Lawrence Berkeley National Laboratory

Brightening the Earth’s surface can increase the outflux of sunlight to space. This diminishes the net (downward minus upward) solar flux into the Earth’s radiative system, providing negative solar radiative forcing that can cool the atmosphere. We are quantifying the extent to which surface-brightening projects, such as installing solar-reflective roofs and pavements, can raise bottom-of-atmosphere (BOA) albedo, increase top-of-atmosphere (TOA) solar outflux, and reduce regional and global atmospheric temperatures. We compared four approaches to assessing BOA albedo: roof- or pavement-product documentation; local measurements with an albedometer (back-to-back pyranometers); extrapolation from extended-color aerial images (blue, green, red, and near-infrared); and multispectral satellite images sharpened by convolution with aerial images, selecting the last. We developed a technique to calculate the atmosphere’s upward solar transmittance anywhere in the Earth’s tropical and temperate zones based on analysis of BOA irradiances reported in the National Solar Radiation Database, then computed radiative forcing factors relating the increase in TOA solar outflux (negative solar radiative forcing) to the rise in BOA albedo (surface brightening). We identified atmospheric modeling experiments using both regional and global circulation models to quantify the atmospheric cooling potential of reflective surfaces at regional and global scales. These activities provide regional estimates of the atmospheric cooling benefits of reflective roofs and pavements.