In the Twin Cities, mapping the warming urban heat island to prepare for an even hotter future

On a sweltering Saturday in late July, Tasha Johnson took her two young daughters to cool off in the wading pool at Rev. Dr. Martin Luther King, Jr. Park in south Minneapolis.

“We had to get out. It’s too hot not to get in some water,” she said as her kids splashed in the water.

It was about 90 degrees and sticky — one of the hottest days of the summer. And this neighborhood, tucked between Nicollet Avenue and I-35W, is particularly hot.

A report released earlier this year found temperatures here are about eight degrees warmer than outlying areas.

“There’s areas in the city, they tend to be areas with more pavement, more buildings and less green space, that are hotter than other parts of the city,” explained Alisa Reckinger, an environmental educator for Hennepin County.

It’s long been known that cities are significantly warmer than outlying areas. Urban heat islands have more roads, buildings and other surfaces that absorb and radiate heat. They also have fewer trees and greenspace that help keep areas cooler.

But within these islands there are huge variations in temperature. There can be as much as a 30 degree difference between the surface temperature of searing asphalt parking lots and cool, shaded parks.

“And we’re trying to get some really detailed data on where the areas are the hottest,” said Reckinger.

So on the same Saturday Johnson’s kids tried to stay cool in a Minneapolis park, Hennepin and Ramsey counties, with federal funding, dispatched dozens of volunteer scientists to collect detailed data from around the metro area, to help create heat maps of the hottest neighborhoods.

The goal is to inform tree planting efforts and other future strategies to provide cooling relief as climate change results in even more extreme heat.

“I think folks generally have a sense that planting trees is a good thing to do,” said Abi Phillips, climate and health planner for Ramsey County. “But using data to build a map to tell that story can be really convincing.”

Tim Bontrager and Jamie Mosel were two of the volunteers who helped collect the data. They attached a small device to the window of their pickup truck, and drove a circuitous route from Richfield, past the airport and the Mall of America, and through tree-lined neighborhoods in Bloomington.

Every few seconds, the satellite sensor recorded the temperature and humidity.

Mosel, who grew up nearby in south Minneapolis, has experienced firsthand the warming of the metro area. The canopies of big elm and ash trees under which she played tag as a kid are now gone.

“So you can really see it on the landscape, and then you can see for me how climate change has become real throughout my lifetime,” she said. “You feel those impacts.”

Dangerous heat

The on-the ground data the volunteers collected this summer will supplement urban heat maps already developed by the Metropolitan Council and others that rely on satellite data.

Those maps, updated by the Met Council in 2022, showed temperatures on a single summer day of 78 degrees in the coolest parts of the metro, compared to a surface temperature of 111 degrees in the hottest areas. That’s a difference of 33 degrees.

“We’ve all experienced that going out to Target on a hot day, park your car and it can be pretty brutal just walking into the store when it’s that hot,” said Eric Wojchik, a planning and climate policy analyst for the Met Council.

The maps also show distinct socio-economic patterns.

The hotter neighborhoods tend to have more lower income residents, and a higher percentage of people of color, said Wojchik. “And we have data that bears that out.”

These neighborhoods were often subjected to discriminatory, race-based housing practices several decades ago. Those policies are still expressed in the tree canopy today.

A 2020 study found that these formerly redlined neighborhoods in Minneapolis were on average 11 degrees hotter than non-redlined neighborhoods. That’s the third largest gap in the country.

And that extra heat can be dangerous. More than 600 people died from the 2021 heat wave that struck the Pacific Northwest and Canada.

The risk is higher for people with conditions such as heart failure, lung disease or kidney disease, and those who are working outdoors.

Minnesotans may be particularly vulnerable.

“There’s research to show that people living in different climates have a different heat tolerance,” said Dr. Laalitha Surapaneni, a Minneapolis physician and University of Minnesota Medical School professor.

“For example, those in the south may be able to tolerate higher temperatures up to 100, whereas in Minnesota, a heat index as low as 86 could put vulnerable people in the hospital,” Surapaneni said.

The power of trees

Urban heat maps also demonstrate the powerful cooling effect of trees. The maps show that the hottest neighborhoods in the Twin Cities tend to have the fewest trees. That’s illustrated by a tornado that tore through north Minneapolis in 2011.

“If you look at the path of that tornado, you can see where it took out the tree canopy in North Minneapolis, because you can see the heat scar. You can actually see that in the map,” said the Met Council’s Wojchik.

“That’s a pretty profound example of just how taking out those trees has affected the heat in that particular neighborhood.”

The Met Council teamed up with two nonprofits, The Nature Conservancy and Tree Trust, to create an online mapping tool called Growing Shade that cities can use to target tree planting in their hottest neighborhoods.

The city of St. Louis Park was one of the first to use the tool. Homes and businesses in environmental justice priority areas can have trees planted for only $35.

“We’re really focused on making this as low-cost as possible so that we can get more of those trees in the ground, in those neighborhoods where they’re desperately needed,” said sustainability manager Emily Ziring.

But there’s an urgency to plant as many trees as quickly as possible, because it takes at least 10 years for a tree to grow tall enough to begin to provide shade, said Wojchik.

And there are a lot of areas to target, not only lower income neighborhoods that historically have fewer trees, but also neighborhoods losing trees to emerald ash borer.

“We’re talking about losing 25, 30, 40 percent of the overall tree canopy in some communities,” said Karen Zumach, director of community forestry with Tree Trust and chair of the Minnesota shade tree advisory committee.

“It’s important to make sure that we’re getting those trees equitably distributed as much as we can, into those areas where we know they’re going to have that biggest impact,” Zumach said.

Cities are also beginning to develop other cooling strategies. St. Louis Park, for example, has created a program to incentivize businesses to remove pavement, and replace it with native plants and trees.

Such efforts are only going to become more critical, as climate change makes extreme heat more common. Currently the Twin Cities experience about 13 days a year of over 90 degrees. By 2050, climate models suggest we could see 40 of those days.

“That’s a lot of days that are very hot,” said Wojchik. “And if we still want to recreate and enjoy the outside, we need to absolutely make sure that we have shade, that we can experience the outside.”

Correction (Aug. 14, 2024): An earlier version of this story misspelled Emily Ziring’s name. The story has been updated.

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