Considering that the measures recommended by the world's politicians to combat global warming will cost tens of trillions of dollars and involve very drastic changes to our way of life, it might be thought wise to check the reliability of the evidence on which they base their belief that our planet is actually getting hotter.

There are four internationally recognised sources of data on world temperatures, but the one most often cited by supporters of global warming is that run by James Hansen of Nasa's Goddard Institute for Space Studies (GISS).

Hansen has been for 20 years the world's leading scientific advocate of global warming (and Al Gore's closest ally). But in the past year a number of expert US scientists have been conducting a public investigation, through scientific blogs, which raises large question marks over the methods used to arrive at his figures.

advertisement

First they noted the increasingly glaring discrepancy between the figures given by GISS, which show temperatures continuing to race upwards, and those given by the other three main data sources, which all show temperatures having fallen since 1998, dropping dramatically in the past year to levels around the average of the past 30 years.

Two sets of data, from satellites, go back to 1979: one produced by Dr Roy Spencer, formerly of Nasa, now at the University of Alabama, Huntsville, the other by Remote Sensing Systems. Their figures correspond closely with those produced by the Hadley Centre for Climate Studies of our own Met Office, based on global surface temperature readings.

Right out on their own, however, are the quite different figures produced by GISS which, strangely for a body sponsored by Nasa, rely not on satellites but also on surface readings. Hansen's latest graph shows temperatures rising since 1880, at accelerating speed in the past 10 years.

Read more from Christopher Booker
The other three all show a flattening out after 2001 and a marked downward plunge of 0.6 degrees Celsius in 2007/8, equivalent to almost all the net warming recorded in the 20th century. (For comparisons see "Is the Earth getting warmer, or colder?" by Steven Goddard on The Register website.)

Even more searching questions have been raised over Hansen's figures by two expert blogs. One is Climate Audit, run by Steve McIntyre, the computer analyst who earlier exposed the notorious "hockeystick" graph that was shamelessly exploited by the Intergovernmental Panel on Climate Change and Al Gore. (This used a flawed computer model to suppress evidence that the world was hotter in the Middle Ages than today.) The other site is Watts Up With That, run by the meteorologist Anthony Watts.

It was McIntyre who last year forced Hansen to publish revised figures for US surface temperatures, to show that the hottest years of the 20th century were not in the 1990s, as Hansen had claimed, but in the 1930s. He has now shown that Hansen had been adjusting almost all his pre-1970 global temperature figures downwards, by as much as 0.5 degrees, and his post-1970 figures upwards.

Although Hansen claimed that this only resulted from more careful calculations, McIntyre pointed out how odd it was that the adjustments all seemed to confirm his thesis.

Watts meanwhile has also been conducting an exhaustive photographic survey of US surface weather stations, showing how temperature readings on more than half have been skewed upwards by siting thermometers where their readings are magnified by artificial heat-sources, such as asphalt car parks or air-conditioning systems.

All this has raised such doubts over the methodology behind the GISS data that informed observers are calling for it to be independently assessed. Hansen himself is notoriously impatient of any criticism of his methods: earlier this month he appealed to Congress that the leaders of those who question global warming should be put on trial.

It is still too early to suggest that the recent drop in temperatures shown by everyone but him is proof that global warming has stopped. But the fact is that not one of those vaunted computer models predicted what has happened to temperatures in recent years. Yet it is on those models (and Hansen's alarmist figures) that our politicians are basing all their proposals for irrevocably changing our lives.