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Article 'Turning Down the Temperature on Urban Heat Islands' by Josh McDaniel. Includes an aerial view image of Oahu and a map of Oahu.

August 31, 2019, tied the record for the hottest temperature ever recorded at the Honolulu airport.

On the same day, volunteers and city workers placed sensors on their vehicles and drove through O‘ahu neighborhoods throughout the day. Staff from the City and County of Honolulu (City) Office of Climate Change, Sustainability and Resiliency (Resilience Office) used the resulting measurements to construct a detailed heat map of the island.

Though it is no surprise that many hotspots were in the concrete jungle of Honolulu’s urban core, certain other windward and leeward locations also registered extreme heat indices, which was both informative and troubling. With the maximum heat index* recorded (107.3 degrees) in Pearl City, several other locations in suburban communities registered heat index readings above 105 degrees.

“We know that it’s getting hot out there,” says Matt Gonser, the executive director of the Resilience Office. “And this has great consequences for people’s public health and safety, and our ability to thrive and stay connected into the future. So, taking no action is not an option.”
The heat mapping project showed localized heat islands, which occur when temperatures in urban areas are higher than the surrounding rural areas. According to the U.S. Environmental Protection Agency, temperatures in urban areas can be nine degrees higher than temperatures in surrounding areas. The primary cause of urban heat islands is the concentration of buildings, roads, parking lots, and other surfaces that absorb and re-emit the sun’s heat more than natural landscapes.

Gonser said that rising air temperatures from climate change are driving the increased frequency of heat island formation on O‘ahu, but there are other factors as well, such as increasing ocean temperatures and changing wind patterns.

Dr. Camilo Mora, a University of Hawai‘i geography professor, has been studying heat waves related to climate change. He says there have been more than 800 deadly heat waves since 1980, including the 2003 European heat wave that killed 70,000 people, and a 2010 Russian heat wave in which more than 10,000 perished.

Mora recently led a group of scientists who completed a comprehensive review of studies looking at climate change effects. They found that heat waves affect food resiliency, water availability, political stability, economic health, and human health.

Green Infrastructure
Urban forests can lessen the effects of extreme heat events by moderating surrounding air temperatures, along with providing a range of benefits such as improving air quality and reducing energy use for cooling. An assessment of Honolulu’s urban tree canopy by the state’s Kaulunani Urban and Community Forestry Program calculated that urban trees provide $90 worth of environmental benefits per tree, and each tree provides about $3 in benefits (electricity savings, carbon storage, air pollution removal, and rain interception) for every $1 spent on tree care.

“Trees should be thought of as infrastructure, like any other utility,” says Shannon Noelle Rivera, the community partnership coordinator for Kaulunani. “That’s especially important as we’re entering this age of climate impacts, whether it’s heat, sea-level rise, or stronger storms. We should start managing for and creating long-term plans for trees.”

Rivera says that will happen through more public education and outreach about the importance of trees in urban areas. She points to the Kaulunani Citizen Forester Program as an example of how people statewide can get involved in advocating for the urban tree canopy while also participating in community science projects.

“Volunteers are trained to assess trees, collecting data on the height, diameter, canopy, and a whole set of measurements for each tree,” Rivera says. “And once that information is entered into inventory software, you can actually see what benefits that tree is producing in carbon sequestration, runoff prevention, and energy saved for the community.”

The extraordinary set of services that trees provide in urban environments has gained attention around the Pacific.

On Palau, a new national Complete Streets project promoting walkability is using trees and greenspace to improve health and wellness, promote commercial activity, and adapt to and mitigate climate change effects such as increased temperatures and flooding. The government has introduced three pilot projects in strategic areas around the nation’s islands to redesign road corridors to promote walking and bike riding.

“The idea is to make these areas beautiful and pleasant so people want to get out of their cars and walk,” says Xavier Matsutaro, the national climate change policy coordinator for the Republic of Palau. “Shade is a big part of making these places attractive as well as functional, because the heat can be unbearable during the mid-day here.”

Design Solutions
Green design and architecture have proven potential to reduce the urban heat island effect. On Oʻahu, urban areas make up about 25 percent of the island, with roads and buildings covering 50 square miles, according to the Hawai‘i Department of Land and Natural Resources. All of that impervious surface hinders the land’s natural ability to absorb and infiltrate rainfall or stormwater and increases the amount of heat absorbed.

One option for reducing impervious surfaces is to install green roofs, which partially or completely add to traditional roofing materials with living green roof systems. They incorporate soil or other growing medium, water drainage and storage layers, waterproofing materials, and a vegetation layer on top.

Dr. Andrew Kaufman, a professor and landscape specialist with the Department of Tropical Plant and Soil Sciences at the University of Hawai‘i at Mānoa, says that green roofs are an untapped resource for reducing heat, driving down energy costs, and providing a wide range of environmental and human health benefits in urban areas.

Kaufman has been developing what he calls “ecosheds” that incorporate different green roof systems along with “living walls” using trays with soil or a compressed peat material on the outside of the home that are much cooler than regular shingled houses (about 6 to 9 degrees). Kaufman says the ecosheds systems may cool the surrounding environment as well.

“Vegetation is going to be a very important tool for Hawai‘i, and the world, to reduce the effects of heat,” Kaufman said. “With climate change, things are just becoming more intense.”

Standing Up for Trees
While Hawai‘i’s relatively mild island climate currently provides a buffer from the worst effects of rising temperatures, episodic heat waves are likely to increase in the future. Planting trees and expanding the use of green design techniques will be crucial in ensuring that urban areas remain livable into the future.

Rivera encourages citizens to get involved in advocating for the urban tree canopy by joining the Citizen Forester Program, or simply being a voice for urban trees by showing up to public meetings and writing to their local officials.

“A few passionate people can make a change,” Rivera says. “Decision-makers will listen to people who have something to say. It’s important to be a loud voice in the room letting them know that trees are important.”


*The heat index combines relative humidity with the actual air temperature as a measure of what the temperature feels like to the human body. For example, high levels of humidity can make a 90-degree day feel like it is well over 100.


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