It is hardly news that in a warming world, there is a greater risk of increased hot temperatures, including truly extreme heat days that push the boundaries of what people are used to experiencing.
“If you’re standing outside, and it’s 110 degrees, and you’re sweating, the heat that gets removed from your body is largely by evaporation. And that’s proportional to the wind strength,” explains geographer Dennis Lettenmaier of the University of California, Los Angeles, one of the study’s co-authors.
The new paper, just out in Environmental Research Letters, examined the changing climates of fully 217 large urban areas between 1973 and 2012, ranging in size from a population of 250,000 all the way up to true megacities featuring populations above 5 million. The work, which was led by Vimal Mishra of the Indian Institute of Technology in Gandhinagar, India, examined long range weather records for temperature, precipitation and wind. That’s how the researchers homed in on these two major trends — which don’t go well together.
For days with extreme temperatures — defined as a temperature exceeding that of 99 percent of days for that particular location — the change was particularly strong at night. Forty-eight percent of the cities showed an increased trend toward more extreme hot days (vs. only 2 percent that saw a decrease). And 63 percent showed the same for hot nights.
Not surprisingly, this also translated into more heat waves, defined as at least aix days in a row in which every single day exceeded the 99th percentile. Heat waves can be deadly — particularly for the elderly. A 2013 summer heatwave in England, for instance, led to an estimated 650 deaths, according to an analysis performed at the London School of Hygiene and Tropical Medicine.
It’s important to emphasize that not all of this trend is global warming related. Part of it is due to the nature of urban areas themselves, which trap heat due to the higher concentrations of surfaces like pavement. These tend to be darker in color, and thus have a lower albedo (or reflectivity), meaning that they bounce away less sunlight than surfaces that are lighter in color — pooling more heat in urban areas.
“We have an overall warming trend, but the central focus of this study is to try to understand the relative contributions in cities of the general warming, and the exacerbation of the cities themselves as heat islands, and it’s clear you’ve kind of got both in there,” said UCLA’s Dennis Lettenmaier.
And then there was the trend in wind. Fully 75 percent of the urban areas saw declines in extreme windy days — while only 10 percent saw an increase. This does not seem to be a change driven by global warming, but rather, by the urban environment which, of course, features numerous very large structures that block the flow of air. “My hypothesis is that it’s increased drag, because of the cities,” said Lettenmaier.
Lettenmaier also thinks that the decrease in wind may explain part of the trends in heat — especially at night. In other words, the two may be interconnected.
The work is particularly crucial because in today’s world, humanity is stampeding into urban areas, which now contain more than half of our global population of 7.2 billion and growing. By 2050, it’s projected that 70 percent of humanity’s masses will live in cities.