Neighborhoods historically marked in red are warmer than others in the Twin Cities


The high temperature in the Twin Cities one afternoon last week was 86 degrees, according to the National Weather Service. But it varied in the metro.

In a parking lot in St. Louis Park, the temperature above an asphalt bed outside a big box store reached 94 degrees by mid-afternoon on August 3. A few miles west, on a tree-lined sidewalk in the South East Isles of Minneapolis neighborhood, it was 82 degrees. On the southeast side of downtown Minneapolis, amid the concrete streets and tall 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 space of a few hours, illustrate what is known as the urban heat island effect.

It is a phenomenon in which urban centers with a lack of vegetation have higher average temperatures than surrounding areas with more trees and plants. But even within cities, the differences between green spaces and those built of concrete are stark. And, because richer neighborhoods tend to be greener than poorer ones, the heat island effect disproportionately affects low-income people and people of color.

In fact, a 2020 study found that there are significant temperature disparities between neighborhoods and that these disparities mirror lines on maps showing where discriminatory housing practices have historically favored white homeowners and prevented people from moving. color to buy. The study, “Undoing the legacies of the landscape ”, published by data firm CAPA Strategies, revealed that Minneapolis had the third largest temperature disparity in the country between neighborhoods formerly marked in red and those where no restrictions were enforced, with a difference of nearly 11 degrees between hottest and coldest areas.

“There is a systemic element at play here,” said Vivek Shandas, a professor at Portland State University, co-author of the study.

With the increased frequency and intensity of heat waves and droughts, due to climate change caused by the burning of fossil fuels, these systemic disparities could continue to widen if governments do not act.

In Minneapolis, the current drought is placing significant stress on the urban forest. According to Minneapolis Park and Recreation Board Forestry Director Ralph Sievert, even mature trees wither and show early fall colors due to dry conditions.

“I think it’s the worst I’ve seen,” said Sievert, who has worked in the forestry department for 27 years. “We had to redirect all our efforts to water the trees.

Understanding the effect

The urban heat island effect is felt around the world, but differs from city to city depending on local climate and history, according to University of Minnesota Associate Professor Tracy. Twine, who works in the Soil, Water and Climate Department and studies the problem. .

Man-made structures built of steel, concrete and asphalt absorb more solar radiation than vegetation during the day, then release that radiation at night, making urban centers hotter at all times. The magnitude of the temperature difference between urban and rural areas varies depending on 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. Large centers like New York are generally about seven degrees warmer than neighboring rural areas.

But the heat island effect is exacerbated during large heat waves. During a heat wave in July 2012, Minneapolis and St. Paul were 14 degrees warmer than the surrounding countryside. For a mid-sized urban core, this is important, Twine said. “These are peak events that are going to hurt people. ”

Minnesota has three main hot spots, Twine discovered while working on a research paper released in 2015 by the Journal of Applied Meteorology and Climatology. Downtown Minneapolis, downtown St. Paul and the area around Minneapolis-St. Paul International Airport are the three hottest places in Minnesota, she reported. Other hot spots are concentrated around major highways and highways, in industrial areas and near large clusters of big box stores in the suburbs.

The Metropolitan Council hunts down heat islands in the Twin Cities using an interactive extreme heat map tool.

A screenshot from the Metropolitan Council’s heat island tool shows hot spots in the Twin Cities, which often reflect historical redlining practices.

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 at a detailed level in Minneapolis over the next ten years.

Disproportionate impact

Historically, when city rulers wanted to build a highway or industrial zone, they targeted low-income neighborhoods, which tended to have less political clout. The Hennepin Energy Recovery Center, a waste incinerator, for example, was built in the 1980s on the northern edge of Minneapolis. In the 1950s and 1960s, Interstate 94 ran straight through the predominantly black neighborhood of Rondo in St. Paul.

It is therefore no coincidence that today’s heat islands are linked to historical practices of redlining and discriminatory land policy, according to experts.

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 scores for these mortgages in a discriminatory manner, granting favorable ratings to neighborhoods with racial discrimination enshrined in local conventions. The government rated neighborhoods on a scale of “A” to “D”: “A” areas prevented people of color and Jews from owning property and were considered the most favorable; The “D” zones allowed these groups, labeled “dangerous” to lenders, to buy homes. the University of Minnesota Prejudice Mapping Project documented this system of discrimination in Minnesota.

When the time came to build new factories or install highways, planners used red cards to place these unwanted 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 excessive air pollution, effects that the targeted areas are still feeling today, according to Shandas of Portland State.

In Minneapolis, he found that “A” neighborhoods, such as those surrounding Lake Harriet and Lake Nokomis on the south side, remain noticeably cooler than “D” areas such as the Near North and East Phillips neighborhoods, which are home to highways and industrial sites.

The temperature difference between districts “A” and “D” of Minneapolis can reach 10.8 degrees, according to the Co-authored Shandas 2020 Report. The city ranked third in the country, behind Portland and Denver, for the biggest difference in heat between red and non-red areas.

“It’s really dramatic,” Shandas said.

These differences become even more striking during heat waves. When Portland and the rest of the Pacific Northwest experienced unprecedented high temperatures from well above 100 degrees at the end of June, Shandas took to the streets and took action. Some city streets in the red zones hit 124 degrees, while others in the greenest sections of the city were 99 degrees. This 25 degree disparity can be 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, zones of concentrated heat exist around low-income residential neighborhoods that are often home to immigrants and people of color like Frogtown and the North End, and in former industrial areas like Midway, according to Russ Stark, manager. of the city’s resilience. .

Potential solutions

The fastest way to combat heat islands, experts say, is to add more green space and vegetation. The Twin Cities have a relatively robust canopy compared to other metropolitan areas, Twine said. Still, there are pockets, especially near industrial areas, which could be improved.

And with the current drought threatening existing trees, especially newly planted ones, the task has become even more difficult, according to Minneapolis Director of Forestry Sievert. The crews redirected all their efforts to watering the saplings and postponed regular pruning and other tasks. But forestry crews are not able to water all the trees on the town’s boulevard, and Sievert said residents can help by watering nearby trees at night, which is best accomplished by letting a hose run over it. the trunk of a tree for an hour.

“Anything that people can do to help is important,” he said.

Growing trees in historically industrialized areas is a challenge, according to Shandas. His study found that Minneapolis is “leveling off,” with smaller gaps between tree canopies in neighborhoods “A” and “D” today than 30 years ago. But areas marked in red still lag behind other neighborhoods.

“We need an increased level of precision and effort to compensate for these huge historical inequalities that have been imposed on our cities,” Shandas said.

The least fertile places for boulevard trees in Minneapolis are downtown and along business 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 built more space is created for the roots to grow underground.

In St. Paul, the forest service analyzes which tree species are the most resilient and can do well when surrounded by concrete and asphalt. The city planted nearly 1,000 trees along the Green Line’s light rail, many of which thrived while others struggled. Watering and protecting trees in their early years is essential for survival, said Resilience Officer Stark. St. Paul is partnering with the non-profit Tree Trust to provide free trees in neighborhoods like Frogtown. He added: “The canopy of trees in the next decades in communities like ours is going to be gold. “

However, it will 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 also help, Twine said.

Cities should engage their residents in heat island conversations and help people take small steps to make a difference, according to Shandas.

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