Hot City or Global Warming?

An analysis of the historical temperature data for the state of Victoria in Australia, including the city of Melbourne, suggests an Urban Heat Island (UHI) effect but no general warming trend.

Urban Heat Island versus Global Warming – A Study of One Region
By Michael Hammer

CITIES represent concentrations of commerce and energy use. This energy release raises the temperatures in the immediate vicinity. Cities are also areas where there is intense development with extensive masonry constructions, skyscrapers, paved surfaces and little vegetation.

Large masses of masonry and paving store heat during the day and release it at night keeping the night time minimum temperatures significantly higher than they would otherwise be. As a result, cities are usually significantly warmer than nearby rural areas, especially at night. This is termed the urban heat island effect or UHI and it can be very large. For large cities such as New York or Tokyo, the UHI has been reported as raising minimum temperatures by up to 6-8C. Even more modest cities like Melbourne show very significant UHI temperature increases.

More significantly, UHI increases as the size of the city increases and as the level of development rises. Both typically increase with time which means the UHI increases with time. This is exactly similar to the claimed global warming signature.

People comment that they have experienced global warming for themselves. That it is now warmer than it used to be and cite examples such as ice covered puddles in the past which they no longer see today. If you live in a city (as the majority of people do) that is quite probably true. However, what you are experiencing is not necessarily global warming but rather the impact of UHI in your immediate environment. As already stated, the impact can be extremely large – several degrees in large cities. Nor is it necessarily limited to just the city area. If there is a prevailing wind and you are living down wind of the city centre then you will be enveloped in the spreading plume of warmth. So, if we see signs of warming in cities is it UHI or is it evidence of global warming?

UHI has minimal impact on global temperatures because the cities represent such a small fraction of the total area of the total planet. However, it can have a large impact on the estimation of global temperatures because so many of the measuring stations are in cities. These stations will show a temperature rise with time which is the sum of any global heating plus the local UHI heating. If the impact of UHI is not allowed for, the result will be an inflated estimate of global warming. Even worse, many measurement stations which were originally sited in very reasonable locations are, through later developments, now severely impacted by nearby heat source such as an air conditioner waste heat vent or on top of bitumen paving. This is yet another factor in addition to UHI adding to the warming bias.

It is not reasonable to assume that this is allowed for by amalgamating data from a very large number of stations worldwide because many of these sites will be tainted by the same problem. Even worse, tainted city sites will probably have the most comprehensive records. Rural sites are more likely to have incomplete records due to the poorer infrastructure. It is natural to place the greatest reliance on those sites with the most nearly complete data.

The IPCC claimed in the past that one of their corrections to the raw temperature data allows for UHI by applying a linear correction with time amounting to 0.06C per century. However, in the latest revision to the historical global temperature record even this minimal correction has apparently been eliminated.

At the same time, measuring stations that have moved from the city to the airport show lower temperatures at their new location and it is claimed that this needs to be compensated for by elevating the airport readings. This would seem to be clear evidence of bias. The airport readings are lower because the station has moved away from the city UHI. Raising the airport readings, while not adding downwards compensation for UHI, results in an overstatement of the amount of warming. It would be more accurate to lower the city readings to match the airport readings rather than vice versa.

It is interesting to explore this issue by looking at the temperature record for the state I live in – Victoria, Australia. The data presented below comes from the Australian Bureau of Meteorology data base published on their website www.bom.gov.au . The annual average maximum and minimum temperatures have been used and it is worth noting that all these numbers are averages over 30 years so they reasonably reflect climate rather than weather.