A letter appeared in The Canberra Times print edition of Saturday 1 August 2009:
Mark Diesendorf begins his article, “It’s power to the people on climate change action” (July 29, p11) with the cliche that all alarmists love, “Global climate change is accelerating”. I assume he means warming. That is simply not true. But as a scientist I cannot accuse him of lying because nothing in science is ever “settled”. But I am entitled to ask him on what evidence he bases his claim, and on behalf of CT readers I do so herewith.
Aert Driessen, McKellar
It refers to an earlier article by Mark Diesendorf (which is not online, due to The Canberra Times‘ ludicrously poor approach to online material: most opinion pieces and a lot of the print content is never made available online).
My response, published on 4 August:
Aert Driessen (Letters, August 1) asks for evidence that global warming is accelerating. Firstly, what does the word “climate” mean? It means the long-term average of the weather, generally defined to be over periods of 30 years or more. Temperatures have a lot of short term variability: they bounce up and down from year to year. The important aspect is the long term trend.
Taking the most commonly cited data for global temperature, from the Hadley Meteorological Centre in the UK, the picture is clear. Annual averages, dominated by short term variability (the “weather”), bounce up and down all the time, but 30-year averages, showing the underlying trend (the “climate”), have been rising since early in the 20th century, and especially since about 1975.
Specifically, the 30-year average shows a warming rate of 0.88 degrees per century in 1989, a rate of 1.10 degrees per century in 1999, and a warming rate of 1.52 degrees per century in 2009. Looks like acceleration to me.
Matt Andrews, Aranda
A note on the details of the above figures: I drew the annual global average temperature data from HadCRUT3 (combined land and marine surface temperatures), then calculated the 30-year averages (what I describe as the “climate” as opposed to the “weather” annual averages). The warming rates I cited are the average for the preceding 10 years in each case. So “0.88 degrees per century in 1989” means that the average annual rate of increase in “climate” temperatures over the ten years to 1989 was at a rate of 0.88 degrees per century. I should probably have made it clearer by rephrasing it like this:
The 30-year average shows a warming rate of 0.88 degrees per century during the 1980s, a rate of 1.10 degrees per century in the 90s, and during this decade we’ve seen a warming rate of 1.52 degrees per century.