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The high temperature in the Twin Cities one afternoon last week was 86 degrees, according to the National Weather Service. But that varied across the metro.
In a St. Louis Park parking lot, the temperature above a bed of asphalt outside a big box store reached 94 degrees in the mid-afternoon Aug. 3. A few miles west, on a tree-lined sidewalk in south Minneapolis’ East Isles neighborhood, it was 82 degrees. On the southeast side of downtown Minneapolis, amid the concrete streets and large buildings of the Elliot Park neighborhood, temperatures averaged around 90 degrees, cooling to 85 degrees in the shade of a local park.
The wide range of measurements, taken by Sahan Journal in the span of a few hours, exemplify what’s known as the urban heat island effect.
It’s a phenomenon in which urban centers with a lack of vegetation average higher temperatures than surrounding areas with more trees and plants. But even within cities, the differences between green areas and those built of concrete is stark. And, since wealthier neighborhoods tend to be greener than poorer ones, the heat island effect disproportionately impacts low-income people and people of color.
In fact, a study from 2020 found that significant temperature disparities exist between neighborhoods, and that those disparities reflect the lines on maps showing where discriminatory housing practices historically favored white homeowners and prevented people of color from buying. The study, “Undoing Landscape Legacies,” published by data firm CAPA Strategies, found that Minneapolis has the country’s third largest temperature disparity between formerly-redlined neighborhoods and those where no restrictions applied, with a nearly 11-degree difference between the hottest and coolest areas.
“There’s a systemic thing at play here,” said Vivek Shandas, a professor at Portland State University, who co-authored the study.
With the increased frequency and intensity of heat waves and droughts, due to climate change caused by the burning of fossil fuels, those systemic disparities could continue to grow if governments don’t take action.
In Minneapolis, the current drought is putting significant stress on the urban forest. According to Minneapolis Park and Recreation Board forestry director Ralph Sievert, even mature trees are wilting and showing fall colors early due to the dry conditions.
“I think this is the worst I’ve seen it,” said Sievert, who has been with the forestry department for 27 years. “We’ve had to redirect all our efforts to keeping trees watered.”
Understanding the effect
The urban heat island effect is felt around the globe, but differs from city to city based on local climate and history, according to University of Minnesota associate professor Tracy Twine, who works in the Department of Soil, Water, and Climate and studies the issue.
Human-made structures built of steel, concrete, and asphalt absorb more solar radiation than vegetation during the day, and then release that radiation at night, making urban centers warmer at all hours. How dramatic the temperature difference is between urban and rural areas varies by the size and scale of the city, Twine said. In Minnesota, the Twin Cities metro is typically about two degrees warmer than the surrounding countryside. Larger hubs like New York City are typically about seven degrees warmer than nearby rural areas.
But the heat island effect is exacerbated in major heat spells. During a July 2012 heatwave, Minneapolis and St. Paul were 14 degrees warmer than the surrounding countryside. For a medium-sized urban core, that’s substantial, Twine said. “Those are peak events that are going to harm people.”
Minnesota has three main hot spots, Twine found when working on a research paper published in 2015 by the Journal of Applied Meteorology and Climatology. Downtown Minneapolis, downtown St. Paul, and the area surrounding the Minneapolis-St.Paul International Airport are the three hottest places in Minnesota, she reported. Other hot spots cluster around major interstates and highways, in industrial zones, and near large groups of big box stores in the suburbs.
The Metropolitan Council tracks heat islands in the Twin Cities using an interactive extreme heat map tool.
A deeper understanding of heat islands in the Twin Cities is on the horizon, Twine said. A new project funded by the National Science Foundation will study the urban environment and temperature differences on a detailed level in Minneapolis over the next ten years.
Historically, when city leaders wanted to build a highway or an industrial zone, they targeted low-income neighborhoods, which tended to wield less political clout. The Hennepin Energy Recovery Center, a garbage incinerator, was built in the 1980s on the edge of north Minneapolis, for example. In the 1950s and ‘60s, Interstate 94 was routed directly through the largely-Black Rondo neighborhood of St. Paul.
It’s no accident, then, that today’s heat islands are tied to historical practices of redlining and discriminatory land policy, experts say.
The New Deal of the 1930s offered government subsidized mortgages, ushering in a new wave of homeownership across the country. The federal government issued credit ratings for those mortgages in discriminatory ways, giving favorable ratings to neighborhoods with racial discrimination written into local covenants. The government ranked neighborhoods on an “A” to “D” scale: “A” areas kept people of color and Jewish people from home ownership and were deemed the most favorable; “D” areas allowed those groups, labeled “hazardous” to lenders, to buy homes. The University of Minnesota’s Mapping Prejudice project has documented this system of discrimation in Minnesota.
When it came time to build new factories or install highways, planners used redlining maps to place these undesirable sources of pollution in neighborhoods where people of color and low-income residents lived. The process led to the removal of trees and vegetation and the creation of excess air pollution, effects targeted areas still feel today, according to Portland State’s Shandas.
In Minneapolis, he found that “A” neighborhoods, like those surrounding Lake Harriet and Lake Nokomis on the southside, remain noticeably cooler than “D” areas such as the Near North and East Phillips neighborhoods, which became home to highways and industrial sites.
The difference in temperature between Minneapolis’s “A” and “D” neighborhoods can be as great as 10.8 degrees, according to the 2020 report Shandas co-authored. The city ranked third in the nation, behind Portland and Denver, for the largest heat difference between redlined and non-redlined areas.
“It is really dramatic,” Shandas said.
Those differences become even more striking during heat waves. When Portland and the rest of the Pacific Northwest experienced unprecedented high temperatures of well above 100 degrees in late June, Shandas hit the streets and took measurements. Some city streets in redlined areas reached 124 degrees, while others in leafier sections of the city were 99 degrees. That 25-degree disparity can mean the difference between discomfort and death.
“Heat waves don’t just kill people by coincidence, there are specific conditions in place,” Shandas said.
In St. Paul, areas of concentrated heat exist around lower-income residential neighborhoods that are often home to immigrants and people of color such as Frogtown and the North End, and in old industrial zones like Midway, according to city chief resilience officer Russ Stark.
The most expedient way to combat heat islands, according to experts, is to add more green space and vegetation. The Twin Cities has a relatively robust tree canopy compared to other metropolitan areas, Twine said. Still, there are pockets, particularly near industrial areas, with room for improvement.
And with the current drought threatening existing trees, especially those that are newly planted, the task has become even more difficult, according to Minneapolis forestry director Sievert. Crews have redirected all their efforts to watering young trees and have postponed regular pruning and other tasks. But forestry crews are unable to water every boulevard tree in the city, and Sievert said residents can help by watering nearby trees in the evenings, which is best accomplished by letting a hose trickle at the trunk of a tree for an hour.
“Anything people can do to help is important,” he said.
Getting trees to succeed in historically industrialized areas is a challenge, according to Shandas. His study found that Minneapolis is “leveling out,” with smaller discrepancies between the tree canopies in “A” and “D” neighborhoods today than 30 years ago. But redlined areas still lag behind other neighborhoods.
“We need a heightened level of precision and effort that goes into making up for these massive historic inequities that have been placed on our cities,” Shandas said.
The least fertile places for boulevard trees in Minneapolis are in downtown and along commercial nodes like Central Avenue, West Broadway, and Washington Avenue, Sievert said. But best practices have evolved in recent years so that when new sidewalks are constructed, more space is created for roots to thrive underground.
In St. Paul, the forestry department is analyzing which tree species are most resilient and can do well when surrounded by concrete and asphalt. The city planted nearly 1,000 trees along the Green Line light rail, many of which flourished while others struggled. Watering and protecting trees in their early years is key to survival, resilience officer Stark said. St. Paul is partnering with the nonprofit Tree Trust to provide free trees in neighborhoods like Frogtown. He added, “Tree canopy in the coming decades in communities like ours is going to be gold.”
However, it’s going to take more than planting trees to eliminate the heat island effect. Replacing black roofs and asphalt with more reflective colors like white and gray can help, too, Twine said.
Cities should engage their residents in conversations about heat islands and help people take small steps to make a difference, according to Shandas.