Medieval castle in Scotland (Image: The Travel Library/Rex Features )

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.

The finding scuppers one of the favourite arguments of climate-change deniers. If Europe had temperature increases before we started emitting large amounts of greenhouse gases, their argument goes, then maybe the current global warming isn’t caused by humans, either.


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.

Temperature over the past 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,” says Trouet.

Warm blast

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 Niño system was in a negative La Niña mode, meaning it was colder than normal,” says Trouet.

Climate loop

El Niño 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 Niña 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.

Trouet thinks external forces like abrupt changes in solar output or volcanism must have started and stopped the cycle, and hopes to pinpoint the most likely candidates in a workshop with other climatologists in May.

‘Profound implications’

Michael Mann at Pennsylvania State University says that based on the analyses and modelling that he has done, increased solar output and a reduction in volcanoes spouting cooling ash into the atmosphere could have not only kicked off the medieval warming, but might also have maintained it directly.

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.

Journal Reference: Science (DOI: 10.1126/science.1166349)