Trees: Helping Cities Solve Climate Change

Quick, name a climate solution for cities which helps lower carbon emissions, protects vulnerable people who live there, and even helps students get better grades.

Give up? The answer is urban forests and you're not alone if you didn't come up with the answer. After all, most of us see the trees woven into our city streets and parks as just a pretty, cinematic backdrop for urban life. Nice to look at, certainly, but not profoundly important when compared to better mass transit and other steps cities can take to help fight climate change.

The strong recognition in the final Paris climate agreement or trees and forests of every kind, from cities to wilderness, is starting to change this perception.

I wrote recently here about the central role America's large rural forests can play in capturing and storing carbon emissions. To provide a sense of scale, our forests in the U.S. now sequester almost 15 percent of our annual carbon emissions. That is roughly equivalent to the projected reductions in emissions from the EPA Clean Power Plan when it hits full stride in 2030.

Urban forests are an important and oft-overlooked piece of the carbon puzzle, providing about 8 percent of the nation's total carbon capture in trees. Certainly this alone would make the case for planting more trees, like New York City's Million Trees campaign, to enhance natural carbon scrubbing power.

But urban forests can do much more to fight climate change and improve our cities aside from simply capturing carbon emissions.

To fully grasp the potential of trees to combat climate change, one has to understand the "Urban Heat Island Effect." This situation occurs when pavement and other built materials in our cities absorb and re-radiate heat, to create an oven-like effect. Heat islands can raise local temperatures as much as five degrees Fahrenheit during the day and as much as 22 degrees at night

From a carbon emissions perspective, heat islands create extra need for cooling on those hot summer days when air conditioning is running full tilt. One analysis from the University of California at Berkeley estimated that 5-10% of peak electricity demand in cities for air conditioning is due to urban heat islands.

These heat islands also worsen one of the greatest public health threats caused by climate change--extreme heat waves. One analysis from the Rollins School of Public Health at Emory University projected a ten-fold increase in heat-related deaths across the eastern states by 2057 thanks to stronger and more frequent heat waves. Cities can't afford to make this problem worse with heat islands.

What is the most effective natural solution to reduce urban heat islands? You guessed it--tree canopy.
Here is where climate justice comes in. In virtually every city in America, tree cover is strongly correlated with income--wealthy neighborhoods generally have significantly more tree cover. This shows the correlation for the Washington, D.C. region. A recent study published in PLOS One examined this correlation for cities more broadly.

Expanding urban tree cover with an emphasis on low-income neighborhoods is a powerful strategy to fight climate change, offering three major benefits. First, more trees will lead to more carbon sequestration and lower energy use for air conditioning--saving money for homes and businesses while lowering carbon emissions. Second, this same cooling effect will protect public health, particularly in low-income neighborhoods where many people face high risks from heat waves due to pre-existing health conditions and lack of air conditioning.

Third , tree canopy and other greenery in cities has been shown to have profound impacts on health and quality of life, touching everything from student achievement to mental health. Thanks to these ongoing quality of life benefits, urban forests are just as valuable on fair weather days as when cities shimmer in the heat.

With so much to gain for relatively small implementation costs, it is understandable why the legions of mayors who rallied behind the Paris agreement last month have joined with Pope Francis in calling for leafier cities. Urban forests simultaneously address our imperative to lower carbon emissions while also meeting our moral obligation to protect the most vulnerable people from climate impacts. The strong emphasis on forest solutions in the Paris climate agreement should be a tailwind to such efforts.