"I am not a climate sceptic," said Senator Nick Xenophon in a recent ABC interview, and went on to explain why. He said he found the case for human-induced global warming generally convincing, though far from certain, and believed governments should take action to reduce greenhouse emissions because of the greater risk of doing nothing.
On most everyday understandings of the term ''scepticism'', the senator was in fact displaying a sceptical attitude towards the issue: he denied that the evidence about global warming was certain and was prepared to entertain doubts about the degree of probability for global warming. His refusal to be labelled a ''climate sceptic'', however, shows how the term has become hijacked in public debate.
''Climate scepticism'' now stands for a policy stance, opposition to the case for emission reduction. It has become detached from its normal sense of reasonable doubt about the science. The confusion is important and reflects a dangerous misunderstanding of how far policy can be based on robust evidence.
In principle, all scientific theories are open to falsification by new evidence and therefore no science can ever be entirely certain. In practice, however, many areas of science are sufficiently well grounded in reliable evidence to be accepted beyond reasonable doubt. But climate science is not among them.
Everyone knows the limitations of short-term weather forecasting. Climate scientists confirm that the large number of independent factors influencing climatic events rules out precise explanation or prediction. With climate change, uncertainty is compounded by the lack of reliable historical data from before the modern period. This does not mean that nothing can be known about climate change or that no predictions are worth making. But it does mean nothing can be known for certain or even with the degree of certainty that can apply in aspects of other sciences, such as physics or chemistry.
Uncertainty pervades the entire field of climate change. Scepticism should therefore be the natural attitude of any intelligent student of the topic.
Proponents of emission reduction policy do their case a disservice by disowning scepticism and reserving ''climate sceptic'' as a term for those who reject their policy. To cast the debate as one between believers and sceptics implies that some sort of faith or belief is needed in order to accept climate change policy. It rules out the more reasoned, sceptical approach that recognises doubts about the evidence for global warming yet decides, on balance, that the risks of inaction are higher than those of inaction.
The faith-versus-scepticism dichotomy also hands an easy propaganda victory to the opponents of climate change policy. Any doubts about the science can be claimed as automatically strengthening the case for inaction. Conversely, supporters of climate change policy are forced into dismissing and disparaging any sceptical voices. But, once the debate is seen to be between various levels of climate scepticism and risk assessment, any new challenge on a point of evidence is simply one more element in the assessment, not a knock-down refutation.
Many proponents of climate change policy are obviously uneasy about admitting the level of doubt that surrounds the science. The recent conclusions of the International Panel on Climate Change, that evidence of climate change is unequivocal and that greenhouse gases are ''very likely'' (90per cent or more probability) to be the cause of such warming, surely overstate the case. The experts clearly fear that no action will be taken unless public opinion believes in the certainty of human-induced global warming.
But hoping for certainty sets the bar for action too high. It also reflects a misunderstanding of the role of knowledge in policy-making. Good policy needs to be informed, where possible, by robust, relevant evidence. But policymakers often have to act without knowing what is happening or what will work. In the current financial crisis, for example, governments find themselves in uncharted waters but cannot afford to delay decisions. No one is requiring certainty before acting or equating uncertainty with inaction.
The same should apply to climate change and environmental policy generally. To look for certainty or near-certainty leads experts into professional dishonesty, forcing them to hide their doubts and the limitations of their evidence.
It also encourages ideological thinking, where public debate becomes polarised between opposing camps unable to admit any contrary evidence that might unsettle their convictions and weaken their advocacy.
Climate change policy, like most major policy, is not a matter of conviction or cast-iron proof but of assessing risks in the context of uncertainty.
Richard Mulgan is a former professor in the Crawford School of Economics and Government at the ANU.