Nature's air conditioners
Tree cover and green spaces can help cities beat the heat
By Aaron Bernstein
July 20, 2010
PEOPLE ARE beating the heat this summer by seeking refuge in air-conditioned buildings. Although this helps keep people cool and may ward off such heat-related illnesses as asthma, heat stroke, and dehydration, it also has untoward side effects. Air conditioners may cool the indoors, but they generate heat outside and may make temperatures climb still further on hot days, especially in cities. They also guzzle electricity, which requires more burning of coal and other fossil fuels that pollute the air and cause yet more suffering for those with heart and lung ailments. The good news is that we can do better to beat the heat.
In Chicago, just prior to a lethal 1995 summer heat wave in which more than 700 people died, a study found that the urban landscape — the assortment of buildings and green spaces in the city — made a difference in how much Chicagoans felt the heat. The 50.8 million trees within the city limits cut air temperature by more than two degrees. Compare that to the warming expected for Boston due to climate change by mid-century, about two to three degrees, and that reduction takes on a new, and impressive, guise.
The study also found that just one shade tree could trim 7 percent from the annual cost of cooling a well-insulated home. Most trees also scrub the air of pollutants, and the study found that trees soaked up more than 1,600 tons worth of air pollution per year, including particles that trigger asthma attacks and heart arrhythmias.
Boston is one of the greenest cities in the United States. In 2007, the city and the US Forest Service conducted a tree inventory and found that tree canopy shades 29 percent of the city, which beats out New York City at 24 percent and Baltimore at 19 percent. However, the survey also revealed very low tree cover in some Boston neighborhoods, particularly in a few densely populated areas with particularly high burdens of respiratory ailments. Mayor Thomas Menino launched a campaign to plant 100,000 additional trees in the city, focusing on such areas, by 2020. The public health value of trees has been one of the reasons for the rapid government response last week to the appearance of the Asian longhorn beetle, which can destroy vast forests in a few months.
Another worthy cooling intervention, particularly for cities, are roofs covered with vegetation. These green roofs provide benefits year-round; they are cooler in summer and hold in heat better in the winter. They also absorb water, thus reducing storm run off and help to reduce the chances for bacterial contamination of beaches.
Buildings account for nearly 40 percent of energy use in the United States and that energy dissipates as heat. To add to their heat concentrating capacity, black roofed buildings (as are most in Boston) make as much sense as wearing a Yankees getup at Fenway, as the darker the surface the more heat it absorbs. Green covered buildings may have temperatures 50 degrees below and cooling loads 50 percent less than those topped with conventional tar and rubber. They also tend to last longer.
Dealing with a few high degree days every summer isn’t too troubling. Dealing with temperatures over 90 degrees every fourth day, and near 100 about every week, which is what may be in store for Boston around mid-century given global warming’s effects, is a bit more unsettling. The major fix, then, for reining in summer heat will be to to reduce fossil fuel consumption and lower greenhouse gas emissions. To abet emissions reduction, we can also harness the capacity of plants to pull carbon dioxide, the most abundant greenhouse gas, from the atmosphere.
Through continuing to green Boston’s landscape and kicking our fossil fuel habit we can keep Boston’s summers filled with the outdoor summertime pleasures we love rather than with advisories to huddle in the air-conditioned quarters.Dr. Aaron Bernstein is on the faculty at the Center for Health and the Global Environment at Harvard Medical School and a physician at Children’s Hospital Boston.