date: Thu, 23 Oct 2008 18:09:59 +0200 from: Stefan Rahmstorf subject: Re: global temperatures to: P.Jones@uea.ac.uk Dear Phil, enviable, to be in Iceland. How are they dealing with their bankruptcy? I take it that you basically agree with the interview text. I have adapted it below, taking some of what you wrote into account. I have cut the bit about the differences between Hadley and GISS, I think that is getting too complex. The change you mentioned that happened in 1990 could not explain why the 1998-2007 trend is so different between GISS and Hadley. My trend calculations you can trust, I computed them myself from http://hadobs.metoffice.com/hadcrut3/diagnostics/global/nh+sh/annual In fact, I just double-checked and recalculated them. Looks like the 2007 value was updated since I last used it, it is 0.01 ēC lower than before. This is why the 9-year trend has changed from 0.22 to 0.21. The 10-year trend is unaffected. Thus the little interview now reads: KlimaLounge: The German journalist Ulli Kulke has repeatedly claimed that global warming has stopped and global temperatures "show no trend in this decade". You are one of the leading experts on global climate data. Has the global warming trend stopped, or at least slowed down? Jones: No. What happens is that some people take the trend starting in 1998, which was an extremely warm year, well above the long-term trend line. This exceptional warmth was due to an El Niņo event in the Pacific ocean. If you take the trend 1998-2007 in our data, it is + 0.09 ēC per decade. But if you take the trend 1999-2007, it is + 0.21 ēC per decade. For comparison: the long-term global warming trend for the past 50 years is 0.13 ēC per decade. Thus, if you start in 1998 you get a below-average trend, if you start in 1999 you get an above-average trend. That simply is a result of natural year-to-year variability, which is always superimposed on the long-term warming trend. There is no indication for a change in the trend. You explained this very nicely in your Realclimate article earlier this year. KlimaLounge: But Kulke claims that his statements are based on you saying that there has been no statistically significant warming since 1998, and that therefore we can relax our mitigation efforts. Jones: That is a serious misinterpretation of what I said. Indeed the trend since 1998 is not statistically significant - simply because of the natural variability just mentioned, the time span of 10 years is too short to reliably determine a trend. The uncertainty on a 10-year trend is +/- 0.2 ēC per decade (2-sigma). Therefore, one certainly cannot claim that the warming trend has slowed down on this basis. This is nonsense and just confuses the signal of global warming with the superimposed noise of natural variability. Such claims are scientifically simply incorrect. KlimaLounge: Thanks for this clarification. Let's hope that Kulke will stop making such false claims in future. How long a period do you need to obtain a reliable trend? Jones: If you take 20-year trends the result is getting more robust, the uncertainty here is only +/- 0.07 ēC per decade. The most recent 20-year trend, 1988-2007, is 0.20 ēC per decade. With this more robust measure, the most recent trends are the highest. Compare this for example with the trend centered ten years earlier, 1978-1997: this is only 0.11 ēC per decade. Thus, on those time scales where one can make scientifically sensible statements, the global warming trend has accelerated and certainly not slowed down. If you can live with this, I'll make a German version. Enjoy the hot springs. Cheers, Stefan -- Stefan Rahmstorf www.ozean-klima.de www.realclimate.org