Last autumn-winter season was Europe's warmest for more than 700 years, researchers say.
The last time Europeans saw similar temperatures to the autumn and winter of 2006-07, they were eating strawberries at Christmas in 1289, according to Jürg Luterbacher at the University of Bern, Switzerland, and colleagues.
European climate measurements and temperature records stretch back several hundred years - UK records are the longest available, going back to 1659. Estimating historical temperatures beyond then involves scrutinising contemporary documents and diaries.
"People in churches, or doctors, wrote diaries, and usually they also included information about weather and climate. Climate historians can use and interpret this information and translate it into a temperature value," explains Luterbacher, who worked with climate historians to compare past and recent temperatures
Early bloomsSeparately, the temperatures experienced during autumn 2006 and winter 2007 are likely to have been the warmest in 500 years, they say. But the sequential combination of two such warm seasons is a still rarer event - probably the first since 1289.
In that year, people in western and central Europe wrote accounts of what they viewed as extremely unusual events.
"Documents report for instance that strawberries were eaten at Christmas, and the [vineyards] produced leaves, stock and even blossoms in the middle of January, and in Vienna fruit trees were flowering like in May," Luterbacher told New Scientist, adding: "This was really extreme, so maybe it can be compared to today in western and central Europe."
Similar unusual events have also been noticed in this recent warm period. For instance, hazel trees and snowdrops in Germany blossomed a full 30 days earlier than at any time in the last 50 years in spring 2007. And in 2006, horse chestnut trees in Switzerland blossomed twice instead of their usual once. "This is really an exceptionally rare event," says Luterbacher.
Human factorIn April, researchers reported that warming climates were causing mushrooms across the UK to reproduce twice a year (see
Warming climate creates mountains of mushrooms).
Luterbacher says the 1289 temperatures may have been caused by a large volcanic eruption in the tropics, but there has been no such event in the past few years. Instead, he and his colleagues found that the warm autumn and winter in 2006-07 were due to warm air moving up from the Atlantic off the coast of North Africa.
They point out that whether or not a single extreme event like this is due to climate change is very difficult to say. Luterbacher told New Scientist that future studies will use computer models to determine how likely it is that such warm temperatures would have been seen without human greenhouse-gas emissions.
In 2004, a similar study estimated that it was "very likely that human influence has at least doubled the risk" of extreme weather events, such as
the 2003 European heat wave which killed 35,000 people.
Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL029951)
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