Climate Change

Climate Change is Already Heating Up the World’s Cities

August 11, 2017

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Sunset in Seattle. Photo © m33gs / Flickr through a Creative Commons license

Around the Mediterranean and southern Europe, the temperature pushed above 100⁰F last week, with some reports of high temperatures in a couple locations reaching 109⁰F (43⁰C). In one notable anecdote told by the New York Times, it was hot enough in Rome that gum was spontaneously melting in its pack. This heatwave, cutely nicknamed “Lucifer” since it feels hellishly hot, has caused all manner of problems, from the minor (Italian art museums closed and Italy’s wine crop could be ruined) to the serious (deaths have been reported in multiple countries). We in the United States have our own heat wave, less well-named. Seattle and Portland, two cities not known for hot summer weather where relatively few people own air conditioners, hit 104⁰F and 107⁰F respectively.

While it is hard to attribute any single event like Lucifer to climate change, new science makes it abundantly clear that climate change has already made our summers hotter and riskier. James Hansen and colleagues just released an update of their curves of summer temperatures across the world (hat to New York Time for an excellent write-up here). Compared with the base period (1951-1980), two-thirds of our summers now are hotter than the previous average, and 15% are much hotter. In other words, what used to be a hot summer heat wave is now just normal summer weather.

A trio of new climate modeling papers show just how hot climate change will make the world’s cities, and just how dire the public health consequences could be.

In May, a paper in Nature Climate Change forecast climate change impacts for major cities globally. There is a figure in this paper, Figure 2, that literally made me gasp when I saw it. It shows the cumulative distribution of temperature changes and increases in the urban heat island (which happens when impervious surfaces store the sun’s energy and then later reradiate it as heat, which is what makes cities hotter than rural areas) in the world’s cities in 2050 and 2100. If I am reading this right, the temperature in cities might increase by 11⁰F (6⁰C) in 2100 on average, but 10% of the time the increase would be greater than 14⁰F (8⁰C). If a typical heat wave in Washington (DC), my home town, is now around 100⁰F, then in 2100 we should be used to 110⁰F heatwaves.

The continuous lines show the estimated temperature increase for 2015 (black), 2050 (orange) and 2100 (red) under the RCP8.5 emissions scenario. Dashed lines include the estimated temperature increase from the UHI effect. Reprinted by permission from Macmillan Publishers Ltd: [Nature Climate Change] (Estrada, F., Botzen, W. W., & Tol, R. S. (2017). A global economic assessment of city policies to reduce climate change impacts. Nature Climate Change, 7, 403-406.), copyright (2017), Figure 2

A recent study by MIT scientists looked specifically at South Asia, focusing in on what is called the wet-bulb temperature. This is simply the temperature a thermometer would give you when wettened and exposed to air. At the wet bulb temperature, evaporation of water, such as the evaporation of sweat, by definition can’t cool someone any more. The MIT study points out that a wet bulb temperature above 35⁰C (95⁰F) would be fatal to most people within a few hours, unless they take steps to cool themselves with fans or air conditioning. Currently, only about 2% of the Indian population experience temperatures above a wet bulb temperature of 32⁰C (90⁰F), whereas by 2100 around 70% of the population will. Crucially, heat waves could bring some of those people very close to the 35⁰C (95⁰F) wet bulb temperature.

Finally, a recent report in the Lancet modeled the toll such hotter temperatures might have one human health. It focused on Europe, and predicted that unless cities begin to adapt, deaths due to warm temperatures could rise from about 2,700 a year now to more than 151,000 per year in 2100. In other words, climate change might increase the risk of mortality from heat waves more than 50-fold.

The key words there is “unless cities begin to adapt.” It is possible for cities to weather quite high temperatures without large increases in mortality, if they are ready. Those of us in the conservation movement have been talking a lot about urban street trees and parks as climate adaptation measures, as ways to reduce air temperatures. And indeed, The Nature Conservancy’s work in the Planting Healthy Air report shows that trees are providing meaningful health benefits to millions in cities. But the average reduction in air temperatures is likely only 1-4⁰F (0.5-2⁰C). This clearly only begins to blunt the risk of higher temperature due to climate change, which could increase temperature 11⁰F (6⁰C) on average by 2100. Cities around the world need to begin actively creating heat action plans, including planning for emergency cooling centers so that those without air conditioning or fans can get to safety.

Climate change is here, and it is only just beginning. It’s time for the world’s cities to get ready.

North Michigan Ave, Chicago. Photo © Samantha Pinkham
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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