Listening to ABC television’s The Insiders yesterday and hearing Andrew Bolt say for the umpteenth time that there had been no increase in global temperatures since 1998 I finally went internet surfing to try and find out what he was talking about. I found the following graph helpful in understanding what he is talking about but also why it is that the overwhelming majority of the scientific experts ignore what he sees as evidence that global warming has stopped or is taking a break.
The authors of this study explain that the red line is the annual global-mean GISTEMP temperature record (though any other data set would do just as well), while the blue lines are eight year trend lines — one for each eight year period of data in the graph. What it shows is exactly what anyone should expect: the trends over such short periods are variable; sometimes small, sometimes large, sometimes negative — depending on which year you start with.
The mean of all the eight year trends is close to the long term trend (0.19ºC/decade), but the standard deviation is almost as large (0.17ºC/decade), implying that a trend would have to be either >0.5ºC/decade or much more negative (< -0.2ºC/decade) for it to obviously fall outside the distribution. Thus comparing short trends has very little power to distinguish between alternate expectations
That study was done in January 2008 but it is not only Andrew Bolt who remained unconvinced by it. The Real Climate website updated the argument last week because “the blogosphere (and not only that) has been full of the ‘global warming is taking a break’ meme lately”.
I wonder what the chances are of Andrew Bolt stopping his “cherry picking”? I can’t wait until his next Insider’s appearance.