Remote sensing data over North America document the ubiquity of secondary aerosols resulting from a combination of primary biogenic and anthropogenic emissions. The spatial and temporal distribution of aerosol optical thickness (AOT) over the southeastern United States cannot be explained by anthropogenic aerosols alone, but is consistent with the spatial distribution, seasonal distribution, and temperature dependence of natural biogenic volatile organic compound (BVOC) emissions. These patterns, together with observations of organic aerosol in this region being dominated by modern ¹⁴ C and BVOC oxidation products with summer maxima, indicate nonfossil fuel origins and strongly suggest that the dominant summer AOT signal is caused by secondary aerosol formed from BVOC oxidation. A link between anthropogenic and biogenic emissions forming secondary aerosols that dominate the regional AOT is supported by reports of chemicals in aerosols formed by BVOC oxidation in a NO _x - and sulfate-rich environment. Even though ground-based measurements from the IMPROVE network suggest higher sulfate than organic concentrations near the surface in this region, we infer that much of the secondary organic aerosol in the Southeast must occur above the surface layer, consistent with reported observations of the organic fraction of the total aerosol increasing with height and models of the expected vertical distribution of secondary organic aerosols from isoprene oxidation. The observed AOT is large enough in summer to provide regional cooling; thus we conclude that this secondary aerosol source is climatically relevant with significant potential for a regional negative climate feedback as BVOC emissions increase with temperature.