Sunday, 12 August 2007

1934 - the hottest year on record

... So what?

There has been a lot of excitement in blogland (1) (a strange place where normal rules do not apply and anything is true if said with enough conviction) about a slight anomaly in temprature data. Basically, after correcting data, it has been found that 1998 was the second warmest year of the twentieth century, not the warmest. That honour now passes to 1934, which had previously languished at number two.

This has lead to some strange conclusions. It would appear the whole theory of anthropogenic global warming has collapsed in light of this change. It hasn't. We always knew that the early twentieth century, up to the end of the 1930s, was warm. Then the Earth cooled off for a while, before heating up again from the 1980s onwards. In the 90s, the global temprature reached levels similar to those of the 1930s. This time, however, there is no indication of the temprature falling away.

The problem is that the re-ordering of hottest years doesn't prove anything beyond the fact that 1934 was marginally - and we're talking marginally - hotter than 1998, contrary to what was previously thought. It is not as if no-one had known that the 30s were a warm decade. The revision of order says nothing about the causes of warming in the 1930s and the 1990s. It is generally accepted that in the 1930s the largest contributor to warming was solar activity. In the 1990s, however, it is thought that human activity is the major conributor. (2)

The nice chaps at Real Climate put the changes into this into context:
The net effect of the change was to reduce mean US anomalies by about 0.15ºC for the years 2000-2006. There were some very minor knock on effects in earlier years due to the GISTEMP adjustments for rural vs. urban trends. In the global or hemispheric mean, the differences were imperceptible (since the US is only a small fraction of the global area).

There were however some very minor re-arrangements in the various rankings (see data). Specifically, where 1998 (1.24 ºC anomaly compared to 1951-1980) had previously just beaten out 1934 (1.23 ºC) for the top US year, it now just misses: 1934 1.25ºC vs. 1998 1.23ºC. None of these differences are statistically significant. (3)
Whether one year was hotter than another is really irrelevant. They were both hot. The crucial issue is what caused the heat, and what the remprature is going to do next. Based on what the Hadley Institute forecast, it is going to get much hotter over the next few years - so quibbling about what year was hottest is going to be rendered irrelevant. they won't be as hot as what is to come. (4)
1 - Like here: Viewed on 12th of August, 2007. Note the wise comment by a level-headed, reasonable sounding poster called lurgee.
2 - As per wikipedia article, Viewed on 12th of August, 2007. Note the quote from Stott et al.
3 - "1934 and all that," on, posted on 10th of August, 2007. (
4 - As discussed just the other day, in this post:

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