A while back I found myself pondering on these two graphs:
The one up top is the Hadcrut mean global anomaly (in this case from Wikipedia); that below is from In Search of Cooling Trends in which Tonyb and I were surprised to find a cyclic pattern matching so closely in diverse stations across the globe. If we had warmth in two periods peaking in the 1930-1945 and peaking now might not it be valuable to compare these around the world? I don’t expect that these cycles would necessarily peak at the same time all around the world, but I wondered if it would be worth comparing the approx. timescales when they peak. Could something as simple as average temperatures in these two periods be useful in some way?
The first three countries I looked at were Greenland, Finland, then Norway. Greenland, out of thirteen stations, has only three stations that have data (in GHCN v3 Beta) during both the 11 year period centred on 1940 (i.e. 1935-1945) and that centered on 200o: Upernavik, Nuuk and Angmagssalik. In Finland, the figure was two, out of eighteen: Sodankyla and Helsinki/Seutula. With the exception of Angmagassilik, in which the two periods had <0.1°C difference, in each of the other four stations the 1995-2005 period was cooler than 1935-1945. In Norway only nine out of thirty-two stations had data in both periods, although the differences in temperature were variable – some stations showed cooling and some warming.
I knew the proportion of long-lived stations in GHCN was quite low, but I found this lack of stability in the temperature record rather shocking. If this was the same across the world, well fragmented records certainly increase the challenge of combining records, but perhaps it would increase the potential for errors creeping in. Would comparing these two periods be a good way of checking sites for the development of UHI? But I didn’t just want to look at individual countries I wanted to look at the whole record.
Well, with a bit of help from another collaborator, I’ve started to do that. Charles Duncan and I have been having fun with pivot tables in Excel (well he’s been producing ever more complex versions). The only thing is that exploring what we can do with them is so much fun that I’m now thinking “Where do I start?” in writing about it.
Digging in the Clay 
Verity
This is an interesting thread.
A couple of observations though.
The two figures have different scales on their x-axes. If you allow for this and look closely it is evident that the MGST anomaly chart (figure 1) shows a marked cooling period from 1880 to 1910 followed by a marked warming period from 1910 to 1940. In contrast the ‘in search of cooling’ chart (figure 2) shows (to my eye) a warming trend for the 1880 to 1900 period followed by a cooling trend from 1900 to 1910 followed by a warming trend from 1910 to 1940. I think contrasting the two figures as you have done is a classic example of why it makes no sense whatsoever IMO to combine anomalies for individual stations into some form of so called ‘global’ average as it is obvious that theer is plenty of cooling in some parts of the world at the same time as there is warming and vice versa.
I presume Figure 2 is raw ‘anomaly’ data where as Figure 1 is homogenised/adjusted anomaly data? This is evident from the fact that in Figure 2 the peak reached in the late 30s/early 1940s is higher than that for the late 20th century (so called ‘modern’) warming period and is a good illustation of how the adjustments (e.g. TOBs and UHI) cause past temperatures to be reduced downwards relative to recent temperatures.
What is clear from both figures is that the rate of warming during the late 20th century warming period is far from ‘unprecedented’ even in the last 100 years let alone the last 1000 to 2000 years as people like Michael Mann etc would have us all believe based on their proxy temperature re-constructions.
Chiefio has an interesting item you may not have seen yet –
http://chiefio.wordpress.com/2011/01/03/coveting-cape/
regarding a referenced paper at –
http://www.agu.org/journals/ABS/2010/2009JD012535.shtml