Volcanic winter in the Garden of Eden: The Toba supereruption and the late Pleistocene human population crash

The Toba eruption (ca. 73,500 B.P., Indonesia) was the largest explosive eruption of the last few hundred thousand years. Several lines of evidence suggest that Toba produced an estimated 10¹⁵-10¹⁶ g of stratospheric dust and H₂SO₄ aerosols, and ice-core data and atmospheric modeling indicate a ca. 6-yr residence time for the dense global aerosol cloud. Such a stratospheric aerosol loading is predicted to have caused a "volcanic winter" with possible abrupt regional coolings of up to 15° C (similar to nuclear winter scenarios), and global cooling of 3-5° C (and possibly greater) for several years. Ice-core data suggest that Toba may have contributed to the initial severe cooling of a millennium-long cold event, suggesting involvement of climate feedback responses such as ocean cooling, and increased sea ice and snow cover. Botanical studies and model simulations suggest that the local and regional effects of the predicted post-Toba cooling would have been disastrous for vegetation. Cold-sensitive tropical vegetation would have been almost totally destroyed, and the predicted reduction of early growing-season temperatures by ≥10° C in higher latitudes could have killed most temperate and subarctic forests, with surviving vegetation severely damaged and recovery times taking decades. Global climate models predict that the Toba atmospheric perturbation would have caused severe drought in the tropical rainforest belt and in monsoonal regions. These results constitute a global ecological disaster, with expected reductions in standing crops of plants and animals especially in the tropics. Evidence for these abrupt environmental changes may be detectable in high-resolution palynological records, coral reefs, and ice cores. Genetic studies indicate that sometime prior to ca. 60,000 yr ago humans suffered a severe population bottleneck (possibly only 3,000-10,000 individuals), followed eventually by rapid population increase, technological innovations, and migrations. The climatic effects of the paroxysmal Toba eruption could have caused the bottleneck, and the event might have been a catalyst for the technological innovations and migrations that followed. The present results as to the predicted environmental and ecological effects of the eruption lend support to a possible connection between the Toba event and the human population bottleneck, and suggest that similar bottlenecks among other organisms might be expected at about the same time. Some chimpanzee populations appear to have undergone such a bottleneck ca. 70,000-60,000 yr ago.