Thoughts From Engineers: Urban Heat Islands Grab the Latest Climate Headlines
Extremely high temperatures searing U.S. and European cities in recent weeks invariably focuses the world’s attention on the “urban heat islands” that cities by and large represent. With rising temperatures of late, cities are literal hotbeds for experiencing the first-hand effects of a changing climate.
According to a number of recent studies, due to a variable combination of site-specific factors—from building materials to placement of buildings and patterns of air circulation to the absence of vegetation that can cool micro-climates—air temperatures in urban areas can be anywhere from 5 to 8 degrees higher than temperatures recorded just outside the city. The U.S. Centers for Disease Control and Prevention claims that heat is the No. 1 cause of death when compared to other fatalities caused by natural hazards, and climate- and weather-related phenomena.
Populations are projected to increase in the world’s cities—by roughly 3 billion by the year 2050—and often the residents least able to help themselves live in the hottest zones. These areas are virtually treeless, void of green spaces and characterized by endless stretches of concrete. We’ve all seen them or driven through these desolate and extremely depressing places—where children and families are somehow expected to live and thrive—or been warned to avoid them altogether due to high crime rates. Lives and well-being are at risk here on so many levels, but especially when temperatures soar. Some cities have taken steps to decrease risk from heat, with mixed results.
Trees Loom Large in Climate-Adaptive Strategies
Climate-adaptive strategies are diverse, including installation of cool pavements and reflective rooftops, revised building codes that incentivize green design principles, and development of vulnerability indices that identify and provide assistance to at-risk populations. Nearly all climate-adaptive strategies on record so far also attempt to bring trees and green space back into the urban picture. Interest in planting trees and creating some form of tree canopy to act as a multi-functional barrier against the sun is receiving renewed attention in numerous parts of the country.
These programs, many of which aim to plant 1 million trees or more within the decade, are ambitious. The notion of tree equity plays a role here as well, as many programs aim to focus on the historically underserved and neglected neighborhoods previously mentioned. Several of the largest cities in the country—from New York City to Baltimore and Miami—have initiated massive tree-planting programs. OneMillionTreesNYC, MillionTreesLA, the Chicago Landscape Median Program, The Canopy Project and TreeBaltimore are some of the initiatives active around the country.
Bringing Out the Trees in Baltimore
Of the programs making headway, the city of Baltimore stands out for an approach that tackles the issue at multiple levels: classes for the public on tree stewardship, free street and yard trees, financial support from city and state government, and targeted programs such as Operation Retree Baltimore County that introduce trees to the most-barren neighborhoods. In addition, the state of Maryland passed legislation in 2021 to support the planting and establishment of 5 million trees within the state by 2031.
This effort is complemented by statewide goals to reduce greenhouse-gas emissions by 50 percent by 2030. The reestablishment of tree cover (www.treebaltimore.org) in Baltimore is tracked meticulously using satellite data, lidar-generated surface models, multispectral imagery and GIS tools through regional partnerships with the U.S. Forest Service, the University of Vermont’s Spatial Analysis Lab and others.
Baltimore’s example and others like it show that while planting trees in urban areas is a worthwhile endeavor, it’s an uphill battle. According to one 2020 study (bit.ly/UrbanTreeCover), tree cover globally is experiencing a statistically significant decline—except for Europe—while the percentage of impervious-surface area in cities is increasing worldwide. According to NPR (n.pr/3Jn2tzS), U.S. cities lose roughly 29 million trees per year. Baltimore’s slight increase in tree cover of 1.4 percent is the lone exception to the vast majority of cities across the country that continue to toe the line and lose valuable tree cover.
These findings make sense in view of how we design and build our cities, which by and large follows the flow of cash. In most cases, a parking lot still has more value than a grove of trees. Parking lots with plenty of space for cars, especially when located next to a profitable big-box retailer, bring in more rent than a park—even if the park in question fulfills multiple ecosystem functions and is a nice place to cool off. This isn’t so much a criticism—because I’m as much a part of the “old guard” as anyone in the engineering community—but an observation that it’s difficult to switch course after decades of working within an entrenched and rigid value paradigm.
I’ve discussed the International Panel on Climate Change (IPCC) conclusions in previous columns. The IPCC’s well-publicized projections on global warming trends and associated ripple effects on ecosystems and life on Earth are key, but its commentary on the state of current climate-adaptation efforts around the world is equally interesting. The IPCC writes in its sixth assessment (2022) that we’re in a period of “rapid urbanization and lack of climate-sensitive planning.” Unfortunately, I couldn’t agree more, but change is difficult.
New Era or Endgame?
Obviously, this column isn’t just about trees and the efforts to reintroduce them into oppressively hot urban spaces. It’s about rethinking which types of infrastructure come out on top in a new age that’s materializing hard and fast—with fires that burn for days and rain events that flood cities in a matter of hours.
In an era where historic benchmarks have uncertain value in charting a future course, we can experiment with alternative forms of infrastructure, as many cities are beginning to do. In combination with standard building and design protocols, the integration of natural systems makes sense on so many levels. Can we shift age-old patterns of thinking quickly enough to embrace them?