Pavement is an urbanite’s worst enemy in the summer. It traps the sun’s boiling rays in two ways: first, by absorbing the incoming sunshine, trapping heat and boosting the daytime temperature, and then releasing that heat as darkness falls, preventing colder nighttime temps from cooling things off. This is the urban heat island effect. It’s a longstanding problem, especially in larger cities with more asphalt, but it’s becoming increasingly problematic as global warming heats the planet even more.
As such, urban planners are looking for ways to mitigate that heat island effect, and one method has come out on top: trees. By blocking the sunlight and using evapotranspiration (a fancy term for evaporating water from their leaves), trees can cool the air around them.
But how many trees do you need?
That might sound like a silly, pedantic question and the answer is simple: More trees is almost always better. If you’re an urban planner, this query is actually quite important. Understanding the relationship between the number of trees in a given area, and the overall cooling effect that amount of urban forest has is necessary. That’s why researchers at the University of Wisconsin, Madison, decided to start measuring. They strapped some sensors to a bike, and sent an able-bodied person around the city of Madison to test temperatures at regular intervals along blocks with varying levels of tree cover. They published their results Monday in the journal PNAS.
What they found is somewhat encouraging. At the city block level, it took at least 40 percent canopy coverage to counteract the warming effect of the asphalt. More coverage obviously induced more of a cooling effect, but even at less than half the street shaded, cities could see real benefits.
Of course, that’s just an overall number. The degree to which a tree can help cool the air depends on the percent of impervious ground cover there is—more pavement works against the trees. In fact, the two variables are highly intertwined. Blocks with more canopy are likely to also have less impervious surfaces. The result is a complicated correlation and a lot of variation in how much trees can help cool a street. With three-quarters of a low-pavement area covered, you could see more than 2.5°C in cooling, but that level was unachievable on blocks with mostly asphalt. Most of the effects seen were smaller, more on the degree of 0.5 to 1.0°C.
But that doesn’t mean adding trees is insignificant. The benefits that various blocks saw increased by 0.2 to 0.6°C on the hottest summer days, amplifying the effect on days when residents would be most likely to have health problems from the heat (and to up their energy consumption). Plus, planting trees had proportionally higher effects than did reducing the impervious surfaces on a street. Most city planners aren’t likely to start ripping up asphalt, so adding canopy coverage could be a good way to beat some of the heat.
The best thing, of course, would be to eliminate as much pavement as possible while also installing more trees. In lieu of that ideal world, the researchers encourage planners to focus on those areas with the lowest canopy coverage right now rather than try to help already-shady blocks. As we combat climate change, they write, we have to be aware of the socioeconomic and racial inequities already plaguing cities. Low income areas with high proportions of minorities generally have less access to green space already, whereas wealthy, white neighborhoods tend to have much higher vegetation coverage (plus the money to pay for air conditioning). If we want to really help people, we’ll plant trees in areas that don’t already have them.
And though the numbers may seem small, the cooling effects are still real. The researchers note that the best strategy would be to cover as much of a city as possible in greenery. But even planting more trees along a single block produced measurable cooling. Trees shade our yards, our walking paths, and even the insides of our homes. A few more strategically placed boughs could make a difference.