Salt Lake City looks to shade trees to mitigate its ‘urban heat island’ effect

This story is part of The Salt Lake Tribune’s ongoing commitment to solving Utah’s greatest challenges through the work of the Innovation Lab.

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Salt Lake City has had its hottest summer on record, and climate reports only forecast it will get hotter.

Along with climate change, many Salt Lake and Utah County’s residents are also grappling with the effects of what are known as urban heat islands.

On the hottest days of summer, you might even feel it yourself – strolling downtown among the concrete buildings and tarmac streets can feel like walking in an oven – but the shady streets of Sugarhouse or The Avenues offer pleasant temperatures.

It’s not just your imagination, either. Temperatures in cities can change dramatically from neighborhood to neighborhood – possibly even from block to block, due to the heat island effect.

Salt Lake City faces some of the hottest temperatures because of this effect, according to a 2015 study by the University of Georgia.

Asphalt and concrete keep the hot temperatures up overnight and offer no respite from the scorching hot days.

Average Minimum Temperature by Month compared to Downtown Salt Lake City

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Average temperature

A couple of degrees doesn’t seem like much of a difference, but when it’s three digits, a couple of degrees have a big impact. Turning up the air conditioning is not an option for many people, and even for those who can, it is an expensive option with a negative impact on the environment.

The cheaper, more environmentally friendly and longer-term solution to lowering temperatures is shade trees.

Plant a solution

Salt Lake City is one of many cities around the world that want to cool their urban environment by planting more trees.

City streets, parking lots and buildings absorb and radiate heat that would otherwise be distracted by vegetation such as grass and trees in rural areas.

Urban forestry can be used to combat heat in a number of ways, said Laura Bandara, an urban planner for the city. They can shade walkways for pedestrians, cool houses, reduce the use of air conditioning, and reflect the sun more than other surfaces like asphalt.

The city is developing a plan to prioritize the planting of trees in the areas that could use the shade most. Mayor Erin Mendenhall has also committed to plant 1,000 trees this year, mainly on the west side of the city.

“The whole idea is to develop a strategy to get more trees – and healthy trees – in the places where they will be most beneficial,” said Bandara.

The plan will have a stock focus, Bandara said. Districts with more colored or poorer residents tend to have higher temperatures than other districts.

“It affects people who are historically marginalized along with children and seniors,” said Bandara.

Excessive heat can be unhealthy, even if it’s just a few degrees. Heat can lead to breathing problems, heat exhaustion, and heat stroke, according to the Environmental Protection Agency. The Centers for Disease Control and Prevention recorded an average of 702 heat-related deaths each year in the United States between 2004 and 2018.

Excessive heat could also add an additional cost burden as residents in hotter areas run their air conditioners more and use more water to keep themselves hydrated and the plants alive.

In addition to the negative health effects of heat, excessive heat can affect critical infrastructure. Record temperatures in the Pacific Northwest this summer have melted power cables and kinked roads.

Water aspects

Trees and other vegetation lower the temperatures of these heat islands, but increasing droughts have made planting difficult. Cities and their residents need to plan where plants have the greatest influence on heat while also taking water consumption into account.

Overall, however, trees use a lot less water than people think, said Diane Pataki, professor of ecology at the University of Utah.

“We’ve found over the years that it’s a pretty good compromise,” said Pataki. “Watering trees is worth it.”

The shade from trees can also help keep the grass alive with less water, she said, as the shade slows water evaporation.

A recent study by Pataki and US researcher Carolina Gomez-Navarro found that temperatures in large, grassy parks were two degrees cooler than paved areas – day and night.

While grass cools the temperature, you may still feel hotter in a field of grass than under a tree. The study only took into account actual temperature, but sun exposure and wind also affect how hot a person feels.

There is evidence that grass can cool areas in the city, Pataki said, but city planners and residents still need to consider the balance with water usage.

“The key is to only have as much turf grass as you need,” said Pataki.

Other solutions

Trees are far from the only solution, said Ladd Keith, assistant professor of planning and sustainable environments at the University of Arizona.

Building multi-storey car parks instead of spacious parking lots can reduce the heat, which the new airport took into account when building its garage, Bandara said. Narrow streets can reduce the heat from the sidewalk. White roofs also divert the heat.

Several efforts to reduce greenhouse gas emissions have the added benefit of reducing excess heat in urban areas, Keith said. Energy efficient buildings do not run air conditioning as much, which reduces the heat emitted by these systems. The increase in local transport and the creation of pedestrian and bike-friendly environments are pulling cars – which can also heat the area – off the streets.

“We already do a lot of things that have to do with heat,” said Keith. “We just don’t recognize them that way.”

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