Exploring the heat island effect

A new summer tradition has developed over the last few years: setting a new record for the hottest day on Earth. According to NASA, the high mark got bumped up twice already this year. The first new high came on July 21, and then pushed higher on July 22. It displaced the previous record, set in July 2023.

But if it's hot enough in your city this summer to fry an egg on the sidewalk, it's not just because the mercury in your thermometer keeps climbing. It's also because of design choices that make metropolitan areas heat magnets. The result is the urban heat island effect, and it is causing a significant increase in the temperatures experienced by people living in densely populated areas.

What is an urban heat island?

Not every 100-degree day is created equal, and it certainly feels different depending on where you are. Rural townships and large, sprawling cities may sit under the same sun, but it can feel significantly hotter in certain parts of the city. These pockets that experience hotter temperatures are known as heat islands, and they are caused in large part by human-created environs.

You know the trope about burning ants by focusing the sun through a magnifying glass? You can think of city infrastructure as the stand-in for the magnifying glass, and humans as the ants. No, no one is getting burned here, but many of the standard structures that we see throughout cityscapes have a tendency to absorb and re-emit the sun's heat.

The result is that urban spaces can experience air temperatures that feel as much as 20°F hotter than surrounding areas.

Where the sun shines brightest

A recent study from Climate Central analyzed heat islands across 65 major U.S. cities that are home to more than 50 million people. It found that nearly 70% of the studied population are exposed to, on average, temperatures of 8°F or higher due to heat island effects.

But that heat does not blanket the city equally. Researchers at Columbia University recently found that areas of cities that were historically subject to redlining, a discriminatory housing practice that kept members of marginalized communities from living in a neighborhood, experience hotter summers than more affluent communities.

"Design choices often impact lower-income communities and communities of color under the guise of urban blight," explained Franklin Forbes, the CEO of Blistery and former teacher of sustainability and urban planning courses at The School of The New York Times.

He pointed to Lincoln Center in New York City, which saw its green spaces stripped away and industrial buildings with metal facades built up, amplifying heat. "In cities like Chicago, Minnesota, and New Orleans, they built highways that bisected Black and brown neighborhoods," Forbes said. "These highways added to the heat island effect by adding congestion, cars, carbon emissions from those cars, and the heat created by the paved roads."

Other factors, from living in buildings with worse insulation and outdated cooling systems to disparities in health care access and health outcomes, create conditions that make exposure to extreme heat much more deadly to these communities that absorb the brunt of the heat island effect.

Decisions on where we have built cities can lead to higher heat exposure, too. Steffen Lehmann, Professor of Architecture and Urbanism at the University of Nevada, Las Vegas and Director of the Urban Futures Lab, said that one billion people across the planet are living in desert cities. Because of the climate surrounding these cities, extreme heat exposure is common, and conditions of city structures exacerbate it.

"Some of those desert cities are the fastest growing cities in the world," he said, pointing to places like Dubai in the United Arab Emirates, Lima in Peru, and Las Vegas in the United States. And while these cities build up quickly, they run the risk of making design and architectural decisions that will worsen the conditions of the urban heat island effect.

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How'd we get here?

There are a confluence of conditions that create the urban island effect, but a few have an outsized impact. At the top of the list is building density and size, which traps heat within tighter quarters and packs people into those hotter areas.

"Many urban planners held Le Corbusier as the standard and followed his ideals of the 'radiant city,' the practice of separating commercial and residential zones and needing more diversity in scale and height," Forbes explained. "This causes heat zones in the cities and their downtown areas and cool zones in lower-density residential areas outside the downtown areas."

The material of these structures matters, too. You may have heard of a city called a "concrete jungle," and the name is apt because all of that concrete helps to create temperatures you'd expect in the tropics. "The city becomes like a baking oven that stores and traps and absorbs the solar radiation," Lehmann explained. "The worst thing you can have is black asphalt," which is commonly used for parking lots and roads but also gets utilized on concrete roofs.

Glass, too, can cause problems. It allows sunlight to pass through, which allows radiant heat to build up. For a period, glass towers were a popular architectural choice for large buildings and skyscrapers, under the guise that lots of glass and windows were needed to pull in natural light and offer better views. "Ten or 20 years ago, a lot of glass towers were built here," Lehmann said. "Nobody would do that today."

Perhaps one of the biggest issues that cities face, though, is what is lost as those roads go in and buildings go up. Green spaces, trees, and vegetation have all been dug out and ditched in an attempt to utilize more space. This, too, can be attributed in part to misguided city planning. "Robert Moses's plans and his creation of significant expressways, which destroyed natural greenways and low-density residential neighborhoods, helped amplify the heat island effect," Forbes said.

Increasingly, finding a park in a city can feel like finding an oasis in the desert. That means many cities are missing out on the evaporative cooling provided by plants and paying the price through hotter temperatures.

Borrowing against the future

Because heat islands are created by infrastructure, the prospect of addressing them isn't really a short-term goal-it is a problem quite literally built into the foundation of many of the places we live. And many of the tools we have for mitigating the effects of these hotter temperatures can contribute to the conditions that invite more extreme temperatures.

Air conditioning, for instance, is the standard frontline of defense against heat in most cities. "Since the middle of the 20th century, the simple solution is air conditioning," Lehmann said. And while the cool air generated is useful for protecting people in their homes from extreme heat, it also contributes to the problem. "The waste heat from the chillers from the air condition is just heating up the city even more," according to Lehmann.

The United Nations found that air conditioning already is responsible for seven percent of all greenhouse gas emissions globally. As the planet continues to warm and hot summers become more common, it's projected that A/C usage will double by 2030 and triple by 2050. That is an unsustainable cycle. The hotter it gets, the more cooling needed, and the more cooling used, the hotter it gets. "It's a vicious circle," Lehmann said.

Solutions for today and tomorrow

Heat islands are one of those problems in which the best time to address the issue was as a city was being planned. Given that time is long gone, the second best time to address it is now. Luckily, there are solutions and techniques available that can both lessen the impact of excessive heat now and work to reduce the amount of heat generated.

According to Lehmann, there are two primary styles of solutions: passive design and active design. The goal of passive design is to use natural elements and architectural features to mitigate urban heat without the need to use systems like air conditioning.

Nature-based solutions like parks and vegetation are ways to use passive design to reduce heat, and they can be implemented both quickly and in a cost-effective manner. "Integrating vegetation and trees wherever possible and creating public parks can cool down the urban microclimate of the city," Lehmann explained. "We measured and found that the integration of greenery and vegetation … can make a temperature difference of around 20°F less hot, which is pretty significant."

Another simple change that can serve as a passive solution is to just give more consideration to color. Dark colors store solar radiation and cause hotter temperatures. "We need brighter, light-reflecting rooftops," Lehmann said. "We need to use 'cool roofs' and facades in white or lighter colored, heat-reflecting surfaces."

Active design solutions typically require a more concerted effort to implement but can make a huge difference over time. Lehmann highlighted photovoltaic solar rooftops, which can not only absorb sunlight but convert it to clean energy that can be used without carbon emissions. Higher-efficiency cooling systems installed in buildings can also significantly reduce the amount of greenhouse gases created by keeping people cooled. Heat pumps, for instance, can cut down on as much as 64% of emissions.

Lehmann also pointed to breakthroughs in nanotechnology development, which can facilitate the creation of "new coatings that keep metal pieces and building products cooler." These breakthroughs can use what is known as the albedo effect, which reflects sunlight off of structures and keeps solar radiation from collecting and producing more heat.

In the long term, Forbes believes that the foundations of city planning need to change. "As we plan our cities, we need to move away from the ideals of Crobusier and Moses and lean into the 'greenification' of our cities with fewer cars, better planning that allows for our cities to be able to cool at night, and making them on a human scale that mixes low density residential and commercial in one place," he said.

Ultimately, all of these solutions will need to be utilized to mitigate the urban heat island effect. Without action, city dwellers will left be stranded without escape from the growing problem of extreme heat.

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