Urban heat island (UHI)
An urban heat island (UHI) is an area within a city that experiences significantly higher air and surface temperatures compared to its surrounding suburban and rural regions. This phenomenon has been observed since the early 19th century, when meteorologist Luke Howard noted warmer nighttime temperatures in London than in the countryside, attributing this to urban activities and fuel combustion. The UHI effect results from various factors, including urban materials like concrete and asphalt that absorb and retain heat, as well as the reduction of natural landscapes and water bodies that typically moderate temperatures.
Additionally, human activities in cities, such as increased energy consumption and vehicle emissions, contribute to higher local temperatures and can exacerbate air pollution, creating a feedback loop that further intensifies the heat. Research indicates that urban areas are experiencing more hot days than nearby rural locations, a trend that is expected to worsen as urban populations grow. Effective strategies to mitigate UHI effects include enhancing urban vegetation and green spaces, which can help cool the environment. Understanding UHI is crucial for accurately assessing historical temperature records and addressing climate change, as urban growth can distort temperature data comparisons over time.
Urban heat island (UHI)
Definition
An urban heat island is a metropolitan area that is significantly warmer in surface and air temperature than its suburban and rural surroundings. English amateur meteorologist Luke Howard, in his book The Climate of London (1818), described temperature differences between London and its surrounding countryside. He noted that it was warmer at night in London than in the country and speculated that the cause might be the burning of fuels. Research over the subsequent two centuries has charted a variety of differences between cities and their surroundings, as distinctive urban landscapes, domestic and industrial structures, and the behavior of urban dwellers affect the ways in which solar and other heat enters and exits the area. Urbanization causes changes to the preexisting natural landscape, as original materials (soil, vegetation, rock, water, and so on) are gradually replaced or modified with materials (concrete, tile, and many others) that, in conjunction with the activities and life patterns of a city’s inhabitants, result in greater heat retention. This heat retention can in turn affect weather patterns.
Significance for Climate Change
Scientists have identified many interconnected causes for the urban heat island effect. During the day, the sun warms buildings and roadways. Pollution from automotive traffic and industrial processes contributes to the formation of clouds and smog, which help trap heat, and tall buildings limit the ability of winds to disperse such formations (the “canyon effect”). The predominance of land structures and the paucity of bodies of water lessen the influence of evaporation, which would use some of the heat energy for the formation of water vapor. Instead, that energy raises the ambient temperature. The concentration of human bodies in cities also contributes to their heat level. The increase in city heat in turn encourages activities, such as the use of air conditioning, that further contribute to localized heat increases, since air conditioners make interiors cooler by making the exterior city warmer. Moreover, increased power plant emissions resulting from the additional consumption of electricity contribute to global warming.
To the extent that urban areas encourage structures or behaviors that stimulate additional activities, increasing emissions of greenhouse gases (GHGs), urban heat islands may be viewed as contributing indirectly to global warming. (If cities are hotter than rural areas, then those who move to cities will be more likely to use energy to cool themselves than they otherwise would be.) Since 2004, according to a 2014 article in Scientific American, cities have more searing hot days each year. In fact, twelve US cities averaged at least twenty more days a year above 90 degrees than nearby rural areas. In two-thirds of sixty cities analyzed, urbanization and climate change appear to be combining to increase summer heat faster than climate change alone is raising regional temperatures. In 2022, cities across the US set records for heat waves of 90-plus degrees, with streaks of 90-plus degree days stretching to nearly seventy in some areas. By 2030, about 89 percent of North America's population was expected to live in urban areas, according to 2021 data from the University of Michigan's Center for Sustainable Systems. As large metropolitan areas fuse into megalopolises or megacities, the problems only become exacerbated. Accordingly, students of the urban heat island effect have seen the need for various degrees of urban redesign as a key element in attempts to mitigate the noxious consequences of the effect. Chief among these is increase in, and optimal distribution of, urban vegetation and green space, including the planting and sustaining of suitable trees.
The urban heat island effect must be taken into account in any attempt to read the historical record of Earth’s temperature. To compare today’s temperature with that of a century ago requires comparability, and that can be affected by urban growth. Places where temperatures were measured in the past often have since experienced significant urban growth, with the associated increased temperature measurements. This growth makes direct comparisons of present and past measurements difficult. In light of this problem, climatologists must adjust the data on the basis of their best guess about distorting factors.
Bibliography
Gartland, Lisa. Heat Islands: Understanding and Mitigating Heat in Urban Areas. Basingstoke, Hampshire, England: Earthscan/James and James, 2008.
Heikkinen, Niina. "How People Make Summer Hotter." Scientific American. Scientific American, 25 Nov. 2014. Web. 24 Mar. 2015.
Kenward, Alyson, Dan Yawitz, Todd Sanford, and Regina Wang. "Heat Islands Cook U.S. Cities Faster Than Ever." Scientific American. Scientific American, 22 Aug. 2014. Web. 24 Mar. 2015.
"Learn about Heat Islands." Environmental Protection Agency, 20 Aug. 2024, www.epa.gov/heatislands/learn-about-heat-islands. Accessed 9 Dec. 2024.
Lee, Derek O. “Urban Climates.” Progress in Physical Geography 8, no. 1 (1984): 1-31.
McKendry, Ian G. “Progress Reports: Applied Climatology.” Progress in Physical Geography 27, no. 4 (2003): 597-606.
Maslin, Mark. Global Warming: A Very Short Introduction. New York: Oxford University Press, 2004.
Parker, David E. “A Demonstration That Large-Scale Warming Is Not Urban.” Journal of Climate 19 (2005): 2882-2895.
Peterson, Thomas C. “Assessment of Urban Versus Rural In Situ Surface Temperatures in the Contiguous United States: No Difference Found.” Journal of Climate 16, no. 18 (2003): 2941-2959.
Pine, Joshua, et al. "Urban Heat Island Effect Solutions and Funding." National League of Cities, 13 Feb. 2023, www.nlc.org/article/2023/02/13/urban-heat-island-effect-solutions-and-funding/. Accessed 9 Dec. 2024.
Souch, Catherine, and Sue Grimmond. “Applied Climatology: Urban Climates.” Progress in Physical Geography 30, no. 2 (2006): 270-279.
Spencer, Roy W. Climate Confusion: How Global Warming Hysteria Leads to Bad Science, Pandering Politicians, and Misguided Policies That Hurt the Poor. New York: Encounter Books, 2008.
Sturman, Andrew P. “Applied Climatology.” Progress in Physical Geography 22, no. 4 (1998): 558-565.
"U.S. Cities Factsheet." Center for Sustainable Systems, 2024, css.umich.edu/publications/factsheets/built-environment/us-cities-factsheet. Accessed 9 Dec. 2024.
Waldrop, Mitchell. "Cities Are Getting So Hot It's Deadly—Can We Fix It?" Inverse, 20 Feb. 2024, www.inverse.com/science/urban-heat-wave-survival-tips. Accessed 9 Dec. 2024.