For scientists who worry about climate change, cities are just plain annoying. The acres of asphalt that cover roads and parking lots and roofs absorb enormous amounts of heat. In the summer, whirring air conditioners channel even more heat out of buildings and into the air. Climate scientists have to subtract this so-called urban heat island effect from their calculations if they want to get a true picture of how greenhouse gas emissions are warming the planet.
For people who actually live in cities however, the urban heat island effect is more than just a mathematical annoyance. If you’re sweltering on a hot summer day, your body doesn’t much care where the heat is coming from. And according to a paper just published in Nature Climate Change, urbanisation alone could drive local temperatures up by a whopping 7 degrees Fahrenheit (about 4 degrees Celsius) by 2050 in some parts of the US – some two or three times higher than the effects of global warming (which would also be going on at the same time).
“If you average this over the whole globe, the effect will be zero,” said lead author Matei Georgescu, of Arizona State University, in an interview. “But people don’t care about global temperatures. They care about conditions where they live.”
In this case, the people live in Arizona’s Sun Corridor, a megalopolis, or band of urbanisation, which stretches from the city of Nogales on the Mexican border to Prescott, about 100 miles north of Phoenix. It’s one of the fastest-growing areas in the country, with up to 16 million residents expected by 2050.
All those people will need roads and roofs and places to live, so Georgescu and several colleagues wondered what the urban heat island effect of all that additional population might be. They discovered that nobody had really thought much about it. “This is an absolutely under-researched area,” Georgescu said, “and it deserves much more attention.”
Georgescu and his team took urbanisation maps created by Maricopa Association of Governments and computerised weather prediction models that take the urban heat island effect into account, and put them all together to figure out where local temperatures might be heading. They focused on summer, when the urban heat island effect is strongest, and when high temperatures can be downright deadly, especially to the elderly who are already suffering from cardiovascular disease, lung diseases or diabetes.
The danger is especially great when nighttime temperatures remain high, which keeps the body from recovering after a scorching day. Unfortunately, the urban heat island effect affects nighttime temperatures the most: that’s when all the heat absorbed by the roads and buildings is re-released.
Georgescu emphasized that the 2050 scenario of a 7°F increase is the worst case: if population growth is more restrained, or if urban planners keep local population densities low and undeveloped open space relatively high, the effect could be much less. Reining in emissions of greenhouse gases would also make a difference. And the simple solution of replacing dark, heat-absorbing asphalt roof shingles with white, reflective roofs could cut the projected temperature increase.
“It’s possible, it’s practical, and it could cut the projected temperature increase in half,” Georgescu said. Unfortunately, he added, it doesn’t help at all with another urbanisation-related problem. When you pave or build over undeveloped land, you seal in whatever moisture there is in the soil. It can no longer evaporate, which cuts off an important source of humidity, and ultimately, of rain.
“So one of our take-home messages,” he said, “is that to be truly sustainable, you can’t just focus on temperatures. The climate system isn’t only about warming.”