It is entirely possible that there would be very little long term impact from such an eruption. The Toba eruption about 70,000 years ago was one of the largest eruptions of the Quaternary (from 2.5 mya to present), and has been linked with climate change and seen as the trigger for the penultimate ice age (Marine Isotope Stage 4). However, sedimentation analysis of the Toba ash in the context of climate indicators suggest that there was approximately 1,000 years of an interstadial event (warm bits between glacial events) after the eruption. (Schulz, H., Emeis, K. C., Erlenkeuser, H., von Rad, U. And Rolf, C. 2002. The Toba Volcanic Event and Interstadial/Stadial Climates at the Marine Isotopic Stage 5 to 4 Transition in the Northern Indian Ocean. Quaternary Research 57:22-31.).