Reason for Concern

A supereruption is extraordinarily rare, but if one came, the effects would threaten civilization. Here is what scientists think we know:

• Odds — the USGS puts the yearly chance of a Yellowstone supereruption at roughly 1 in 730,000, about the same as a large (~1 km) asteroid striking Earth — extraordinarily rare either way
• Warning — the system would likely give months to years of notice through earthquake swarms, ground swelling, and rising gas emissions
• Reach — ash could blanket much of North America, while a global cooling spell could cripple farming for years