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Urban Heat Islands

Urban Heat Islands


Urban heat islands are areas of increased warmth in built-up areas such as cities and suburbs. As areas become covered with buildings and pavements, moist soil, vegetation, and open water tend to be replaced with stony surfaces that absorb solar energy effectively. Such surfaces also dry more quickly after precipitation, reducing evaporation, which has a cooling effect in green areas. Fuel-burning also adds heat to the local urban environment (with very slight direct effect on global warming). The net effect is that built-up or urban areas are warmer than country or rural areas.

Some skeptics of global warming have argued that the world only appears to have become warmer because too many temperature-measuring stations have been located near urban areas. As urban areas have grown over the last century, temperatures at urban or near-urban measuring stations have risen, giving a false numerical appearance of global warming. In effect, these critics say, scientists have measured temperature in places that have warmed for reasons that have nothing to do with global climate—urban heat islands.

Urban heat islands are real, and climatologists agree that temperature data must be adjusted for urban heat-island effects. However, such adjustments are standard

practice in climate measurement. Today, there is no scientific standing for the claim that global warming is an illusion created by expanding urban heat islands. Global warming has been established by numerous, independent, convergent lines of evidence, including over 540 million separate readings of ground and sea surface temperature and more than 30 years of satellite measurements scanning the whole surface of Earth.

Historical Background and Scientific Foundations

One of the first scientists to propose that temperature records showed a global warming trend, and that a human-caused greenhouse effect might be the cause of that trend, was British engineer Guy Stewart Callendar (1898-1964). In the 1930s, Callendar analyzed thousands of temperature records, most from Europe and North America, and concluded that the world had warmed by about one degree Fahrenheit (about half a degree Celsius) from 1890 to 1935. Scientists now know that Callendar was approximately right about the warming, but that the warming he observed was due mostly to natural variations, not to the greenhouse effect; global warming due to the human-caused greenhouse effect did not become definitely measurable until after 1980.

Callendar's claim of a global warming trend was disputed by many weather scientists, some of whom pointed to the well-known fact that temperatures in urban areas tend to be higher than those in rural areas—namely, the urban heat-island effect. Callendar and scientists supporting his view argued that they had taken such effects into account in their temperature calculations. Through the 1960s and 1970s, this point continued to be debated, with scientific opinion sharply divided over the question of whether heat-island effects were creating an illusion of warming by systematically biasing temperature numbers.

It was not until about 1990 that meticulous re-analysis of historical climate records, with care taken to compensate for urban heat-island effects, convinced the great majority of climate scientists that the global warming trend was real. The issue has been continually re-studied since that time, with results consistently showing that urban heat islands have not significantly biased estimates of global warming. According to the 2007 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), urban heat islands and other land-use changes have biased the land-based temperature record only by .012°F (.006°C)—an amount of warming at least a factor of 10 smaller than the observed trends in global temperature.

Impacts and Issues

One effect of urban heat islands is that as global climate continues to warm and heat waves become more severe, prolonged, and common (a predicted effect of global climate change), city environments, already warmer than rural areas, will be exposed to especially high temperatures. A 2005 study projecting the likely effects of climate change on New York City found that the city's urban heat-island effect would be worsened both by increasing baseline temperatures and slower local winds. Increased urban heat-island effects could likely entail increased air conditioning costs, heat stress on human health, formation of smog, and degradation of plant areas.

Despite the achievement around 1990 of broad consensus about the reality of global warming, some scientific debate about the effect of urban heat islands on global temperature estimates persisted into the early 2000s. However, continued studies have shown that the observed overall warming of surface air temperatures is real, not an artifact of urban development. In particular, a 2004 study published in Nature announced that temperatures over land—according to data corrected for urban heat-island effects—had increased as much on windy nights as on calm nights, even though urban heat islands are cooler on windy nights. This showed that the corrections applied for urban heat-island effects were accurate: scientists were not being fooled by the urban heat islands.


CALLENDAR EFFECT: Global warming due to atmospheric carbon dioxide released by burning fossil fuels. Named after Britsh engineer Guy Stewart Callendar (1898–1964), who proposed in the 1930s (erroneously) that weather records already showed such warming.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

SOLAR ENERGY: Any form of electromagnetic radiation that is emitted by the sun.

See Also Climate Change Skeptics; Global Warming;Temperature Record.



Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of theIntergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Weart, Spencer. The Discovery of Global Warming. Cambridge, MA: Harvard University Press, 2004.


Foley, Jonathan A., et al. “Global Consequences of Land Use.” Science 309 (2005): 570-574.

Hopkin, Michael. “Climate Skeptics Switch Focus to Economics.” Nature 445 (2007): 582-583.

Parker, David E. “Large-Scale Warming Is Not Urban.” Nature 432 (2004): 2990.

Rozenweig, C., et al. “Characterizing the Urban Heat Island in Current and Future Climates in New Jersey.” Global Environmental Change 6 (2005): 51-62.

Web Sites

Schmidt, Gavin A. “No Man Is an (Urban Heat) Island.”, July 2, 2007. <> (accessed October 30, 2007).

Larry Gilman

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