Is It Hot In Here, Or Is It Just Me?
Today we’ll get familiar with the urban heat island effect, and then we’ll look into some fascinating research about how it affects real people’s lives. My career goal is to influence policy decisions related to the environment so that I can improve people’s lives. This topic really speaks to me because it’s a direct application of my environmental science background to my community and communities around the country. Urban heat islands negatively impact people in measurable ways, and the more research we have the more likely it is that we can form policies to help! Today I’ll educate you Scouts, and daydream about the day that I can educate congresspeople. With that bit of background, let’s jump in!
What is the urban heat island effect?
At its most basic level, the urban heat island effect is the phenomenon of cities being hotter than the surrounding rural areas, because of human activity.
Why are cities hotter than the countryside?
Here are some of the main reasons in no particular order:
One reason that cities are hotter has to do with their large amount of concrete and asphalt. We all know that dark colors tend to have lower albedos, or absorb more heat (one of the reasons Texans never wear black in the summertime). Now think of all the dark asphalt roads and parking lots in cities - they absorb heat from sunshine all day long, and hold onto that heat into the night. The rural areas surrounding cities tend to have higher albedos - asphalt only reflects back 5-10% of sunlight, while grass and soil both reflect 10-25%. Because rural areas bounce back that sunlight instead of absorbing it, they’re able to cool down faster when the sun goes away.
Cities experience a lot of air pollution from human activities (like driving), and when greenhouse gases are pumped into the air they help retain heat. Since wind is often blocked in the city by large buildings, it’s hard for the air pollution to dissipate, so it just hangs around.
Both humans and machines create “waste heat.” When you go for a jog, your body releases heat. The same thing happens when you use a machine - just think how hot the hood of your car is after you’ve driven it. All that heat has to go somewhere, and in cities it just gets trapped. This is caused by the urban canyon effect - when big tall buildings line streets, heat energy gets bounced around from surface to surface and has a hard time escaping. You can think of this like a sweatier version of Pong.
Cities have fewer trees than the surrounding countryside. We all know how shade works - that’s why I’d way rather park far from the grocery store under a tree than right up front in the open. In addition to shade, trees also perform the important function of evapotranspiration. Evapotranspiration is the process in which leaves convert ground water into water vapor and it works together with the evaporation of bodies of water to cool temperatures. And just because trees are so kind, they throw in a third benefit for free: elimination of GHGs. Trees gobble up carbon dioxide and give back oxygen, helping to clean the air of some pollutants that human activities emit.
How much hotter?
According to the EPA “The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C).” That’s a lot!
Why is the difference bigger at night?
The long answer gets a little complicated, so for now I’ll stick with the simple version. Basically, materials like concrete can retain heat very well. In fact, concrete can retain about 2,000 times more heat than the same volume of air. Because cities are built of materials like concrete, it takes longer to radiate all their heat back out into space. The process is made even harder because tall buildings block wind that could otherwise help cool things off through convection. Oh yeah, and that urban canyon effect makes buildings and the ground absorb some of the heat that each other radiates, taking even longer for heat to actually escape the city into space. If you’re interested in meteorology, I’d recommend doing some more research into this diurnal (night/day) pattern of urban heat islands. The science is counterintuitive, which I think makes it extra interesting!
I’ll note that the difference in temperatures is also bigger in winter than in summer, and without digressing, albedo is part of the answer again. The snow that builds up in rural areas has a very high albedo and reflects a lot of solar energy, while humans tend to plow snow in cities and cause it to melt faster. This again makes the albedo of city vs. country very different and can exacerbate the heat island effect.
Do urban heat islands have any larger effect on regional climate?
Yes, although the research is mixed as to how big the effect is. Heat islands can definitely change local heat and precipitation patterns, and can affect the amount of rain in surrounding areas. Some studies show that heat islands only affect local climate, but others suggest that heat islands could play a larger role by interfering with the jet stream. There is not a strong consensus as to the role heat islands play in contributing to climate change, but several studies have shown that the severity of the effect is increasing with climate change.
How do urban heat islands affect humans?
Man oh man, let me tell you. First of all, urban heat islands can create positive feedback loops that double down on the problem. If a city is feeling extra hot, especially if there’s no cooling relief at night, lots of people want to turn on their air conditioners. That uses a lot of electricity, which strains resources and increases energy demand, which have their own negative externalities. Air conditioners also create waste heat when they’re turned on, which gets trapped in the city for all the same reasons that the heat island occurred in the first place. Now even more people want air conditioning. The electricity demand can get too high, causing rolling blackouts across the city when energy providers can’t keep up. Now people feel even hotter and stickier because they’re stuck back in the heat they wanted to escape. National Geographic states that “the urban heat island around Los Angeles, California, costs the city $100 million a year in energy!” This phenomenon wades into economic territory and energy budgets, not just people feeling sticky and uncomfortable.
Heat can get personal too - and dangerous. Emergency calls spike when the heat index does, especially in Northern cities where people are less acclimated to heat and not everyone has air conditioning. Heat stroke itself can be fatal, especially in the elderly, and extreme heat can also exacerbate other health conditions like asthma and heart disease. NPR conducted a study of the 97 most highly-populated cities in America, and the results showed that in 3/4s of those cities the hottest areas were also the poorest. Temperature data was mapped over socioeconomic data, and showed that poorer neighborhoods are bearing the brunt of the urban heat island effect in America. And not only are these hotter neighborhoods in low socioeconomic areas, they also tend to be populated by people of color. The people who are being most affected by heat islands are the least equipped to cope: if you have a low income, you likely have poor access to healthcare in the first place. Now what can you do if your health problems are made worse by the heat you live in? You don’t have the luxury to move to a cooler (and more affluent) area of town. You don’t have the ability to pay for medical expenses or to miss work. In many places in America, you don’t even have an air conditioner in your home to help you cool off. This area of climate change activism and research regarding the disparate effects on different (and primarily disadvantaged) social groups is called environmental justice. The urban heat island effect is clearly a public health problem, it disproportionately affects already vulnerable social groups, and it’s only going to get worse with climate change.
Enter: public policy. People need help. And right now, unsurprisingly, lower income people need help the most. So what can be done, particularly at the local level? Planting more trees in cities is one option. Trees provide shade, evapotranspiration helps to cool things off, and removing CO2 from the air helps with one of the root causes. Many cities are investing in expanding their tree canopy, but this can be an expensive and long-term project. Increasing public transit is another strategy cities can use. This won’t actively work to cool things down like trees can, but taking cars off the road can at least help stop contributing to the problem. Some cities are designing roofs with higher albedos that reflect more sunlight, or rooftop gardens that have naturally higher albedos and come with the other benefits of plants. These strategies boil down to some main categories: increasing solar reflectivity, increasing evapotranspiration, and behavioral changes to mitigate heat emissions. Different options make more sense for different cities, and there’s plenty of room for more innovation and invention.
The one thing that’s set in stone? We need action. Fellow Americans and human beings with just as much inherent worth as you and I need to be protected by those that have power to make changes. Climate change isn’t like a chess opponent that will stop to let us react and take a turn. We need to act, and we need to act fast. Write to your city councilors and mayor, write to your congresspeople, organize community gardens with your neighbors, start a compost pile - work on whatever scale you can manage. Let your reps know that you’re watching and you care. And importantly, stay educated. I highly recommend the NPR article below for a human interest-style presentation of this problem. Want to learn more about mitigation strategies for cities? Check out the researchgate.net link. Keep your eyes peeled for trustworthy reporting about this problem, and share what you learn with friends so that we can all be involved in improving the lives of people negatively affected by urban heat islands.
Here’s what I read before writing this article. Check out the links if you want to learn more!
https://www.nationalgeographic.org/encyclopedia/urban-heat-island/
https://www.epa.gov/heat-islands
https://www.npr.org/2019/09/03/754044732/as-rising-heat-bakes-u-s-cities-the-poor-often-feel-it-most
https://www.epa.gov/heat-islands/heat-island-cooling-strategies
https://science.howstuffworks.com/nature/climate-weather/storms/trees-affect-weather1.htm
