Green spaces in cities

 By RP Siegel

Well before anyone was talking about global warming, cities were already heating up…

That’s not to say that planetary warming, driven by the greenhouse effect, isn’t also happening. It is. But cities are warming even more, due to something that scientists call the urban heat island effect (UHI). The effect shows consistently higher temperatures in cities, compared to nearby outlying areas. It’s a big problem that not enough people are talking about. When combined with the fact that the planet is warming and considering that recent forecasts estimate that nearly 70 percent of the world population will live in cities by 2050, it’s a combination that could become deadly.

Simple explanation of urban heat island (Joanna Kośmider, Wikimedia)

Hot cities will spend more

Cities cover only around one percent of the Earth’s surface, but they produce 80 percent of Gross World Product, consume about 78 percent of the world’s energy and are already home to over half of the world’s population. They will suffer more than their rural brethren because of this effect, and they will pay more, too. According to Richard S.J. Tol, Professor of Economics at the University of Sussex, the costs of climate impacts in cities could be as much as 2.6 times higher than previously thought, once these localized effects are taken into account. The major drivers behind these costs are energy used for cooling, and a variety of health impacts along with associated lost productivity.
The effect, which is strongest in the fastest-growing cities in the hottest regions, is primarily driven by the replacement of vegetated areas, which help to cool their surroundings, with buildings and roads that tend to absorb heat. In those worst cases, costs could add up to 10.9 percent of GDP, compared to a global average of 5.6 percent. Altogether, says Tol, over the next century, these local impacts will be roughly equal to, and additive to, those being experienced globally.

Islands of heat

People have been studying climate in cities going back at least as far as 1833, when Luke Howard published his book The Climate of London, which included his daily observations over several decades. The subject of urban heat islands was first introduced by T.R. Oke in the 1982. Among its causes, he also included waste heat, additional heat trapping due to air pollution, and geometric obstruction to wind flow. Clearly, this is a complex subject.
If there is any good news in all this, it’s that local leaders can take action to adjust the thermostat in their cities by addressing these drivers. A number of countermeasures can make a difference. Professor Brian Stone, who runs the Urban Climate Lab at Georgia Tech, evaluated three U.S. cities: Atlanta, Philadelphia and Phoenix for heat-related mortality rate. His team found that in some cases, reductions of 25-33 percent in heat-related deaths can be achieved through a variety of countermeasures. It’s worth noting that when it comes to weather, more people die from heat than from hurricanes, tornadoes, and all other forms of extreme weather combined.

NASA thermal data on metropolitan Atlanta, Georgia (NASA, Wikimedia)

Two problems with the same history

Dr. Patricia Culligan, a member of Columbia University’s Urban Design Lab regularly looks at questions like coastal cities resiliency, green and sustainable infrastructure, and climate change adaptation. Her primary interest as a Civil Engineer is storm water management, a major challenge in a coastal city at a time of rising seas and runaway storms.
That’s the doorway Culligan came through into the world of green infrastructure. It’s a problem that shares a great deal with the heat island effect, since both are understood to be the results of large scale replacement of vegetated areas in cities, with what Culligan calls “hardscape.” That’s because the same ecological assets, namely living plants and healthy soil, that kept a city cool before it was a city, also absorbed a great deal of rainwater. With those assets now largely gone, cities have both problems to deal with.
Culligan is one of many researchers seeking to understanding to what extent these problems might be alleviated by re-vegetating the city in an attempt to return to something resembling the “pre-development hydrological cycle.”

Plant two trees and call me in the morning

Re-vegetating a place like New York City is not easy, but then it’s not impossible either. After all, you have the parks. You have the tree-lined streets, and you have recently-added bioswales installed in right-of-ways. Perhaps the biggest opportunity, though, is the roughly 30 square miles of flat rooftops.

Green roof in Arlington County (Arlington County Flickr)

Green roofs have fostered a worldwide movement, one in which the U.S. is lagging behind Europe. Their many benefits include not only storm water management, and mitigation of the urban heat island effect, but also improved air quality, improved energy efficiency, reduced heating and cooling costs, fire prevention, reduced penetration of radiation and noise, aesthetic impact, and a sanctuary for birds.
They cool using several mechanisms. Most researchers focus on two: the fact that they absorb considerably less heat than conventional roofing materials, referred to as “black roofs,” and through a moisture exchange process between plants and the atmosphere. Lighter, more reflective materials absorb less heat based on the albedo effect. The moisture exchange process is known as evapotranspiration, which refers to the movement of water vapor in and out of plants as they breathe, as well as evaporation of water from the ground, both of which cause cooling.

Stuart Gaffin, another member of Columbia University’s Green Infrastructure team, spoke with Eniday about his work on heat island countermeasures and the mechanisms behind them. The effectiveness of these measures can be seen in the 843-acre Central Park, where, says Gaffin, the air temperature, “is demonstrably cooler than the surrounding streets, during the day, and certainly during the night,” though the actual differences are surprisingly difficult to measure.
Plants, says Gaffin, are fairly dark in color, so why don’t they burn up in the hot sun? Evapotranspiration, is one reason. But there is another reason, too. We’ll get to that a little later.

How much greening is enough?

With his team, Franco Montalto, a Civil Engineering professor at Drexel University conducted measurements of the 6-acre green roof on the Javits Center, the second largest green roof in the U.S., at a time when only a portion of the green roof was completed. This made comparisons easier. As expected, the surface of the green roof was up to 33˚F cooler than the pre-existing conventional roof, and air temperatures above the green roof was up to 3.2˚F cooler.

Javits Center (Wikimedia)

He told Eniday, “the green roof is definitely reducing the air temperature, but not enough to fully offset the heat island effect at street level.” He does think that “green infrastructure can help to mitigate the heat island effect, but local factors will determine by how much, and widespread greening of the city is going to be required to generate significant improvements at the urban scale.” The fact that the green roof has also reduced the air conditioning load on the 840,000-square foot building will also help keep the city cooler by reducing the amount of waste heat emitted by the massive cooling system. As Oke pointed out, waste heat from buildings and vehicles are certainly a factor, and in place like NYC it can be a considerable one.
It’s a complicated question. The fact that the surface of the roof is cooler, means that some of the heat is being reflected back into space, which could be benefiting the global warming problem as much or more than it is helping to keep local conditions bearable. A similar result can be seen with white roofs. A number of projects that have installed white coatings on city rooftops found similar temperature differences.

Can I get that in white?

Unlike green roofs, which provide multiple cooling mechanisms, white roofs count solely on their high albedo to reflect solar heat back into space. Both, as it turns out, provide similar results.
Former NYC mayor, Michael Bloomberg wrote, in the book Climate of Hope,” In 2009, I joined former Vice President Al Gore on a rooftop in Queens to launch a volunteer program called for NYC Cool Roofs.” By painting roofs white, they helped “many New Yorkers save on their utility bills while also contributing to the fight against climate change.” NYC has 1.6 billion square feet of roofs, equal to 28,000 football fields.

Something to reflect on

If the path to cooling our cities lies in replacing dark buildings, streets, and sidewalks with bright, white, shiny ones, what is it going to be like to live there? People don’t always like super-bright surfaces, because of the glare, and because darker colors tend to be soothing to the eye. Concerns about this has led researchers at the Lawrence Berkeley Lab, to investigate a wide variety of building materials, such as roofing, in order to gain an understanding of how some non-white materials could reflect heat as effectively or nearly as effectively as white, while providing a more pleasing visual appearance.
The lab maintains a database of “cool colors.” The reason this is possible is because most of the heat in sunlight is contained in a portion of the spectrum called the near-infrared (near-IR), which is not visible to the human eye. Therefore, it is possible for some materials to appear dark in color, while still remaining cool in the sun as long as they are reflective in the infrared range. Whether that can be done affordably, especially for sidewalks and streets, remains another question.
This, in fact, is the other reason plants can withstand hours on end in the hot sun. While mainly using the visible portion of the sunlight for photosynthesis, according to Stuart Gaffin, they actually reflect as much as half of the near-IR energy that falls on them. So, nature has already succeeded in designing such a material, an example we would do well to follow.

This is why green matters

In closing, it’s worth looking at what happens when a city fails to consider the urban heat island effect. The city of Louisville, Kentucky, was ranked by Brian Stone as the fastest warming city in the U.S. In rising 1.67˚F per decade since 1961, Louisville had almost twice the increase of 2nd ranked Phoenix, which rose 0.96˚F. Primary drivers are likely the city’s expansion, and lack of tree cover. Trees cover a mere 27 percent of the county in which Louisville resides, compared to an average of 40-50 percent for most southern U.S. cities. While Stone found that countermeasures could reduce heat-related mortality in Louisville by 25-33 percent, Atlanta, which has 50-60 percent tree cover, can, with proper actions, avoid any increased mortality from the heat island effect. Stone works with cities like Louisville to identify specific hot spots and offering advice on where countermeasures will be most effective.
As we move forward into warmer days, plans to put more trees in your city and plants on local roofs, will be a welcome relief, not just to your eyes, but to your well-being as well.

SEE MORE: Climate Fighters by Michelle Leslie

about the author
RP Siegel
Skilled writer. Technology, sustainability, engineering, energy, renewables, solar, wind, poverty, water, food. Studied both English Lit.and Engineering at university level. Inventor.