Anticipating future Volcanic Explosivity Index (VEI) 7 eruptions and their chilling impacts

Worst-case or high-end subduction-related earthquakes and tsunamis of 2004 and 2011 are painfully fresh in our memories. High-end subductionrelated volcanic eruptions have not occurred in recent memory, so we review historical and geologic evidence about such eruptions that will surely recur within coming centuries. Specifically, we focus on Volcanic Explosivity Index (VEI) 7 eruptions, which occur 1–2 times per thousand years. A variety of environmental changes followed the VEI 7 eruption of Rinjani (Samalas), Indonesia, in CE 1257 and several more eruptions of VEI 6 or 7 that occurred in the succeeding few centuries. The Rinjani eruption and its impacts are relatively well documented, modeled, and, for the purposes of attribution, uncomplicated by antecedent eruptions. It seems likely that the Rinjani eruption triggered the onset of the Little Ice Age, and subsequent large eruptions sustained it. Although climatic effects of eruptions like Pinatubo (Philippines) and even Tambora (Indonesia) lasted only a few years, it seems that coupling of oceans, sea ice, and atmosphere after larger eruptions can force decadeto century-long cooling, decreased precipitation, and major impacts on crops. The next VEI 7 will affect a world very different from that of CE 1257. Today, populations within 100 km of candidate volcanoes range from fewer than 1000 people in remote areas to between 20 and 30 million people near several candi dates in Indonesia and the Philippines. If a VEI 7 eruption occurs, those populations will be at dire risk, and eruptions in some locations could destabilize financial centers, national economies, and even peace between nations. Distal effects on air travel, the global positioning system, and climate will be felt by a high-technology, globally interdependent world. We suggest and apply criteria to identify candidates for future VEI 7 eruptions, and discuss likely challenges for short-range forecasting of such events. Preparation for such low-probability but high-consequence events is difficult to imagine, yet some modest early measures can be considered. Volcanologists should refine geologic histories and ensure at least baseline monitoring of candidate volcanoes, and consider how they will judge the likelihood that an impending eruption will be of VEI 7. Forward-thinking governments and industries would be wise to consider how a proximal or distal VEI 7 eruption will affect their jurisdictions, and what responses make the most economic and sociopolitical sense. INTRODUCTION Explosive volcanic eruptions occur in a wide range of sizes, and the modern world has not yet seen an eruption of either Volcanic Explosivity Index (VEI) 7 or 8, the highest 2 orders of magnitude known from written and geologic history (Newhall and Self, 1982; Mason et al., 2004; Global Volcanism Program, 2013). Most, although not all, of these very large eruptions have occurred above subduction zones. The latest VEI 7 event, at Tambora, Indonesia, in CE 1815, was at the low end of VEI 7 eruptions. Its global effects were widely reported (Stommel and Stommel, 1983; Stothers, 1984; Harington, 1992; de Jong Boers, 1995; Oppenheimer, 2003a; Raible et al., 2016). In this paper we list all known Holocene VEI 7 eruptions (Table 1) and discuss effects from that of Rinjani (Samalas) Volcano in Indonesia in CE 1257. We then discuss effects more generally, including how a VEI 7 today will have very different effects than those of Rinjani (CE 1257) or Tambora, on a world that now has much higher populations, higher technology, and more global interdependence than in those earlier years. Our emphasis on VEI 7 eruptions is to call attention to this class of eruptions that are considerably more likely than a VEI 8 supereruption, yet still have remarkable global impacts. For long-range forecasting we suggest characteristics of volcanoes that might produce future VEI 7 eruptions. Experiences at Pinatubo (Philippines), Rabaul (Papua New Guinea), and Campi Flegrei (Italy) illustrate the challenges of short-range forecasting at those volcanoes. Neither forecasting nor mitigation will be easy, and there are special challenges for low-probability, high-consequence events. No volcano observatory or community will stay on high alert for events like VEI 7 eruptions that occur globally only once or twice per millennium and locally far less often. However, because the next VEI 7 eruption could occur at any time, and affect the entire world, we recommend some modest new monitoring and response planning. SIZES OF EXPLOSIVE VOLCANIC ERUPTIONS An imperfect but handy measure of explosive magnitude is the VEI (Newhall and Self, 1982), a broadly logarithmic scale similar to Richter magnitude for earthquakes and to a more precise but also more data-demanding magniGEOSPHERE GEOSPHERE; v. 14, no. 2 doi:10.1130/GES01513.1