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Thursday, 26 April, 2001, 18:01 GMT 19:01 UK
Records 'show strong recent warming'
Planet earth
Scientists have analysed climate data for the 20th century
By BBC News Online's environment correspondent Alex Kirby

UK scientists say a thousand years' climate records show the last three decades were the millennium's warmest.

They also conclude that natural phenomena like El Nino are unlikely to have caused the unprecedented recent warming.

Their findings strengthen the argument that climate change is not produced by natural causes alone.

The scientists are Professor Phil Jones, Dr Tim Osborn, and Dr Keith Briffa, all from the Climatic Research Unit at the University of East Anglia. They report their work in the journal Science.

Their analysis included instrumental and documentary records, and also other "proxies" of past climate variability - evidence from tree rings, corals and ice cores.

Warmest century

For the northern hemisphere, their temperature reconstructions show that "the recent 30-year period is likely to have been the warmest (about 0.2 degrees Celsius above the 1961 to 1990 average) of the millennium, with the warmest century (by about 0.1 degrees C) likely to have been the 20th".

Forest fires, Greece
Greece has been hit by forest fires in recent years
The authors say the first half of the millennium was milder than the 1500 to 1900 period. The coolest century was the 17th., followed by the 19th., with a milder interval between.

They add that their work provides some support for the idea that there were two epochs in the last millennium, the medieval warm period, spanning roughly 900 to 1200, and the little ice age from about 1550 to 1900.

The authors say: "The southern hemisphere temperature reconstructions are shorter and less reliable; they do indicate cooler conditions before 1900, but not the same inter-centennial variation evident in the north.

"The average shows greater recent warming than earlier in the 20th century, and there is no evidence of the slight 1945 to 1975 cooling seen over many northern hemisphere land areas.

"Instrumental data from Antarctica show a temperature rise until the early 1970s, with little change since then."

Greatest warming

Professor Jones said: "The accuracy of records for the first half of the millennium is sometimes queried. We have calculated errors, and the picture is clearer. All records show that the 20th century experienced the greatest warming of the millennium.

"Examining this broad span of records from all parts of the world, we see that the North Atlantic Oscillation, which is responsible for the UK's recent milder, wetter winters, has behaved in this unusual way before, notably in the 1730s, the mid-19th century, and the early 1900s.

"Similarly, we find elevated activity of El Nino events in some earlier periods. Some people have attributed global warming to these two phenomena. But the records show that their past activity did not result in significant warming."

No freeze

The scientists say it is important to recognise the dangers of taking documentary sources at face value.

They say accounts of the Thames freezing over in the past are often cited as proof that winters were colder then. But they say a significant factor in the freezing of the river was the way the old London Bridge was built with a number of piers, encouraging a process known as "ponding". In the winter of 1962/63, the third coldest since 1659, the river did not freeze at all.

There has been no complete freezing since the bridge was rebuilt to a different pattern between 1825 and 1835.

Professor Jones told BBC News Online: "Our work is part of the jigsaw, narrowing down the range of possible past climates. It shows that it is more likely that the underlying trend in global warming is the result of human influence."

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See also:

19 Apr 01 | Sci/Tech
Beyond Kyoto's sound and fury
03 Apr 01 | Americas
Anger as US abandons climate treaty
28 Oct 00 | Sci/Tech
Global warming 'worse than feared'
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