As extreme heat breaks records around the world, a little-reported fact offers some hope for cooling cities: Even during the most intense periods of extreme heat, some city blocks never experience heat wave temperatures.
How is this possible?
Civilizations have recognized the power of cities to heat and cool themselves for centuries. City architects in ancient Rome called for narrowing streets to lower late afternoon temperatures. It was discovered that narrow streets cooled the air by limiting the area exposed to direct sunlight.
The whitewashed architecture of the Greek islands demonstrates another long-practiced strategy. Light-colored walls and roofs can help cool cities by reflecting incoming sunlight.
In hot, humid areas of the southern U.S., Thomas Jefferson proposed a different approach to cooling: have all new settlements use a checkerboard pattern of heavily vegetated city blocks interspersed with dense development. This could promote cooling by convective air movement between cool and warm zones.
As I describe in my recent book, “Radical Adaptation: Transforming Cities for a Climate Changed World,” modern cities are inadvertently raising their own temperatures, creating the so-called “urban heat island effect.”
How cities warm themselves
Cities are raising their temperatures in four key ways:
Combined, these four factors that cause the urban heat island effect can raise city temperatures by 10 to 20 degrees Fahrenheit (5.6 to 11 degrees Celsius) on a hot summer afternoon — a significant, man-made change in the weather that can pose a serious health risk to anyone who doesn’t have air conditioning.
In combination with the design of the built environment, a city’s natural topography can further accentuate temperature differences between neighborhoods. For example, San Francisco’s hills and fog patterns consistently divide the city’s neighborhoods into distinct climate zones. And the extensive use of garden irrigation systems in hot and dry climates can result in lower temperatures in the city than in the surrounding desert, sometimes referred to as urban cool islands.
Simple steps to cool down cities
Understanding the extent to which cities can warm themselves will help us cool them down, as human-caused global warming is raising the base temperature.
First, it is essential that cities drastically reduce their greenhouse gas emissions to stop fueling the global phenomenon of climate change. Globally, urban areas, with their industries, vehicles and buildings, are responsible for more than 70% of greenhouse gas emissions from energy use, and their populations are growing rapidly. Even globally coordinated reductions in greenhouse gas emissions will take many decades to measurably slow warming trends, so cities will still need to adapt.
Cities can also slow the pace of urban heat island-driven warming trends by taking sometimes simple steps. Research shows that the health benefits of urban heat island reduction can be substantial.
At the Georgia Tech Urban Climate Lab, my colleagues and I work with city governments to estimate the cooling potential of urban heat management – sets of strategies designed to reverse the urban heat island effect. To do this, we measure the direct health benefits of actions such as expanding tree cover and other green infrastructure and using cool materials for roads and roofs.
Our research shows that planting trees in just half the space available to support tree canopy – such as along streets, in parking lots, and in residential areas – can reduce summer afternoon temperatures by 5-10 F (2.8-5.6 C), reducing heat-related deaths by 40%-50% in some neighborhoods.
Recognizing these significant benefits, New York City set a goal to plant 1 million trees in five boroughs.
Cool roofing and light-colored surfaces can also help lower the temperature. 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. Likewise, light-colored building materials, roofing materials, and shingles reflect more of the incoming solar heat than darker ones, and absorb less of that heat. This is especially effective during the heat of the day, when the sun’s radiation is strongest.
To take advantage of this cooling effect, in 2013 Los Angeles became the first major city to require cool roofs on all new homes.
What cities can do now
Aggressive strategies to increase green tree cover in cities, a rapid transition to cool roofing materials, and even replacing some on-street parking strips and other underutilized impervious areas with vegetation-filled bioswales can significantly reduce urban temperatures. In this way, a city’s resilience to rising temperatures can be increased.
Our urban heat risk assessments in several US cities, including Atlanta, Dallas, Louisville (Kentucky), and San Francisco, show that a combination of urban heat management strategies can reduce neighborhood temperatures on hot days by more than 5.6°C and reduce premature heat-related deaths by 20%-60%.
A cooler city is a safer city, and that is something the community itself can create.
This article is republished from The Conversation, an independent nonprofit organization that brings you facts and analysis to help you understand our complex world.
It was written by: Brian Stone Jr., Georgia Institute of Technology.
Read more:
Brian Stone Jr. is not an employee of, an advisor to, an owner of stock in, or a recipient of financial support from any company or organization that would benefit from this article, and he has disclosed no relevant affiliations beyond his academic appointment.