cc: "Rayner, Nick" , Wallace@metoffice.gov.uk, John , Jones@metoffice.gov.uk, Phil , Kennedy@metoffice.gov.uk, John date: Wed, 14 Nov 2007 08:40:02 +0000 from: David Parker subject: Re: A discontinuity in surface temperature observations to: David Thompson Dave Thanks. I'm familiar with COWL from your published papers. The plots are very interesting and I hope John Kennedy, who is working on marine data biases, will be able to contribute in due course regarding the 1945 dip. Regards David On Wed, 2007-11-14 at 17:44 +1100, David Thompson wrote: > Dear David, Phil and John, > > > (This is a bit of a long email, so you might want to grab a cup of > coffee - or tea - before reading on...) > > > Thanks again for the quick and helpful responses last week. > > > Mike and I would be happy to include John as a coauthor on our paper. > And David, Phil: we understand if you are too busy to join another > project. But if you are interested in joining the paper, too, that > would be great. The goal of the paper is to clarify some key aspects > of 20th century temperature variability, and the study would certainly > benefit from your expertise. > > > Before I get too far ahead of myself, let me review the main points of > the paper as it currently stands. I've attached 3 pages of figures > (the figures will evolve as the writing evolves, but as of now it > appears the paper will end up being short and punchy) > > > Figure 1 includes 2 panels. The top time series in the top panel shows > the global-mean temperature time series. The next two time series show > the linear fit of ENSO and the COWL (cold-ocean/warm-land) time series > to the global-mean. The ENSO time series is found as a damped thermal > response to variations in the cold-tongue (this gives a marked > improvement in the representation of ENSO in global-mean > temperatures). The COWL time series represents the effects of random > fluctuations in climate acting on the different heat capacities of the > ocean and land (eg: periods of warm advection over land/cold advection > over the ocean lead to warmer than normal global-mean temperatures by > virtue of the fact that the continents have a lower heat capacity). > > > I'll provide more details of the ENSO and COWL methodologies in a > future email, but the main point is that a lot of the high frequency > 'noise' in global-mean temperatures can be accounted for on the basis > of two simple, physically based time series. > > > The bottom panel in Fig. 1 includes a reproduction of the global-mean > time series (top) and also shows the residual global mean time series, > which is found by removing the ENSO and COWL time series from the > global-mean time series. > > > The bottom panel of Fig. 1 is the key figure in the paper. We think > it's remarkable how well the fitting 'cleans up' the global-mean time > series. If you look closely, you'll see that the major volcanoes of > the past century (marked by solid vertical lines) are much, much > clearer in the residual time series. But the fitting not only isolates > the volcanoes, it also isolates the very large drop in Aug 1945. The > Aug 1945 drop is about 0.3 K, almost twice as large as the response to > Pinatubo. > > > The residual time series also suggests a slightly different view of > 20th century temperature variability. The canonical view is that > temperatures warmed in the 20s, settled from the 40s-70s, and warmed > from the 70s-the present. But if you stare at the residual time > series, you get the impression that global-mean temperatures have > actually risen steadily over the past century, but that the warming > has been disturbed by several discrete and abrupt drops in > temperature. > > > As for the volcanos: > In figure 2 we're exploiting the fitting procedure to provide a > 'cleaned up' version of the volcanic response in surface temperatures. > The figure shows the composite temperature response for the 4 largest > tropical volcanoes of the 20th century. The composite is done such > that the 10 year period before each volcano has a trend of zero and > mean of zero. (If you don't remove the trend then the long term > warming biases the composite). The response in the residual data is > surprisingly coherent (much more so than in raw data). But we think > what's most striking is that surface temperatures do not appear to > fully recover for up to a decade after each volcano (the composite is > limited to 9 years after the eruptions since the eruption of Pinatubo > occurred 9 years after the eruption of El Chichon). > > > You can see the long timescale of the volcanoes in the residual > global-mean time series: if you follow the temperature time series > before, say, Agung or Pinatubo, you can see that it takes a long, long > time for global-mean temperatures to catch up to where they would have > been, assuming they would have continued to warm... > > > And as for the dip in 1945: > Fig 2 shows the residual (ie with the COWL or ENSO time series > removed) global-mean land and ocean time series. The point here is > that the large dip in Aug 1945 does not show up in the land data. > > > My impression from your emails last week is that the dip is almost > certainly due to changes in instrumentation during the war. But it's > also my impression that the specific reasons for the dip are not yet > known. It's possible that the dip is offset by spurious rises in SSTs > at the start of the war. But this isn't certain. And even so, there is > a large drop in SSTs between the period before the war (1939) and the > period after the war (1945). To my eye, the residual ocean time series > in Fig. 3 suggests temperatures ratcheted downwards spuriously in > 1945. > > > In our view, the dip in Aug 1945 is very important and warrants being > highlighted in the literature. In fact, once you know it's there, it's > hard to view any time series of global-mean temperatures and not > wonder how different it would appear if the dip was not there. For > example, Fig. 4 shows the raw and residual global-mean temperature > time series assuming the 0.3 K drop in Aug 1945 is spurious. I realize > the figure is crude, and it might not make it into the paper. But the > point is that you would get a very, very different impression of 20th > century temperature trends if the dip proves to be an artifact of the > end of the war. > > > So our main point regarding the drop in Aug 1945 is this: if we assume > the dip is spurious, then the global-warming of the past century would > be at least ~0.3 K larger than currently thought, and global-mean > temperatures would have risen steadily throughout the past century. > > > That's enough for now. I'm working on the writing and our goal is to > submit something by Xmas. Please let me know what you think, and if > you are interested in continuing to interact with Mike and I on the > paper. > > > And again: thanks for your time and interest. > > > -Dave > > > > -------------------------------------------------------------------- > -------------------------------------------------------------------- > David W. J. Thompson > www.atmos.colostate.edu/~davet > > > Dept of Atmospheric Science > Colorado State University > Fort Collins, CO 80523 > USA > > > Phone: 970-491-3338 > Fax: 970-491-8449 > > > -- David Parker Met Office Hadley Centre FitzRoy Road EXETER EX1 3PB UK E-mail: david.parker@metoffice.gov.uk Tel: +44-1392-886649 Fax: +44-1392-885681 http:www.metoffice.gov.uk