evidence: Here we present a complementary, solar ultraviolet (UV) radiation volcanic catastrophe theory, which highlights that the volcanic impact is represented by a spectrum of environmental stresses rather then by climate cooling and drying only. We show that the Toba volcanic plume caused massive ozone (O3) loss and produced hostile UV radiation conditions at the Earth’s surface, particularly in the tropics. To account for uncertainties, we cover the range of sulfur emissions (1–100× Pinatubo) to illustrate that these environmental and evolutionary stress factors are general and do not strongly depend upon the magnitude of the model simulated or sediment-analyzed volcanic winter effects.