The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (August 2018)
An urban heat island (UHI) is an urban area or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the urban heat island effect is from the modification of land surfaces. Waste heat generated by energy usage is a secondary contributor. As a population center grows, it tends to expand its area and increase its average temperature. The term heat island is also used; the term can be used to refer to any area that is relatively hotter than the surrounding, but generally refers to human-disturbed areas.
Monthly rainfall is greater downwind of cities, partially due to the UHI. Increases in heat within urban centers increases the length of growing seasons and decreases the occurrence of weak tornadoes. The UHI decreases air quality by increasing the production of pollutants such as ozone, and decreases water quality as warmer waters flow into area streams and put stress on their ecosystems.
Not all cities have a distinct urban heat island, and the heat island characteristics depend strongly on the background climate of the area in which the city is located. Mitigation of the urban heat island effect can be accomplished through the use of green roofs and the use of lighter-colored surfaces in urban areas, which reflect more sunlight and absorb less heat.
Concerns have been raised about the possible impact of urban heat islands on climate change. While some lines of research did not detect a significant impact, other studies have concluded that heat islands can have measurable effects on climate phenomena at the global scale.
The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. Prior to the 1990s, relatively few studies of UHI had been published. Between 1990 and 2000, about 30 studies were published annually; by 2010, that number had increased to 100, and by 2015, it was more than 300.
There are several causes of an urban heat island (UHI); for example, dark surfaces absorb significantly more solar radiation, which causes urban concentrations of roads and buildings to heat more than suburban and rural areas during the day; materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy budget of the urban area, often leading to higher temperatures than surrounding rural areas. Another major reason is the lack of evapotranspiration (for example, through lack of vegetation) in urban areas. The U.S. Forest Service found in 2018 that cities in the United States are losing 36 million trees each year. With a decreased amount of vegetation, cities also lose the shade and evaporative cooling effect of trees.
Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and prevents pollutants from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. UHI not only raises urban temperatures but also increases ozone concentrations because ozone is a greenhouse gas whose formation will accelerate with the increase of temperature.
For most cities, the difference in temperature between the urban and surrounding rural area is largest at night. While temperature difference is significant all year round, the difference is generally bigger in Winter. The typical temperature difference is 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, as in "68 °F (20 °C) downtown, 64 °F (18 °C) in the suburbs". "The annual mean air temperature of a city with 1 million people or more can be 1.8-5.4 °F (1.0-3.0 °C) warmer than its surroundings. In the evening, the difference can be as high as 22 °F (12 °C)."[unreliable source?]
The UHI can be defined as either the air temperature difference (the canopy UHI) or the surface temperature difference (surface UHI) between the urban and the rural area. These two show slightly different diurnal and seasonal variability and have different causes 
The IPCC stated that "it is well-known that compared to non-urban areas urban heat islands raise night-time temperatures more than daytime temperatures." For example, Barcelona, Spain is 0.2 °C (0.36 °F) cooler for daily maxima and 2.9 °C (5.2 °F) warmer for minima than a nearby rural station. A description of the very first report of the UHI by Luke Howard in the late 1810s said that the urban center of London was warmer at night than the surrounding countryside by 2.1 °C (3.7 °F). Though the warmer air temperature within the UHI is generally most apparent at night, urban heat islands exhibit significant and somewhat paradoxical diurnal behavior. The air temperature difference between the UHI and the surrounding environment is large at night and small during the day. The opposite is true for skin temperatures of the urban landscape within the UHI.
Throughout the daytime, particularly when the skies are cloudless, urban surfaces are warmed by the absorption of solar radiation. Surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, urban surfaces act as a giant reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. As a result, the large daytime surface temperature within the UHI is easily seen via thermal remote sensing. As is often the case with daytime heating, this warming also has the effect of generating convective winds within the urban boundary layer. It is theorized that, due to the atmospheric mixing that results, the air temperature perturbation within the UHI is generally minimal or nonexistent during the day, though the surface temperatures can reach extremely high levels.
At night, the situation reverses. The absence of solar heating leads to the decrease of atmospheric convection and the stabilization of urban boundary layer. If enough stabilization occurs, an inversion layer is formed. This traps urban air near the surface, and keeping surface air warm from the still-warm urban surfaces, resulting in warmer nighttime air temperatures within the UHI. Other than the heat retention properties of urban areas, the nighttime maximum in urban canyons could also be due to the blocking of "sky view" during cooling: surfaces lose heat at night principally by radiation to the comparatively cool sky, and this is blocked by the buildings in an urban area. Radiative cooling is more dominant when wind speed is low and the sky is cloudless, and indeed the UHI is found to be largest at night in these conditions.
The urban heat island temperature difference is not only usually larger at night than during the day, but also larger in winter than in summer. This is especially true in areas where snow is common, as cities tend to hold snow for shorter periods of time than surrounding rural areas (this is due to the higher insulation capacity of cities, as well as human activities such as plowing). This decreases the albedo of the city and thereby magnifies the heating effect. Higher wind speeds in rural areas, particularly in winter, can also function to make them cooler than urban areas. Regions with distinct wet and dry seasons will exhibit a larger urban heat island effect during the dry season. The thermal time constant of moist soil is much higher than that of dry soil. As a result, moist rural soils will cool slower than dry rural soils and act to minimize the nocturnal temperature difference between urban and rural regions.
If a city or town has a good system of taking weather observations the UHI can be measured directly. An alternative is to use a complex simulation of the location to calculate the UHI, or to use an approximate empirical method. Such models allow the UHI to be included in estimates of future temperatures rises within cities due to climate change.
Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969. His paper surveys UHI and criticizes then-existing theories as being excessively qualitative. A general purpose, numerical energy budget model is described and applied to the urban atmosphere. Calculations for several special cases as well as a sensitivity analysis are presented. The model is found to predict the correct order of magnitude of the urban temperature excess. The heat island effect is found to be the net result of several competing physical processes. In general, reduced evaporation in the city center and the thermal properties of the city building and paving materials are the dominant parameters. It is suggested that such a model could be used in engineering calculations to improve the climate of existing and future cities.
Species that are good at colonizing can utilize conditions provided by urban heat islands to thrive in regions outside of their normal range. Examples of this include grey-headed flying fox (Pteropus poliocephalus) and the common house gecko (Hemidactylus frenatus). Grey-headed flying foxes, found in Melbourne, Australia, colonized urban habitats following increase in temperatures there. Increased temperatures, causing warmer winter conditions, made the city more similar in climate to the more northerly wildland habitat of the species.
With attempts to mitigate and manage urban heat islands, temperature changes and availability of food and water are reduced. With temperate climates, urban heat islands will extend the growing season, therefore altering breeding strategies of inhabiting species. This can be seen the best in the effects that urban heat islands have on water temperature. With the temperature of the nearby buildings sometimes reaching over 50 degrees different from the near-surface air temperature, precipitation will warm rapidly, causing runoff into nearby streams, lakes and rivers (or other bodies of water) to provide excessive thermal pollution. The increase in the thermal pollution has the ability to increase water temperature by 20 to 30 degrees. This increase will cause the fish species inhabiting the body of water to undergo thermal stress and shock due to the rapid change in temperature to their climate.
Urban heat islands caused by cities have altered the natural selection process. Selective pressures like temporal variation in food, predation and water are relaxed causing for a new set of selective forces to roll out. For example, within urban habitats, insects are more abundant than in rural areas. Insects are ectotherms. This means that they depend on the temperature of the environment to control their body temperature, making for the warmer climates of the city perfect for their ability to thrive. A study done in Raleigh, North Carolina conducted on Parthenolecanium quercifex (oak scales), showed that this particular species preferred warmer climates and were therefore found in higher abundance in the urban habitats than on oak trees in rural habitats. Over time of living in urban habitats, they have adapted to thrive in warmer climates than in cooler.
The presence of non-native species is heavily dependent on the amount of human activity. An example of this can be seen in the populations of cliff swallows seen taking nests under the eaves of homes in urban habitats. They make their homes using the shelter provided by the humans in the upper regions of homes, allowing for an influx in their populations due to added protection and reduced predator numbers.
Aside from the effect on temperature, UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the humidity, and the rates of precipitation. The extra heat provided by the UHI leads to greater upward motion, which can induce additional shower and thunderstorm activity. In addition, the UHI creates during the day a local low pressure area where relatively moist air from its rural surroundings converges, possibly leading to more favorable conditions for cloud formation. Rainfall rates downwind of cities are increased between 48% and 116%. Partly as a result of this warming, monthly rainfall is about 28% greater between 20 miles (32 km) to 40 miles (64 km) downwind of cities, compared with upwind. Some cities show a total precipitation increase of 51%.
Research has been done in a few areas suggesting that metropolitan areas are less susceptible to weak tornadoes due to the turbulent mixing caused by the warmth of the urban heat island. Using satellite images, researchers discovered that city climates have a noticeable influence on plant growing seasons up to 10 kilometers (6.2 miles) away from a city's edges. Growing seasons in 70 cities in eastern North America were about 15 days longer in urban areas compared to rural areas outside of a city's influence.
Research in China indicates that urban heat island effect contributes to climate warming by about 30%. On the other hand, one 1999 comparison between urban and rural areas proposed that urban heat island effects have little influence on global mean temperature trends. One study concluded that cities change the climate in area 2-4 times larger than their own area. Other suggested that urban heat islands affect global climate by impacting the jet stream. Several studies have revealed increases in the severity of the effect of heat islands with the progress of climate change.
UHIs have the potential to directly influence the health and welfare of urban residents. Within the United States alone, an average of 1,000 people die each year due to extreme heat. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. Research has found that the mortality rate during a heat wave increases exponentially with the maximum temperature, an effect that is exacerbated by the UHI. The number of individuals exposed to extreme temperatures is increased by the UHI-induced warming. The nighttime effect of UHIs can be particularly harmful during a heat wave, as it deprives urban residents of the cool relief found in rural areas during the night.
Research in the United States suggests that the relationship between extreme temperature and mortality varies by location. Heat is more likely to increase the risk of mortality in cities in the northern part of the country than in the southern regions of the country. For example, when Chicago, Denver, or New York experience unusually hot summertime temperatures, elevated levels of illness and death are predicted. In contrast, parts of the country that are mild to hot year-round have a lower public health risk from excessive heat. Research shows that residents of southern cities, such as Miami, Tampa, Los Angeles, and Phoenix, tend to be acclimated to hot weather conditions and therefore less vulnerable to heat related deaths. However, as a whole, people in the United States appear to be adapting to hotter temperatures further north each decade. However, this might be due to better infrastructure, more modern building design, and better public awareness.
Increased temperatures have been reported to cause heat stroke, heat exhaustion, heat syncope, and heat cramps. Some studies have also looked at how severe heat stroke can lead to permanent damage to organ systems. This damage can increase the risk of early mortality because the damage can cause severe impairment in organ function. Other complications of heat stroke include respiratory distress syndrome in adults and disseminated intravascular coagulation. Some researchers have noted that any compromise to the human body's ability to thermoregulate would in theory increase risk of mortality. This includes illnesses that may affect a person's mobility, awareness, or behavior. Researchers have noted that individuals with cognitive health issues (e.g. depression, dementia, Parkinson's disease) are more at risk when faced with high temperatures and "need to take extra care" as cognitive performance has been shown to be differentially affected by heat. People with diabetes, are overweight, have sleep deprivation, or have cardiovascular/cerebrovascular conditions should avoid too much heat exposure. Some common medications that have an effect on thermoregulation can also increase the risk of mortality. Specific examples include anticholinergics, diuretics, phenothiazines and barbiturates. Not only health, but heat can also affect behavior. A U.S. study suggests that heat can make people more irritable and aggressive, noting that violent crimes increased by 4.58 out of 100,000 for every one degree increase in temperature.
A researcher found that high UHI intensity correlates with increased concentrations of air pollutants that gathered at night, which can affect the next day's air quality. These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. The production of these pollutants combined with the higher temperatures in UHIs can quicken the production of ozone. Ozone at surface level is considered to be a harmful pollutant. Studies suggest that increased temperatures in UHIs can increase polluted days but also note that other factors (e.g. air pressure, cloud cover, wind speed) can also have an effect on pollution. Studies from Hong Kong have found that areas of the city with poorer outdoor urban air ventilation tended to have stronger urban heat island effects and had significantly higher all-cause mortality compared to areas with better ventilation.
The Centers for Disease Control and Prevention notes that it "is difficult to make valid projections of heat-related illness and death under varying climate change scenarios" and that "heat-related deaths are preventable, as evidenced by the decline of all-cause mortality during heat events over the past 35 years". However, some studies suggest that the effects of UHIs on health may be disproportionate, since the impacts may be unevenly distributed based on a variety of factors such as age, ethnicity and socioeconomic status. This raises the possibility of health impacts from UHIs being an environmental justice issue.
In recent years, researchers have discovered a strong correlation between neighborhood income and tree canopy cover. In 2010, researchers at Auburn University and University of Southern California found that the presence of trees are "highly responsive to changes in [neighborhood] income." Low-income neighborhoods tend to have significantly fewer trees than neighborhoods with higher incomes. They described this unequal distribution of trees as a demand for "luxury," rather than "necessity." According to the study, "for every 1 percent increase in per capita income, demand for forest cover increased by 1.76 percent. But when income dropped by the same amount, demand decreased by 1.26 percent."
Trees are a necessary feature in combating most of the urban heat island effect because they reduce air temperatures by 10 °F (5.6 °C), and surface temperatures by up to 20-45 °F (11-25 °C). Researchers hypothesized that less-well-off neighborhoods do not have the financial resources to plant and maintain trees. Affluent neighborhoods can afford more trees, on "both public and private property." Part of this is also that wealthier homeowners and communities can afford more land, which can be kept open as green space, whereas poorer ones are often rentals, where landowners try to maximize their profit by putting as much density as possible on their land.
UHIs also impair water quality. Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Additionally, increased urban water body temperatures lead to a decrease in diversity in the water. In August 2001, rains over Cedar Rapids, Iowa, led to a 10.5C (18.9F) rise in the nearby stream within one hour, which led to a fish kill. Since the temperature of the rain was comparatively cool, it could be attributed to the hot pavement of the city. Similar events have been documented across the American Midwest, as well as Oregon and California. Rapid temperature changes can be stressful to aquatic ecosystems. Permeable pavements may mitigate these effects by percolating water through the pavement into subsurface storage areas where it can be dissipated through absorption and evaporation.
Another consequence of urban heat islands is the increased energy required for air conditioning and refrigeration in cities that are in comparatively hot climates. The Heat Island Group estimates that the heat island effect costs Los Angeles about US$100 million per year in energy. Conversely, those that are in cold climates such as Moscow, Russia would have less demand for heating. However, through the implementation of heat island reduction strategies, significant annual net energy savings have been calculated for northern locations such as Chicago, Salt Lake City, and Toronto.
The temperature difference between urban areas and the surrounding suburban or rural areas can be as much as 5 °C (9.0 °F). Nearly 40 percent of that increase is due to the prevalence of dark roofs, with the remainder coming from dark-colored pavement and the declining presence of vegetation. The heat island effect can be counteracted slightly by using white or reflective materials to build houses, roofs, pavements, and roads, thus increasing the overall albedo of the city. Relative to remedying the other sources of the problem, replacing dark roofing requires the least amount of investment for the most immediate return. A cool roof made from a reflective material such as vinyl reflects at least 75 percent of the sun's rays, and emit at least 70 percent of the solar radiation absorbed by the building envelope. Asphalt built-up roofs (BUR), by comparison, reflect 6 percent to 26 percent of solar radiation.
Using light-colored concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature. A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat creating warmer near-surface temperatures. Paving with light-colored concrete, in addition to replacing asphalt with light-colored concrete, communities may be able to lower average temperatures. However, research into the interaction between reflective pavements and buildings has found that, unless the nearby buildings are fitted with reflective glass, solar radiation reflected off light-colored pavements can increase building temperatures, increasing air conditioning demands.
A second option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs. Green roofs are excellent insulators during the warm weather months and the plants cool the surrounding environment. Air quality is improved as the plants absorb carbon dioxide with concomitant production of oxygen. The city of New York determined that the cooling potential per area was highest for street trees, followed by living roofs, light covered surface, and open space planting. From the standpoint of cost effectiveness, light surfaces, light roofs, and curbside planting have lower costs per temperature reduction.
A hypothetical "cool communities" program in Los Angeles has projected that urban temperatures could be reduced by approximately 3 °C (5 °F) after planting ten million trees, reroofing five million homes, and painting one-quarter of the roads at an estimated cost of US$1 billion, giving estimated annual benefits of US$170 million from reduced air-conditioning costs and US$360 million in smog related health savings.
Mitigation strategies include:
Assembly Bill (AB) 32 required the California Air Resources Board to create a scoping plan. This plan is California's approach on how to carry out their goal of combating climate change by reducing greenhouse emissions by 2020 to levels from the 1990s. The scoping plan had four primary programs, advanced clean cars, cap and trade, renewables portfolio standard and low-carbon fuel standard all geared toward increased energy efficiency. The plan has main strategies to reduce green house gases such as having monetary incentives, regulations and voluntary actions. Every five years the scoping plan is updated.
Clean Air Act
The EPA has initiated several air quality requirements that help reduce ground-level ozone that leads to urban heat islands. In the Clean Air Act, one of the EPA's chief policies, there are certain regulations that are put in place to ensure the state's emissions stay below a certain level. Included in the Clean Air Act, all states must set forth a State Implementation Plan (SIP) which is designed to guarantee all states meet a central air quality standard.
State implementation plans and policies
Athens, the capital of Greece, has undertaken initiatives to mitigate the urban heat island effect and reduce the impact of pollution from vehicles. To create green spaces that offer cooling, small unused plots of land are being reconfigured into pocket parks.
The Seattle Green Factor, a multifaceted system for urban landscaping, has seen much success in the mitigation of urban heat islands. The program focuses on areas that are prone to high pollution, such as business districts. There are strict guidelines for any new construction that exceeds roughly 20 parking spaces, and this platform helps developers physically see their levels of pollution while trying different methods of construction to figure out the most effective course of action. Seattle has correspondingly produced a "score sheet" for cities to use in their city planning.
This compendium focuses on a variety of issues dealing with urban heat islands. They describe how urban heat islands are created, who is affected, and how people can make a difference to reduce temperature. It also shows examples of policies and voluntary actions by state and local governments to reduce the effect of urban heat islands.
The Weatherization Assistance Program of the U.S. Department of Energy helps low income recipients by covering their heating bills and helping the families to make their homes energy efficient. In addition, this program allows states to also use the funds to install cooling efficiency measures such as shading devices.
Voluntary green building programs have been promoting the mitigation of the heat island effect for years. For example, one of the ways for a site to earn points under the US Green Building Council's (USGBC) Leadership in Energy and Environmental Design (LEED) Green Building Rating System is to take action that reduces heat islands, minimizing impacts on microclimates and human and wildlife habitats. Credits associated with reflective roofing or planted roofs can help a building achieve LEED certification. Buildings also receive credits by providing shade. Similarly, The Green Building Initiative's Green Globes program awards points to sites that take measures to decrease a building's energy consumption and reduce the heat island effect. As many as 10 points may be awarded to sites with roof coverage from vegetation, highly reflective materials, or a combination of the two.
Moreover, some scholars advocate that microclimate moderation should be included to foster, or at least be harmless to, the initial goals of GB in energy consumption reduction, carbon emission reduction and indoor environmental quality. Based on this, they argue the next generation of green building should be GB-based UHI mitigation system, or 'zero UHI impact building', or 'zero-heat building' or 'microclimate neutral building', aiming to achieve the zero-heat impact on surrounding environments through reasonably designing and operating buildings, or depending on innovative techniques to eliminate the excessive heats, on the basis of GB's goals.
Every year in the U.S. 15% of energy goes towards the air conditioning of buildings in these urban heat islands. According to Rosenfeld et al., "the air conditioning demand has risen 10% within the last 40 years." Home and business owners alike can benefit from building a cool community. A decrease in energy usage directly correlates to cost efficiency. Areas with substantial vegetation and reflective surface materials used for roofs of houses, pavement, and roads are proven to be more effective and cost efficient.
In a case study of the Los Angeles Basin, simulations showed that even when trees are not strategically placed in these urban heat islands, they can still aid in minimization of pollutants and energy reduction. It is estimated that with this wide-scale implementation, the city of Los Angeles can annually save $100M with most of the savings coming from cool roofs, lighter colored pavement, and the planting of trees. With a citywide implementation, added benefits from the lowering smog-level would result in at least one billion dollars of saving per year.
The cost efficiency of green roofs is quite high because of several reasons. According to Carter, "A conventional roof is estimated to be $83.78/m2 while a green roof was estimated at $158.82/m2."[clarification needed] For one, green roofs have over double the lifespan of a conventional roof, effectively decelerating the amount of roof replacements every year. In addition to roof-life, green roofs add stormwater management reducing fees for utilities. The cost for green roofs is more in the beginning, but over a period of time, their efficiency provides financial as well as health benefits.
In Capital E Analysis' conclusions of the financial benefits of green buildings, it was determined that green roofs successfully lowered energy usage and raised health benefits. For every square foot of green roof used in one study the savings amounted to $5.80 energy-wise. There were also savings seen in the emissions, water, and maintenance categories. Overall, the savings amounted to $52.90-$71.30 on average while the cost of going green totaled -$3.00-$5.00.
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. Such effects are removed by homogenization from the raw climate record by comparing urban stations with surrounding stations. While the "heat island" warming is an important local effect, there is no evidence that it biases trends in the homogenized 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 siting 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. The effects of the urban heat island may be overstated. One study stated, "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 in elevation, and thus cooler, than urban areas). 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 microscale and local-scale impacts dominate the mesoscale impact of the urban heat island. Many sections of towns may be warmer than rural sites, but surface weather observations are likely to be made in park "cool islands."
Not all cities show a warming relative to their rural surroundings. After trends were adjusted in urban weather stations around the world to match rural stations in their regions, in an effort to homogenise the temperature record, in 42 percent of cases, cities were getting cooler relative to their surroundings rather than warmer. One reason is that urban areas are heterogeneous, and weather stations are often sited in "cool islands" - parks, for example - within urban areas.
Studies in 2004 and 2006 attempted 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 one study said that "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."
A view often held by those who reject the evidence for 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 of this paper, including Gavin A. Schmidt, have said the results can be explained away as an artifact of spatial autocorrelation. McKittrick and Nicolas Nierenberg stated further that "the evidence for contamination of climatic data is robust across numerous data sets."
The preliminary results of an independent assessment carried out by the Berkeley Earth Surface Temperature group, and made available to the public in October 2011, found that among other scientific concerns raised by skeptics, the urban heat island effect did not bias the results obtained by NOAA, the Hadley Centre and NASA's GISS. The Berkeley Earth group also confirmed that over the past 50 years the land surface warmed by 0.911 °C (1.640 °F), and their results closely matched those obtained from earlier studies.
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.
A 2014 study published in the Proceedings of the National Academy of Sciences of the United States of America looks at the potential of large-scale urban adaptation to counteract the effects of long-term global climate change. The researchers calculate that without any adaptive urban design, by 2100 the expansion of existing U.S. cities into regional megalopolises could raise near-surface temperatures between 1 and 2 °C (1.8 and 3.6 °F) over large regions, "a significant fraction of the 21st-century greenhouse gas-induced climate change simulated by global climate models." Large-scale adaptive design could completely offset this increase, however. For example, the temperature increase in California was calculated to be as high as 1.31 °C (2.36 °F), but a 100% deployment of "cool roofs" would result in a temperature drop of 1.47 °C (2.65 °F)--more than the increase.
The same urban area that is hotter in the day can be colder than surrounding rural areas at ground level at night, leading to a new term urban cold island. Snow cover in rural areas, for example, insulates plants. This was an unexpected discovery when studying the response of plants to urban environments. The urban cold island effect takes place in the early morning because the building within cities block the sun's solar radiation, as well as the wind speed within the urban centre. Both the urban heat island and urban cold island effects are most intense at times of stable meteorological conditions. Several other studies have observed the urban cool island in semi-arid or arid regions. The reason for this phenomenon is the availability of water and vegetation in the urban region as compared to the surroundings.