This article is part of Nature's Climate Change 2007 special.

Reports by the Intergovernmental Panel on Climate Change (IPCC) are held out as a model of consensus science, with thousands of international scientists coming together to present the most detailed look ever at a single scientific topic. Yet a consensus among most of the world's researchers does not mean that everybody agrees.

Climate Change 2007

“I am one of the 2,000 with their names on [the assessment], but don't sign me up for that catastrophic view of climate change,” says John Christy, a climatologist at the University of Alabama in Huntsville and a contributing author to the report.

And outside the IPCC process there remains a dwindling band of climate sceptics, those who argue that global warming is not linked to human activity and that it would be rash to take drastic action to cut carbon emissions. The focus of these arguments, however, has shifted noticeably since the previous IPCC report was published six years ago. Many of the scientific uncertainties the sceptics have seized on are no more.

Their argument continues to shift. That makes it clear that the issue is not the science.

“Their argument continues to shift,” says Naomi Oreskes, a geologist and science historian at the University of California, San Diego. “That makes it clear that the issue for them is not the science. Whatever the science is, they will try to find ways to question it.”

The previous IPCC report, for instance, gained infamy for featuring in its summary for policymakers the 'hockey stick' palaeotemperature graph. This shows a sharp rise in temperatures at the end of the last millennium that forms the blade of the hockey stick. Sceptics, notably economist Ross McKitrick of the University of Guelph in Canada and minerals consultant Steven McIntyre, have spent years working to discredit the statistical analysis and temperature proxies that were used to create the graph.

But a host of other studies, including a 2006 review by the US National Academy of Sciences, has reaffirmed that the past decade has seen an unprecedented rise in global temperatures. “This is just one of many lines of evidence,” Michael Mann of Pennsylvania State University in University Park told an audience of congressional aides in Washington DC last week. Mann is the originator of one version of the graph.

Another key argument for sceptics has been the apparent discrepancy between warming at Earth's surface and temperatures in Earth's lower atmosphere, which satellite records suggested have cooled over the past several decades. But in 2005, a major US report, commissioned specifically to look into this question, concluded that the original data had been analysed incorrectly, and when corrected do indeed show a slight warming.

The Sun has set on most scientific objections to the evidence for global warming. Credit: S. T. SMITH/CORBIS

Other favourite arguments of climate sceptics are also dismantled in the latest IPCC report. Urban heat islands — the fact that cities tend to heat the air above them — do exist, the report says, but have a negligible effect on global temperatures. And solar variability — natural fluctuations in the amount of the Sun's radiation reaching Earth — does affect climate, but to a far smaller extent than the burning of fossil fuels.

With less to argue about on the scientific front, climate sceptics have been turning their attention to the economics of adapting to a changing climate.

Christy believes that fostering innovation is the way to decrease reliance on fossil fuels. “We're going to look back in a century and say 'wasn't it quaint, we burned carbon',” he says. “I'm very optimistic; I see the wealth of the Earth continuing to rise. But suppressing energy is not the way. Keep energy inexpensive and affordable and allow people to do research.” As for the Kyoto Protocol on climate change, the international agreement to reduce carbon emissions, Christy calls it “sinister”.

Economic arguments also play a strong role in the views of sceptic Patrick Michaels, an environmental scientist at the University of Virginia who argues that taking action on climate change can have dire economic consequences. He sees the current US move towards embracing biofuel as causing corn prices to surge, triggering inflation and leaving many poor people, particularly in Mexico, struggling to buy food. “Small changes in policy can lead to a recession,” he says.

It remains to be seen whether these arguments will gain much traction. Alan Thorpe of Britain's Natural Environment Research Council, which hosted an online debate to canvass climate sceptics on their views, says that such views range from lazy to devious.

“I think there is a degree to which there is mischievous use of scepticism,” he says. “Sceptics want to accuse scientific society of wanting a particular policy outcome, but actual policy is up to governments.”