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Al Gore once said, “The best available technology for pulling carbon dioxide from the air is something called a tree.”

True, Mr. Gore. Trees provide cooling at a global scale by sequestering carbon dioxide, the greenhouse gas most responsible for global warming. Trees are also the smartest available technology at the local level for cooling our homes, schools, and parks through evaporation and shading.

Yet Honolulu is losing trees. Why? It’s a loss of a tree here, a tree there. In short, lots of little things that collectively add up to a big problem. 

This is what the youth, in a Trees for Honolulu’s Future initiative called “Heat Island Investigators,” discovered over the past year. Heat islands are locales often associated with urban development that experience higher temperatures than surrounding areas.

Honolulu has many such locations where buildings and roads absorb and reemit the sun’s heat. Urban areas, where human-built infrastructure like this is concentrated and greenery is limited, become “islands” of higher temperatures relative to outlying areas.

Why are trees so good at mitigating heat islands?

• Trees act like pumps as roots gather water from the soil and release it to the atmosphere where it evaporates, cooling the air in a process called transpiration.
• Trees act like umbrellas as leaves in the canopy intercept and reflect solar radiation keeping surfaces below cooler because they are shielded from direct sun.

Groups of trees are most impactful. Isolated trees provide some, but limited cooling as wind passes warmer air from adjacent areas beneath the canopy in a process called advection. Quite simply, more trees equals more cooling.

The Heat Island Investigators initiative included two sites, both of which are underserved areas that are heat islands: Aiea Intermediate School and Makalapa Neighborhood Park (via Parents and Children Together program at Puuwai Momi public housing).

Taking The Temperature

At each site student scientists developed maps of current conditions, including measuring and plotting trees and temperatures, and identified solutions by applying experiential place-based learning using an array of sophisticated equipment and a rigorous testing regime.

Temperature, light, and canopy conditions were measured under and adjacent to a number of trees at several local parks. Measurements in the shade and sun were taken simultaneously at cardinal directions to account for differences in canopy density as well as time of day and year.

Now here’s where findings got interesting. Measurements of surface temperature and light associated with sun and shade under and around a Monkeypod tree illustrate the relationship of sun exposure to surface temperature. Sensors in the sun were about 12 degrees Fahrenheit warmer and it was about 20 times brighter whereas under a partial gap in the canopy light and temperature were intermediate.

It’s no big surprise that shaded surface temperatures were cooler, yet the ambient air temperature, measured in a solar radiation shield, showed only a 0.5 degrees Fahrenheit difference between sun and shade.

The conclusion? Air isn’t necessarily cooler in the shade, but surfaces that are in the shade stay cooler.

Even without trees, in areas getting the same amount of solar radiation, such as a grassy field versus on an outdoor basketball court, it will feel cooler on the grass. There are other factors such as wind and humidity as well — the takeaway here is air temperature isn’t the only factor in how hot, or cool, one feels.

It’s not just the youth who have figured this out. The City and County of Honolulu Climate Change Commission has identified heat impact as both a stress through urban heat islands and as a shock through heat waves.

Honolulu has a multitude of heat islands that cause serious human impacts such as heat-related mortality and morbidity, as well as economic costs such as increased peak electricity demand. Trees aren’t the only answer to this situation, but they sure are a smart, cost-effective, response.

Fortunately, change at Makalapa Neighborhood Park, featured in a May 2023 Civil Beat story, is underway. In September 2023, after Noah Kindaro, a 10-year-old youth, testified at the Honolulu City Council Committee on Parks, the park’s lights were turned on the next evening.

A Friends of Makalapa Park community group is in the process of being established. And the city’s Department of Parks and Recreation is exploring planting shade trees. Read on to discover why trees are an excellent way to help this, and other areas of Honolulu, reduce how hot it feels.

How To Help

How can you make a difference in reducing the urban heat island effect?

Plant a tree in your own yard. Myriad resources for advice abound, including at , local nurseries, and certified arborists to help you select the right tree in the right place and provide the right care.

Volunteer! Numerous organizations actively plant and maintain trees. Where to start? The Events listing at TreesForHonolululu.org is a good place.

Request a street tree. Call the City’s Division of Urban Forestry at (808) 971-7151 or email DUF@honolulu.gov. Keep in mind that not all requests will be approved as the location must meet various standards, however, staff will review and consider your request. If approved, the city will plant and prune the tree, but you will need to commit to watering the trees to help get it established.

Advocate for urban trees as if your life depends on it, because it does. This initiative received financial support from the United States Environmental Protection Agency under an Assistance Agreement. For more, .