Radiative forcing and temperature response to changes in urban albedos and associatedCO2 offsets

Abstract

The two main forcings that can counteract to some extent the positive forcings fromgreenhouse gases from pre-industrial times to present day are the aerosol and relatedaerosol-cloud forcings, and the radiative response to changes in surface albedo. Here, wequantify the change in radiative forcing and land surface temperature that maybe obtained by increasing the albedos of roofs and pavements in urban areas intemperate and tropical regions of the globe by 0.1. Using the catchment landsurface model (the land model coupled to the GEOS-5 Atmospheric GeneralCirculation Model), we quantify the change in the total outgoing (outgoingshortwave+longwave) radiation and land surface temperature to a 0.1 increase in urban albedos for allglobal land areas. The global average increase in the total outgoing radiation was0.5 W m−2, and temperaturedecreased by ∼0.008 K for an average 0.003 increase in surface albedo. These averages represent all global landareas where data were available from the land surface model used and are for the borealsummer (June–July–August). For the continental US the total outgoing radiation increased by2.3 W m−2, and land surface temperature decreased by∼0.03 K for an average 0.01 increase in surface albedo. Based on these forcings, the expected emittedCO2 offset for a plausible 0.25 and 0.15 increase in albedos of roofs andpavements, respectively, for all global urban areas, was found to be∼57 Gt CO2. Amore meaningful evaluation of the impacts of urban albedo increases on global climate and the expectedCO2 offsets would require simulations which better characterize urban surfaces and represent thefull annual cycle.