Did global warming stop in 1998? No!
I just noticed a friend's status update questioning whether global warming has stopped since 1998. This shocked me a little, since it is a patently ridiculous claim, but apparently one being made by the group of nutjobs and the willingly ignorant who style themselves "global warming skeptics". I think I prefer the term "reality denialists". I've just done a quick search on Google News, amd this claim is everywhere. Fortunately most of these claims turn out to be in the Letters to the Editor section of newspapers, but it has managed to wiggle its way into articles too. A few years ago, in 2006, the Telegraph ran a story with the headline "There IS a problem with global warming... it stopped in 1998". I guess this isn't surprising, since it is after all the Telegraph, but even so I am shocked that such a claim is propogating.
So, I thought I should at least look at the data to see where the claim was coming from. This chart taken from Wikipedia seems to explain it all:

In 1998 there was an enormous spike in the average global temperature far exceeding the temperature in either 1997 or 1999-2001. As you can see, it is an outlying data point about as far from the 5-year average as any point on the graph. The problem with taking yearly temperatures is that the data is very noisy and the temperatures jump about quite a lot from year to year, obscuring the trend at least over short intervals. The running 5-year average makes it clear that the average temperature is increasing sharply. So where does this claim about no global warming since 1998 come from? Well, if you look at the temperature in 1998 and compare it to the average temperature from 1999 onwards then they are about equal. But this is complete nonsense since we've cherry picked the outlying 1998 data point as our starting position. If we start on the 5 year average, then we see a dramatic increase. In fact we could play the same game in reverse and say that the average global temperature has rocketed up since 1999 at almost double the actual rate. The claim that the average global temperature has not increased since 1998 is a flagrant abuse of statistics. It works simply by picking the hottest year on record as the starting point and then only considering a short time period so that the regression to the mean increase more or less cancels the increase in the average temperature. It's a trick that preys on an ignorance of statistics. In five years the trick won't work, because the increase in the 5-year average will have brought us far above the 1998 data point. So the claim is bullshit, pure and simple.
What is particularly incidious about this clasim is that it has almost certainly been composed deliberately to trick people. It often comes accompanied by an assertion that climate models fail to predict this halt to global warming, which is of course true. It's true that none of the main climate models predict a halt or pause in global warming for the last decade, but that's probably just as well, since there hasn't been one. Should a climate model predict how people will abuse statistics? For me, predicting the climate change is enough and we can leave the precrime detection for the movies.




