Published online 4 December 2006 | Nature | doi:10.1038/news061204-2

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Results hint that Europe may be in for a warm winter.

Blooming warm: a late onset of winter can cause horse chestnuts to burst into flower too soon. Alamy

Do you still have roses in bloom in your English garden? Then you might not be surprised to hear that Europe is experiencing the warmest autumn since Columbus first sailed to America.

Preliminary analysis shows that continental mean temperatures in September and October were 11°C — that's 1.8 °C higher than the long-term average for these months. November was 2.5 °C higher than the average. The results show that 2006 has beaten the 'hottest' autumns of 1772, 1938 and 2000 by about a degree.

Previous research has shown that spring seems to be coming earlier around the world (see 'Warming planet shifts life north and early'). But autumn climate trends have been generally less well investigated.

That's partly because warm autumns pose less stress on plants and animals than do temperature anomalies in spring, says Annette Menzel, a phenologist at the Technical University of Munich, Germany. But warm autumns come with their own problems.

Some flowering trees, such as horse chestnuts, may spring into blossom before winter comes, causing problems later in the year. And butterflies and other animals may face trouble if they miss the signal to reduce their activity for the winter. A disruption in the usual pattern between temperature and food availability can cause starvation, Menzel says.

History lesson

Finding data to support seasonal trends can be tricky, however. The instrumental record doesn't date back much further than the onset of the twentieth century.

To get around this, Elena Xoplaki, a climate historian at the University of Bern in Switzerland, has looked at historic sources in Europe going back to the 1500s, such as weather observations recorded by monks, doctors and scholars1.

She has now updated her reconstruction with the latest temperature data from NASA's Goddard Institute for Space Studies. The unpublished study reveals that the past three months have been uniquely warm in the context of the past half millennium, even when the uncertainties related to the historic data are taken into account.

"Exceptionally warm autumns in one region or another wouldn't be so telling," says Juerg Luterbacher, a climatologist at the University of Bern. "But the signal is consistent over the whole European land mass, from Iceland to Greece."

Going up

The change matches a trend. Autumn temperatures in Europe have been rising by about 0.45 °C per decade over the past three decades. The new data show that the past 30 years has been the warmest such period on record, and the past 10 years has been the warmest decade.

The strongest warming has been observed over the British Isles and Scandinavia.

Hot times: NASA data shows that Europe is abnormally warm, the United States mainly cool. NASA

NASA data show an even more extreme autumn temperature anomaly for parts of the Arctic. By contrast, large parts of the United States have been below average (see map). These data only go back to the 1950s.

Cause and effect

Weather is in essence a random phenomenon. So extreme anomalies during a single season, such as the European summer heat wave of 2003 or this warm autumn of 2006, can't be pinned to a single cause. However, the autumn warming trend over the past few decades does strongly point to a significant human influence, says Luterbacher.

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So what can we expect next? Xoplaki and Luterbacher have also looked at what winters were like after the 30 warmest autumns in their data set. They found that Decembers and Februaries were generally warmer than the 1971-2000 European average, whereas Januaries were slightly cooler.

Seasonal climate forecasts by the British Met Office and the International Research Institute for Climate and Society predict a slightly higher probability of warm rather than cold winter conditions over parts of Europe this year. However, cautions Lutenbacher, the skill of seasonal forecasts is low, and uncertainties rather large.

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