Climate Change and the Urban Heat Island Effect

Have you ever walked across a hot parking lot in the summer and noticed how much hotter it feels than standing under a shady tree? Cities can be a lot like that—they have tons of concrete and buildings that trap heat, and not always enough trees to cool things down. Scientists call this the urban heat island effect, and it is becoming a big problem as our planet warms.

Urban heat islands are parts of cities that are hotter than the surrounding countryside because they have lots of pavement and buildings that absorb heat and fewer trees to provide cooling shade. Hotter temperatures in urban heat islands can be dangerous, causing many people to get sick with heart and breathing problems. Most cities around the world are getting hotter due to climate change, meaning the problems associated with urban heat islands are becoming worse over time. This is a worldwide concern because over half of the world’s population—more than 4 billion people—lives in cities, many of which are becoming dangerously hot due to global warming [1].

Tropical cities, like Miami, Florida, are often the hottest. Working outside in Miami puts some people at risk of heat stress. Now, due to global warming, Miami is getting even hotter. By the end of the century, Miami’s maximum air temperatures are expected to rise by almost 4°C (7°F). We need to do everything we can to cool Miami’s urban heat islands, to keep the city livable for everyone!

Trees Help Cool Cities

Trees provide a lot of benefits to all the people who live and work in cities. Trees make cities better by helping to filter pollution, reducing floods, and letting people connect with nature. Trees also help to cool cities by providing shade and transpiring (releasing water through tiny holes in their leaves—like sweating for trees). Maintaining healthy trees in cities can make a big difference in keeping temperatures pleasant.

Trees Can Feel the Heat, Too!

To maintain a healthy urban tree community that keeps the city cool, urban foresters and the experts in charge of planting trees in cities must consider the many different factors that can stress trees, like drought, saltwater, and strong winds. This is especially true in Miami, where hurricanes, sea-level rise, and extreme heat can make it a hard place for trees to live. Sadly, as climate change raises temperatures, trees are at an even greater risk of getting heat stressed, which can damage their leaves and make them more vulnerable to diseases and fungal and insect attacks, and may eventually cause some trees to die. To help keep cities cool, foresters should make sure the tree species they plant can survive the hotter future temperatures, especially since trees can live many decades or even centuries. Unfortunately, not a lot of cities are paying attention to future temperatures when they decide which trees to plant along their streets.

Choosing the Right Trees for the Future

Based on careful measurements of some trees, we believe that air temperatures in Miami are already approaching the upper heat tolerance for some species [2]. Heat-stressed trees may become less healthy, and less-healthy trees will be less able to cool the city. In their new study, Kullberg et al. [3] tried to determine which types of tree species will still be healthy when Miami is even hotter in the future. We had three goals for this study. The first was to figure out how climate warming will affect Miami’s urban trees during this century. The second goal was to determine whether native tree species in Miami are more or less tolerant to warming than the introduced tree species that people have brought to Miami either accidentally or on purpose from other places. Our third goal was to predict which tree species in Miami will be most resilient to multiple stress factors (including heat, drought, wind, and salt).

Estimating Trees’ Heat Tolerances and Thermal Safety Margins

The first step was to make a list of all the kinds of trees that are planted in Miami. We found 249 tree species that are commonly planted in the Miami area. For each of these species we noted whether it is native or introduced. When possible, we also took note of each species’ drought, wind, and salt tolerance, based on information we found through tree nurseries, planting guides, or in scientific papers.

The next step was to estimate the heat tolerance of each of the 249 tree species (Figure 1). In other words, we wanted to know the maximum temperature that each type of tree can survive in. To do this, we used observations from scientists that are available in online databases to map all the places in the world where each tree species is known to grow naturally.

In each of these places, we then checked the air temperatures to find the hottest temperature that each species typically experiences, and we used this as an indicator of each species’ heat tolerance. The difference between a tree species’ heat tolerance and the temperature in Miami was then used to estimate its thermal safety margin (Figure 1), which indicates how much hotter it can get in Miami before that tree is expected to start overheating. If the thermal safety margin is positive (greater than 0), it means that the tree species is okay and can tolerate some additional warming, but if the thermal safety margin is negative (less than 0), the tree species might be at risk of heat stress. We also figured out the thermal safety margin for each species in the future, when climate change is expected cause Miami’s temperature to rise by almost 4°C (7.2°F).

Native Vs. Introduced Species

In our study, we found that with 4°C warming, as expected by 2100, more than 40% of Miami’s tree species will have negative thermal safety margins. In other words, by the end of this century, almost half of the tree species in Miami will be at risk of heat stress. Another research team conducted a similar study looking at different cities around the world and they found similar results. Specifically, they found that urban trees in tropical regions are generally more vulnerable to rising temperatures, meaning that trees in cities like Miami are probably at a greater risk due to climate change than urban trees in milder climates [4]. This is worrisome since billions of people live in tropical cities and tropical cities are growing much faster than cities in other places.

Unfortunately, we predict that Miami’s native tree species will be more impacted by climate warming than introduced species. If the temperature rises 4°C in Miami by 2100, then 36% of introduced species and 48% of native species will have negative thermal safety margins (Figure 2). Some of the native species that will have negative thermal safety margins in the future include Florida’s state tree, the cabbage palm, as well as slash pine and southern live oak. These native trees are currently planted widely throughout the city, and as temperatures rise, they will be heat stressed more often. Introduced species already account for a large amount of the trees in Miami, and urban foresters may need to rely on planting even more of these trees in the future because they tend to be more heat tolerant. However, some of the introduced species can harm local plants and animals, so urban foresters will have to be very careful when deciding which tree species to plant. Some of the introduced species with especially high thermal safety margins (for example, tea tree, Australian pine, and banyan fig) are known to be very harmful to native ecosystems and therefore should not be planted.

Can Trees Get Used to the Heat?

Even though many trees, especially native species, are at risk, it is possible that some species could adjust, or acclimate, to warmer conditions over time. Acclimation means that trees might slowly change how they grow or use water and energy so they can better handle hotter weather, for example, by producing fewer, smaller, or thicker leaves. If this happens, certain trees could survive in Miami longer than scientists currently predict. But acclimation is limited and does not always work if the environment changes are too extreme. This idea connects to a bigger environmental challenge: many plants and animals around the world will also need to adapt to climate change. Whether it is trees in cities, coral reefs in oceans, or animals in forests, the ability to adjust to new conditions could make the difference between survival and decline.

Building A Strong Urban Tree Community For Miami

Along with the heat, trees in Miami face stress from many other things like hurricanes, drought, pollution, and exposure to saltwater from sea-level rise. Tree species that are resilient to rising temperatures may not be resilient to these other stressors. Some native trees in Miami—like American mahogany, black olive, wild tamarind, seagrape, button mangrove, and gumbo limbo—are really tough and are naturally good at dealing with all of these stressors. These species are already commonly planted in Miami, so as other trees die or are removed, foresters can plant more of these resilient species. Choosing the right trees for current and future conditions is essential for keeping urban trees healthy, which in turn will help the billions of people who live and work in cities.

Glossary

Urban Heat Island: ↑ A developed area that is significantly hotter than surrounding rural areas due to the replacement of natural, vegetated areas with impervious surfaces like pavement and buildings, which absorb solar radiation.

Heat Stress: ↑ When plants struggle to function in high temperatures. Heat stress can affect plant growth or fruit production, and it can make their leaves wilt or even turn brown.

Heat Tolerance: ↑ The temperature beyond which a plant can become seriously heat stressed. Heat tolerance differs between plant species, meaning some plants are better adapted to handle the heat than others.

Native: ↑ Species that naturally grow in a certain place without being brought there by people. They have adapted to local climate, soil, and wildlife over many years.

Introduced: ↑ Species that people bring to a new place, either on purpose or by accident.

Resilient: ↑ Able to handle challenges like high temperatures, drought, wind, or diseases. Resilient plants survive tough conditions and can grow back after being damaged.

Thermal Safety Margin: ↑ The “extra room” a plant has to survive hotter temperatures before it gets too hot for the plant to handle (i.e., above its heat tolerance).

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

AI Tool Statement

The author(s) declared that generative AI was used in the creation of this manuscript. A full original draft of the article was written by AK. Then AK fed the first draft through ChatGPT to provide suggested changes to the language to make it more understandable for the intended age group. The resulting suggestions were used piecemeal to simplify language and sentence structure. Generative AI was only used as a tool to help edit existing text.

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