cc: Aad van Ulden , "Klein Tank, Albert" date: Fri, 19 Jan 2007 18:34:41 +0100 from: Geert Jan van Oldenborgh subject: Re: Precipitation trends statement IPCC 4AR SPM to: Phil Jones , David.Easterling@noaa.gov Dear Phil, David, thank you you for the figure, which clarifies a lot. Phil Jones wrote: > If you've just seen the SPM, then you will not know about a figure > within Chapter 3. These are figures 3.14 and 3.15. I'm not supposed to > send these out, so you got them from Albert. Don't pass on to > anyone else. > > So the SPM bullet points are based on these. There are also trend > maps by seasons for 1979-2005 and the year for 1901-2005 and 1979-2005, > and global land series time series for 1901-2005 from various databases > - many more than just GHCN and CRU. However, the reader of the SPM will not know these maps either, and assume something else from the names than you mean. Also you do not follow this consistently: "Western Africa" with a clear trend is arbitrarily replaced by the Sahel, with only a trend in the western half, and "Southern South America" is replaced by "eastern SOuth America". > We chose the regions in the chapter to show precip differently from > how it > had been done in previous IPCC reports. The regions were defined in a > paper > by Giorgi and someone else (from about 2001/2002) that is used in Ch 11 > (Table 11.1). Why are the regions not defined based on the signal? This way one groups together regions with and without trends (e.d., in the Mediterranean, with no significant trends on the European side). > The regions are large, take no account of rainy seasons or rainfall > regimes, > so they have very little climatological content. They use a lot of the > gridded > data though and there are some surprising similarities and dissimilarities > between them. We chose annual, as we only had space for one Figure. This seems like a wise choice, especially since some of the observed changes are shifts in the rain seasons, as in the southern African rain season moving backwards. A fixed window like JJA would show a decrease when in fact there is none. > Now the important point - the SPM. If you've read the SPM you'll have > noticed that hardly any country is mentioned. This is deliberate and we > refer to large regions. This is because we would likely not get the > text past > the govts in Paris the week after next if we were that specific. I agree that this makes sense, however, I disagree with choosing the regions first and making statements as if the observed trend applies to the whole region, rather than parts of it. The reader of only the SPM will conclude that rainfall has decreased everywhere in the Mediterranean, when in fact it has not in half; same with Central Asia, Eastern North America, etc. Coming back to some individual regions mentioned in the SPM I still do not understand most of the claims made in the SPM statement. 1) Eastern North America: your figure shows as well as my maps that a significant increase is only seen in the easternmost provinces of Canada. This should not be labelled "Eastern North America"; there is no trend in New York City and Washingtonn D.C. to name a few populous and politically important places, whereas to the reader this is implied. If this small a region cannot be mentioned it should be left out. 2) Eastern South America is not even defined in your figure. Southern South America is, with a clear trend in Fig 3.14 (which is much weaker in the GPCC data), but this is not included in the SPM. 3) There is a clear trend in northern Europe, but as we all know that it is only in winter and the summer has in fact an opposite trend, would it be possible to add the word "winter"? 4) The North Asian trend. Looking at the data from individual GHCN stations, almost all of them have lots of missing data around 1940, when the averaged series shows a big jump. What is the evidence that this is not caused by chances in the observing system? I find step function always quite suspect. The VasclimO dataset, which the authors claim has better homogenization, has a decreasing trend for the period 1951-2000! This does not seem the kind of certainty that warrants inclusion in the SPM. 5) I still see no significant trends in Central Asia except for 3 stations in the far west of China and in Russia (see plot). Do you want to make a sweeping statement "Central Asia is getting wetter" based on these three station series? Wulomoi shows 1.5 decadal cycle that imitates a trend, Dulan and Irtyssk have barely significant trends (p=0.04). There are many other stations with no trends. 6) I do not see an area labelled Sahel on your Figure in Chapter 3. Why is it then included here? The trend in the Sahel is only significant when you start late and finish early; rainfall has increased substantially again since 1995. Given the large decadal variability in the first half of the century, and the attribution to aerosols of the drought in the 1970s and 1980s, I would hesitate to call the remaining trend "significant". Also, it is only the western Sahel that has a trend, not the eastern Sahel. 7) In the Mediteranean there is only a significant trend in North Africa, there is no significant trend on the northern shores. The trend in the time series of Fig 3.14 is not very convincing by eye, it is much better if you take only the southern half, i.e., North Africa. Claiming the "Mediterranean" is receiving less rainfall as a whole is again misleading. 8) From your plot (and mine on www.knmi.nl/adrica_cenarios) I see very strong decadal variability in southern Africa, and no significant trend. We could just happen to have had a downward cycle near the end. What value for the autocorrelation was used to determine the significance of the trend? The judgement by eye agrees with the map, which does not show strong brown colours either. 9) From your map, this concerns Butan/Assam only; the rest of the subcontinent is getting wetter. I see why the restriction on naming countries causes problems here... In the GHCN dataset I find only one station with >70 years of data there with a significant downward trend, Darjeeling, and only a half dozen with >50 years between many more stations with no trend. Again, you are basing a very important statement on very little actual data, and this statement will doubtless be interpreted to mean that large parts of teh subcontinent are drying. I think it would be better to leave it out. To my great surprise "Western Africa" is included in Fig. 3.14, with a steep decline, but this is not mentioned in the SPM! Western Australia also shows up very clearly in the colours, but is ignored. Why? Because Giorgi used northern and southern Australia? So, in spite of the background information I still do not understand how this statement follows from the observations. Greetings, Geert Jan Attachment Converted: "c:\eudora\attach\time series per yr0.png" Attachment Converted: "c:\eudora\attach\Re Precipitation trends statem.png" Attachment Converted: "c:\eudora\attach\time series per yr01.png" Attachment Converted: "c:\eudora\attach\time series per yr02.png" Attachment Converted: "c:\eudora\attach\Re Precipitation trends statem1.png"