Climate Change

Hot Times, Summer in the City: Understanding the Urban Heat Wave

July 15, 2015

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A heat mirage: a common sight in many cities this summer. Photo: Brocken Inaglory under a Creative Commons license
A heat mirage: a common sight in many cities this summer. Photo: Brocken Inaglory under a Creative Commons license

London, that city of perennial fog and drizzle, has been baking this summer. In June, at the peak of the heatwave, temperatures topped 98˚F.

Those without air conditioning huddled in malls or other places that did, or searched out a shaded spot with a bit of breeze. More than 400 people likely died in the UK due to heat-related ailments, like heat exhaustion or heat stroke.

While 98˚ may not seem especially hot in some cities – like Washington, DC, where I live — the heat wave was a real surprise for Londoners.

As I read all the news reports of London, I felt a deep sense of déjà vu. When I researched urban heat waves for my book, Conservation for Cities, I was struck by how frequently big cities get hit by a bad heat wave: from Chicago in 1995 to Moscow in 2010 to London today.

While for any particular city bad heat waves occur only infrequently, there are several cities globally facing one every summer. This year, it was London’s turn.

Perhaps the most well-researched heat wave in history was the European heat wave of 2003.

In Paris, its epicenter, daily temperatures were 15˚F greater than usual, topping 100˚F, and nighttime temperatures did not fall much below the day’s highs.

The beautiful stone buildings of Paris and other French towns heated up in the morning and stayed hot, becoming ovens that were slowly baking their residents.

In France, the most impacted country, there were 11,000 deaths. The total for all Europe was greater than 70,000 dead.

When I asked John Tagliabue, the New York Times reporter in Paris that summer, what he recalled about the heatwave, he said simply “The main thing I remember were the bodies.”

So many bodies that the morgues were full, requiring other refrigerators in a vegetable market to be commandeered.

As morbid as that sounds, there is also something immensely hopeful to me about the European response to the heatwave.

At multiple levels of government, officials learned from their mistakes and put in place better heat action plans.

In big heat waves, emergency cooling centers would be set up. Doctors and police had lists of elderly residents without nearby caregivers, so that they could be escorted to the cooling centers.

It’s not rocket science, but it works. When another heat wave, smaller but still significant, hit in 2006, the new procedures likely saved thousands of lives.

A second, perhaps surprising, finding from the research into the European heat wave is that street trees, parks, and other green space can save urban residents.

Neighborhoods with more greenness were cooler, and each 1˚C (around 1.8˚F) decline in temperature reduced the odds of death by 21 percent.

In other words, by providing shade the trees reduced the “urban heat island” effect, the tendency for cities to be generally hotter than the surrounding countryside, and thus saved lives.

Trees and other vegetation also cool the air in cities because they transpire water into the atmosphere, and this water can store heat that otherwise would make the water warmer (this is the same phenomenon that makes it fell cooler when your sweat evaporates on a hot day).

London won’t be the last city to have a bad heat wave. Indeed, Lahore in Pakistan also had a hot June, with serious consequences for its residents.

Climate change means these kind of extreme heat waves will become more common. One study for the U.S. predicted a four-fold increase in the number of days with extreme heat by 2050.

For most big cities globally, it is now not a question of if they will get hit by a heat wave, but when.

The good news is that the world’s cities are increasingly aware of this threat.

Groups like C40 and ICLEI have formed collective teams, with staff from multiple cities sharing ideas and techniques about how to reduce the effects of urban heat waves.

And conservation scientists are increasingly ready to do their part in helping cities adapt to climate change.

We can quantify the benefit of tree planting to air temperatures and human health, and we are increasingly helping cities plan where to plant trees to get maximum benefit.

While urban heat waves will be a more and more frequent summer problem, I am optimistic the world’s cities will be ready.

Rob McDonald

Dr. Robert McDonald is Lead Scientist for the Global Cities program at The Nature Conservancy. He researches the impact and dependences of cities on the natural world, and help direct the science behind much of the Conservancy’s urban conservation work. More from Rob

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