Urban Heat Island (UHI) refers to the fact that human-built structures are generally warmer [REWORD TO SAY "retain/absorb more heat"?] than the natural items they replace. Examples of such structures include concrete buildings and asphalt roads that absorb heat and raise temperatures (especially at night). However, there are exceptions like irrigated fields, trees planted and watered in deserts, etc. that can cool an area.
Surface temperature measurements are used to assess global and regional warming/cooling trends. Therefore, it is necessary to make an adjustment to historical temperature measurements that accounts for UHI at the individual surface station level. Determining the size of corrections which should be applied to the surface temperature record is a controversial subject. [Items should be added here to discuss this topic]
The GISS assumes that there is a positive correlation between population and UHI and adjusts their surface station temperature histories accordingly. The larger the human population in an area, the larger the number of structures and thus the larger the ability to store heat during the day and to release heat during the night (i.e. greater UHI effect).
One would expect the largest cities to have the largest UHI effects. However, it should also be noted that relatively small towns show an urban heat island effect (ref Oke??).
Peterson (2003) states "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 meteorological observations are likely to be made in park "cool islands." (summarized from wikipedia)
Parker 2004, 2006
A study by David Parker published in Nature in November 2004 and in Journal of Climate in 2006 attempts to test the urban heat island theory, by comparing temperature readings taken on calm nights with those taken on windy nights. If the urban heat island theory is correct then instruments should have recorded a bigger temperature rise for calm nights than for windy ones, because wind blows excess heat away from cities and away from the measuring instruments. There was no difference between the calm and windy nights, and the author says: we show that, globally, temperatures over land have risen as much on windy nights as on calm nights, indicating that the observed overall warming is not a consequence of urban development. (from wikipedia)
Jones et al. 1990
Relation to Global Warming
Some feel that increased urbanization may contribute to higher local temperatures which may be (then falsely) interpreted as being caused by global warming.
IPCC stance on the UHI
IPCC 2007, chpt 3, p.244 says (broken up by me):
Studies that have looked at hemispheric and global scales conclude that any urban-related trend is an order of magnitude smaller than decadal and longer time-scale trends evident in the series (e.g., Jones et al., 1990; Peterson et al., 1999). This result could partly be attributed to the omission from the gridded data set of a small number of sites (<1%) with clear urban-related warming trends. In a worldwide set of about 270 stations, Parker (2004, 2006) noted that warming trends in night minimum temperatures over the period 1950 to 2000 were not enhanced on calm nights, which would be the time most likely to be affected by urban warming. Thus, the global land warming trend discussed is very unlikely to be influenced significantly by increasing urbanisation (Parker, 2006).
Over the conterminous USA, after adjustment for time-of-observation bias and other changes, rural station trends were almost indistinguishable from series including urban sites (Peterson, 2003; Figure 3.3 (sic), and similar considerations apply to China from 1951 to 2001 (Li et al., 2004). One possible reason for the patchiness of UHIs is the location of observing stations in parks where urban influences are reduced (Peterson, 2003). In summary, although some individual sites may be affected, including some small rural locations, the UHI effect is not pervasive, as all global-scale studies indicate it is a very small component of large scale averages.
Accordingly, this assessment adds the same level of urban warming uncertainty as in the TAR: 0.006°C per decade since 1900 for land, and 0.002°C per decade since 1900 for blended land with ocean, as ocean UHI is zero.