Do you know that GISS sometimes correct temperature data for supposed urban cooling? Let us look at examples from southern Sweden.
The three towns Karlstad, Jönköping, and Visby all have small airports. Of these towns, Visby is located on Gotland, the largest island in the Baltic Sea. Karlstad lies on the northern shores of the largest lake of Sweden, while Jönköping lies on the southern shores of the second largest lake. These two lakes are nearby to each other, and it seems unlikely that there could be any climate change at one of them not affecting the other.
These three airports also have associated weather stations, included in the GISS/GHCN global network. Since truly rural sites are non-existent, a small airport station may be one of the best locations to measure climate change. There is often an open horizon for the thermometer and not too much concrete and asphalt nearby, since people do not want to live or work near an airport, due to the noise from the planes and the risk of accidents. If there are changes in the location environment, however, I think it is more likely to have changes that cause urban warming near the weather station than the opposite.
Evidently, GISS does not agree. GISS has corrected the temperature records for the above-mentioned small airports in southern Sweden for urban cooling. Most remarkable is the correction for the Jönköping record (Fig 1). The uncorrected temperature data show no trend at all for the period 1950-2005. The corrected data have a warming trend of slightly more than 2 C/century.
The uncorrected data for Visby shows a barely significant warming trend of 1.4 C/century (Because of the year-to-year variability, a lower limit for a detectable and significant trend is 1.3 C/century), while the corrected version has a clearly significant warming trend of 2.7 C/century. For Karlstad, both the uncorrected and corrected data set show a significant warming trend, but the result of the correction is to increase it from 2.9 to 3.6 C/decade.
Are those corrections realistic? If you ask me, I say no. Maybe it is possible for one out of three to really have an urban cooling trend. Maybe. But three out of three? No way. Furthermore, Karlstad is about 185 km north of Jönköping and both lie beside a large lake. They should have a similar climate and the same climate change. Neither the uncorrected nor the corrected GISS data sets show similar trends at all. Only if we compare the uncorrected data set for Karlstad and the corrected data set for Jönköping do we get something similar.
If we look at the difference in uncorrected annual mean temperatures between Karlstad and Jönköping (Fig 2), we see that it varied around 0.5 C up to about 1980, with Karlstad as the somewhat cooler. Then something happened and since about 1992, Karlstad has been warmer than Jönköping. This something may have started with a jump upwards by half a degree between 1979 and 1980.
Maybe you have realized something important by now. If we believe the uncorrected temperature record from Jönköping airport, there has been no temperature change in southern Sweden since 1950. A correction for urban cooling is needed to get a warming.
But if Jönköping shows the correct trend, then there must be urban warming in the temperature data from Karlstad airport. Visby is located at the coast of the Baltic Sea, so maybe it has reacted to a warming of the seawater (I don’t know if there has been any), which has not affected the inland towns Jönköping and Karlstad.
If we look at the difference between uncorrected annual mean temperatures for Visby and Jönköping (Fig. 3), we see that something happened between 1978 and 1979, as the difference jumps about 1 C in just one year. Maybe there is also a cooling trend in the period 1963-1978. The warming of Visby relative to Jönköping is clearly caused by the 1978-79 jump and not by any gradual trend.
Karlstad airport was moved in 1997, from a location a few kilometres from the centre to about 18 km away. I don’t know if the weather station moved with it, but probably it did. There are 260,000 passenger/year passing the airport.
The airport in Jönköping moved to its present location in 1961. The runway was extended in 1984 and a new terminal was built in 1991. About 140,000 passengers/year pass the airport. I am uncertain whether any of these changes are seen in Fig 2.
Visby airport was ready in 1942 and has 194,000 passengers/year. Since Gotland is an island, you either have to go there by boat or by aeroplane, which is an important reason for the rather large number of passengers for this small airport. It is quite close to the centre of the town, since it takes only 5 minutes to travel by bus or car.
Jönköping is the largest of the three towns, with a population of about 85,000. This is in fact more people than living on the entire island of Gotland, and about 23,000 of those live in Visby. Karlstad has a population of about 58,000, but the movement of the airport in 1997 actually placed it in a nearby town with about 8,000 people.
The latter fact is an indication that there could be an urban cooling in the temperature record for this airport in the last decade, but hardly any support for such a trend all the time since 1950. Karlstad, Jönköping, and Visby all have had slowly growing populations since 1969, which is the oldest census data I have been able to find.
Most southern Swedish cities and towns grew in the 1950’s and 1960’s, both due to an increasing population in general and due to migration from the northern parts of the country. The municapilities of which these towns are center have more than doubled their populations since 1950, and usually urban centers grow faster than the surroding rural areas.
Thus, I can find no support at all for the idea that any of these three temperature records should suffer from urban cooling in the period since 1950. If any urban trends exist in these data sets, it should be urban warming not urban cooling.