I just found this draft paper. I hadn’t heard of Professor Bill Johns before, but his credentials seem respectable, and the paper is fascinating. It summarises the evidence and science behind each of the arguments that carbon dioxide is the main cause of the current global warming. In each case, it finds that the evidence is weaker than appears at first sight. The main arguments are as follows.
1. The Vostok argument. This is named from the Vostok ice core drilled by the Russians through Antarctic ice, and a subsequent study finding a strong correlation between the concentration of carbon dioxide and temperature over a period of 450,000 years. The data is available here. Johns notes that the correlation is striking but does not prove that increasing carbon dioxide causes increasing temperatures (medieval logicians called this the ‘fallacy of false cause’ - fallacia secundum non-causam ut causam). It is possible that the correlation simply results from the fact that when the global temperatures are low, carbon dioxide dissolves in the oceans and its concentration in the atmosphere is correspondingly reduced.
2. The exceptional rise argument. The argument is that the current rate of temperature rise is higher than any rise experienced on Earth for at least 10,000 years. This suggests that industrialisation is responsible for climate change. He concludes, using statistical analysis of earlier changes in temperature over long periods, that there may be something unusual in the current rate of global warming. “However, the statistics give only weak support to the hypothesis that there is something climatically unusual”.
3. The current correlation argument. The argument is that the current correlation between atmospheric carbon dioxide concentration and global warming is significant. Johns concludes that, from statistics alone, there is no reason to believe that the correlation between global warming and increased carbon dioxide concentrations is other than coincidence. “It follows that this coincidence cannot form part of the science supporting the Carbon Hypothesis”.
4. The simple science argument. Much of the support for the Carbon Hypothesis (as our commenter Belette noted) is that mathematical models of climate change predict temperatures rises with catastrophic results (i.e. 3 degrees C or more). Johns argues that mathematical models were originally designed for chemical or physical processes that are well understood, but the physics and chemistry of climate processes is not so well understood. “Modellers do not have a set of similar planets to test their models on, or to evolve their modelling tools”. His comments about the interaction between water vapour (which is the most significant greenhouse gas), and carbon dioxide, are interesting. In the presence of excess water vapour the absorptivity of carbon dioxide is strongly suppressed, and at sea level the contribution of carbon dioxide to infra-red absorption is negligible compared to the natural variability of water vapour concentration in the atmosphere. Thus the science is not so simple.
5. The consensus argument. The IPCC has a number of reliable climate models from which it concludes that there is a 90% probability that carbon dioxide is causing global warming. How can all of those scientists be wrong? Johns notes that few of these climate scientists are “main-stream scientists”, and that most come from a weather-forecasting (meteorology) background. They are not used to formulating and testing models like other scientists because they cannot go back to the laboratory to test the various elements of the model; they have to wait for the climatic conditions to arise that test the models. We should therefore read ‘scientific consensus on global warming’ as ‘meteorologists consensus on global warming’.
Please note that I am simply summarising his arguments. Note also that this is a logic blog. Arguments like ‘X doesn’t understand climate science’ or ‘Y is a global warming denier’ are logical fallacies. From a logical point of view, there are essentially only two replies to an argument of the form “p and p implies q, therefore q”. The first is that p does not imply q. The second is that p is not true.