Inclusion of isoprene as a source of secondary organic aerosol (SOA) in a global model increases the global burden of SOA from all sources by more than a factor of two. The isoprene source substantially increases SOA concentrations in the free troposphere, because isoprene, and, more importantly, isoprene's oxidation products, have much greater concentrations at higher altitudes than other biogenic SOA precursors, highlighting the importance of semi-volatile organics for SOA formation. These results are robust with respect to increases in partitioning of non-isoprene oxidation products at higher altitudes and increased wet removal of isoprene oxidation products. This additional source of SOA enhances production of SOA from other parent hydrocarbons by 17%, and leads to an overall distribution of SOA that differs enough from previous predictions to warrant reevaluation of the radiative effects of organic carbon aerosol.