User:Tillman/UHI adds

Relation to global warming
Because some parts of some cities may be hotter than their surroundings, concerns have been raised that the effects of urban sprawl might be misinterpreted as an increase in global temperature. While the "heat island" warming is an important local effect, there is no evidence that it biases trends in historical temperature record; for example, urban and rural trends are very similar.

The Third Assessment Report from the IPCC says:


 * However, over the Northern Hemisphere land areas where urban heat islands are most apparent, both the trends of lower-tropospheric temperature and surface air temperature show no significant differences. In fact, the lower-tropospheric temperatures warm at a slightly greater rate over North America (about 0.28°C/decade using satellite data) than do the surface temperatures (0.27°C/decade), although again the difference is not statistically significant.

Ground temperature measurements, like most weather observations, are logged by location. Their citing predates the massive sprawl, roadbuilding programs, and high- and medium-rise expansions which contribute to the UHI. More importantly, station logs allow sites in question to be filtered easily from data sets. Doing so, the presence of heat islands is visible, but overall trends change in magnitude, not direction.

A view often held by skeptics of global warming, is that much of the temperature increase seen in land based thermometers could be due to an increase in urbanization and the siting of measurement stations in urban areas. For example, Ross McKitrick and Patrick J. Michaels conducted a statistical study of surface-temperature data regressed against socioeconomic indicators, and concluded that about half of the observed warming trend (for 1979-2002) could be accounted for by the residual UHI effects in the corrected temperature data set they studied -- which had already been processed to remove the (modeled) UHI contribution. . Critics, including Gavin A. Schmidt,  have said the results can be explained away as an artifact of spatial autocorrelation. McKitrick & Nicolas Nierenberg have submitted a rebuttal defending their results.

Climate Change 2007, the Fourth Assessment Report from the IPCC states the following. 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). ... 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.

As the Fourth assessment hints, oceanic data is in hand from a wide variety of different data collection methods, taken by both civil and national defense groups, as well as multiple subsurface readings, in addition to lower-, middle-, upper-, and ultrahigh-atmosphere datasets.