Guest Post by Willis Eschenbach
Anthony has highlighted a study by Coughlin and Butler. Their study says that there is little or no urban warming (urban heat island, or UHI) in the temperature record from the Armagh Observatory in Ireland. They say:
It is concluded that temperature observations made at Armagh Observatory have been unaffected by rapid urbanisation over the past three decades.
Why is Armagh important? And is there really no UHI in Armagh?
The Armagh record is very valuable because it is one of the longest well-documented temperature series in existence. Here is the monthly mean temperature record from Armagh. (NOTE: I have replaced the earlier Figures 1 and 3, which only went up to the year 2004, with updated figures which now include 2005-2010. My thanks to those who wrote in with the location of the post-2004 data.)
Figure 1. 209 years of monthly temperatures at Armagh, Ireland. Pale blue is monthly surface air temperatures. Dark blue is Gaussian average of the temperature. Photo is noctilucent clouds over Northern Ireland.
My conclusions from Figure 1?
1. First, one single temperature station says nothing about the temperature of the planet. However, this one says a lot about century-long temperature changes in the North of Ireland.
2. The most striking thing to me is the slow regularity of the two-century-long temperature trend. Yes, there are decadal swings. But they don’t stray far from a simple trendline.
3. The recent warming from ~ 1980 on is not particularly unusual or anomalous compared to earlier periods of warming. From this, however, we can’t tell if there is a heat island signal in the record.
4. The Armagh data shows the same 0.6°C temperature trend over the 20th century that is shown by the global record. It also shows the same features as the global record, warming to the late 1940′s, cooling for thirty years or more, recent warming.
5. There is no sign of any acceleration, and indeed little change at all, in the long slow two centuries of warming.
Oddly, the Armagh Observatory data does not form part of the GHCN dataset that is used by all parties to create global temperature datasets. But I digress. Onward to the UHI.
First, some terminology. “UHI” stands for “Urban Heat Island”. Bad name. There’s lot’s of heat islands that are not urban. Trees, changes in the vegetation of the site, hedges, all of these can cause heat islands. I prefer the term “LHI”, for “Local Heat Island”. I know, I’m swimming uphill, so I call it UHI like everyone else does. But remember it doesn’t have to be urban.
The question of whether Armagh contains a heat island signal is an important one. Casting around for a way to determine the amount (if any) of heat island signal in the Armagh data, I decided to look at the relationship between Armagh temperature and the sea surface temperature (SST) of the North Atlantic and the Irish Sea. I reasoned that for an island on the edge of the North Atlantic, the SST would determine the land temperature. Here are the areas I used to see if my reasoning was correct:
Figure 2. Areas of ocean used for the comparison with the Armagh temperatures. Armagh Observatory is at the center of the yellow house. Left gridsquare is the North Atlantic area. Right gridsquare is the Irish Sea area.
I took the anomalies of the HadISST sea surface temperatures for each of those areas, and of the Armagh temperatures. Here are the results:
Figure 3. Temperature anomalies around Ireland. Monthly averages have been removed. Note that the vertical scale is different from Figure 1. Pale colored lines are actual monthly anomalies, heavy solid color lines are Gaussian averages. Sea surface temperatures (SSTs) are HadISST temperatures from KNMI.
Now, that’s pretty interesting. My observations, in no particular order, are:
1. As I suspected, the ocean temperature around the island of Ireland determines the Armagh temperature. The island is ruled by ocean winds and breezes.
2. The Irish Sea and the North Atlantic temperatures are quite similar. This increases confidence in the precision of the sea surface temperature data.
3. As you would expect, the swings in the land temperature extremes are greater than those of the sea surface temperature.
4. From 1900 to 1986, the averages of all three records are generally all quite close to each other. I always like seeing such a close correspondence of two entirely separate and discrete natural records. It increases the confidence in both datasets. In particular, the wiggle-match between the North Atlantic (heavy red line) and Armagh (heavy blue line) is quite impressive.
5. From 1986 onwards, the Armagh and the ocean datasets diverge in a significant manner.
6. The size of the divergence from 1986 to the end of the record in July 2010 is about a degree.
The Coughlin and Butler paper says:
The grounds surrounding the Observatory and its climate station have remained relatively unchanged over the past 200 years. However, in that time, the town of Armagh has spread in several directions, including to the north and east, past the Observatory site. Much of the development around the site has been in the form of housing built over the past 20-30 years and this development still continues.
Does this mean that Armagh is showing urban or site-specific warming over the last quarter century? I don’t know. But I find it mighty suspicious that after 85 years of running right in sync with both the North Atlantic Ocean and the Irish Sea, the Armagh temperature should suddenly strike out on its own towards new heights, just when the town starts building up around it.
As a result, I’m not prepared to agree with Coughlin and Butler that there is no UHI signal in the Armagh data. They say:
However, recent research into the historical temperature records and comparisons with present day data from rural weather stations indicate that any temperature differences which existed between the Observatory site and the countryside 20-30 years ago have not increased over the intervening years.
Comparison of Armagh with ocean data, however, clearly shows increasing temperature differences in the exact time frame which they have used in their paper to discriminate a valid signal.
My regards to all,
PS – I can’t find any Armagh data after 2004 … does anyone know where it might be available? (Solved, thanks to those who wrote in.)