The entire northern hemisphere is experiencing a sustained period of warming that is unprecedented in the past millennium, a study has found.

A review of a range of temperature records, from tree rings and ice cores to historical documents, has found that at no time since the 9th century have temperatures been so consistently high. The study, published in the journal Science, found that the late 20th century was the warmest period for the northern hemisphere since at least 800AD, eclipsing the well-known medieval warm period when vines were cultivated successfully in northern Europe and the Vikings exploited the ice-free seas to colonise Greenland.

Seasonal surface melt extent on the Greenland Ice Sheet has been observed by satellite since 1979 and shows an increasing trend. The melt zone, where summer warmth turns snow and ice around the edges of the ice sheet into slush and ponds of meltwater, has been expanding inland and to record high elevations in recent years (source: Arctic Impacts of Arctic Warming, Cambridge Press, 2004).

Timothy Osborn and Keith Briffa, climate scientists from the University of East Anglia in Norwich, analysed 14 sets of temperature records from America, Europe and East Asia. Each record covered a relatively wide region, such as northern Sweden or the low countries of the Netherlands and Belgium, and extended back at least several centuries.

Ten of the 14 records were based on tree-ring data, which went back as far as 800AD, one measured ice cores from Greenland, one involved historical documents from Europe and one covered the chemical composition of sea shells on the east coast of the US. The final set of records came from China and Japan and used a variety of records, from ice cores to historical documents.

"Our results show that, during the late 20th century, warming affected the entire northern hemisphere and that at no point in the past 1,000 years has the northern hemisphere experienced the same widespread warming," Dr Osborn said.

The study showed that the medieval warm period ran from about 890 to 1170 and that this was later followed by a significant period of cooling between 1580 and 1850, which included the period known as the "little ice age" when frost fairs were held on the River Thames.

"The key conclusion was that the 20th century stands out as having unusually widespread warmth, compared to all of the natural warming and cooling episodes during the past 1,200 years," Dr Osborn said.

Climate scientists have in the past found evidence to suggest that the late 20th century was warmer than at any time in the past millennium but this study is the first to look at a variety of temperature records from across the entire northern hemisphere.

© 2006 Independent News and Media Limited

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