By Camryn Fujita
A Heatwave in Queens, New York.
Source: Chris Goldberg
Picture yourself walking around a city on a hot summer day.
Your vision is blurred by sweat dripping down into your eyelashes. Mirages make solid ground look like a shimmery pool of water. You can definitely feel the heat of the pavement searing through your shoes and frying your tender feet. Finally, it comes to the point where you are desperately looking for an air-conditioned corner store or source of shade and are now regretting whatever influenced you to step outside in the first place.
For many people, this whole experience might amount to a mere inconvenience and irritation. However, for those who do not have immediate access to shelter and must spend long periods of time outside for essential activities such as work or school, excessive heat could easily create a very dangerous situation. In dense urban areas that suffer from a lack of trees, an urban heat island could form. This phenomenon is caused by a lack of trees and an abundance of asphalt and concrete in a dense city. These surfaces absorb and trap heat from the sun, cars, machines, and factories, raising “ambient temperatures” up to “10°F warmer than the surrounding natural land cover.” Urban heat islands can greatly impact people and natural environments within cities by causing heat related injuries and deaths during heat waves, increasing energy consumption, decreasing in air quality, and warming urban ecosystems and bodies of water, which can disturb wildlife.
Heat related illness, such as heat strokes, heat cramps, and heat exhaustion, is the leading cause of death from natural weather or environmental events. Populations such as the elderly, infants, people with pre-existing health conditions, people living in poverty, and non-U.S. citizens in America are the demographics most vulnerable to dying from heat related illness. Extreme changes in daily temperature has also been known to increase the amount of heart attacks according to research presented to the American College of Cardiology. In July of last year, a massive heat wave killed at least 6 people in the United States, triggered declarations of weather-related emergency, and forced sporting events such as Major League Baseball and the NYC Triathlon to either cancel or change schedules. Outside of the U.S., the 2019 European Heatwave was responsible for the deaths of over 30,000 people and 13 billion Euros in damage. The heatwave was the hottest recorded temperature in Europe since the 16th century.
Overall, scientists have tracked statistically significant, rapid warming over the contiguous U.S. land surface over the past several decades. Experts say that 50 years ago, extreme heat waves were a rarity. However, that seems to be changing. Research has shown that the bell-shaped curve of observed temperatures has already shifted by one standard deviation interval warmer, meaning extreme heat occurrences now make up about 7% of observed temperatures. Periods of extreme heat are outpacing periods of extreme cold at a rate that could become 50 to 1 by the end of the 21st century.
The consequences of urban heat islands are important to recognize because it is a tangible example of how human activity contributes to global climate change and can negatively impact our daily lives. Although climate change and urban heat islands are two distinct phenomena, they have the potential to create a positive feedback loop and intensify, which could accelerate the pace of global climate change patterns mentioned previously. For example, when “summertime heat islands contribute to global warming by increasing demand for air conditioning, which results in additional power plant emissions of heat-trapping greenhouse gases.” In fact, research shows that warmer air temperature “is responsible for 5-10% of urban peak electric demand for air conditioner use, and as much as 20% of population-weighted smog concentrations in urban areas.” Even though warmer winters and nighttime temperatures may allow for conserving energy on heating, trends suggest that warmer summer periods cause increases in net energy use. Thus there is a serious risk of straining our nation’s energy supply and infrastructure resilience if extreme heat waves continue, not to mention the financial costs of this burden.
Observed and projected changes in near-surface air temperature for Hawai‘i. Observed data comes from 1905–2014. Projected changes for 2006–2100 predict two possible futures: one where greenhouse gas emissions continue to increase (higher emissions) and another where greenhouse gas emissions increase at a slower rate (lower emissions).
Source: CICS-NC and NOAA NCEI
What can we do to address urban heat islands? Besides offering shade to filter direct sunlight, trees provide a natural cooling effect from photosynthesis. The transpiration of water from leaves into the atmosphere cools the surrounding air. Therefore the U.S. Environmental Protection Agency and other experts recommend planting more trees and vegetation on roofs as an effective solution. Trees also address the concerns around air pollution in urban centers. Trees are some of the most efficient vehicles of carbon sequestration. Furthermore, tree planting strategies such as planting certain species that are fast-growing, planting a variety of species, and turning trees that are cut down into lumber instead of allowing it to decompose can maximize carbon-capture and storage.
Potential solutions also lie in developing proactive building practices and in passing legislation as well. Minimizing the use of black asphalt and painting dark surfaces white or installing cool roofs and pavement reflect the sun’s heat instead of storing it. According to the Yale School of Forestry and Environmental Studies, “fresh asphalt reflects only 4% of sunlight compared to as much as 25% for natural grassland and up to 90% for a white surface such as fresh snow” and lighter colored surfaces can lower extreme temperatures by 2-3°C. New construction codes that maintain variability in building height allow for increased airflow down streets. Mixing land use for residential, commercial, and recreational use and intentionally designing walkable neighborhoods allows more space for trees. Responsible development and consciousness in urban planning can yield results that minimize the damaging effects of the urban heat islands on our most vulnerable communities.
In 2019, Honolulu’s Office of Climate Change, Sustainability, and Resiliency completed Oahu’s first Community Heat Mapping Campaign. Honolulu was one of 10 cities selected to conduct the study funded by the National Oceanic and Atmospheric Administration’s Climate Program Office. Volunteers aimed for a detailed heat map by fixing sensors that take temperatures and record humidity while driving along predetermined routes throughout the island at various times during the day to measure changes in heat. The final results are available in this interactive map. People living in Hawaii might falsely believe that the urban heat island phenomenon is not a problem here, because we are different from the mainland. We do not have immense urban sprawl such as the size and type you will find in Los Angeles or New York City. However, the final report of the project shows that volunteers detected afternoon temperatures of as high as 107°F, and, as expected, some of the hottest areas of the island are in low lying, urban areas in downtown Honolulu.