The United Nations climate panel faces a new challenge with scientists casting doubt on its claim that global temperatures are rising inexorably because of human pollution.
In its last assessment the Intergovernmental Panel on Climate Change (IPCC) said the evidence that the world was warming was “unequivocal”.
It warned that greenhouse gases had already heated the world by 0.7C and that there could be 5C-6C more warming by 2100, with devastating impacts on humanity and wildlife. However, new research, including work by British scientists, is casting doubt on such claims. Some even suggest the world may not be warming much at all.
“The temperature records cannot be relied on as indicators of global change,” said John Christy, professor of atmospheric science at the University of Alabama in Huntsville, a former lead author on the IPCC.
The doubts of Christy and a number of other researchers focus on the thousands of weather stations around the world, which have been used to collect temperature data over the past 150 years.
These stations, they believe, have been seriously compromised by factors such as urbanisation, changes in land use and, in many cases, being moved from site to site.
Christy has published research papers looking at these effects in three different regions: east Africa, and the American states of California and Alabama.
“The story is the same for each one,” he said. “The popular data sets show a lot of warming but the apparent temperature rise was actually caused by local factors affecting the weather stations, such as land development.”
The IPCC faces similar criticisms from Ross McKitrick, professor of economics at the University of Guelph, Canada, who was invited by the panel to review its last report.
The experience turned him into a strong critic and he has since published a research paper questioning its methods.
“We concluded, with overwhelming statistical significance, that the IPCC’s climate data are contaminated with surface effects from industrialisation and data quality problems. These add up to a large warming bias,” he said.
Such warnings are supported by a study of US weather stations co-written by Anthony Watts, an American meteorologist and climate change sceptic.
His study, which has not been peer reviewed, is illustrated with photographs of weather stations in locations where their readings are distorted by heat-generating equipment.
Some are next to air- conditioning units or are on waste treatment plants. One of the most infamous shows a weather station next to a waste incinerator.
Watts has also found examples overseas, such as the weather station at Rome airport, which catches the hot exhaust fumes emitted by taxiing jets.
In Britain, a weather station at Manchester airport was built when the surrounding land was mainly fields but is now surrounded by heat-generating buildings.
Terry Mills, professor of applied statistics and econometrics at Loughborough University, looked at the same data as the IPCC. He found that the warming trend it reported over the past 30 years or so was just as likely to be due to random fluctuations as to the impacts of greenhouse gases. Mills’s findings are to be published in Climatic Change, an environmental journal.
“The earth has gone through warming spells like these at least twice before in the last 1,000 years,” he said.
Kevin Trenberth, a lead author of the chapter of the IPCC report that deals with the observed temperature changes, said he accepted there were problems with the global thermometer record but these had been accounted for in the final report.
“It’s not just temperature rises that tell us the world is warming,” he said. “We also have physical changes like the fact that sea levels have risen around five inches since 1972, the Arctic icecap has declined by 40% and snow cover in the northern hemisphere has declined.”
The European Centre for Medium-Range Weather Forecasts has recently issued a new set of global temperature readings covering the past 30 years, with thermometer readings augmented by satellite data.
Dr Vicky Pope, head of climate change advice at the Met Office, said: “This new set of data confirms the trend towards rising global temperatures and suggest that, if anything, the world is warming even more quickly than we had thought.”