Temperature trends in Garnaut 2011

I remain unconvinced by Dr Garnaut’s arguments that global average temperature trends in the past decade should, if anything, add to a sense of alarm about man-made “greenhouse-gas” emissions.

There is much in the updated review that I have not had an opportunity yet to consider. (Update: There’s a more comprehensive critique of the updated review here.) But on one topic where I believed Dr Garnaut must have been kidding us in his 2008 report—the expert statistical review of the (now known to have been ‘doctored‘) temperature trend since 1860—he continues, in my view, to overstate the conclusions of the statistical advice he has been given.

Dr Garnaut wants us to be even more alarmed than we were in 2008. He says that the updated statisticians’ report he obtained for his latest review demonstrates that there remains a rising trend in temperatures over “recent years”:

The statistically significant warming trend has been confirmed by observations over recent years: global temperatures continue to rise around the midpoints of the range of the projections of the Intergovernmental Panel on Climate Change (IPCC) and the presence of a warming trend has been confirmed;

But in my reading of the paper from the two academic statisticians I cannot find any direct support for that assessment. What Breusch and Vahid say in their updated report—as in their original report—is more cautious and leads to a weaker conclusion.

The statisticians say three things; first, something no-one, whether warmist or sceptic, disputes:

“We conclude that there is suffcient statistical evidence in the temperature data of the past 130-160 years to conclude that global average temperatures have been on a warming trend”
They say, second, that the statistics themselves have no meaning without some hypothesis arising from an understanding of the underlying physical reality, and on this point they have no contribution to make:
…[W]e do not claim that we have uncovered the nature of the trend in the temperature data. There are many mechanisms that can generate trends and linear trends are only a first order approximation (see Granger 1988). It is impossible to uncover detailed trend patterns from such temperature records without corroborating data from other sources and close knowledge of the underlying climate system
Third, the statisticians agree that the overall 1860-2010 warming trend seems to continue up to 2010 if your perspective is that of a person in 1980 who looks forward over 30 years. That person would be uncertain about future trends but the current temperature would fall more than the 1-standard-deviation above the expected trend as seen from the 1980s. In statistical terms, “there is sufficient evidence in all three temperature series to reject the hypothesis of no drift in favour of a warming trend in global temperatures.”

But this last point, that Dr Garnaut makes much of, says nothing at all about the temperature statsis over the last decade. Almost all of the warming trend since 1980s took place prior to 2001. You don’t need a statistical model to see this. You only have to look at the temperature record.

ChristyLowerTropTempGraph tmb Here, for example, is a graph taken from this week’s U.S. Congressional testimony of John Christy, Distinguished Professor of Atmospheric Science, Alabama’s State Climatologist and Director of the Earth System Science Center at The University of Alabama in Huntsville. Dr Christy has been a Lead Author and Contributing Author to the UN IPCC Assessments.

The graph shows, from top-to-bottom, the “anomalous” excess of lower-troposphere (air) temperature over the 1980-1990 level (TLT); the sea-surface temperature anomalies (SLT); the air-temperature minus the influence of the sea-surface temperature anomalies; the influence of volcanic ash on the cooling of air temperatures (VOL); and, finally, at the bottom, the air temperature anomalies minus both sea-surface and volcanic influences. The horizontal ‘slices’ in the graph are each 1°C in height.

What you see in both the top and bottom lines, without the help of any statistical model, is that there has been no net warming since 1999 (although the amplitude of the variations is much smaller after confounding factors are removed and the 1998 El-Nino spike in TLT is smaller when the SST is subtracted ). This is hardly grounds for the increased alarm that Dr Garnaut urges.

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