Your phone dings. It’s an emergency alert.

It reads: “Conserve energy: NYC is urging all households and businesses to immediately limit energy usage to prevent power outages as the intense heat continues. Please avoid the use of energy-intensive appliances such as washers, dryers and microwaves. Limit unnecessary use of air conditioning.”

Everyone in New York City received that emergency alert in  June 2021. The city was pleading with residents to lower their energy consumption. The heat wave was causing an increase in electricity usage by people desperate to get cool and as a result, the city’s power grid was in danger of failing. 

What happened this summer in New York is not a one off event. It is a harbinger of what our urban areas will continue to look like if left unchecked. 

What is an urban heat island?

Heat islands are a well-studied phenomena where urban spaces are hotter than surrounding suburban or rural areas. Cities become pockets of heat compared to cooler surrounding areas, hence the term “islands.” Even within cities themselves, temperature can shift widely based on the presence of things like trees and bodies of water.

Not by coincidence, urban heat islands are more pronounced in historically Black and Brown neighborhoods. This disparity is a legacy of our nation’s history of redlining, racially restrictive land covenants, and the ongoing lack of investment into those communities. This is seen across the country, including in my own Massachusetts backyard. Temperatures in Chelsea, Everett, and poorer areas of Boston can be 10 to 40 degrees higher than wealthier suburban towns like Newton, which are lined with plentiful trees and green spaces.

What causes heat islands?

Great question! The answer has two main parts: the absence of green spaces and the presence of asphalt and concrete. 

Green spaces cool down cities by providing shade and evapotranspiration– a fancy term for when evaporating water from trees and other plants cool the air around it. Asphalt and concrete on the other hand are absorbent surfaces rather than reflective surfaces. This means that they absorb heat and gradually release it, raising temperatures around them. 

These two factors combine to create heat islands, like in Chelsea versus Newton in Massachusetts. Buildings and road surfaces make cities hot, the lack of green spaces keep it so.

So heat islands aren’t caused by climate change?

Correct. Urban heat islands would exist with or without climate change. But the warming of our planet only makes situations like heat islands worse. In some cities, the rate of warming will be double the global average. This means that cities would see the effects of climate change more so than anywhere else, making them the hotbed (pun intended) of a public health crisis.

Okay, it’s hotter… so what?

Heat is a silent killer. Every year, more people die from heat than from all other natural disasters combined including seemingly more deadly natural disasters such as tornadoes, hurricanes, and floods. Scientists estimate that heat kills around 12,000 Americans each year. By 2040 this number is predicted to rise to 30,000 deaths annually.

And this is much more than just heat strokes. When we think of heat-related deaths, we may solely envision the elderly dying from heat strokes but this is an incomplete picture. But these deaths are not just from heat strokes. Periods of extreme heat increase mortality from all causes. In other words, if you have any pre-existing conditions, exposure to heat is likely to make it worse. 

Heat has other surprising effects on health, including on the entire reproductive cycle from conception to delivery. High temperatures negatively impact sperm production, hindering people’s efforts at conception. Extreme heat also contributes to higher levels of oxytocin, the hormone that triggers labor contractions in pregnant women. This can lead to a higher frequency of preterm births, a condition that can be dangerous to the child and portend life-long adverse health consequences. 

City residents’ experience of heat islands is also driven by socioeconomic status. Heat doesn’t affect all residents equally, as not everyone has access to air conditioning or cars to get them to air-conditioned places.  In some major cities, like San Francisco and Seattle, less than half of all residents have air conditioning. In other cities, even though a majority of residents may have an air conditioning unit they may not be able to use it due to electricity shortages, as was the case in New York City this past summer. This means that summertime heat is inescapable–and therefore deadly–for far too many. 

What’s the solution?

Trees. 

Their benefits are everywhere. Trees protect biodiversity, reduce runoff in storms, reduce noise and air pollution, and give us the oxygen we need to breathe. But trees also provide another key benefit, something that is especially important for urban heat islands: air conditioning. 

Trees are mother nature’s very own cooling system. 

The presence of trees in urban spaces can lower temperatures by 15-20 degrees. And it gets better. Surface temperatures in the shade of a tree canopy can be 20-45 degrees lower than surrounding areas! This can make a drastic difference in reducing morbidity and mortality.

Trees are cool. Literally.

By planting trees in our cities we can reduce the overall temperature, protect ourselves from the dangers of heat waves and make our neighborhoods prettier while we’re at it! The benefits of planting trees in our urban areas can’t be understated. Planting trees can and should be part of our urban public health strategies. 

This is already happening in many cities across the nation. For example, a 2017 tree planting campaign in Dallas reduced the average temperature in some neighborhoods by 15 degrees, providing much-needed relief for residents living in a city famous for its sweltering summers. 

Trees are not a panacea for all of a city’s heat-related woes. There are other strategies too, like adding green walls and roofs, incorporating cooler materials into building design, painting rooftops white to reflect heat, or making buildings more energy-efficient to reduce waste heat. These strategies have a crucial role in cooling down our cities, like in narrow streets that don’t have the space to accommodate trees or areas where it would be difficult to maintain tree growth.

But trees, as simple as they are, remain one of the most effective strategies. It is not a cure all, but it is the best place to start. In this way, planting trees is not just an environmental issue, but an explicit urban planning, public health, and health equity strategy. 

Is there something I can do?

Yes! Residents in major cities like New York City, Los Angeles, San Francisco, Boston and others can request free trees, a feature that is part of many cities’ larger urban reforestation campaigns. Even if your city doesn’t have such a campaign (yet) requesting a tree might be easier than you think! Residents can also advocate for more green spaces and the protection of already existing green spaces from development.