UHI In Greece

By Paul Homewood

http://www.cato.org/blog/progressive-increase-urban-heat-islands-influence-temperature-records

Cato report on a new study on UHI in Greece:

Perhaps no other climatic variable receives more attention in the debate over CO 2 -induced global warming than temperature. Its forecast change over time in response to rising atmospheric CO 2 concentrations is the typical measure by which climate models are compared. It is also the standard by which the climate model projections tend to be judged; right or wrong, the correctness of global warming theory is most often adjudicated by comparing model projections of temperature against real-world measurements. And in such comparisons, it is critical to have a proper baseline of good data; but that is easier acknowledged than accomplished, as multiple problems and potential inaccuracies have been identified in even the best of temperature datasets.

One particular issue in this regard is the urban heat island effect, a phenomenon by which urban structures artificially warm background air temperatures above what they normally would be in a non-urbanized environment. The urban influence on a given station’s temperature record can be quite profound. In large cities, for example, urban-induced heating can be as great as Tokyo’s 10°C, making it all the more difficult to detect and discern a CO 2 -induced global warming signal in the temperature record, especially since the putative warming of non-urbanized areas of the planet over the past century is believed to be less than 1°C. Yet, because nearly all long-term temperature records have been obtained from sensors initially located in towns and cities that have experienced significant growth over the past century, it is extremely important that urbanization-induced warming – which can be a full order of magnitude greater than the background trend being sought – be removed from the original temperature records when attempting to accurately assess the true warming (or cooling!) of the natural non-urban environment. A new study by Founda et al. (2015) suggests this may not be so simple or straightforward a task.

Working with temperature records in and around the metropolitan area of Athens, Greece, Founda et al. set out to examine the interdecadal variability of the urban heat island (UHI) effect, since “few studies focus on the temporal variability of UHI intensity over long periods.” Yet, as they note, “knowledge of the temporal variability and trends of UHI intensity is very important in climate change studies, since [the] urban effect has an additive effect on long term air temperature trends.”

To complete their objective the four Greek researchers compared long-term air temperature data from two urban, two suburban and two rural stations over the period 1970-2004. The UHI was calculated as the difference between the urban and suburban (or rural) stations for monthly, seasonal and annual means of air temperature (max, min, and mean).

Among their several findings, the authors report notable differences in the UHI’s intensity across the seasons and in comparing the UHI when calculated using maximum, minimum, or mean temperatures. Of significance to the discussion at hand, however, the authors note “the warming rate of the air temperature in Athens is particularly large during [the] last decades,” such that the “difference of the annual mean air temperature between urban and rural stations exhibited a progressively statistically significant increase over the studied period.” Indeed, as shown in the figure below for the stations (a) National Observatory of Athens (NOA) in the center of Athens and Tanagra (TAN), approximately 50 km north of the city, as well as for (b) the coastal urban station of Hellinikon (HEL) and again the rural station of Tanagra, the anthropogenic influence of urbanization on temperatures at these two urban stations is growing in magnitude with time such that “the mean values of UHI magnitude [calculated across the entire record] are not quite representative of the more recent period.”

Interdecadal variation and annual trends of the Athens, Greece UHI calculated between two urban and one rural station using mean annual temperatures over the period 1970-2004. The two urban stations were the National Observatory of Athens (NOA) in the center of Athens and Hellinikon (HEL), located near the urbanized coast. The rural station Tanagra (TAN), was located approximately 50 km north of the city. Adapted from Founda et al. (2015).

Such findings as these are of significant relevance in climate change studies, for they clearly indicate the UHI influence on a temperature record is not static. It changes over time and is likely inducing an ever-increasing warming bias on the temperature record, a bias that will only increase as the world’s population continues to urbanize in the years and decades ahead. Consequently, unless researchers routinely identify and remove this growing UHI influence from the various temperature data bases used in global change studies, there will likely be a progressive overestimation of the influence of the radiative effects of rising CO 2 on the temperature record.

According to the graph, the influence of UHI has been steadily growing since 1970. Hellinikon, by the way, was Athens’ airport until 2001, since when it has been redeveloped. The rate of divergence between Athens and the rural site at Tanagra looks to be the best part of a degree since 1970. Indeed, the Abstract quotes a figure of 0.2C /decade.

This leads us to the question, how much allowance are GISS making for UHI in Athens and other locations in Greece?

Astonishingly, at Athens Observatory, far from allowing for UHI, GISS are doing the opposite by reducing historic temperatures by 0.1C. For instance, the annual temperature in 1970 was 18.66C, but GISS have adjusted this down to 18.56C.

Altogether, there are six operational sites in Greece that are used by GISS, of which none are rural:-

Population x 1000 Airport? Allowance for UHI

Since 1970

Degree C Athens Observatory 2567 -0.1 Athens Airport 2567 Y 0.1 Thessaloniki 482 Y 0.2 Kerkyra 29 Y 0 Larissa 72 0.39 Kalamata 39 Y 0

It is clear that the adjustments made are woefully short of what is needed to reflect the reality of the situation. As a result, and given the lack of any rural stations, the GISS analysis grossly overstates climatic warming in Greece.

Indeed, Founda et al find that UHI accounts for almost half of Athens’ warming since 1970. This statement takes on even more significance when we see that the familiar cyclical pattern, with a much colder interval during the 1970’s , squeezed in between the warm 1930’s – 40’s and recent warming.

The reality is that there has been little if any warming in Athens since the 1930’s. Take away the UHI effect, and we are probably looking at an underlying cooling trend.

http://data.giss.nasa.gov/cgi-bin/gistemp/show_station.cgi?id=618167140000&dt=1&ds=12

It is becoming painfully apparent that the GISS system for dealing with UHI is pitifully inadequate, a fact that I suspect they already know. It is no wonder that they say:

The homogeneity adjustment procedure [Hansen et al., 1999, Figure 3] changes the long-term temperature trend of an urban station to make it agree with the mean trend of nearby rural stations. The effect of this adjustment on global temperature change was found to be small, less than 0.1°C for the past century.

http://onlinelibrary.wiley.com/doi/10.1029/2010RG000345/full

despite the fact that the vast majority of global temperature stations are urban/airport based.

The GISS dataset is quite simply unfit for purpose.