prevails in the future”, he says.”

At the current time, it appears that La Niña is becoming less prevalent, and this is what is usually stated in research :-

http://data.giss.nasa.gov/gistemp/graphs/Fig.E.lrg.gif

However, we could see a radical flip in any Earth system, as the average global temperature is hotter than at any time in the last 1,000 years :-

http://www.newscientist.com/data/images/ns/cms/dn11648/dn11648-2_726.jpg

Previous research on dry/wet periods in various parts of the World show that there are general prevailing conditions for definite periods of time measured in multiple decades :-

http://www3.interscience.wiley.com/journal/72508871/abstract

“Rainfall variability in southern Spain on decadal to centennial time scales : Abstract : In this work a long rainfall series in Andalusia (southern Spain) is analysed. Methods of historical climatology were used to reconstruct a 500-year series from historical sources. Different statistical tools were used to detect and characterize significant changes in this series. Results indicate rainfall fluctuations, without abrupt changes, in the following alternating dry and wet phases: 1501-1589 dry, 1590-1649 wet, 1650-1775 dry, 1776-1937 wet and 1938-1997 dry. Possible causal mechanisms are discussed, emphasizing the important contribution of the North Atlantic Oscillation (NAO) to rainfall variability in the region…”

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