Cooling cities with trees and other greenery

Scientists find out more about how to keep cities cool

As recent headlines from Portland and Seattle testify, global warming is now making things unbearably hot. A week ago, after two days of triple-digit sweltering heat, Portland, Oregon, set a record of 115 degrees, 30 to 40 degrees hotter than what Portlanders normally experience this time of year.

Climates have changed around the world. Throughout the globe, spring occurs earlier and fall comes later. Less snow falls and it melts more quickly. Droughts and extreme weather, once rare, are now common.

And there will be more extreme heat, just like Portland last week. Folks can head for the hills or crank up the air conditioning, but the root problem will not go away, and it will only get worse.


Cities can mitigate summertime heat and heatwaves by planting trees along streets or ensuring that they have parks and open spaces among their buildings and boulevards. Besides shading streets and buildings, and thus cooling them, the natural process of greenery pulling up water from the ground and releasing it through their leaves, called evapotranspiration, acts as a natural outdoor swamp cooler. Trees are thought to reduce city dwellers’ exposure to ultraviolet radiation from the sun, which can lead to skin damage and skin cancer. It is also assumed that trees lessen ground-level ozone and other common city pollutants.

These things are intuitively clear, and there has been scientific research examining the benefits of urban greenery. But there are still questions. What would be an optimal amount of greening for a city? Can you have too much of a good thing? Too many trees?

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A team of researchers has just reviewed the scientific literature on this subject, examining 308 studies of what is called “urban greening.” Topics included how trees and other plants can cool urban settings, as well as how greenery can mitigate UV radiation, ground-level ozone, and contribute to public health. The study is an update of a review that was performed by many of the same team members in 2010.

The heat island effect

Concrete, tarmac, and other man-made city stuff are warmed by the sun. These surfaces then radiate their warmth to the surrounding air, leading to what is called the “urban heat island effect.” The result is that temperatures in cities are warmer than those in the surrounding countryside. Folks have known about this for a long time. Ancient Romans noted that their imperial city was usually warmer than the fields and forests nearby.

With a warming world, the heat islands of cities will worsen. More people will also be affected, as more populations migrate to cities.

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Health effects of excessive heat in cities

Extreme heat can affect health and can even result in the loss of life. In 2017, the Lancet Commission estimated that, worldwide, between the years 2000 and 2016, an additional 125 million vulnerable adults, such as the elderly, were exposed to heatwaves that posed serious health risks due to human-induced climate change.

Excessive heat can also increase ground-level ozone and nitrogen oxides in city air. These gases can damage vegetation and soil. They irritate lungs and make people cough. They can also exacerbate symptoms for people with asthma, bronchitis, or emphysema. Nitrogen oxides come from the burning of fossil fuels, and in cities come most commonly from motor vehicle exhaust. Ground-level ozone is a similar product of chemical reactions of pollutants produced by cars, refineries, and power plants.

Trees to the rescue (maybe?)

It has been thought that trees and other greenery can reduce ozone levels by absorbing the pollutants that lead to the production of ozone. But trees also emit what are called biogenic volatile organic compounds (BVOCs). These are alcohols and other substances. The best way to understand BVOCs from a layman’s point of view is to take a whiff in a pine forest. That smell is mostly BVOCs. These chemicals can produce ozone under the right conditions, so the picture of urban greening grows a little complicated.

The study

The research team examined studies and data from cities all over the globe, from the tropics to the far north and south. They also looked at temperature variations within cities, comparing the temperature of a city street with the temperature within a park or a garden. They also looked at the temperatures over the course of a day or over the course of months or years. They compared gardens, grass-covered fields, and forested areas.

Their findings, published in the peer-reviewed journal Environmental Evidence on June 5th, confirmed the prevailing thought on urban greening. Importantly, the team also found some more effective ways to green up a city. They found that green areas, tree-lined streets and parks could be almost four degrees Fahrenheit cooler than spaces without greenery. An important part of the study found that trees are great for daytime cooling, but they tend to hold heat at night. Conversely, grassy areas are cooler than trees at night, and less cool than shaded tree areas during the day. Both areas, as expected, are cooler than city streets. The authors suggest these findings might form the basis for planning urban gardens and parks with a mix of trees and open grass to maximize cooling during the day and cooling at night.

Some studies examined by the scientists found that larger gardens and parks increased cooling, while other studies found no such correlation. The authors suggest that the shape of the green space within the urban environment may have a lot to do with this. The cooling effect of a green space can reduce the temperature of a city or neighborhood up to three-quarters of a mile away. Thus cities can maximize their urban cooling by planning effective spacing of parks and city gardens.

The scientists further found that greenery reduced nitrous oxides but did not reduce the amount of ground-level ozone.


Moving forward

Currently, San Diego, where I live, incorporates urban greening into its Climate Action Plan, as do many other cities throughout the world. From what I’ve read of the Plan, the city intends to plant a bunch of trees to fight our urban heat island effect. I don’t believe the planners know about the finer points spelled out by the study authors. Findings from this study and others like it should be incorporated into all climate action plans. The heat waves are coming, again to Portland, as well as every other city on the globe. We all need the most effective cooling possible.

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