Providence Needs to Reduce the Heat-Island Effect


Although city living is largely considered more environmentally friendly, that doesn’t mean that cities are going to be less impacted by climate change. In fact, cities become significantly warmer than surrounding rural areas on hot summer days, because of their manmade surfaces, heat- and exhaust-producing machinery, and reduced trees and vegetation.

This urban phenomena is referred to as the heat-island effect.

Summers in Providence, for example, are projected to be warmer by 6 to 14 degrees Fahrenheit by 2100. These increased temperatures will bring with them an increase in the frequency and severity of heat waves, which can be deadly for older people and people who live without cooling systems.

There are various ways in which these warming trends might be alleviated in Providence, including planting trees, increasing green groundcover, and implementing white, reflective roofs.

Providence currently has 23 percent tree cover. Trees cool by shading structures and paved surfaces, absorbing carbon dioxide for photosynthesis and through evapotranspiration, a process by which solar energy is used by plants to release water vapor. Thus, trees are invaluable to mitigating the heat-island effect.

In 2008, the Providence mayor at the time, David Cicilline, released Greenprint: Providence, a comprehensive report on the city’s efforts to mitigate the effects of climate change and adopt more environmentally conscious practices. One of the initiatives, Trees 2020, aims for 30 percent  tree cover by planting a total of 40,000 trees, or 3,333 trees annually, from 2008 to 2020. Providence buys trees in bulk and subsidizes the cost for people to plant them on private property. However, current funding constraints have only allowed for 1,100 trees to be planted per year since 2008.

A study out of Modesto, Calif, found that peak summer air temperatures are reduced by about 0.2 degrees for every 1 percent increase in canopy cover. Using this model, summers in Providence are projected to be 3.6 to 11.6 degrees warmer by 2100 — a change from the original projection by minus 2.4 degrees — under the current pattern of tree planting, which illustrates that trees can significantly reduce urban temperatures.

Another primary cause of the heat-island effect is the density of manmade, impermeable surfaces. Providence’s groundcover is currently 59 percent impermeable, with 21 percent buildings, 30 percent asphalt and 8 percent cement.

A United Kingdom study found that increasing urban green space by 10 percent in areas with little or no green cover led to a 4.5 degree decrease in air temperatures. Using this model, converting 4 percent of the groundcover from asphalt to green space would lead to Providence summer temperature increases of 1 to 9 degrees by 2100 — a change of minus 5 degrees — in areas that currently have little or no green cover.

So, Providence neighborhoods with low canopy cover such as downtown, Olneyville, Federal Hill and upper South Providence would see significant cooling with added green space.

It’s also possible to change how much heat is absorbed by some impervious  surfaces. Most roofs and roads are dark, which means they absorb a lot of solar energy and produce heat. High-albedo (high-reflectance) roofs are lighter and diminish this problem. It has been suggested that buildings with light roofing use up to 40 percent less electricity for cooling than those with dark roofs.

In a typical downtown cluster of office buildings, the maximum temperature increase caused by the sum of their cooling systems has been shown to be about 1.3 degrees on a summer day. Using this model, light roofs in downtown Providence would provide a maximum temperature decrease of minus 0.52 degrees, with smaller decreases in residential areas. While light roofing’s direct effect on the heat-island effect is minimal, it indirectly mitigates the effect by significantly reducing the electricity needed for cooling.

High-albedo roads are another possible solution. Asphalt and cement concrete have albedos of 0.1 and 0.35, respectively, and it has been suggested that this increase in pavement reflectance in an urban environment yields a 1 degree reduction in air temperatures. However, light roads cause glare and present a hazard to drivers, so they are not very feasible.

The heat-island effect can also be mitigated indirectly by reducing carbon dioxide and other greenhouse-gas emissions and thus slowing global warming’s gradual rate of average temperature increase. Providence’s trees remove 4,030 tons of carbon dioxide annually, and planting more trees would increase this amount.

Greenprint: Providence also outlines the Distributed Generation Growth Act, a bill renewed in June that provides funding for 200 megawatts of renewable-energy projects to be built and connected to the grid from 2014-2019. Its primary intention is to facilitate residential and small-business solar rooftop projects.

The heat-island effect in Providence poses a significant threat to at-risk populations. However, there are certain steps the city can take to mitigate this effect, such as incentivizing landlords and homeowners to turn asphalt into green space, better promote energy efficiency and plant more trees.

Providence, like all other urban centers, is susceptible to the heat-island effect, but it also is equipped to better address the problem.

Selena Buzinky is a senior at Brown University. Click here to see her calucations and the works she cites, and here to see the project she presented to her class.