Global Cooling: Policies to Cool the World and Offset Global Warming fromCO2 Using Reflective Roofs and Pavements

Global Cooling: Policies to Cool the World and Offset Global Warming from CO 2 Using Reflective Roofs and Pavements Hashem Akbari* 1 and Ronnen Levinson, Heat Island Group, Lawrence Berkeley National Laboratory, Berkeley, CA, USA Arthur Rosenfeld, California Energy Commission, Sacramento, CA, USA Matthew Elliot, California Environmental Associates, San Francisco, CA, USA *Corresponding author email: HAkbari@ENCS.Concordia.ca ABSTRACT Increasing the solar reflectance of the urban surface reduce its solar heat gain, lowers its temperatures, and decreases its outflow of thermal infrared radiation into the atmosphere. This process of “negative radiative forcing” can help counter the effects of global warming. In addition, cool roofs reduce cooling-energy use in air conditioned buildings and increase comfort in unconditioned buildings; and cool roofs and cool pavements mitigate summer urban heat islands, improving outdoor air quality and comfort. Installing cool roofs and cool pavements in cities worldwide is a compelling win-win-win activity that can be undertaken immediately, outside of international negotiations to cap CO 2 emissions. We propose an international campaign to use solar reflective materials when roofs and pavements are built or resurfaced in temperate and tropical regions. Introduction As the threat of climate change becomes more pronounced, a number of scientists have proposed supplementing the full range of mitigation efforts with geo-engineering (manipulation of the Earth’s environment) to quickly respond to this threat (AMS 2009). Many proposed geo- engineering techniques are novel and unproven. One simple technology has been in practice for thousands of years: changing the solar reflectance (albedo) of the built surface. “Cool roofs” and “cool pavements” should be among the first geo-engineering techniques used to combat global warming. Increasing the solar reflectance of the urban surface reduces its solar heat gain, lowers its temperatures, and decreases its outflow of thermal infrared radiation into the atmosphere. This process of “negative radiative forcing” effectively counters global warming. Most existing flat roofs are dark and reflect only 10 to 20% of sunlight. Akbari et al. (2008) have shown that resurfacing conventional dark roofs with a cool white material that has a long-term solar reflectance of 0.60 or more increases its solar reflectance by at least 0.40. Retrofitting 100 m 2 of roof has an effect on radiative forcing equivalent to a one-time offset of 10 tonnes of CO 2 . Given that CO 2 is currently traded in Europe at $20/tonne, the value of this change could be worth up to Present Address: Department of Buildings, Civil and Environmental Engineering, Concordia University, Montreal, Canada. Tel: 514-848-2424 x3201.

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