Professor Don Aitkin’s sceptical view of global warming, presented (PDF 258KB) to the Planning Institute of Australia on April 2, 2008, has been widely publicised. He claims that the current level of warming is not historically unprecedented, that the link between warming and greenhouse gas emissions is weak, and that we should not do anything to restrict emissions until it is “absolutely plain” that there is no alternative. He says the global warming issue is a distraction from the water and peak oil crises facing Australia.
Having failed to understand why climate scientists are advocating urgent action (“stridently”, he claims), Professor Aitkin proceeds to play the people instead of the ball by questioning their motives, claiming, among other things, that having set up the IPCC the scientists have to keep justifying it by claiming there is warming.
Professor Aitkin poses several questions, the three most immediate being whether the Earth is warming, whether its present warming is unprecedented, and whether our burning of fossil fuels is causing the present warming. However there are other equally important questions lurking behind this debate, such as whether it would really be so hard for us to change our lifestyle, whether such changes would have other benefits, and whether we must change anyway, for reasons other than global warming.
Professor Aitkin makes much of the uncertainty of climate observations, and claims scientists don’t discuss them, particularly singling out the IPCC. In this he misunderstands or misconstrues the role of the IPCC reports: they are entitled “Assessment” reports. They are not just the science, they are an assessment of the science, using a process I will explain more later. Evidently he denies IPCC the role (I would say the responsibility) of assessing the state of the evidence. Nor does he note its consensus procedure and the political vetting of its final texts, both of which tend strongly to make its assessments conservative.
Professor Aitkin presents himself, on the other hand, as a paragon of disinterested enquiry. However, he inevitably makes his own selections and judgments of the evidence. For example, he queries the claim that the present warming is unprecedented and cites one study arguing the Medieval Warm Period in Europe was warmer than at present. However, a number of temperature proxies show the temperature to have been 0.0-0.3C cooler than the mid-20th century (and thus 0.6-0.9C cooler than now - you can see the graph on Wikipedia). Also, according to the US National Oceanic and Atmospheric Administration there is no clear evidence that the warm period extended outside Europe.
More egregiously, Professor Aitkin makes no significant mention of dramatic changes in the Arctic, or of pervasive and rapid retreats of mountain glaciers. These are noted only as “the evident melting of sea ice and the retreat of some glaciers”. In this statement he misrepresents the observations of mountain glaciers, which are indisputable, and glosses over perhaps the most dramatic and disturbing symptom of warming.
Professor Aitkin claims there is no reasonable evidence that greenhouse gas emissions are the cause of the present warming (though he acknowledges well-established basic physics). One of his arguments is that there is “no dramatically linear relationship” between the two during the 20th century. But of course there are natural short-term fluctuations in temperature (which sceptics like to emphasise) but not in the rise of carbon dioxide, so we don’t expect any simple linear relationship, particularly in the short term. This is a superficial and uncompelling argument.
Another argument, frequently raised by sceptics, is that during the ice ages temperature rises preceded carbon dioxide rises. Professor Aitkin leaves the implication dangling that we don’t understand why, and that this also shows that carbon dioxide levels do not determine temperature.
In fact this topic is well (if not widely) understood. The ice age fluctuations were triggered by fluctuations in the amount of heat received from the sun (due to slow gyrations of Earth in its orbit around the sun), but the temperature fluctuations would have been minor were they not strongly amplified by the carbon dioxide released as temperature rose. The amplification also explains the strongly asymmetric temperature fluctuations (slow cooling, rapid warming). The conclusion is that carbon dioxide was the dominant factor, though not the trigger. The ice age observations thus strongly support the hypothesis that a rise in carbon dioxide levels causes a rise in temperature.
Our present situation is different from the ice age situations. Solar heating has not changed significantly, but carbon dioxide levels have gone up a lot - higher than they’ve been for at least three million years. Our understanding of the ice age fluctuations gives us strong reason to expect that temperatures will increase, with a time lag of a few decades. The fact that the mean temperature has increased by at least 0.6C over the past few decades (a conclusion Professor Aitkin does not dispute) is then reason for serious concern.
There is additional, longer-term geological evidence in which carbon dioxide levels correlate well with surface temperature. The implied temperature variation is, if anything, greater than computer climate models suggest. Thus, contrary to Professor Aitkin’s claim, there is independent and strong evidence that greenhouse gases cause warming. Climate models, for all their imperfections, confirm this, which is one reason to take them seriously, if cautiously.
Professor Aitkin asserts that “there is simply no evidence” that polar ice will melt and that sea levels will rise. The only citation he makes is to a claim that recent evidence may show a slight cooling of the oceans during the past five years. Even if substantiated, that would be no basis for his grand assertion.
This is an edited version of a longer article, the full version of which is available here (PDF 44KB).