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Ericson: How to make cities green

Dense living has both positive and negative effects on the environment.
Image by Mary Ellen Ritter

Close your eyes. Picture an environmentally friendly home. What do you see?

Maybe you see a remote cabin in the woods, totally off-grid, plastered with solar panels.

But, what if there was another way?

According to some experts, dense, urban living can actually be good for the planet — as long as you do it correctly.

“Density and climate are related in quite simple ways,” said Shlomo Angel, professor of city planning and leader of the New York University Urban Expansion Program. “Density, basically, is the number of people per acre, or per square kilometer or hectare, whatever.”

While increasing the number of people per acre might sound like it would only drive out plants and animals, it can sometimes have the opposite effect.

“The higher the density, in general, the less space the city takes up, so the less of nature is disturbed,” Angel said. “The smaller the city for a given population, the better it is for climate. First of all, because it leaves a lot of greenery for sequestration of carbon.”

There are other reasons, too, he said.

“The tighter the city, the shorter the distances between different locations, the less vehicle miles traveled to reach the same level of destinations,” Angel said.

Density also has an impact on transport.

“If you get to high enough densities, then public transport becomes viable and public transport uses less energy than cars,” Angel said. “Also, we get higher density, more people can walk, more people can take bicycles and less carbon.”

Density can also make buildings more energy efficient.

“Multi-story structures are better insulated than single-story structures; there’s less energy loss,” Angel said. “Higher density is associated with taller buildings that are more efficient in terms of their insulation, in terms of their use of energy.”

With density, “we’re able to reduce the energy needed to keep cool, because we have less windows that are facing the exterior; the heat transfer of the building is less,” said Jay Arehart, an assistant teaching professor at the University of Colorado-Boulder and co-author of a 2021 paper on density, tall buildings and carbon emissions.

However, density isn’t always a good thing. Tall buildings require different types of materials, some of which require lots of carbon emissions to make.

“It’s not only the energy that we generate in these buildings, but the energy that it takes to build them,” Angel said. “Up to a certain height, you can build with renewables.”

Tall buildings, he continued, especially ones that are taller than eight stories, require energy-intensive materials like steel, concrete and aluminum.

“If you start to use steel and concrete and aluminum, when you look at the amount of energy that it takes to produce these things, no density will cover for it,” Angel said. “If you just keep building in timber, it’s a lot more efficient, and you can still get a lot of density.”

Arehart also said building too tall can have adverse impacts. For example, tall buildings need larger columns to support them, and use more energy to bring water up to higher floors.

In addition, there can sometimes be a trade-off between urban green space and dense construction. Robert McDonald, a lead scientist at the Nature Conservancy, is also a co-author of a recent study about this that showed there is typically a trade-off between density and greenery. However, the two don’t have to be mutually exclusive, he said in an email to the Minnesota Daily.

“There are plenty of examples of bright spots that manage to be both,” McDonald said.

Angel said it’s usually worth it to build more densely. But, he warned there is a trade-off between lowering carbon emissions and adapting to the impacts of climate change.

“Density is mostly about mitigating climate change. Green space is mostly about adapting and dealing with climate change. So what you need is to preserve enough area in greenery to absorb precipitation, for example,” he said.

Arehart said, in many cases, it’s worth it to build more densely at lower heights — usually six to eight stories.

“There’s no need for us to build more than six to eight stories,” he said.

So, while density has real environmental benefits, we also need to be wary of trade-offs in terms of tall buildings and green space.

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