The Garnaut Climate Review Interim Report—I’m not convinced

The Garnaut Review’s Interim Report accepts the IPCC’s evaluation of the climate risk (and the carbon-forcing theory) ‘on the balance of probabilities’. The Review team’s report says that they have neither the skills nor resources to re-visit the climate science. They acknowledge controversy anduncertainty in this science but they decide—not unreasonably—that they have little option but to work with the projections of the IPCC, which purport1 to summarize the recent views of a large number of experts.

Most of us will be tempted to do just what the Garnaut Review team has done and assume that, despite controversies and uncertainties, the IPCC scenarios are a safe bet to be an accurate assessment ‘on the balance of probabilities’. But this isn’t a reason to ignore the evidence for or against the IPCC story. For my part, I’m increasingly uneasy with the IPCC assessments. There are too many grounds for reasonable doubt about the central propositions of a worrying temperature rise and the carbon-forcing theory.

I’ll confine myself to the first ground for doubt—because it’s the easiest to illustrate—about the basic story that there is a worrying trend in global temperatures. Also, I’ll keep this very brief. I have two concerns:

  1. There’s no evidence from the reconstructions of the paleo-climate (that is, temperature estimates drawn from proxy sources such as ice-cores going back thousands of years) that recent global temperature variations are outside the range of historical, and evidently not man-made, climate variation. Here’s a picture of a reconstruction that you can read all about on WikiPedia (click). It shows that the proxies reveal recent temperature levels are close to, or possibly below, the average over the period since the last ice-age 12,000 years ago
  2. Here’s a second graphic that I constructed my self (in Excel) from data that’s readily available on the web. This is a set of estimates of global temperature anomalies : that is, deviations from average temperatures in the period 1961-1990. The estimates combining actual measured land, sea and air temperatures are from the UK Meteorological Research laboratory at Hadley. You can get the same data here
    Click for full size

What I notice in the second picture is:

  1. That the range of variations measured over this period of time—one and two-third centuries—is about 0.7 degrees celsius, maybe a touch more. Not very big at all and not much out of line with the secular variation in the first picture.
  2. That there has been a jump in the temperature in the late 1990s (1998 was an el Nino year) but that after that the temperature anomaly didn’t have any trend at all, up or down, for most of the past 7 years (inset: just an extract from the same data).
  3. There has been a surprising drop in January 2008—the coldest northern winter for many years—that is as big as the jump in the late 1990s.

Now none of my observations here is new or a scientific basis for identifying a trend in an obviously volatile series such as global temperature anomalies. But the story of climate change here does not look so worrying to me that I want to rush into any endorsement of theories of big impending threats or calls for drastic mitigation strategies. It just doesn’t make sense

Although the Garnaut Review takes the assertions of IPCC on face value, some key assertions it accepts don’t seem to fit with the facts in the data reproduced here. Let’s take the statement that the interim report quotes from a paper dated 2007 from a group of climate scientists associated with the IPCC.

Global mean surface temperature increase since 1990 has been measured at 0.33ºC, which is in the upper end of the range predicted by the IPCC in the Third Assessment Report in 2001 (page 21 of the Garnaut Review Interim Report, emphasis added)

In fact, the HadCRUT3 data in the second picture above apparently contradict this. If we take the global anomaly as measured in January 1990 and January 2007 we see that they are both 0.22 degrees celsius. In other words, precisely the same deviation from the average temperature in the period 1961-1990. There is no apparent increase over this period of time.

Of course, variation from one fixed point to another is not sound statistical practice, particularly given the range of measurement error in the anomalies (my plot shows only the Hadley Center’s best estimate). But it certainly weakens the case for extreme action; at least for anyone who remembers the awful panic over Y2K.

The role of citizens who are not climate scientists is to help the government decide what levels of risk mitigation are proportional to the problem. That is a political question as much as a scientific one. Considering the potential costs of the mitigation strategies being considered—the Garnaut Review Interim Report talks of cutting Australian per-captia emissions by 70% to 90%—you owe it to your self, and your children, to question whether these dramatic measures are proportionate to the risk as you see it.

1. The representative character of the IPCC-4 report has been challenged by an Australian researcher who examined the review records

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