A reader asked this question, and I thought it was fair and that I should answer it. I’ve been asked it before but until now only at question time after a talk. Several years ago I gave such a talk at the Planning Institute of Australia, and in it I set out the basic AGW proposition like this:
Human activity in burning coal and oil, and clearing forests has, over the past century, put an enormous amount of carbon dioxide into the atmosphere where it has combined with water vapour and other gases like methane to increase global temperatures in an unprecedented way. The evidence that this has occurred is clear-cut, and the increase in temperature will have, according to our computer models, dire effects on the planet, causing the melting of polar ice, the raising of sea levels, droughts, floods, storms and desertification. We must put an end to this prospect by changing our way of life lest catastrophe strike us. It may already be too late.
The rest of my talk, which you can read here, was an examination of each aspect of the AGW proposition, using the evidence that was currently available, including that in the IPCC’s AR4 of 2007. I have said recently somewhere that the evidence that has come available since 2007 makes me even less prepared to accept the AGW scare, but that doesn’t answer the question in the title. So here goes.
1. That the earth has been warming over the last century. I think that this is very likely to be the case. Temperature data are rubbery, and the uncertainty increases rapidly the further back we go. But I think that I would agree that the earth has warmed, in fits and starts.
2. That the warming is unprecedented. Compared to when? At present I think the evidence points to earlier warming periods in human history, conventionally called the Mediaeval, the Roman and the Minoan Warm Periods. I would be more interested in the AGW scare if there were very strong evidence that the present period is significantly warmer than these eras.
3. That the warming is strongly amplified by water vapour and clouds. This is a necessary element of the scare, because radiative physics says that doubling the carbon dioxide proportion in the atmosphere will, other things being equal, lead to an increase in temperature of about 1.1 degrees Celsius, and that the increase is logarithmic. If we go back to the start of the Industrial Revolution, when the CO2 proportion has been estimated at 280 ppm, then a doubling would be to 560 ppm, and most of the increase will have already occurred. In short, there is a while to go before we get there. The orthodox use a notion called ‘climate sensitivity’ to argue that water vapour and clouds will amplify the ordinary increase by many times, which could lead to an increase in temperature of perhaps 6 degrees C. There is no observational evidence, or laboratory experimentation, that supports any particular amplification, and estimates of the amplification range from less than 1 through neutral to large numbers. I would take the AGW scare much more seriously if it could actually be shown that the amplification was of the order of, say, three times.
4. That human activity has been responsible, or largely responsible, for the warming we have been having. This is probably the key element in the AGW scare, and the orthodox have been trying for more than twenty years to nail it down. In the beginning it was sufficient to point to the correlation between rising CO2 levels and rising temperature. But after 1998, and the failure of temperature to keep rising, the orthodox had to find reasons for the failure of air temperature to rise. The current orthodox explanation is that the warming has disappeared into the deep ocean. That has problems too. I would take the AGW scare more seriously if the human ‘signal’ were really distinguishable in the climate noise, and if it were significant.
5. That the evidence is clear-cut (= the science is settled). From what I have written above, it is clear, at least to me, that ‘the science’ of climate is simply equivocal about the AGW scare. Almost anyone can pick up a bundle of papers that support his or her position. I have done my best to look at both (all) sides of the issue, and have emerged as a ‘lukewarmer’ — that is, the earth is warming and will continue to warm, but slowly and without obvious threats either to humanity or the environment. I would become much less agnostic if there were more and more scientific papers that dealt with the issues I have raised above, and produced conclusions that showed the warming was unprecedented, caused by human activity, and dangerous — and these papers survived critical scrutiny.
6. That the computer models used correctly foretell future climates. To the best of my knowledge, none of the GCMs that have been used by climate modellers has been both verified and validates, none has correctly predicted the passage of temperature change over the past twenty years, and virtually all their projections for the future seem way too high. In short, the models are running too hot, and one reason for that could be that they greatly over-rate the importance of carbon dioxide. I would begin to take serious notice of model outputs when it is agreed that one (or more) of the models have been properly validated and verified, and that they have correctly predicted what has happened. If it is in fact the case that climate is chaotic, and does not operate in a linear fashion, getting any model to that state would seem to me extraordinarily difficult.
7. That we need to change our way of life to deal with this threat. I would heed this injunction if I ceased to be agnostic.