Atlantic dynamo turned up the heat over Medieval Europe

Apr 03, 2009

(PhysOrg.com) -- In the April 3rd edition of Science a collaborative group of scientists from Switzerland, California and the UK report that medieval climate over Europe was heated by the North Atlantic Oscillation (NAO). This oscillation pattern, defined as the pressure difference between the Icelandic Low and the Azores High, also influences modern-day weather conditions and has contributed to the recent droughts in North Africa and floods in North-Central Europe.

A comparison of tree rings from 1000-year old trees in Morocco and growth layers in a stalagmite from a cave in Scotland now reveal the mechanism behind the ‘Medieval anomaly’ - a period of global warmth between 1000 and 1400 AD. During this period, the pressure difference between the Azores High and the Icelandic Low was large and, by driving warm Atlantic winds over the cold European continent in winter time, was heating the European mainland.

Trees and stalagmites are “proxy archives”, meaning that they are natural data sources from which past climatic conditions can be derived. Old cedar trees from the Atlas Mountains in Morocco grew extremely slowly during Medieval Times and thus reflect much drier conditions during this period compared to following centuries. These dry conditions, in turn, are an indicator for a strong Azores High. Opposite to the African tree rings, the Scottish stalagmite shows that during the same period it was much wetter than normal in northern , reflecting a strong Icelandic Low.

Scientists from the Dendro Sciences Unit at the Swiss Federal Research Institute have teamed up with experts from the USA and the UK, including Royal Society-Leverhulme Trust Senior Research Fellow Professor James Scourse of the School of Ocean Sciences at Bangor University, to compile and to develop the first NAO reconstruction to extend back to the Middle Ages. By comparing these proxy archives to modelled climate simulations they were able to analyse temperature, precipitation, and wind conditions across Europe over the last millennium and to test the reliability of their results. Comparisons with other terrestrial and marine proxy archives from across the globe suggest that the changes seen in the NAO are part of a global reorganization of the ocean-atmosphere system.

Valerie Trouet, first author of this study, points out that “the modern-day effects of the NAO are relatively small and short-lived compared to those during the Middle Ages”. This study demonstrates that climate has undergone large changes long before humans started releasing large amounts of greenhouse gases into the atmosphere, indicating that natural forcings should be taken into account when trying to understand the climate of the future.

Provided by Bangor University

Explore further: Quakes destroy or damage 83 houses in Philippines

add to favorites email to friend print save as pdf

Related Stories

Wind patterns could mask effects of global warming in ocean

Feb 07, 2008

Scientists at the University of Liverpool have found that natural variability in the earth’s atmosphere could be masking the overall effect of global warming in the North Atlantic Ocean. The research is published in journal ...

The grass is greener after a cold winter

Jan 25, 2006

We may well be shivering through an unusually chilly winter, but the dip in temperature is not all bad news, at least for your lawn. Researchers at Harper Adams University College, Shropshire, believe a cold winter leads ...

Natural climate change may be larger than commonly thought

Feb 10, 2005

A new study of climate in the Northern Hemisphere for the past 2000 years shows that natural climate change may be larger than generally thought. This is displayed in results from scientists at the Stockholm University, made ...

Recommended for you

Kiribati leader visits Arctic on climate mission

Sep 20, 2014

Fearing that his Pacific island nation could be swallowed by a rising ocean, the president of Kiribati says a visit to the melting Arctic has helped him appreciate the scale of the threat.

NASA catches a weaker Edouard, headed toward Azores

Sep 19, 2014

NASA's Aqua satellite passed over the Atlantic Ocean and captured a picture of Tropical Storm Edouard as it continues to weaken. The National Hurricane Center expects Edouard to affect the western Azores ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

theophys
5 / 5 (3) Apr 03, 2009
That's pretty interesting, but I would like to know a little more about the Azores High and Icelandic Low. What exactly caused these pressure system to be do strong? How long did it last? What did the weather systems look like in other parts of the world around this time?
dachpyarvile
not rated yet Apr 08, 2009
Valerie Trouet, first author of this study, points out that "the modern-day effects of the NAO are relatively small and short-lived compared to those during the Middle Ages". This study demonstrates that climate has undergone large changes long before humans started releasing large amounts of greenhouse gases into the atmosphere, indicating that natural forcings should be taken into account when trying to understand the climate of the future.


Amen to that! :)