Medieval warming 1000 yrs ago was a result of natural climate mechanism
According to a report in New Scientist, the research was carried out by Valerie Trouet at the Swiss Federal Institute for Forest, Snow, and Landscape Research in Birmensdorf.
It suggests that the medieval warm period was mainly a regional phenomenon caused by altered heat distribution rather than a global phenomenon.
To work out what the global climate was doing 1000 years ago during the so-called "Medieval Warm Period", Trouet and colleagues started by looking at the annual growth rings of Moroccan Atlas cedar trees and of a stalagmite that grew in a Scottish cave beneath a peat bog.
This revealed how dry or wet it has been in those regions over the last 1000 years.
The weather in Scotland is highly influenced by a semi-permanent pressure system called the Icelandic Low, and that in Morocco by another called the Azores High.
"So by combining our data, which showed a very wet medieval Scotland and very dry Morocco, we could work out how big the pressure difference between those areas was during that time," said Trouet.
This pressure difference in turn revealed that the medieval period must have experienced a strongly positive North Atlantic Oscillation (NAO) - the ocean current that drives winds from the Atlantic over Europe.
The more positive the NAO is, the more warm air is blown towards the continent.
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 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 Nino system was in a negative La Nina mode, meaning it was colder than normal," said Trouet.
El Nino and the NAO are connected by a process called thermohaline circulation, which drives the "ocean conveyor belt" that shuttles sea water of different density around the world’s oceans.
According to Trouet, a Pacific La Nina mode and a positive NAO mode could have reinforced each other in a positive feedback loop - and this could explain the stability of the medieval climate anomaly.