It’s 86°F outside the city, but not far away, in the downtown area without trees, temperatures are significantly hotter—in the 90’s—people are sweltering, air conditioners are on full blast, sidewalks are being damaged, air pollution is up. This is becoming an increasingly common phenomenon as forecasts expect increasingly warmer, drier summers. Cities and urban areas can easily become “heat islands” when the asphalt in cities heats up faster and longer than green areas.

It’s been found that the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F warmer than its surroundings. And in the evening, the difference can be as high as 22°F warmer!

hyperhydrosisAnd that causes a host of heat-related problems…
Heat islands can negatively affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality. One recent report predicts 150,000 additional heat-related deaths will occur in U.S. cities by 2100. What’s more, the increased power plant loads lead to additional air pollution and increased landscape watering depletes aquifers.

Young_Milkwood_trees_in_Cape_Town_city_centre_-_Sideroxylon_inerme_2Trees can be valuable assets in keeping our cities cool.
Trees can help alleviate the extra heat. How do they help? First and most obviously, trees provide direct shade which cools asphalt, roofs, and concrete parking lots, preventing the initial heating and storage. The net cooling effect of a young, healthy tree is said to be equivalent to ten room-size air conditioners operating 20 hours a day. Second, trees are efficient at dissipating the heat received by the sun by transpiring water from leaf surfaces. This action cools the air by taking “heat” out of the air to evaporate the water.

The “Jewel of the Midwest” uses trees to abate heat
Recognizing the significant costs of urban heat islands, cities throughout the country are being creative in finding solutions to combat the effects. Chicago, for example, launched a great new program– called Sustainable Backyards–to leverage residents’ abilities in helping to cool their city’s heat island.  The city provides financial assistance in the form of rebates that reimburse citizens for up to 50 percent of the cost of installing trees, native plants, compost bins, and/or rain barrels on their property.  (There are reasonable limits based on the value of the ecosystem services provided by each product: you can get a rebate for up to $100 for planting a tree, for example, or up to $40 for installing a rain barrel.) The net effect is meant to be cooler, more sustainable neighborhoods even in an urban environment.

Investing in Trees is a Cost-effective Solution
In sum, trees help abate the variety of problems caused by the increase heat of non-natural urban surfaces. In fact, a five-city study found cities spent roughly $15–$65 annually per tree, with net annual benefits ranging from approximately $30–$90 per tree. Proving that it makes sense to invest from the ground up!