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How to Cool Our Sweltering Cities Without Air Conditioners

We can do more than battle heat waves with armies of air conditioners.
Photo via jerryfergusonphotography/Flickr

New York City's 90 degree-plus temperatures last week may not have set new records for the month of May, but it was certainly a sign of the sweltering months to come. Even though we deal with this every year—temperatures that are often 5 degrees Fahrenheit higher during the day than nearby non-urban areas, and up to 22 degrees higher at night, due to the urban heat island effect—what can be done about it?

It doesn't take deep thinking to understand why built-up urban areas are notably hotter than surrounding, less developed places: It's a combination of asphalt, pavement, and buildings, absorbing and retaining heat, furthered by a lack of trees and planted surfaces, plus traffic and exhaust from building cooling systems venting waste heat. We experience the urban heat island both in terms of increased surface temperatures (roofs and pavements can be 50-90°F hotter than the air) and increased air temperatures (the 5-22°F temperature differences mentioned above).

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All of it adds up to increased energy demand: Up to 10 percent of peak energy demand in the summer comes from increased demand for cool buildings.

Image: EPA

The effect is only going to get worse with increasing global warming and urbanization. Last August a study in Nature Climate Change showed that, while the urban heat island effect doesn't contribute much to global temperature rise, local temperature rise near our expanding urban areas could be two to three times higher than average by 2050.

Solutions to reducing these conditions, confining ourselves to the built environment and not how we can adapt our personal lives and routines to the heat, fall into two broad categories: Changes that we can make in the built environment today and changes in how we build in the future.

Though trees seem to be benefiting some from urban heat islands, they are also one of the better solutions to reducing the effect. According to the EPA's summary of their benefits, trees and vegetation reduce urban temperatures in two main ways: Though direct shading, where in summer just 10-30 percent of solar radiation reaches the areas beneath a tree, reducing surface temperatures 20-45°F; and through increasing evapotranspiration, decreasing air temperatures up to 9°F underneath trees.

Placement of trees and vegetation matters. Planting trees along western and eastern exposures both cools buildings and reduces energy demand. Planting on southern exposures does this as well, but must be balanced against reducing the ability of a building to take advantage of solar heating in cooler months. Vines can be used for similar results. The EPA says planting trees on the west and southwest exposures of a building can reduce energy needed for cooling by 7-47 percent. In parking lots and along streets, trees obviously provide many cooling benefits as well.

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Beyond providing shade, trees actively reduce air pollution directly, decreasing particulate air pollution, nitrogen oxides, sulfur dioxide, carbon monoxide, and ground-level ozone. Due to increase carbon storage potential, urban trees also contribute to slowing climate change.

Photo: kafka4prez

Green roofs offer similar benefits as do trees and vegetation in reducing temperatures, though there have been some genuine questions raised about their effectiveness. A green roof can reduce the energy demand of a building, reduce air pollution in the same manner as vegetation planted on the ground, as well as help manage stormwater runoff.

A far less expensive option targeting roofs, cool roof techniques radically reduce surface temperatures of a building (50-60°F lower than conventional roofing), as well as reducing energy demand, both by changing the reflectivity of the roof. Overall, a cool roof can reduce yearly energy demand of a building by 50 cents per square foot.

All of things can be implemented fair quickly, acknowledging that trees take years to reach mature growth and maximum cooling effect. But it's also worth looking to how cities and buildings were built when there was no option of just flipping a switch to turn on the AC.

Photo: Selmer van Alten

India's traditional stepwells (above) first appeared more than 1000 years ago, producing a microclimate that can be 20°F cooler than the surrounding area just by virtue of below-ground construction, thick walls, and water. A study from Arizona State University looking at traditional urban planning and construction techniques in Yazd, Iran, highlights how houses were built to maximize cooling and work with the local conditions, rather than fight against them. The report describes how it works:

The hot and arid air is caught by the high adobe wind catcher and is led into the house. On its way through the long adobe chimney and with the help of evaporation principle, the air becomes moist and cool. Through the adobe wind canals inside the house, a part of the air is led into the basement and the other part into the summer residence and courtyard. The first part passes through the basement's adobe wind canals under the courtyard and absorbs moisture from the canal walls. Accordingly, it blows to the courtyard through the small openings on the surface. The whole evaporation and also air circulation processes cause a more comfortable climate in house and in courtyard."

These structures cause outside heated air, entering at over 100°F, to cool down to a far more comfortable 73°F by the time in passes through the building.

Image: Herberger Institute

Now, both these examples are from arid climates, unlike the far more humid climates of New York and many major cities in the US. Southern California and the Southwest, however, could benefit. Regardless, the myriad passive cooling solutions out there are proof that, in our modern cities, we can do more than battle heat waves with armies of air conditioners.