Hot City or Global Warming?
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.
Starting with Melbourne city centre the data is as shown in Table 1.
This data shows clear signs of warming – 1.7C since 1861 in the minimum temperatures. Furthermore, the warming is far from linear. Over the first 40 years the minimums rose 0.5C while over the last 40 years they rose 1.5C. The maximiums were steady over the first 40 years yet rose 0.4 C over the last 40. This pattern is extremely similar to the pattern of global warming claimed by AGW proponents. However are we seeing global warming or increasing UHI?
We could differentiate by looking at temperature records from locations which are nearby by not in the city centre. Since these would still be in metropolitan Melbourne, some UHI could be expected but it should be less. Unfortunately the Bureau does not list any complete temperature records for nearby locations (exactly the incomplete record problem I mentioned earlier). However the Bureau does list some data for several Melbourne suburbs and these are useful comparisons.
The thing that immediately stands out is that the latest temperatures from all of these sites is very close to the temperatures for Melbourne city centre in the late 19th century: 9.5 minimum and 19.7 maximum. These sites are all reasonably close to Melbourne central and cover essentially all directions. It is hard to believe that they were all cooler than Melbourne central in the 19th century. Nothing in Melbourne’s topography would suggest that.
Only 3 sites have enough data to form any opinion about temperature trends and these all show some increase but far less than the city centre which is what one might expect from UHI.
We could get a stronger confirmation by looking at some site in regional Victoria. These are likely to show much less UHI since the population centres are far smaller. Again one is hampered by the lack of comprehensive data for most locations but there are a few sites for which the Bureau gives reasonably comprehensive data. These are shown in table 3.
The data for Ballarat, Alexandra and Mildura suggest no temperature rise at all over the measurement period and especially (Ballarat and Mildura) over the last 3 decades. Horsham data suggests some temperature rise early in the century but if anything cooling over the last 3 decades. Cape Otway is questionable, there is an 0.7C rise in minimums but a 2.1C fall in the maximums most of which occurred in the early part of the century before the rapid rise in carbon dioxide. The last 3 decades have seen a return to the temperatures of the 1920’s.
Overall one would have to say the Bureau data suggests no significant warming over the last century and in particular the last 3 decades in Victoria. On the other hand it does show significant UHI. Consider that an increase of 1.5C in the minimums for Melbourne over the last 3 decades corresponds to 5C per century. Averaging the minimum and maximum readings yields a rise from 14.7C to 15.7C over 30 years equating to 3.3C per century and Melbourne is far from the worst city in the world for UHI. Look also at how non linear the UHI rise really is. Compare that with a linear allowance of 0.06C per century. For that to be appropriate it would mean that only 1 in 50 sites is showing the same degree of UHI as does Melbourne and the other 49 show no UHI at all, even then the linear allowance is not correct.
The data analysed here is real, high quality, data from Australia’s premier weather/climate organisation. It is completely at odds with the claims of about 0.6C of warming over the 20th century predominantly in the last 3- 4 decades. Is Victoria really so anomalous or are the AGW claims questionable?
Michael Hammer graduated with a Bachelor of Engineering Science and Master of Engineering Science from Melbourne University. Since 1976 he has been working in the field of spectroscopy with the last 25 years devoted to full time research for a large multinational spectroscopy company.
Other articles by Michael Hammer can be found here: http://jennifermarohasy.com/blog/author/michael-hammer/
The picture was taken in Melbourne in 2008 by Jennifer Marohasy.