Cool Cities

Ever wonder why cities are warmer than their surrounding towns? Think it might have an adverse affect on your health?… The answers are Keep reading for heat island basics. and Yes, in cities with fewer trees.



Urban forestry is simply about trees in places where people live.
Ultimately, conservation is about empowering citizens to improve the communities where they live and work. The Alliance for Community Trees is the only national organization working to improve the urban forests where 80% of Americans live– our cities, towns, and villages. ACT’s national office assembles coalitions that drive broad environmental success for our more than 180 organizations in 41 states in the pursuit of Clean Air, Green Streets, and Healthy Neighborhoods.
About Urban Heat Islands
Cities average 2 to 10 degrees Fahrenheit warmer than their rural surroundings. This marked increase in city temperatures is known as urban heat island effect (UHI). The effect is largely due to replacing natural green infrastructure with concrete and gray infrastructure. Under sunlight, building rooftops and roads trap and release a large quantity of heat. Cities that have been paved over do not benefit from the natural cooling effect of vegetation.
Adverse Health Effects
Summer– and even spring– temperatures across the U.S. can reach over 90 degrees in many cities already. Heat exhaustion is quickly turning into a more major cause of death, especially in seniors. UHI only exacerbates these already high temperatures. On average, more U.S. deaths are attributed to high temperatures than to any other weather related event. Ozone concentrations also increase with heat and can cause severe respiratory problems and death.
Increased Energy Demand
As air temperatures rise, so does the demand for air-conditioning (A/C). This leads to higher emissions from power plants, as well as increased smog formation, ground ozone, and acid rain. In the United States, urban heat island effect is responsible for 5–10% of peak electric demand and as much as 20% of population-weighted smog concentrations in urban areas due to A/C use alone.
In the U.S., significantly more energy is consumed in the summer for cooling than in the winter for heating. It is estimated that the rising temperatures cause by UHI will increase A/C use by 3–8 percent more just to counterbalance UHI, and more than 20 million more barrels of oil at a cost of $2 billion annually. Ironically, all of those fossil fuels that we burn to stay cooler… they are warming the planet further through the emission of CO2 and NOx, causing a cascading loop. The Intergovernmental Panel on Climate Change actually predicts more frequent, intense, and lengthy heat waves in cities that are already at risk.
Green Roofs and Green Buildings
Average summer temperatures in major cities across the U.S. have been on the rise over the past decade. These artificially high summer temperatures have a range of direct and indirect negative impacts on our quality of life. Green roofs are ideal for urban settings in which high-density developments offer few opportunities for mitigating increasing electricity consumption as air conditioners run longer, heat-related illnesses, and the rate at which ground level ozone forms. Like urban forests and reflective roofing surfaces, greenroofs absorb and/or deflect solar radiation so that it does not produce heat.
Cool Cities– Plant a Tree
One simple way to cool cities is to plant trees along sidewalks and turn rooftops into greenroofs. The evapotranspiration from vegetation and natural absorption and reflection of solar radiation will cool a community by a few degrees in the summer. Studies suggest that if 10% of city roofs were greenroofs, that the ambient temperature would be lowered by 2–4 degrees Fahrenheit.
Heat island mitigation is an effective air pollution control strategy, more than paying for itself in cooling energy cost savings. The cooling energy savings in the U.S. from cool surfaces and shade trees, when fully implemented, is about $5 billion per year (about $100 per air-conditioned house). Another way to look at is, reducing air temperatures in Los Angeles by just 2–3 degrees would reduce urban smog exposure by roughly the same amount as removing all vehicle exhaust in the entire LA basin.
Increasing the amount of trees by even a small margin would help. The US Forest Service has found increasing the urban tree canopy by just 1% would bring maximum midday city temperatures down by .07 to .36 degrees Fahrenheit.
At this moment, the nation wants action to secure real energy and climate security, action that goes beyond buzzwords such as green and sustainable. Healthy urban forests are key to helping our growing cities and towns to address climate concerns.
Find Out More:
Trees and Energy Savings
Trees and Climate Change
Tales from the Urban Forest- Urban Heat Island and Human Health
Tales from the Urban Forest- Urban Health Island Problems and Solutions
The Alliance for Community Trees is a 501©3 tax exempt organization (EIN # 68–0319301), and also participates in the Combined Federal Campaign (CFC # 12402). To discuss planned giving opportunities, call us at 301–277-0040.


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