Temperatures are rising and cities, in particular, are feeling the heat as a result of a phenomenon known as “Urban Heat Island effect.” An urban heat island (UHI) is a metropolitan area that’s much warmer than surrounding rural areas. According to the Environmental Protection Agency, temperatures in US cities can get as much as 10 degrees higher than their surrounding areas.
There are many reasons that temperatures soar in UHIs: buildings are close together; building materials hold in heat; and urban areas are densely populated. The energy created by people, vehicles, and factories escape in the form of heat. Cities also typically have fewer trees to provide shade, and there is a lot of black pavement that absorbs heat from the sun.
Tokyo, in the midst of a record-breaking heat wave, has growing concern about the 2020 summer Olympics. Tokyo’s original bid for the 2020 games claimed that July 24 to August 9 is a great time “with many days of mild and sunny weather, this period provides an ideal climate for athletes to perform at their best.” Yet last month (July 23, 2018), just short of two years before the 2020 Tokyo Olympics, Tokyo experienced the hottest day in the city’s history; it was 104 degrees with 74% humidity.
There are particular concerns about the marathon. Last week the International Olympic Committee approved starting the marathon at 7:00 am, and is considering a 2:00 am start. In addition, they may move the marathon north of Tokyo to a cooler area, install cooling mist showers, and pave the marathon course with heat resistant materials that will emit less surface heat. The Public Works Research Institute in Japan has experimented with paints containing pigments that reflect 86% of infrared light, which helps keep the surface cool, yet reflect just 23% of visible light, to keep down glare. Organizers have said “if this heat-blocking pavement is covering the asphalt then, on average, there will be a temperature suppression of eight degrees Celsius (46.4 degrees F.)”
Los Angeles is taking a similar approach in battling its heat wave. Mayor Eric Garcetti intends to cut the average temperature in LA by 3 degrees over the next two decades. As part of the plan, the city is coating some of its roads in CoolSeal, a gray paint that keeps streets and parking lots 10 to 15 degrees cooler than black asphalt. The test program has been so successful that there have been stories of dogs leaving the sidewalk for the cooler coated pavement while out on walks. The city will also be planting more trees and painting rooftops white since light colors reflect more sunlight and trap less heat.
Computer simulations of LA show that resurfacing about two-thirds of roads and rooftops with reflective surfaces, as well as planting more trees, could cool the city by about 37 degrees. That would reduce LA smog as much as a total ban on cars and trucks. And cooler roofs would also save a fortune in electric bills. Los Angeles is the first major city with a requirement for “cool roofs.”
Phoenix is also an urban heat island with temperatures that are rising nearly 1 degree per decade, consistently placing it alongside Dallas and Louisville as some of the fastest-warming US cities. It is warming at three times the rate of the planet as a whole, according to Brian Stone of the Georgia Institute of Technology’s Urban Climate Lab. Phoenix is also considering using lighter-colored asphalt for its streets.
Roads and roofs are estimated to cover more than half the available surfaces in urban areas, which have spread to roughly 3% of the Earth’s land surface according to the Global Rural Urban Mapping Project (GRUMP). Dark roofs and roads reflect about 10-20% of sunlight, while white surfaces tend to send back at least half. In 2009, Hashem Akbari, (an engineering professor at Concordia University in Montreal who specializes in research on the effects of urban heat islands, cool roofs and paving materials, energy efficiency, and advanced integrated energy optimization in buildings) calculated that a mass movement to change the color of roads and roofs would increase the amount of sunlight bounced off our planet by 0.03%, which would cool the Earth enough to cancel out the warming caused by 44 billion tons of CO2 pollution. And according to Building Energy Resilience, one ton of CO2 could fill a modest house of approximately 1,300 square feet. Imagine the negative warming effects of 44 BILLION houses going away instantaneously. It’s a step, no doubt, in the right direction to addressing global warming.
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