Urban Heat Islands: A Peek into Portland's Shady History
Temperature disparities across Portland. Source: Sustaining Urban Places Research (SUPR) Lab, Portland State University, 2018.
Blog post by Thomas Meinzen, Program Associate
As temperatures in Portland climbed over 100 degrees this summer, the heat probably felt oppressive. But it may have felt much worse, depending upon where you were in the city. Some neighborhoods, like those in Northeast Portland along Interstate 5, typically rise to a full 10 degrees warmer than other neighborhoods, such as those west of the Willamette River. This difference can have serious consequences for residents, who face higher temperatures and often have fewer resources to beat the heat and keep cool.
By collecting temperature readings across Portland, researchers from Portland State University found in a 2018 study that certain parts of the city remain significantly cooler than others, and these differences are strongly correlated with race and income status. This means that while some residents might be enjoying a 75-degree day in Portland, others could be sweltering in nearly 90-degree heat. How does this happen?
Urban Heat Islands: How they work
Parts of cities that retain high levels of heat are known as urban heat islands. These areas have disproportionately high amounts of impermeable surfaces, such as black rooftops and asphalt, which absorb the energy of sunlight instead of reflecting it away. Consider how blisteringly hot a parking lot is on a summer afternoon compared to a nearby park or grassy field. And the problem only expands: after capturing heat, surfaces like asphalt and concrete release it slowly over time. Ever notice how a parking lot remains warm long after the sun sets, while a park or undeveloped field becomes noticeably cooler as the night progresses? All this heat retention in pavement adds up to higher nighttime and daytime temperatures.
Tree canopy cover percentage across Portland, grouped by census block. Note the correlation between disparities in tree canopy cover and disparities in temperature shown in the map above. Source: Portland Parks & Recreation Urban Forestry, 2014
Urban heat islands are also created by a scarcity of trees, water, and green spaces, elements which help keep cities cool. Trees and other plants reflect sunlight and release moisture through evapotranspiration, keeping heat from accumulating in an area and cooling the air. Without trees, parks, and other green spaces, neighborhoods become much hotter and more dangerous places to live during heat waves. Many Portland neighborhoods with the least tree canopy cover and fewest parks also have the most impermeable surfaces, compounding the heat island effect.
Portland: Not So Green inside the Red
Portland’s reputation as a green city draws many to the region, and its urban forests and parks make that reputation more than a metaphor. However, Portland’s greenery is not distributed evenly across the city—not even close. West of the Willamette River, for example, most neighborhoods have more than twice the tree canopy cover of neighborhoods in North and East Portland. As of 2018, 80% of Portland’s population lives east of the Willamette, where canopy cover is only 20.5%—a lower rate than New York or Los Angeles. The number of parks, natural areas, and shady yards which serve to reduce urban heat accumulation are similarly skewed toward western Portland’s wealthier and whiter neighborhoods.
As research by PSU’s Vivek Shandas and colleagues has shown, neighborhoods which accumulate the most heat are also home to the highest proportion of low-income people and people of color. And despite living in parts of the city that are consistently hotter, residents of these areas often face more barriers to keeping cool, whether due to the financial challenges of paying higher electric bills, landlord restrictions on A/C units, or limited access to cooling locations like pools and shady parks.
Redlined neighborhoods, shown by red border and crosshatch, overlaid with temperature disparities in Portland. Darker red represents warmer areas. Neighborhoods outlined in blue were rated as the best investment by the Home Owners’ Loan Corporation. Source: Sustaining Urban Places Research (SUPR) Lab, Portland State University, 2018
These correlations are no accident, but rather the result of generations of racist and discriminatory urban planning practices, which barred access to home loans to those living in certain neighborhoods (a practice, shown at right, known as redlining) and prevented people of color from moving into many of Portland’s wealthier neighborhoods. Redlined neighborhoods received less public investment and were often zoned to border industrial areas, resulting in fewer street trees and public green spaces. Because of these planning practices, such neighborhoods are now some of the region’s hottest urban heat islands.
Moving Forward
Measures can be taken to combat urban heat islands and make these areas cooler, safer, and more livable for all residents. One of the simplest is to plant trees and invest in parks and green spaces in neighborhoods that have been overlooked, including in north Portland and elsewhere in the Columbia Slough Watershed. Trees reduce temperatures while absorbing carbon dioxide, meaning that planting and saving trees can help us stay comfortable in the summer heat and reduce our carbon footprint. In addition to being very useful, trees increase property values; an urban planning study found that street trees increased Portland houses’ property values on average by $8,870! Through targeted tree-planting efforts, the City and Metro-area government are working to address the racial and socioeconomic disparities in urban temperature and tree cover, but there’s a lot of catchup to do. A recent study of 108 cities around the U.S. found that Portland had the highest temperature discrepancies between its wealthy and poor neighborhoods—nearly 13 degrees Fahrenheit!
Much of the Columbia Slough Watershed consists of industrial and commercial properties. Limited investment in trees, habitat enhancement, and permeable surfaces in these areas causes heat to accumulate, sometimes to dangerous levels. This affects the adjacent residential communities, including Gresham and Fairview, and jeopardizes worker health and wellbeing. Thousands of people spend 40 or more hours a week working in these industrial areas, and they brunt the full effects of urban heat islands while doing so. To address this, the Council is working with industrial landowners across the watershed to enhance not only the ecological value of their properties, but also the working conditions for their employees.
As temperatures continue to rise due to climate change, it is critical that we reduce urban heat island effects wherever possible. Multnomah County offers resources and recommendations to help residents stay safe when it’s hot, and research continues to inform how Portland’s urban planning decisions affect temperature disparities and racial justice.
In the meantime, YOU can help community groups and agencies and make the Portland metro area cooler and greener for everyone:
-> Want to improve native habitat and help plant trees around the watershed? Sign up to volunteer with the Council!
-> Want street trees in front of your house or business? Contact Friends of Trees!
-> Want a free tree for your yard? Portland Parks & Recreation Urban Forestry is giving away 1,200 free trees for Portland yards this fall! Giveaway opens September 8!
-> Looking to learn more about how to plant and care for a new tree on your property—and get a rebate on your water bill? Check out these resources from Metro and the City of Portland Environmental Services!
