It’s getting hot in here: The role of Urban Heat Islands

by Julie Cook

February 11th, 2008 in Community, Environment, News & Info

What is an Urban Heat Island? And why should you care? Depends on who you ask.

An Urban Heat Island (UHI) is a metropolitan area that is significantly warmer than its surroundings. The buildings, concrete, asphalt, and human and industrial activity of urban areas have caused cities to maintain higher temperatures than their surrounding countryside. The air in a UHI can be as much as 20°F higher than rural areas surrounding the city.

As with many environmental claims, UHI’s impact on global warming varies depending on who you ask. Regardless, it is a proven fact that city centers, which are packed with more people, buildings, cars, and developments per square foot than their suburban counterparts, contribute in some measure to negative environmental factors.

Impact

Urban heat islands affect air quality, pose numerous risks to public health, demand higher energy use, and contribute to global warming.

Poor Air Quality: Hotter air in cities increases both the frequency and intensity of ground-level ozone (the main ingredient in smog) and can push metropolitan areas out of compliance with federal air quality standards. Smog is formed when air pollutants such as nitrogen oxides (NOx) and Volatile Organic Compounds (VOCs) are mixed with sunlight and heat. The rate of this chemical reaction increases when temperatures exceed 7°F.

Risks

Public Health: The UHI Effect prolongs and intensifies heat waves in cities, making residents and workers uncomfortable, and putting them at increased risk for heat exhaustion and heat stroke. In addition, high concentrations of ground-level ozone aggravate respiratory problems such as asthma, putting children and the elderly at particular risk.

High Energy Use: Hotter temperatures increase demand for air conditioning, increasing energy use when demand is already high. This, in turn, contributes to power shortages and raises energy expenditures at a time when energy costs are at their highest.

Global Warming: Global warming is in large part caused by the burning of fossil fuels to produce electricity for heating and cooling buildings. Urban Heat Islands contribute to global warming by increasing the demand for electricity to cool our buildings. Depending on the fuel mix used in producing electricity in your region, each kilowatt hour of electricity consumed can produce up to 2.3 pounds of carbon dioxide (CO₂), the main greenhouse gas contributing to global warming.

Interesting fact: Cooling Los Angeles by 5 – 7° F can reduce smog formation by 20% — the equivalent of taking all cars out of LA’s airshed for one day.

How you can help

Various environmental and governmental agencies are working to decrease the temperatures of Urban Heat Islands. The most prominent changes are switching dark surfaces to light reflective surfaces and by planting trees. Dark surfaces, such as black roofs on buildings, absorb much more heat than light surfaces, which reflect sunlight. Other options include creating green spaces like parks and recreation areas and installing “green roofs” that use living vegetation on roofs in order to help reduce heat accumulation of buildings.

Relevant links:

• Land-Surface Air Temperature — from the IPCC
• Urban Heat Islands and Climate Change - from the University of Melbourne, Australia
• The Surface Temperature Record and the Urban Heat Island from RealClimate.org
• NASA Earth Observatory: The Earth’s Big Cities, Urban Heat Islands
• Research and mitigation strategies on UHI; U.S. EPA designated, National Center of Excellence on SMART Innovations at Arizona State University
• Urban Heat Island research group; NSF project, Department of Geography, Indiana State University