If you have ever visited a city in the summer, you most likely have experienced the effects of urban heat islands without even knowing.
An urban heat island is an area in a city, typically downtown, that experiences, absorbs and releases excessive heat at a higher rate than its nearby rural regions. As a result, the increased heat stress in one centralized area causes considerable damage to the climate, increases energy consumption, and exacerbates heat-related illness and death.
This summer in Duke's Climate+ Program, Zach Calhoun, a third-year Duke PhD student, and undergraduates Claudia Rodriguez and Harry Ge worked together for 10 weeks to tackle the issue of urban heat islands in Duke's hometown of Durham, North Carolina. As the project manager and expert in the subject matter, Calhoun worked with his team daily to keep his students organized and provide necessary help as they used data-driven research to understand and find solutions to the problem.
Like most urban cities, downtown Durham is filled with impermeable surfaces like concrete sidewalks and steel buildings, which absorb and trap heat more than permeable ones like natural land-trees and grass. These surfaces then re-emit the sun's heat into the Earth's atmosphere at night, making the atmosphere hotter during both the day and night and resulting in less relief from heat for urban residents.
This excessive heat is detrimental to those living in the area, as it contributes to heat-related illnesses like stroke and heat exhaustion.
"The heat can also exacerbate already pre-existing conditions. It is very dangerous in terms of heat and how much the body can constantly handle being exposed to," Rodriguez said.
As a result of the increased temperature, energy consumption levels in cities increase dramatically due to the higher demand for air conditioning and cooling systems. Not only does this demand make energy costs rise for those in the community, but more electricity demands lead to higher greenhouse gas emissions and air pollution, creating respiratory problems for the locals and directly contributing to global climate change.
As the team continued to analyze the data, they came across important relationships that further explained the depth and severity of the issue and the urgency needed to take action.
"We found that the areas in Durham that have the lowest socioeconomic status tend to have higher temperatures and lower levels of vegetation. This discovery reiterates the idea that this whole issue is centered around how it disproportionately impacts marginalized communities," Rodriguez said.
With all this knowledge, the team got to work to figure out real-world interventions to reduce the urban heat island effect.
The researchers used a 2021 weather data set from the National Oceanic and Atmospheric Administration (NOAA) that compared urban temperatures to rural ones in Durham as their "ground truth." They then compared it to other data and collected their own, looking at three main independent factors: vegetation, land use and albedo. Albedo is the fraction of radiation from the sun that is reflected by the Earth's surface, where white surfaces are a "1" and reflect radiation, and black surfaces are a "0" and absorb radiation.
The team discovered a direct correlation between these three factors and temperature, with vegetation being the most effective solution to decreasing excess heat. They created their own statistical model that used the factors to predict temperature on a hot day in Durham, which gave useful insight into how to tackle planting initiatives.
"With a city like Durham, our main focus is going to be on increasing vegetation to reduce the urban heat islands effect," Calhoun said.
Calhoun said that with vegetation, planting more trees adds more shade and moisture to the environment, working to help reduce the heat island effect. Trees also use solar radiation for photosynthesis, decreasing the amount of heat emitted. Reducing land use, thus preserving vegetation, would also benefit the cause. Lastly, Calhoun said that increasing albedo by painting roofs white in Durham would be effective, as it would reduce the amount of heat absorbed and re-emitted into the atmosphere at night since white surfaces reflect radiation.
However, while the group has made tremendous progress, there are still some questions left to be answered.
"We generally know that trees and increasing albedo are effective strategies to reduce the heat island effect," Calhoun said. "But, what we think is the missing gap is a really clear analysis of determining how cities decide where to plant trees and when it makes sense to paint a bunch of roofs white. While we know these strategies work, we don't know if the cost is worth it."
To figure this out, in terms of tree planting, Ge further explained that the team is using different methods to hopefully determine how successful planting trees at a specific building, for example, would be in decreasing the temperature in the building's location.
At the end of the project, the group plans to publish their results to the leaders and city planners of Durham and get feedback from the urban climate community.
