In 2010, a study by Berkeley Lab using data from NASA estimated that using “cool roof ” techniques on the roofs and pavement of all cities in the northern hemisphere could have a one-time effect of removing more than 44 billion tons of carbon dioxide — 24 billion tons from roofs and 20 billion tons from pavements. That’s more than the total of all global carbon emissions each year.
“If all eligible urban flat roofs in the tropics and temperate regions were gradually converted to white (and sloped roofs to cool colors), they would offset the heating effect of the emission of roughly 24 gigatons of CO2, but one-time only,” said Art Rosenfeld, a physicist at Berkeley Lab. “However, if we assume that roofs have a service life of 20 years, we can think of an equivalent annual rate of 1.2 Gt per year. That offsets the emissions of roughly 300 million cars (about the cars in the world) for 20 years!”
OK. Are you sitting down? While the world is frantically pushing the transition to electric vehicles in order to lower carbon emissions from the transportation sector, and scientists are pursuing research into geoengineering strategies that could cost trillions of dollars, here is a technique that is available and quite inexpensive that could offset the carbon emissions from 300 million cars and trucks. Why are we just hearing about this now? (Actually, CleanTechnica did report on this back in 2012.)
The Washington Post did a feature story on cool roof technology on June 20th, in which it said that cities, on average, are now 2 to 5 degrees warmer than rural or natural areas. On hot summer days, that difference can be as much as 20 degrees. A black roof in summer in New York City can get hot enough to literally fry an egg — 190º F.
Cities suffer from what is known as the “heat island” effect. They get so hot during the day that they can’t fully cool down overnight, meaning they start each new day with excess heat left over from the previous day. There were more than 10,000 heat related deaths in the United States between 2004 and 2018, according to the Centers for Disease Control and Prevention.
Passive Cooling & The Solar Roof
All those charming villages along the shores of the Mediterranean are not painted white so tourists can take pretty pictures of them. White reflects more sunlight than darker colors. That’s called solar reflectance, and it applies to the spectrum of visible light. But sunlight is also composed of infrared radiation, the energy we feel from a hot oven but cannot see. A white roof will reflect visible light, but not infrared radiation. That’s where cool roof research becomes important.
A “cool” roof should have both high solar reflectance and high thermal emittance, says Berkeley Lab. “We note that since the spectrum of thermal radiation (4-80 microns) does not overlap with that of solar radiation (0.3-2.5 microns), thermal emittance is independent of solar reflectance. Thus, we distinguish between the two when defining a cool roof.”
Berkeley Lab explains that substituting a cool roof for a warm roof reduces conduction of heat into the building, convection of heat into the outside air, and thermal radiation of heat into the atmosphere. This benefits our buildings, our cities, and our planet.
Cooler outside air. Cool roofs lower urban air temperatures by reducing the amount of heat transferred from roofs to the air, mitigating the urban heat island effect.
Fewer power plant emissions. Reflective roofs reduce cooling energy demand in air conditioned buildings, decreasing emissions of greenhouse gases and other air pollutants at power plants that burn fossil fuels.
Better air quality. Cool roofs decrease urban air temperatures and thus slow the formation of ground level ozone. Ozone, the primary component of smog, can aggravate respiratory illness and can act as a greenhouse gas.
Slowed climate change. Cool roofs decrease heat absorbed at the Earth’s surface and thus can lower surface temperatures. This decrease in surface temperatures reduces the flow of heat into the atmosphere, offsetting warming caused by greenhouse gases.
Energy and cost savings. During hot summer months, cool roofs reduce the need for cooling in air conditioned buildings, which saves energy and money.