The radiative forcing benefits of “cool roof” construction in California: quantifying the climate impacts of building albedo modification
Exploiting surface albedo change has been proposed as a form of geoengineering to reduce the heating effect of anthropogenic increases in greenhouse gases (GHGs). Recent modeling experiments have projected significant negative radiative forcing from large-scale implementation of albedo reduction technologies (“cool” roofs and pavements). This paper complements such model studies with measurement-based calculations of the direct radiation balance impacts of replacement of conventional roofing with “cool” roof materials in California. This analysis uses, as a case study, the required changes to commercial buildings embodied in California’s building energy efficiency regulations, representing a total of 4300 ha of roof area distributed over 16 climate zones. The estimated statewide mean radiative forcing per 0.01 increase in albedo (here labeled RF01) is −1.38 W/m2. The resulting unit-roof-area mean annual radiative forcing impact of this regulation is −44.2 W/m2. This forcing is computed to counteract the positive radiative forcing of ambient atmospheric CO2 at a rate of about 41 kg for each square meter of roof. Aggregated over the 4300 ha of cool roof estimated built in the first decade after adoption of the State regulation, this is comparable to removing about 1.76 million metric tons (MMT) of CO2 from the atmosphere. The point radiation data used in this study also provide perspective on the spatial variability of cool roof radiative forcing in California, with individual climate zone effectiveness ranging from −37 to −59 W/m2 of roof. These “bottom-up” calculations validate the estimates reported for published “top down” modeling, highlight the large spatial diversity of the effects of albedo change within even a limited geographical area, and offer a potential methodology for regulatory agencies to account for the climate effects of “cool” roofing in addition to its well-known energy efficiency benefits.
I would like to thank the California Air Resources Board for supporting this research, and the efforts of one reviewer whose close reading and timely suggestions greatly improved this paper.
The contents of this report and opinions expressed herein are the work of the author alone, and do not represent official statements or policies of the California Air Resources Board or of the State of California. Any mention of commercial products does not imply an endorsement by California Air Resources Board or of the State of California.
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