New research on keeping cities cool

Planting trees is smart, planting them in the right places is smarter.

Cities around the world are responding to the climate crisis by greening up, cooling their avenues with trees and placing public green spaces amid their buildings of steel and concrete.

It might be desirable for cities to create tree canopies throughout all of their neighborhoods, but greening a city is a big task, and it can be expensive. Ottawa has a tree-planting budget of $1.9 million. The Cleveland area has tree-planting grants totaling almost $1 million. Planting trees also entails adapting infrastructure to accommodate the trees, things like ensuring that tree roots don’t upend city sidewalks and their branches don’t entangle power lines.

One of the reasons I enjoy living in my neighborhood is the trees along the streets. This photo was taken just outside my favorite coffeeshop.

These trees shade a large sidewalk where diners enjoy brunch on weekends.

Cities thus need to be prudent in the way they green themselves up. To that end, a research team based at the University of Manchester in Great Britain undertook a GIS-based analysis of Lilongwe City in Malawi to find the optimal placement and area for urban greening to benefit the most residents in the best way. They chose to study Lilongwe City because of its recent urbanization growth spurt and the city’s scarcity of green spaces. The study was published in the journal Environmental Systems Research in October.

Why cities need to cool down

Cities are greening up to combat the urban heat island effect. Anyone who has ever strolled along a city street in summer knows about this phenomenon. In summer, cities are warmer than the surrounding countryside. This happens in cities big and small, and people have known of it for millennia. During Rome’s imperial days, residents of the Eternal City noticed that their streets and promenades were hotter than the nearby fields and forests. The heat island effect can make cities from 1 to 7 degrees Fahrenheit warmer during the day and 2 to 5 degrees warmer at night. With climate change, cities all over the world are going to get hotter and hotter.

If you’ve ever stepped barefoot from a path onto a sidewalk, you’ve probably noticed the concrete can be considerably warmer than the earth you’ve just stepped from. Sunshine warms the man-made stuff in cities, the tarmac, concrete, steel, etc. As these objects warm, they radiate their heat to the surrounding air, and the whole city gets warmer. Greenery reduces this heat island effect, first by shading sidewalks and streets. Trees and shrubs also release water from their leaves and act like outdoor swamp coolers.