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An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings.
As population centers grow in size from village to town to city, they tend to have a corresponding increase in average temperature which is more often welcome in winter months than in summertime.
The EPA says: "On hot summer days, urban air can be 2-10°F [2-6°C] hotter than the surrounding countryside. Not to be confused with global warming, scientists call this phenomenon the "urban heat island effect."
There is no controversy about cities generally tending to be warmer than their surroundings. What is controversial about these heat islands is whether, and if so how much, this additional warmth affects the (global) temperature record.
Scientists compiling the historical temperature record are aware of the UHI effect, but they vary as to how significant they think it is. Some scientists (see Peterson, below) have published peer reviewed papers indicating that the effect of the UHI has been overestimated, and that it does not affect the record at all. Other scientists have used various methods to compensate for it.
Some advocates change that temperature data from heat islands has been mistakenly used as evidence for the global warming theory.
There are several causes of UHI. The principle reason for the night-time warming is (comparatively warm) buildings blocking the view to the (radiatively cold) night sky. Replacement of high-albedo natural ground with low-albedo asphalt and buildings also has an effect, though natural forest has a low albedo already. Also the cooling effect of evaporation is absent from built-over areas.
Different climatic regions may have very different experiences of UHI's. In an already warm area they will be unwelcome; in a cold area they might be beneficial.
The EPA says:
"Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb – rather than reflect – the sun's heat, causing surface temperatures and overall ambient temperatures to rise." (ibid)
The lesser-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area.
Some cities exhibit a heat island effect, largest at night (see below), and particularly in summer , or perhaps in winter , with several degrees between the center of the city and surrounding fields. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports: e.g., "68 degrees downtown, 64 in the suburbs".
One consequence of urban heat islands is the increased energy required for air conditioning and refrigeration, but only for cities that are already in comparatively hot climates. Those that are in cold climates would presumably need somewhat less in the way of heating. The Heat Island Group estimates that the heat island effect costs Los Angeles about $100 million per year in energy.
The heat island effect can be counteracted slightly by using white or reflective materials to build houses, pavements, and roads. This is a long established practice in many countries. Another option is to increase the amount of well watered vegetation.
The IPCC states that "it is well-known that compared to non-urban areas urban heat islands raise night-time temperatures more than daytime temperatures." . For example, Moreno-Garcia (Int. J. Climatology, 1994) found that Barcelona was 0.2°C cooler for daily maxima and 2.9°C warmer for minima than a nearby rural station. A description of the very first report of the UHI in 1820 says:,
Howard was also to discover that the urban center was warmer at night than the surrounding countryside, a condition we now call the urban heat island. Under a table presented in The Climate of London (1820), of a nine-year comparison between temperature readings in London and in the country, he commented: "Night is 3.70° warmer and day 0.34° cooler in the city than in the country." He attributed this difference to the extensive use of fuel in the city. .
The explanation for the night-time maximum is that the principal cause of UHI is blocking of "sky view" during cooling: surfaces lose heat at night principally by radiation to the (comparitavely cold) sky, and this is blocked by the building in an urban area. Radiative cooling is more dominant when wind speed is low and the air is cloudless, and indeed the UHI is found to be largest at night in these conditions .
Because some parts of some cities may be several degrees hotter than their surroundings, a difference double or triple the warming observed over the past, say, 50 years (see historical temperature record), there is a risk that the effects of urban sprawl might be misinterpreted as an increase in global temperature. Evidence of UHI could be mistaken for proof of the global warming hypothesis. However, the fact that the UHI is so large is, paradoxically, evidence that it largely absent from the record, otherwise warming would be shown as much larger in the record. The 'heat island' warming does unquestionably affect cities and the people who live in them, it is not so clear that it biases the historical temperature record.
If thermometers located in cities suffered from increased UHI and recorded significantly higher temperatures (increasing over time as urbanisation progressed), this could contaminate the temperature record. However, this does not seem to have occurred - see the Peterson study, discussed below.
The IPCC says:
The maximum difference in temperature between urban areas and their rural surroundings is often several times higher than the cumulative rise in temperature recorded by land-based thermometers since 1880, although urban areas are heterogeneous, and weather stations are often sited in "cool islands" - parks, for example - within urban areas. Some (which?) sources claim that most of the "global warming" detected by thermometers worldwide is due chiefly to the urban heat island effect. That is, these (unnamed) sources assert that cities are growing and getting warmer, and this warming is falsely being extrapolated to the entire globe.
Note that not all cities show a warming relative to their rural surroundings. For example, Hansen et al. (JGR, 2001) adjusted trends in urban stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record. Of these adjustments, 42% warmed the urban trends: which is to say that in 42% of cases, the cities were getting cooler relative to their surroundings rather than warmer.
The Intergovernmental Panel on Climate Change, which has issued several influential reports on climate trends, says that the effects of urban heat islands on the recorded temperature "do not exceed about 0.05°C over the period 1900 to 1990". This rests on various sources, contributing reasons being :
A recent paper ("Assessment of urban versus rural in situ surface temperatures in the contiguous United States: No difference found"; J climate; Peterson; 2003) indicates that the effects of the urban heat island may have been overstated, finding that "Contrary to generally accepted wisdom, no statistically significant impact of urbanization could be found in annual temperatures.". This was done by using satellite-based night-light detection of urban areas, and more thorough homogenisation of the time series (with corrections, for example, for the tendency of surrounding rural stations to be slightly higher, and thus cooler, than urban areas). As the paper says, if its conclusion is accepted, then it is necessary to "unravel the mystery of how a global temperature time series created partly from urban in situ stations could show no contamination from urban warming". The main conclusion is that micro- and local-scale impacts dominate the meso-scale impact of the urban heat island: many sections of towns may be warmer than rural sites, but meteoroloical observations are likely to be made in park "cool islands".
Another view is that nearly all warming recorded by land-based thermometers is due either to (a) the heat island effect and other artifacts of the recording process and (b) solar variability - indeed, to almost any effect but global warming.
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