"The fact that there has been no recorded global warming trend for the past 15 years or so, during which CO2 emissions have grown very rapidly indeed, thanks in large part to remarkable economic growth in China, suggests that the climate sensitivity of carbon is in fact less than is incorporated in the models on which, for example, the IPCC relies."
The opening phrase of that quote surprised me. I was under the impression that global warming had continued, albeit with a brief downward trend in the mid noughties. Figure 1 is a graph of the raw figures, from the HADCRUT3 dataset, for the past 15 years with the trend line shown in orange. As you can see, the trend line has only a very shallow slope, probably insignificantly different from zero.
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| Figure 1 - Source: HADCRUT3 dataset |
Does this mean that global warming has ground to a halt? I don't think so, and to be fair to Lawson, he doesn't claim that. He merely says that this indicates that temperature is at the low end of estimates for climate sensitivity to carbon and he then accepts that this is too short a period over which to draw firm conclusions. That doesn't stop a graph of recent global temperatures appearing in the masthead of his Global Warming Policy Foundation's website, without any caveats about its significance, but with an orange line helpfully inserted to emphasise the appearance of flatlining.
Now with only 15 data points, my calculation of the trend is bound to be subject to wide margins of error. Also, if my understanding of climate science is correct on this, the data is so "noisy" that valid comparisons can only be made between periods of at least a decade. Hence, for graphing purposes, I prefer to look at a ten-year moving average over several decades. Figure 2 shows the ten-year moving average since 1982 (and so incorporates data going back to 1973).
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| Figure 2 - Source: HADCRUT3 dataset |
As is obvious, there is a strong upward trend but we are currently experiencing a dip - the fourth such dip over this period. The data also tell us that the decade to 2011 was warmer by 0.175 degC than the previous decade. The decade to 2001 was warmer by only 0.144 degC than the one before, so the 10-year average suggests a longer term trend of accelerating global warming. This is what we would expect, given the rising level of CO2 in the atmosphere.
On the basis of that comparison, the rate of increase in warming is 21.7% per decade. That rate of increase, if continued to the end of this century, would imply 5 degC of warming - two and a half times the 2 degC warming generally deemed by politicians to be the "safe" limit. But don't take too much notice of my quick calculation, which grossly oversimplifies the issues and compares an arbitrary set of three decades. It has no more significance than the flatlining trend over the past 15 years. However, it suggests to me that the recent temperature record gives little support for the sort of complacency on global warming that Lawson seems to want to encourage.
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The HADCRUT3 dataset can be downloaded from here:-
http://www.metoffice.gov.uk/hadobs/hadcrut3/data/download.html
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