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.

New research casts doubt on such claims, however. 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 urbanization, 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, 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 industrialization and data quality problems. These add up to a large warming bias," he said.

Such warnings are supported by a study of U.S. 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His study, which has not been peer reviewed, is illustrated with photographs of weather stations in locations where their readings are distorted by heat-generating equipment.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: a weather station next to a waste incinerator.

For more on this story, see The Sunday Times.