On Sunday a colleague and I discussed the general issue of correlation versus causation in science. He suggested that 1. There must be a body of work establishing a causal link between anthropogenic carbon dioxide and warming, and also 2. Some work that quantifies the extent of the warming from the anthropogenic carbon dioxide. He assumed as much because our government, the Australian government, is planning major perturbation to our economic system on the basis that carbon dioxide from the burning of fossil fuels is contributing to dangerous global warming. He assumed that the Australian government would not undertake such an action lightly, indeed that such an action would be premised on good evidence establishing a proven causal link between anthropogenic carbon dioxide and global warming. I replied that I was not so sure.
He said he would do a search of the scientific literature on Monday. I said it would be interesting to compare what he turned up from this systematic search of library databases with a few random requests on the internet.
Perhaps not surprisingly one of the first to claim that a body of work existed was John Quiggin; a well known climate alarmist. Professor Quiggin went as far as to claim that there are “hundreds of papers on both the causal link and the question of sensitivity” but could only cite a few papers which dabble with the issue of sensitivity later in that blog thread.
By early Monday evening (when I started writing this blog post) the thread at Professor Quiggin’s blog had thrown up only three papers that I thought could potentially provide a causal link and a quantification of the extent of warming. Interestingly one of them was published as long ago as 1938. I had listed the papers earlier in the day at the threads at both Professor Quiggin’s and my blogs to see if other papers were put forward in preference to these, but they weren’t. The papers are:
1. Callendar, G.S., 1938. The artificial production of carbon dioxide and its influence on temperature. Q. J. R. Meteorol. Soc., Vol 64, 223–237.
2. Hofmann, D.J., J. H. Butler, E. J . Dlugokencky, J . W. Elkins, K. Masarie, S. A. Montzka and P. Tans, 2006. The role of carbon dioxide in climate forcing from 1979 to 2004: Introduction of the Annual Greenhouse Gas Index, Tellus B, Vol 58, 614-619.
3. Crowley, T. 2000. Causes of Climate Change Over the Past 1000 Years. Science Vol 289: 270-277.
The Hofmann et al. paper initially looked the most promising to me, is available on the internet (click here) and is the first that I shall consider. (I plan to post comment on the Crowley or Callendar paper tomorrow.)
While the title of the Hofmann et al paper does suggest a “role” is established for carbon dioxide as a forcing/warming agent, in fact it is just assumed in the body of the paper.
The authors simply taken a set of gases (including carbon dioxide) and ascribed an effect: climate forcing. They do not demonstrate a mechanism, or even shown a correlation, between levels of anthropogenic carbon dioxide and global warming.
So, I shall have to conclude that this paper fails the criteria (1. There must be a body of work establishing a causal link between anthropogenic carbon dioxide and warming, and also 2. Some work that quantifies the extent of the warming from the anthropogenic carbon dioxide).
Reading the paper I was struck by the extent to which the authors assume that the increase in atmospheric levels of carbon dioxide since 1750 is primarily a result of “anthropogenic emissions” without explaining why.
There is an extensive scientific literature discussing the notion of causation and how this might be established, for example between environmental stressors and observed effects in natural systems. This literature emphasises that where there are potential alternative explanations for an observed correlation these should be considered.
In the case of carbon dioxide and global warming, Lance Endersbee (former Dean of Engineering and Pro-Vice Chancellor of Monash University) has suggested a direct physical relation between increasingly levels of atmospheric carbon dioxide and increasingly average global sea surface temperatures over the last few decades (click here) suggesting the carbon dioxide is being released from the warming oceans.
Thanks to everyone who has posted and/or emailed papers and comment following my three random requests on the internet last night.
I intend this to be just the second in the series. I shall endeavour to consider the submissions beginning with those that appear the most promising and as a series of blog posts. Of course guest posts, and with alternative opinions, are welcome.