GISS UHI Adjustments Add To Warming Trend

By Paul Homewood

I ran a post yesterday about GISS’s UHI adjustments for Reykjavik, which had added to the warming trend instead of reducing it.

https://notalotofpeopleknowthat.wordpress.com/2012/11/09/giss-double-up-on-reykjavik-temperatures/

So I thought I would ask GISS what the hell was going on. I was frankly astounded by the reply I received from Reto Ruedy :-

Locally, especially where station density is low, any UHI effect can easily be dwarfed by weather noise. Looking at all the 2933 “non-dark” stations that were adjusted, we find that the trend was reduced in 1711 cases, did not change in 18 cases, and increased in 1204 cases including Reykjavik. Averaged over the globe, our adjustment does slightly reduce the global warming trend. In this case the local noise (real or artificial) cancels out to the point that the UHI effect becomes noticeable. As far as Reykjavik is concerned, our adjustment is actually minimal compared to the effect of the NCDC homogenization (check the GHCN v2 data before and after our adjustment – currently still available on our web site).

So, there are nearly as many urban sites, that have warming trends added, as there are that work in the opposite way. It is no wonder that Hansen et al 2010 tells us “The effect of urban adjustment on global temperature change is only of the order of 0.01°C [ for 1900-2009]”. Most adjustments cancel each other out!

Let’s start by being clear about one thing. The urban heat island is very real and can have a significant effect on temperature. Richard Muller, for instance, talks about Tokyo, where there has been nearly 2C of excess warming over the last century, compared to the rest of Japan. He goes on to point out :-

“Urban areas are heavily overrepresented in the siting of temperature stations: less than 1% of the globe is urban, but 27% of the Global Historical Climatology Network Monthly 40 (GHCN-M) stations are located in cities with a population greater than 50,000”

And Hansen et al 2010 tells us “Human made structures and energy sources can cause a substantial local warming that affects measurements in the urban environment. “

It is important to remember, though, that it is not the absolute UHI effect we are interested in, but the change in the UHI effect over the period that we are monitoring temperatures that we need to consider. How much has UHI increased temperatures in Reykjavik, for instance, since 1940? Or 1970?

Hansen et al 2010 do offer a possible explanation as to why the overall UHI adjustment they make is so small, when they say:-

“The urban influence on long‐term global temperature change is generally found to be small. It is possible that the overall small urban effect is, in part, a consequence of partial cancellation of urban warming and urban cooling effects. A significant urban cooling can occur, for example, if a station is moved from central city to an airport and if the new station continues to be reported with the same station number and is not treated properly as a separate station in the global analysis. “

There are, however, two rather large drawbacks to this argument :-

1) GHCN tell us that their own adjustments, which have added 0.13C per century to the warming trend, “may be due in part to systematic changes in station locations from city centres to cooler airport locations”. (GHCN tech log). Bear in mind that GISS start with the adjusted GHCN data, so it appears they are both adjusting for the same thing.

2) A relocation such as this may cause a step change in temperature at the time, but as the airport develops from a small grass strip into the sort of busy affairs we see today, there must surely be artificial warming introduced as a result.

Overall the two factors may cancel each other out, in which case the UHI adjustment would be zero. However, actual temperatures recorded since the move to the airport would be showing a warming trend that was purely due to local factors. This is an important point, because many such relocations appear to have taken place decades ago, thus bringing into question the true temperature trend since.

A look at the GHCN station database shows just how significant this whole issue is.

Type of Station Number of Stations % Rural <10000 population 3992 54 Small Town <50000 population 1420 19 Urban >50000 population 1952 27 Total 7364

http://data.giss.nasa.gov/gistemp/station_data/v3.temperature.inv.txt

Nearly half the stations are classified as “non rural” . Furthermore many of the “rural” sites will, in reality, still be subject to some UHI effect, even with populations below 10000. Now let’s add in the number of sites that are airport based.

Type of Station Total Stations Number at Airports % at Airports Rural 3992 1005 25 Small Town 1420 504 35 Urban 1952 910 47 Total 7364 2419 33

[ I should point out that not all of the above stations are currently live. The latest information from GISS suggests a figure of 6028. It would not seem unreasonable to assume that most of the drop outs are at non airport sites, and probably off the beaten track.]

So clearly :-

1) Sites that are subject to UHI are heavily overrepresented in the coverage, particularly taking into account towns below the 10000 population cut off.

2) Airport sites, even at “rural” locations, are also heavily overrepresented .

If the GISS homogenisation checks trends at urban sites against rural ones, as they claim, how reliable are the rural figures themselves? In any event, we have already seen that the adjustment in Reykjavik simply does not stand up under scrutiny. How many others would fail the test?

Let’s finish by revisiting the comment Reto made.

“Locally, especially where station density is low, any UHI effect can easily be dwarfed by weather noise”

We know that the UHI effect is real, but it only increases gradually over periods of decades. Reto is effectively admitting that it is unrealistic to expect their algorithm to pick this up amongst all the other weather noise that goes on.

If he is right, the homogenisation adjustment is not fit for purpose. And if so, the same can be said of the whole GISS Surface Temperature Analysis, relying as it does on a preponderance of stations which may be affected by UHI.