Has global warming gone on vacation ? Headlines to that effect have appeared on the basis of one very legible graph of one eight year statistical trend. But does it depict Weather or Climate ?
Both sides in the Climate Wars devote a lot of time and vitriol to accusing each other of Cherry Picking- the statistical crime of choosing particular data intervals in order to amplify or play down whatever point they are trying to make. One sovereign remedy for this sort of statistical Gamesmanship is to bloody well lay out all of the choice intervals the players choose on one unforgiving graph, to let disinterested eyeballs do the walking through recent climatic history and arrive at whatever judgment they may.
This is the core of a current disputation between Gavin Shmidt and
Roger Pielke Jr, Schmidt havibf fielded the graph below- Pielke
delivers a running commentary in his replies on the link at the end of
the caption by Schmidt:
"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 8-year trend lines - one for each 8-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 8 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."
So, it should be clear that short term comparisons are misguided, but the reasons why, and what should be done instead, are worth exploring.