Millions Are Sweating Through July on Urban ‘Heat Islands’

July is creeping closer to finishing as the hottest month in Earth’s recorded history, and if you live in a city, you’ve probably been exposed to even higher temperatures than your rural neighbors. That means you’ve faced higher health risks and higher cooling costs, with fewer options to protect yourself or your family from the dangers of extreme heat.

Those are the inescapable facts that emerge from my organization, Climate Central’s analysis of the urban "heat island” effect in 44 large U.S. cities. Some neighborhoods were literally built hotter. People who live there are taking some of the harshest punishments that global warming is dishing out.

Today, that unequal impact is most acute as a human health crisis. People are dying. A broken air conditioner is a life-threatening event in some cities. An outdoor job is a risk. Life on the streets is increasingly deadly.

In the future, over the long term, the inequitable distribution of summer heat could shape urban development patterns and housing markets, steering investment away from hot neighborhoods, leaving their communities to face the urban heat island effect with even fewer options.

Urban heat islands are areas made hotter by the built environment, like buildings and streets. Just about all cities, by virtue of population density — lots of people in a confined area generating heat by doing things like running air conditioners — are hot. But some neighborhoods are practically designed to radiate heat. Dark, impermeable surfaces, from parking lots and sidewalks to walls and rooftops, absorb heat instead of reflecting it. Tall buildings and narrow streets block airflow. Parks, trees and even patches of grass are rare. These heat island characteristics boost summer temperatures in the most impacted neighborhoods by at least 12 degrees Fahrenheit.

By applying a model of how these characteristics influence temperatures to census tract-level data that catalogs them for every city, our analysis showed that Miami, San Diego, San Francisco — and a host of other cities — have densely populated hot zones that hit this range.

Meanwhile, more than 2 million residents of Houston, Los Angeles, Dallas and Chicago experience temperature increases of at least 8 degrees attributable to their neighborhoods’ development patterns.

One city stands alone atop the rankings, though.

New York City is in a class by itself when it comes to urban heat islands in the United States. Across the entire city, the built environment exposes New Yorkers to an average temperature increase of almost 10 degrees. Nearly 3.8 million people — 41% of the population —live in neighborhoods whose characteristics can be expected to generate an even bigger temperature increase. A census tract heat map of Manhattan looks like a column of fire, except for a keyhole of modest relief squarely in the middle: Central Park.

That doesn’t mean an 800-acre park is the only solution to New York’s urban heat island effect. Replacing strips of pavement with grass can make a difference. Planting trees to shade buildings and sidewalks, as well as places where people wait for buses, can make a difference. Light-colored building materials, reflective roofs and rooftop gardens can make a big difference.

Even then, even if all of those measures reduced New York’s (or any city’s) urban heat island effect by half, residents would still live in a warming world. New York’s average summer temperatures have inched upward by more than 1 degree since the 1970s. Detroit’s have shot up almost 4 degrees since then. In Las Vegas, the jump is closer to 6 degrees. As long as Earth keeps warming, efforts to temper cities’ tendencies to amplify heat will be swimming against an accelerating tide. Of course, those efforts might save lives, nonetheless.

But if you live in a city, the real solution to protect you and your community from the risks of the urban heat island effect is to rein in the warming. That means cutting emissions globally to the point where the blanket of pollution wrapped around our planet stops getting thicker. Until that happens, life beneath that blanket will only get hotter. And people in urban heat islands will feel the heat even more.

Jen Brady is the lead data analyst at Climate Central, identifying significant trends, patterns and notable climate events. Brady previously worked at the U.S. Environmental Protection Agency evaluating the climate change impacts of waste and contaminated land management.