Letter to the editor: Climate change, urban heat waves, and the natural solution


Robert McDonald is a Vanguard Scientist with The Nature Conservancy, based at the Conservancy’s Worldwide Office in Arlington, Virginia. PHOTO CREDIT: Mark Godfrey © The Nature Conservancy

To the editor:

As Washington faces what looks to be a very hot summer, Astrid Joehnk can peer out from her 1923 row house in Brookland. She knows her street is a little less hot and glaring than it used to be.

With help from the nonprofit Casey Trees, Jöehnk and her neighbors have advocated for, planted, and watered a dozen trees on their block. Those trees include three graceful yellowwoods, three American hornbeams, and a redbud. Her side of the street had no trees at all in 2011. “It needed help,” said the 52-year-old radio producer.

It’s a simple, old-fashioned act. Planting a tree lies at the forefront of a very modern battle facing cities: the struggle against climate change.

Tornadoes and hurricanes may generate more fear and media coverage. But heat waves are humanity’s biggest weather-related public health threat. In a typical year, around 12,000 people die globally from heat waves. The elderly or those in fragile health are often the individuals who succumb to heat-related heart attacks, strokes, and renal failure.

Many Washingtonians, like most Americans, are lucky enough to have air conditioning — which reduces the health risk during a heat wave. An average of 1,300 people die per year in the U.S. from heat exposure. As the demand for air conditioning increases, utility managers must deal with massive spikes in electricity use and the threat of power outages.

Heat waves will be more frequent, with events that previously only occurred once every 20 years now happening every two to four years. According to the United States National Centers for Disease Control and Prevention’s report, “What the public now considers to be an exceptional event could become routine across much of the country.”

Heat waves will also be longer and more intense, with hot weather lasting 10-20 more days per summer than before. Peak temperatures in DC likely will likely push above 110 degrees Fahrenheit. One study suggests that unless cities and societies begin to adapt, deaths from heat waves could exceed 250,000 globally every year by 2050.

Cities around the world have to begin planning for unimaginably hot temperatures they have never experienced before. There are lots of ways cities can prepare for this coming hotter world. Many cities around the world are beginning to craft heat action plans. These often center on having public cooling centers and making sure vulnerable residents can get to them.

What Astrid Joehnk was doing in Brookland is increasingly seen as a key part of the fight against climate change among forward-thinking planners. The humble street tree is in vogue as an old-fashioned solution to the newly pressing problem of extreme heat waves.

Indeed, trees cool the air mostly by shading impervious surfaces like concrete and asphalt. This prevents these surfaces from absorbing energy from incoming solar radiation — which they would later release as heat into the surrounding air. The heat makes cities hotter than the surrounding countryside, a micro-climate phenomenon known as the urban heat island effect.

All our streets have micro-climates. Urban trees that shade asphalt or concrete can keep these surfaces a remarkable 18-36 degrees cooler on a day. This leads to air temperature along those fortunate streets that are two to four degrees Fahrenheit lower.

A recent study by Robert McDonald shows trees can be a cost-effective way to cool the air. The study looked at 27 major U.S. cities, quantifying the benefits from targeted tree planting in the sites that had the highest return on investment.

It found that strategic tree planting could reduce summer temperatures by more than four degrees Fahrenheit for up to 13 million people. The annual cost for planting and maintenance would be eight dollars per person.

Temperature reductions in this range won’t solve the climate change challenge, but it could save thousands of lives. One study in Paris found that each two-degrees Fahrenheit reduction in air temperature reduced the risk of death by 21 percent.

At the same time, cooler air temperatures reduce air conditioning use, with each increase of two degrees Fahrenheit in air temperature. This causes an increase of up to five percent in peak electricity use.

There are numerous other benefits to living among trees. One is to help reduce the anxiety that comes from living in a hotter world. Trees help lower our blood pressure and boost our human connections to each other by uniting communities around their public spaces.

“I sit on the porch and hear the birds and see the calm movement of the leaves in the wind,” says Joehnk. In front of her house, the yellowwood she has affectionately named Robert Redford has grown from seven to fifteen feet tall since she planted it seven years ago. “I talk to the people who walk by,” she says. “The trees have helped us get to know one another better.”

Dr. Robert McDonald is Lead Scientist for the Global Cities program at The Nature Conservancy. He has published more than 50 peer-reviewed publications, and a recent book, entitled Conservation for Cities: How to Plan & Build Natural Infrastructure (Island Press 2015).

Florence Williams is the author of The Nature Fix: Why Nature Makes us Happier, Healthier and More Creative (W.W. Norton 2017).