Prof Fred Singer, a meteorologist at the University of Virginia, used temperature data assembled by James Hanson of Nasa, who first highlighted the problem of climate change, to challenge the findings of the Inter-governmental Panel on the subject which underpin the Kyoto climate treaty. 

He said: "The climate has warmed in the last century but this took place before 1940. The hottest years in America were around 1940. We don't know the cause of the warming but we don't think it was human activity." 

Mr Singer says he has found no evidence suggesting future extreme weather events, such as severe storms or droughts, increases in infectious diseases, or changes to forests and other ecosystems. He accepts there has been an increase in greenhouse gases but believes this has led to "a greening of the planet, improved agricultural yields and more vigorous forest growth". 

He also accepts evidence from temperature records all over the world that there appears to have been a pronounced warming since 1975, with some of the hottest years in the 1990s. But he says that satellite records of the temperature three miles up, which should show a warming, do not show a warming at all. 

He said: "One explanation is that the satellites are wrong. The other explanation - that is my hypothesis - is that the surface appears to be warming but isn't really warming at all." 

Bob Watson, chairman of the IPCC, has used the surface temperature records of the past 20 years to claim that the 20th century is the warmest for 1,000 years, but Mr Singer disagrees. He places greater faith in the "proxy" records of temperature, contained in tree rings, ocean sediments, ice cores and so on, which he says show no warming since 1940. 

He said: "Thermometers may not be quite correct. Proxy records say the global temperature has not increased in the past 20 years." He believes that "heat islands" caused by urbanisation have distorted thermometer readings. He produced graphs from research conducted by the University of East Anglia and analysis of Greenland ice cores over 100,000 years published in scientific papers to support his point. 

Mr Singer is one of several scientists to challenge the broad conclusions of the IPCC, a distillation of the work of 3,000 scientists from most of the leading meteorological institutes. He shared a platform with Richard Coutney, from Britain's Institute of Economic Affairs, who suggested that the summary and conclusions of the IPCC's assessment of the climate had been manipulated by politicians.