Sunday, May 6, 2012

3851.txt

date: Thu, 23 Oct 2008 18:09:59 +0200
from: Stefan Rahmstorf <rahmstorfatXYZxyzan-klima.de>
subject: Re: global temperatures
to: P.JonesatXYZxyz.ac.uk

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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 �C 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 Ni�o event in the Pacific
ocean. If you take the trend 1998-2007 in our data, it is + 0.09 �C per
decade. But if you take the trend 1999-2007, it is + 0.21 �C per decade.
For comparison: the long-term global warming trend for the past 50 years
is 0.13 �C 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 �C 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 �C per decade. The most recent 20-year
trend, 1988-2007, is 0.20 �C 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 �C
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

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