As intense temperatures break records around the world, a little-known fact offers hope for cooling cities: Even during the height of extreme heat, some city blocks never experience a heat wave.
How is this possible?
Civilizations have recognized for centuries the power of cities to heat and cool themselves. City architects in ancient Rome wanted streets to be narrower to reduce afternoon temperatures. It was found that narrow streets cooled the air by limiting the area exposed to direct sunlight.
The whitewashed architecture of the Greek Isles illustrates another long-practiced strategy: Light-colored walls and roofs can help cool cities by reflecting incoming sunlight.
In the hot, humid regions of the southern United States, Thomas Jefferson suggested another approach to cooling: that all new settlements use a checkerboard pattern of densely vegetated city blocks interspersed with dense buildings. This could promote cooling through convective air movement between cool and warm zones.
As I explore in my recent book, Radical Adaptation: Transforming Cities for a Climate-Changing World, modern cities are unintentionally raising their own temperatures, creating what is known as the “urban heat island effect.”
How do cities heat themselves?
Cities increase their temperatures in four primary ways:
Combined, these four drivers of the urban heat island effect can increase urban temperatures by 5.6 to 11 degrees Celsius (10 to 20 degrees Fahrenheit) on a hot summer afternoon—a significant human-caused weather change that can pose a serious health risk to anyone without air conditioning.
When combined with the design of the built environment, a city’s natural topography can make temperature differences from one neighborhood to another even more pronounced. For example, San Francisco’s hills and fog patterns consistently divide the city’s neighborhoods into distinct climate zones. And in hot, dry climates, the widespread use of garden irrigation systems can provide lower urban temperatures than the surrounding desert, sometimes called urban cool islands.
Simple steps to cool cities
Understanding the extent to which cities can heat themselves provides powerful tools for cooling them as human-caused global warming increases base temperatures.
First, it is essential that cities sharply reduce their greenhouse gas emissions to stop fueling global climate change. Globally, urban areas, with their industries, vehicles and buildings, account for more than 70% of greenhouse gas emissions from energy use, and their populations are growing rapidly. Even coordinated global reductions in greenhouse gas emissions would require decades to measurably slow warming trends, so cities will still need to adapt.
Cities can sometimes slow the pace of urban heat island-related warming trends by taking simple steps. Research shows that urban heat island reduction can have significant health benefits.
At the Georgia Tech Urban Climate Lab, my colleagues and I partner with cities to estimate the cooling potential of urban heat management—strategies designed to reverse the urban heat island effect. To do this, we measure the direct health benefits of actions like expanding tree cover and other green infrastructure and using cool materials for roads and roofs.
Our study shows that planting trees on just half of the available area to support tree cover (along streets, in parking lots, and in residential yards) could lower summer afternoon temperatures by 5–10 F (2.8–5.6 C) and reduce heat-related deaths by 40%–50% in some neighborhoods.
To realize these significant benefits, New York City set and achieved a goal of planting 1 million trees across its five boroughs.
Cool roofing materials and light-colored surfaces can also help reduce heat. If you wear a black shirt in the sun on a hot day, you will heat up more than if you wear a white shirt. Similarly, light-colored building materials, roofing, and shingles reflect more incoming solar heat than darker-colored ones and absorb less of it. They are especially effective in the heat of the day when the sun’s radiation is strongest.
To take advantage of this cooling effect, Los Angeles became the first major city to mandate cool roofs on all new homes in 2013.
What can cities do now?
Aggressive strategies to increase green tree cover across cities, a rapid transition to cool roofing materials, and even replacing some street parking lanes and other underused impervious areas with vegetation-filled bioswales can significantly reduce urban temperatures and, in doing so, increase a city’s resilience to rising temperatures.
Our urban heat risk assessments in numerous U.S. cities, including Atlanta, Dallas, Louisville, Kentucky, and San Francisco, show that a combination of urban heat management strategies can reduce neighborhood temperatures by more than 10 F (5.6 C) on hot days and reduce premature heat-related deaths by 20%–60%.
A cooler city is a safer city, and it is in the hands of communities to create this.
This article is republished from The Conversation, a nonprofit, independent news organization that delivers facts and analysis to help you understand our complex world.
Written by: Brian Stone Jr. Georgia Institute of Technology.
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Brian Stone Jr. does not work for, consult, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no affiliations beyond his academic appointment.