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Scientists Discover Yet Another Reason Cities Need More Trees

Using a computer model, scientists quantified how trees impact local weather and save energy.

Trees make neighborhoods look beautiful and feel safe. They can also play a key role in reducing the impact of wind in cities.

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That might seem inconsequential at first. But wind can affect things like humidity, temperature, how pleasant it is to walk around outside, and even the pressure gradient between the interior and exterior of buildings. When there’s a larger gradient, more air from outside will enter a building through small gaps and openings, requiring greater energy consumption to heat or cool the building. Researchers from the University of British Columbia created a computer model showing that when there are no trees in a neighborhood, that pressure gradient goes up by a factor of four. Because wind pressure can be the root cause for about a third of a building’s energy consumption, the finding means that if there are no trees around, the cost to maintain a building’s temperature can rise. The researchers estimate that trees in the Vancouver neighborhood they modeled save 15% on your energy bill in the summer and 10% in the winter.

[Photo: courtesy University of British Columbia]
To study the effect of trees on wind, the researchers built an incredibly detailed model of a Vancouver neighborhood by scanning every building, street, and tree using LIDAR technology and combining that with wind data that’s accurate down to a 1.6 feet from a University of British Columbia research tower. Using the data and an algorithm they developed to model wind speed, the scientists were able to create a virtual model using supercomputers in Switzerland. They found that when running the model without any trees, the wind speed increased by a factor of two; the study was published in the journal Advances in Water Resources earlier this month.

The researchers chose the neighborhood in Vancouver, because they had high-resolution wind data and geographic data from the scan, and they believe it’s representative of much of suburban Canada and the U.S. because the height of the buildings is similar or slightly less than the height of the trees. The drastic increase in wind speed occurs in the model partially because the researchers chose a neighborhood where most of the trees were taller than the buildings, leading to a far greater impact when the trees were removed.

[Image: Marco Giometto/University of British Columbia]
Marco Giometto, a postdoctoral fellow in civil engineering, says that they also found that in areas where trees are shorter than buildings, they can still reduce wind speed for pedestrians in street canyons, where buildings are tightly packed together on both sides of the street. Giometto says that having vegetation that’s taller than people in street canyons can reduce about 30 mile-per-hour winds to between three and six mile-per-hour winds for pedestrians, but it has a far smaller impact on buildings’ pressure gradients, and thus their energy consumption.

This is the first step toward a tool that could help architects and urban planners take some of the guesswork out of where to plant trees. The tool he envisions would enable the user to choose an end goal–like lowering a building’s energy consumption, or reducing the amount of air pollution (trees also act like a natural filter)–and then let them play around with adding different kinds of trees, all without having to code. “Hopefully within a couple of years, there will be a more user-friendly interface and computing power will have increased and it might really become a powerful tool for architects and urban designers to decide where to place vegetation within a city,” Giometto says.

[Image: Marco Giometto/University of British Columbia]
The biggest hurdle remains the resources needed to run the simulation in the first place–Giometto says it would take your laptop 21 years to complete the simulations they used in the study. As he waits for computing power to catch up, Giometto plans to include the ratio of building to vegetation height in the model and expand to different neighborhoods and different cities. He’s particularly interested in modeling New York City’s downtown area because the buildings are so tall. Then, he hopes to add other factors, like temperature and humidity, in order to give planners the fullest sense of impact planting trees will have. This kind of model could be used in hyperlocal weather forecasts as well as in urban planning.

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Giometto also hopes that the research could be used to inform public policy. If there’s a quantifiable economic impact that trees have on the urban environment, then perhaps laws requiring developers to plant a certain number of trees would make sense. “This should also then be used by policy makers, especially to determine the effective values of trees, and to create laws for the benefit of citizens and the environment,” he says.

About the author

Katharine Schwab is a contributing writer at Co.Design based in New York who covers technology, design, and culture.

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