Urban heat island

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Example of dense urban living without green spaces which leads to a pronounced urban heat island effect (Milan, Italy) Milan skyscrapers aerial view.jpg
Example of dense urban living without green spaces which leads to a pronounced urban heat island effect (Milan, Italy)
Example of an inner city green space which can reduce the urban heat island effect (Central Park, New York) New York City-Manhattan-Central Park (Gentry).jpg
Example of an inner city green space which can reduce the urban heat island effect (Central Park, New York)

Urban areas usually experience the urban heat island (UHI) effect, that is, they are significantly warmer than surrounding rural areas. The temperature difference is usually larger at night than during the day, [1] and is most apparent when winds are weak, under block conditions, noticeably during the summer and winter. The main cause of the UHI effect is from the modification of land surfaces while waste heat generated by energy usage is a secondary contributor. [2] [3] [4] Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population. [5] 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. [6]

Contents

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. [7] The impact in a city can change a lot based on its local environment. Heat can be reduced by tree cover and green space which act as sources of shade and promote evaporative cooling. [8] Other options include green roofs, passive daytime radiative cooling applications, and the use of lighter-colored surfaces and less absorptive building materials. These reflect more sunlight and absorb less heat. [9] [10] [11]

Climate change is not the cause of urban heat islands but it is causing more frequent and more intense heat waves which in turn amplify the urban heat island effect in cities. [12] : 993  Compact, dense urban development may increase the urban heat island effect, leading to higher temperatures and increased exposure. [13]

Definition

A definition of urban heat island is: "The relative warmth of a city compared with surrounding rural areas." [14] : 2926  This relative warmth is caused by "heat trapping due to land use, the configuration and design of the built environment, including street layout and building size, the heat-absorbing properties of urban building materials, reduced ventilation, reduced greenery and water features, and domestic and industrial heat emissions generated directly from human activities". [14] : 2926 

Description

Mechanism of the urban heat island effect: the densely-built downtown areas tend to be warmer than suburban residential areas or rural areas. Urban heat island.svg
Mechanism of the urban heat island effect: the densely-built downtown areas tend to be warmer than suburban residential areas or rural areas.
Tokyo, an example of an urban heat island. Normal temperatures of Tokyo go up higher than those of the surrounding area. HeatIsland Kanto en.png
Tokyo, an example of an urban heat island. Normal temperatures of Tokyo go up higher than those of the surrounding area.

Diurnal variability

Cities often experience stronger urban heat island effects at night; effects can vary with location and topography of metropolitan areas. Urban heat island variation.jpg
Cities often experience stronger urban heat island effects at night; effects can vary with location and topography of metropolitan areas.

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 reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air.[ citation needed ] As a result, high daytime surface temperatures within the UHI can be 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. 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, keeping surface air warm from the still-warm urban surfaces, resulting in warmer nighttime air temperatures within the UHI.

Generally speaking, the difference in temperature between the urban and surrounding rural area is more pronounced at night than in daytime. [15] For example, in the United States, the temperature in urban areas tends to be warmer than the surrounding area by about 1–7 °F (0.6–4 °C) during the daytime, and about 2–5 °F (1–3 °C) warmer at night. [16] However, the difference is more pronounced during the day in arid climates such as those in southeastern China and Taiwan. [17] [18] Studies have shown that diurnal variability is impacted by several factors including local climate and weather, seasonality, humidity, vegetation, surfaces, and materials in the built environment. [19] [16] [20] [21]

Seasonal variability

Seasonal variability is less well understood than diurnal variability of the urban heat island temperature difference. [22] Complex relationships between precipitation, vegetation, solar radiation, and surface materials in various local climate zones play interlocking roles that influence seasonal patterns of temperature variation in a particular urban heat island. [22] [23] [24] [25]

Measurements and predictions

Urban Heat Island Index (UHII)

One method to quantify the UHI effect within urban areas is the UHI Index created by the Californian EPA in 2015. It compares the temperature of a surveyed area and rural reference points upwind from the surveyed area, at a height of two meters above ground level. The difference in temperature in degrees Celsius is taken hourly and differences with an increased urban temperature compared to the reference points are summed up, creating an amount of degree-Celsius-hours, which is the UHI Index of the surveyed area. The measure of Celsius-hours might be averaged over many days, but is specified as Celsius-hours per averaged day. [26] [27] [28]

The index was created to estimate the expected use of air conditioning and resulting greenhouse gas emissions in California. [27] The index does not consider values of or differences in wind-speed, humidity, or solar influx, which might influence perceived temperature or the operation of air conditioners. [28]

Models and simulations

If a city or town has a good system of taking weather observations the UHI can be measured directly. [29] An alternative is to use a complex simulation of the location to calculate the UHI, or to use an approximate empirical method. [30] [31] 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. [32] The heat island effect was 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. [32] Modern simulation environments include ENVI-met, which simulates all interactions between building and ground surfaces, plants and ambient air. [33]

Causes

Example of dense urban living: High-rise buildings of Manhattan during sunset High-rise buildings of Manhattan during sunset.jpg
Example of dense urban living: High-rise buildings of Manhattan during sunset
Thermal (top) and vegetation (bottom) locations around New York City via infrared satellite imagery. A comparison of the images shows that where vegetation is dense, temperatures are lower. Newyork heat island.jpg
Thermal (top) and vegetation (bottom) locations around New York City via infrared satellite imagery. A comparison of the images shows that where vegetation is dense, temperatures are lower.

Urban design

There are several causes of an urban heat island (UHI) related to common urban design aspects. 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; [2] 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. [34]

Pavements, parking lots, roads or, more generally speaking transport infrastructure, contribute significantly to the urban heat island effect. [35] For example, pavement infrastructure is a main contributor to urban heat during summer afternoons in Phoenix, United States. [35]

Another major reason is the lack of evapotranspiration (for example, through lack of vegetation) in urban areas. [36] The U.S. Forest Service found in 2018 that cities in the United States are losing 36 million trees each year. [37] With a decreased amount of vegetation, cities also lose the shade and evaporative cooling effect of trees. [38] [39]

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. [4] [40] [41]

Heat islands can be affected by proximity to different types of land cover, so that proximity to barren land causes urban land to become hotter and proximity to vegetation makes it cooler. [42]

Air pollution

High levels of air pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. [34] 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. [43]

Climate change as an amplifier

Climate change is not a cause but an amplifier of the urban heat island effect. The IPCC Sixth Assessment Report from 2022 summarized the available research accordingly: "Climate change increases heat stress risks in cities [...] and amplifies the urban heat island across Asian cities at 1.5 °C and 2 °C warming levels, both substantially larger than under present climates [...]." [44] : 66 

The report goes on to say: "In a warming world, increasing air temperature makes the urban heat island effect in cities worse. One key risk is heatwaves in cities that are likely to affect half of the future global urban population, with negative impacts on human health and economic productivity." [12] : 993 

There are unhelpful interactions between heat and built infrastructure: These interactions increase the risk of heat stress for people living in cities. [12] : 993 

Impacts

Example of urbanization: Dubai Dubai skyline 2015 (crop).jpg
Example of urbanization: Dubai

On weather and climate

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. [45] 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. [46] 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 and 40 miles (32 and 64 km) downwind of cities, compared with upwind. [47] Some cities show a total precipitation increase of 51%. [48]

One study concluded that cities change the climate in area two–four times larger than their own area. [49] One 1999 comparison between urban and rural areas proposed that urban heat island effects have little influence on global mean temperature trends. [50] Others suggested that urban heat islands affect global climate by impacting the jet stream. [51]

On human health

Image of Atlanta, Georgia, showing temperature distribution, with blue showing cool temperatures, red warm, and hot areas appear white Atlanta thermal.jpg
Image of Atlanta, Georgia, showing temperature distribution, with blue showing cool temperatures, red warm, and hot areas appear white

UHIs have the potential to directly influence the health and welfare of urban residents. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. The number of individuals exposed to extreme temperatures is increased by the UHI-induced warming. [52] 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. [53]

Increased temperatures have been reported to cause heat illnesses, such as heat stroke, heat exhaustion, heat syncope, and heat cramps. [54] [55]

Extreme heat is the deadliest form of weather in the U.S. Per a study by Professor Terri Adams-Fuller, heat waves kill more people in the U.S. than hurricanes, floods, and tornadoes combined. [56] These heat illnesses are more common within medium-to-large metro areas than the rest of the U.S., largely in part due to UHIs. Heat illnesses can also be compounded when combined with air pollution which is common in many urban areas. [55]

Heat exposure can have adverse effects on mental health. Increases in temperature can contribute to increased aggression, as well as more cases of domestic violence and substance abuse. [57] Greater heat can also negatively impact school performance and education. According to a study by Hyunkuk Cho of Yeungnam University, an increased number of days with extreme heat each year correlates to a decrease in student test scores. [58]

High UHI intensity correlates with increased concentrations of air pollutants that gathered at night, which can affect the next day's air quality. [59] These pollutants include volatile organic compounds, carbon monoxide, nitrogen oxides, and particulate matter. [60] [61] The production of these pollutants combined with the higher temperatures in UHIs can quicken the production of ozone. [59] Ozone at surface level is considered to be a harmful pollutant. [59] 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. [59]

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 [62] and had significantly higher all-cause mortality [63] compared to areas with better ventilation. Another study employing advanced statistical methods in Babol city, Iran, revealed a significant increase in Surface Urban Heat Island Intensity (SUHII) from 1985 to 2017, influenced by both geographic direction and time. This research, enhancing the understanding of SUHII's spatial and temporal variations, emphasizes the need for precise urban planning to mitigate the health impacts of urban heat islands. [64] Surface UHI's are more prominent during the day and are measured using the land surface temperature and remote sensing. [65]

On water bodies and aquatic organisms

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 biodiversity in the water. [66] For example, in August 2001, rains over Cedar Rapids, Iowa led to a 10.5 °C (18.9 °F) rise in the nearby stream within one hour, resulting in a fish kill which affected an estimated 188 fish. [67] Since the temperature of the rain was comparatively cool, the deaths 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. [68] Rapid temperature changes can be stressful to aquatic ecosystems. [69]

With the temperature of the nearby buildings sometimes reaching a difference of over 50 °F (28 °C) from the near-surface air temperature, precipitation warms rapidly, and run-off into nearby streams, lakes and rivers (or other bodies of water) to provide excessive thermal pollution. The increase in thermal pollution has the potential to increase water temperature by 20 to 30 °F (11 to 17 °C). This increase causes the fish species inhabiting the body of water to undergo thermal stress and shock due to the rapid change in temperature of their habitat. [70]

Permeable pavements may reduce these effects by percolating water through the pavement into subsurface storage areas where it can be dissipated through absorption and evaporation. [71]

On animals

Species that are good at colonizing can use conditions provided by urban heat islands to thrive in regions outside of their normal range. Examples of this include the grey-headed flying fox (Pteropus poliocephalus) and the common house gecko (Hemidactylus frenatus). [72] Grey-headed flying foxes, found in Melbourne, Australia, colonized urban habitats following the 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 temperate climates, urban heat islands will extend the growing season, therefore altering breeding strategies of inhabiting species. [72] This can be best observed in the effects that urban heat islands have on water temperature (see effects on water bodies).

Urban heat islands caused by cities have altered the natural selection process. [72] Selective pressures like temporal variation in food, predation and water are relaxed causing 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 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 urban habitats than on oak trees in rural habitats. Over time spent living in urban habitats, they have adapted to thrive in warmer climates than in cooler ones. [73]

On energy usage for cooling

Images of Salt Lake City show positive correlation between white reflective roofs and cooler temperatures. Image A depicts an aerial view of Salt Lake City, Utah, site of 865000 sqft white reflective roof. Image B is a thermal infrared image of same area, showing hot (red and yellow) and cool (green and blue) spots. The reflective vinyl roof, not absorbing solar radiation, is shown in blue surrounded by other hot spots. NASA SLC Urban Heat Island Effect Roof.jpg
Images of Salt Lake City show positive correlation between white reflective roofs and cooler temperatures. Image A depicts an aerial view of Salt Lake City, Utah, site of 865000 sqft white reflective roof. Image B is a thermal infrared image of same area, showing hot (red and yellow) and cool (green and blue) spots. The reflective vinyl roof, not absorbing solar radiation, is shown in blue surrounded by other hot spots.

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 effect costs Los Angeles about US$ 100 million per year in energy (in the year 2000). [74] 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. [75]

Every year in the U.S. 15% of energy goes towards the air conditioning of buildings in these urban heat islands. It was reported in 1998 that "the air conditioning demand has risen 10% within the last 40 years." [76]

Increases in air conditioning use also serve to worsen the effects of UHIs at night. While cooler nights are often a reprieve from heat waves during the day, the residual heat created by the use of air conditioning systems can lead to higher nighttime temperatures. According to a study by Professor Francisco Salamanca Palou and colleagues, this residual heat can cause nighttime increases of up to 1 °C in urban areas. [77] Increased energy use from air conditioners also contributes to carbon emissions, which doubly exacerbates the effects of UHIs.

Options for reducing heat island effects

Botanical Garden in Lublin, Poland Ogrod botaniczny UMCS.png
Botanical Garden in Lublin, Poland

Strategies to improve urban resilience by reducing excessive heat in cities include: Planting trees in cities, cool roofs (painted white or with reflective coating) and light-coloured concrete, green infrastructure (including green roofs), passive daytime radiative cooling. [78] [61]

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-coloured 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. [79]

Concentric expansion of cities is unfavourable in terms of the urban heat island phenomenon. It is recommended to plan the development of cities in strips, consistent with the hydrographic network, taking into account green areas with various plant species. [80] In this way, it was planned to build urban settlements stretching over large areas, e.g. Kielce, Szczecin and Gdynia in Poland, Copenhagen in Denmark and Hamburg, Berlin and Kiel in Germany.

Planting trees in cities

Planting trees around the city can be another way of increasing albedo and decreasing the urban heat island effect. [81] [82] It is recommended to plant deciduous trees because they can provide many benefits such as more shade in the summer and not blocking warmth in winter. [83] 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), [84] and surface temperatures by up to 20–45 °F (11–25 °C). [85] Another benefit of having trees in a city is that trees also help fight global warming by absorbing CO2 from the atmosphere.

Cool roofs and light-coloured concrete

Green roof of Chicago City Hall 20080708 Chicago City Hall Green Roof.JPG
Green roof of Chicago City Hall

Painting rooftops white has become a common strategy to reduce the heat island effect. [86] In cities, there are many dark coloured surfaces that absorb the heat of the sun in turn lowering the albedo of the city. [86] White rooftops allow high solar reflectance and high solar emittance, increasing the albedo of the city or area the effect is occurring. [86]

Additionally, covering rooftops with a reflective coating, has shown to be an effective measure to reduce solar heat gain. A study led by Oscar Brousse from University College London, which simulated the impact of various cooling measures in London found that rooftops, which were either painted white or had reflective coating, proved to be the most effective solution for reducing outdoor temperatures at the pedestrian level, outperforming solar panels, green roofs, and tree cover. The study simulated the impact of various cooling measures in London during a 2018 heatwave, finding that the so-called cool roofs could reduce average outdoor temperatures by 1.2 °C, and up to 2 °C in certain areas. In comparison, additional tree cover reduced temperatures by 0.3 °C and solar panels by 0.5° C. [78]

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. [87]

Using light-coloured concrete has proven effective in reflecting up to 50% more light than asphalt and reducing ambient temperature. [88] A low albedo value, characteristic of black asphalt, absorbs a large percentage of solar heat creating warmer near-surface temperatures. Paving with light-coloured concrete, in addition to replacing asphalt with light-coloured concrete, communities may be able to lower average temperatures. [89] 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-coloured pavements can increase building temperatures, increasing air conditioning demands. [90] [91]

There are specific paint formulations for daytime radiative cooling that reflect up to 98.1% of sunlight. [92] [93]

Green infrastructure

Grassed tramway track in Budapest, Hungary Budapest, CAF Urbos 3, 4.jpg
Grassed tramway track in Budapest, Hungary

Green roofs are excellent insulators during the warm weather months and the plants cool the surrounding environment. Plants can improve air quality as they absorb carbon dioxide and concomitantly produce oxygen. [94] Green roofs can also have positive impacts on stormwater management and energy consumption. [95] Cost can be a barrier to implementing a green roof. [96] [97] Several factors influence the cost of a green roof, including design and soil depth, location, and the price of labor and equipment in that market, which is typically lower in more developed markets where there is more experience designing and installing green roofs. [98] The individualized context of each green roof presents a challenge for making broad comparisons and assessments, and focusing only on monetary costs may leave out the social, environmental, and public health benefits green roofs provide. [97] Global comparisons of green roof performance are further challenged by the lack of a shared framework for making such comparisons. [97]

Stormwater management is another option to help mitigate the effect of the urban heat island. Stormwater management is the controlling the water produced by the storm in a way that protects property and infrastructure. [99] Urban infrastructure like streets, sidewalks, and parking lots do not allow for water to penetrate into the earth's surface causing water to flood. By using stormwater management, you can control the flow of the water in ways that can mitigate UHI effect. One way is using a stormwater management technique called pervious pavement system (PPS). This technique has been used in over 30 countries and found to be successful in stormwater management and UHI mitigation. The PPS allows water to flow through the pavement allowing for the water to be absorbed causing the area to be cooled by evaporation. [100]

Green parking lots use vegetation and surfaces other than asphalt to limit the urban heat island effect.

This section is an excerpt from Green infrastructure.[ edit ]
Runoff from the vicinity flows into an adjacent bioswale Meridian Hill Bioswale.jpg
Runoff from the vicinity flows into an adjacent bioswale
Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature. [101] The main components of this approach include stormwater management, climate adaptation, the reduction of heat stress, increasing biodiversity, food production, better air quality, sustainable energy production, clean water, and healthy soils, as well as more anthropocentric functions, such as increased quality of life through recreation and the provision of shade and shelter in and around towns and cities. [102] [103] Green infrastructure also serves to provide an ecological framework for social, economic, and environmental health of the surroundings. [104] More recently scholars and activists have also called for green infrastructure that promotes social inclusion and equity rather than reinforcing pre-existing structures of unequal access to nature-based services. [105]

Passive daytime radiative cooling

A passive daytime radiative cooling roof application can double the energy savings of a white roof, attributed to high solar reflectance and thermal emittance in the infrared window, [106] with the highest cooling potential in hot and dry cities such as Phoenix and Las Vegas. [107] When installed on roofs in dense urban areas, passive daytime radiative cooling panels can significantly lower outdoor surface temperatures at the pedestrian level. [10] [11]

Society and culture

History of research

The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. [108] A description of the very first report of the UHI by Luke Howard said that the urban center of London was warmer at night than the surrounding countryside by 2.1 °C (3.7 °F). [109]

Investigations of the urban atmosphere continued throughout the nineteenth century. Between the 1920s and the 1940s, researchers in the emerging field of local climatology or microscale meteorology in Europe, Mexico, India, Japan, and the United States pursued new methods to understand the phenomenon.

In 1929, Albert Peppler used the term in a German publication believed to be the first instance of an equivalent to urban heat island: städtische Wärmeinsel (which is urban heat island in German). [110] 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. [111]

Leonard O. Myrup published the first comprehensive numerical treatment to predict the effects of the urban heat island (UHI) in 1969. [32] His paper surveys UHI and criticizes then-existing theories as being excessively qualitative.

Aspects of social inequality

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, [60] [112] ethnicity and socioeconomic status. [113] This raises the possibility of health impacts from UHIs being an environmental justice issue. Studies have shown that communities of color in the United States have been disproportionately affected by UHI. [114] [115] [116]

There is a correlation between neighborhood income and tree canopy cover. [117] Low-income neighborhoods tend to have significantly fewer trees than neighborhoods with higher incomes. [118] 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". [119] One reason for this discrepancy is that wealthier homeowners and communities can afford more land, which can be kept open as green space, whereas poorer housing often takes the form of rentals, where landowners try to maximize their profit by putting as much housing density as possible on their land. [120]

Chief heat officers

Beginning in the 2020s, a number of cities worldwide began creating Chief Heat Officer positions to organize and manage work counteracting the urban heat island effect. [121] [122]

Examples

United States

Bill S.4280, [123] introduced to the U.S. Senate in 2020, would authorize the National Integrated Heat Health Information System Interagency Committee (NIHHIS) to tackle extreme heat in the United States. [124] Successful passage of this legislation would fund NIHHIS for five years and would instate a $100 million grant program within NIHHIS to encourage and fund urban heat reduction projects, including those using cools roofs and pavements and those improving HVAC systems. As of July 22, 2020 the bill has not moved past introduction to Congress.

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. [125]

Los Angeles

A hypothetical "cool communities" program in Los Angeles has projected in 1997 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. [83]

In a case study of the Los Angeles Basin in 1998, 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. [76]

Los Angeles TreePeople is an example of how tree planting can empower a community. TreePeople provides the opportunity for people to come together, build capacity, community pride and the opportunity to collaborate and network with each other. [126]

Los Angeles has also begun to implement a Heat Action Plan to address the city's needs at a more granular level than the solutions provided by the state of California. The city uses the LA Equity Index in an effort to ensure that the effects of extreme heat are mitigated in an equitable manner. [127]

Virginia

In 2021, Climate Adaptation Planning Analysis (CAPA) received funding from the National Oceanic and Atmospheric Administration to conduct Heat Mapping across the United States. [128] Ten areas from Virginia – Abington, Arlington, Charlottesville, Farmville, Harrisonburg, Lynchburg, Petersburg, Richmond, Salem, Virginia Beach and Winchester – participated in the heat watch campaign. This campaign consisted of 213 Volunteers brought together by campaign organizers who made 490,423 Heat Measurements across 70 Routes total. After taking measurements throughout the day, equipment and data was sent back to CAPA where it was analyzed using machine learning algorithms. After analysis of the data, CAPA came back together with campaign organizers from each area to discuss potential plans for each town in the future.

New York

New York City implemented its "Cool Neighborhoods NYC" program in 2017 intending to mitigate the effects of extreme urban heat. One of the plan's ambitions was to increase funding for the city's Low-Income Home Energy Assistance Program. Specifically, the plan sought to increase funding for cooling solutions for lower-income families. [129]

India

Several cities in India experience significant urban heat island effects due to rapid urbanization, loss of green cover, and extensive concretization. A report by The Hindu highlights that metropolitan areas like Delhi, Bengaluru, Chennai, Jaipur, Ahmedabad, Mumbai, and Kolkata have seen temperature differences ranging from 1°C to 6°C compared to their rural surroundings. These urban heat islands not only increase the local temperatures but also exacerbate the impacts of heatwaves, leading to higher energy consumption for cooling and posing health risks to vulnerable populations. [130]

Mumbai

Mumbai, India's financial hub and one of the most densely populated cities globally, is significantly affected by the urban heat island effect. Rapid urbanization, extensive concretization, and loss of green spaces have led to higher temperatures in the city compared to its surroundings. According to a report, Mumbai is projected to spend twice as much as New York City to manage urban heat generated due to concretization. This increased expenditure highlights the severity of the urban heat island effect in Mumbai and its impact on the city's infrastructure and residents. [131]

See also

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Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 to 1. Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time.

<span class="mw-page-title-main">Microclimate</span> Local set of atmospheric conditions that differ significantly from the surrounding area

A microclimate is a local set of atmospheric conditions that differ from those in the surrounding areas, often slightly but sometimes substantially. The term may refer to areas as small as a few square meters or smaller or as large as many square kilometers. Because climate is statistical, which implies spatial and temporal variation of the mean values of the describing parameters, microclimates are identified as statistically distinct conditions which occur and/or persist within a region. Microclimates can be found in most places but are most pronounced in topographically dynamic zones such as mountainous areas, islands, and coastal areas.

Energy-efficient landscaping is a type of landscaping designed for the purpose of conserving energy. There is a distinction between the embedded energy of materials and constructing the landscape, and the energy consumed by the maintenance and operations of a landscape.

In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation.

The climate in urban areas differs from that in neighboring rural areas, as a result of urban development. Urbanization greatly changes the form of the landscape, and also produces changes in an area's air. The study of urban climate is urban climatology.

Urban climatology is the study of urban climate. It is a branch of climatology that concerns interactions between urban areas and the atmosphere, the effects they have on one another, and the varying spatial and temporal scales at which these processes occur.

<span class="mw-page-title-main">Reflective surfaces (climate engineering)</span>

Reflective surfaces, or ground-based albedo modification (GBAM), is a solar radiation management method of enhancing Earth's albedo. The IPCC described this method as "whitening roofs, changes in land use management, change of albedo at a larger scale ."

<span class="mw-page-title-main">Waste heat</span> Heat that is produced by a machine that uses energy, as a byproduct of doing work

Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.

<span class="mw-page-title-main">Sustainable drainage system</span> Designed to reduce the potential impact of development

Sustainable drainage systems are a collection of water management practices that aim to align modern drainage systems with natural water processes and are part of a larger green infrastructure strategy. SuDS efforts make urban drainage systems more compatible with components of the natural water cycle such as storm surge overflows, soil percolation, and bio-filtration. These efforts hope to mitigate the effect human development has had or may have on the natural water cycle, particularly surface runoff and water pollution trends.

<span class="mw-page-title-main">Urban green space</span> Green area planned in an urban location

In land-use planning, urban green spaces are open-space areas reserved for parks and other "green spaces", including plant life, water features – also referred to as blue spaces – and other kinds of natural environment. Most urban open spaces are green spaces, but occasionally include other kinds of open areas. The landscape of urban open spaces can range from playing fields, to highly maintained environments, to relatively natural landscapes.

An urban thermal plume describes rising air in the lower altitudes of the Earth's atmosphere caused by urban areas being warmer than surrounding areas. Over the past thirty years there has been increasing interest in what have been called urban heat islands (UHI), but it is only since 2007 that thought has been given to the rising columns of warm air, or ‘thermal plumes’ that they produce. Common on-shore breezes at the seaside on a warm day, and off-shore breezes at night are caused by the land heating up faster on a sunny day and cooling faster after sunset, respectively. Thermals, or warm airs, that rise from the land and sea affect the local microscale meteorology; and perhaps at times the mesometeorology. Urban thermal plumes have as powerful although less localized an effect.

<span class="mw-page-title-main">Urban resilience</span> Ability of a city to function after a crisis

Urban resilience has conventionally been defined as the "measurable ability of any urban system, with its inhabitants, to maintain continuity through all shocks and stresses, while positively adapting and transforming towards sustainability".

<span class="mw-page-title-main">Low-impact development (U.S. and Canada)</span>

Low-impact development (LID) is a term used in Canada and the United States to describe a land planning and engineering design approach to manage stormwater runoff as part of green infrastructure. LID emphasizes conservation and use of on-site natural features to protect water quality. This approach implements engineered small-scale hydrologic controls to replicate the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating, and detaining runoff close to its source. Green infrastructure investments are one approach that often yields multiple benefits and builds city resilience.

Cool pavement is defined by the United States Environmental Protection Agency as pavement that implements technologies contributing to heat island effect reduction efforts. Most cool pavements either increase pavement albedo to reflect shortwave radiation out of the atmosphere and reduce heat transfer to the Earth’s surface, or use evaporative water cooling through the pavement to lower ambient temperatures. Newer technologies involve energy harvesting, heat storage methods, and phase change materials. Cool pavements are commonly made with reflective coatings or aggregates applied to conventional pavements or incorporation of porous or permeable materials. While pavement load capability may be reduced with more frequent maintenance requirements, cool pavements show promising results in reducing ambient temperatures and reducing energy usage.

<span class="mw-page-title-main">Oasis effect</span> Alteration of a surrounding environment by the presence of a water source

The oasis effect refers to the creation of a local microclimate that is cooler than the surrounding dry area due to evaporation or evapotranspiration of a water source or plant life and higher albedo of plant life than bare ground. The oasis effect is so-named because it occurs in desert oases. Urban planners can design a city's layout to optimize the oasis effect to combat the urban heat island effect. Since it depends on evaporation, the oasis effect differs by season.

<span class="mw-page-title-main">Heat dome</span> Weather phenomenon

A heat dome is a weather phenomenon consisting of extreme heat that is caused when the atmosphere traps hot air as if bounded by a lid or cap. Heat domes happen when strong high pressure atmospheric conditions remain stationary for an unusual amount of time, preventing convection and precipitation and keeping hot air "trapped" within a region. This can be caused by multiple factors, including sea surface temperature anomalies and the influence of a La Niña. The upper air weather patterns are slow to move, referred to by meteorologists as an Omega block.

<span class="mw-page-title-main">Passive daytime radiative cooling</span> Management strategy for global warming

Passive daytime radiative cooling (PDRC) is the use of unpowered, reflective/thermally-emissive surfaces to lower the temperature of a building or other object.

Transpirational cooling is the cooling provided as plants transpire water. Excess heat generated from solar radiation is damaging to plant cells and thermal injury occurs during drought or when there is rapid transpiration which produces wilting. Green vegetation contributes to moderating climate by being cooler than adjacent bare earth or constructed areas. As plant leaves transpire they use energy to evaporate water aggregating up to a huge volume globally every day.

Urban forest inequity, also known as shade inequity or tree canopy inequity, is the inequitable distribution of trees, with their associated benefits, across metropolitan areas. This phenomenon has a number of follow-on effects, including but not limited to measurable impacts on faunal biodiversity and the urban heat island effect. Urban heat inequity occurs when intra-urban heat islands, with their associated negative physical and emotional health consequences, are more common and more intense in lower-income communities.

Urban heat inequity, also termed thermal inequity, is an unequal distribution of heat in urban areas or neighborhoods within them, which causes disproportionate impacts to people living in those communities. Unequal threat of heat stress in urban environments is often correlated with differences in demographics, including racial and ethnic background, income, education level, and age. While the general impacts of urban heat inequity depend on the city studied, negative effects typically act on historically marginalized communities. The idea is closely tied to the urban heat island effect, where a major cause to urban heat inequity is increased urbanization.

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