We don't want to alarm you.... Ok actually we do, but only a little. Super-eruptions, so large they can cancel multiple summers and blanket a continent with ash, happen more frequently than we thought. That doesn't mean we should expect one next Tuesday, and the risk remains much lower than for human-induced threats, but the methods used to determine this might reveal some other lurking dangers.
In 2004, geologists estimated that volcanic eruptions where more than 1,000 gigatons of material is released happen somewhere between once every 45,000 and once every 714,000 years. That's a very wide range of uncertainty, but even at the lower end, it's substantially longer than the time since the invention of agriculture, making it unsurprising that an eruption hasn't come along to throw our civilization off course.
Professor Jonathan Rougier of the University of Bristol, UK, has challenged that estimate in Earth and Planetary Science Letters. He thinks the true range is 5,200-48,000 years, with the most likely figure being 17,000 years. It's been longer than that since the Earth last experienced such an event, with the most recent occurring over 20,000 years ago.
However, such eruptions don't occur on a reliable cycle. Rougier's team found that there were two in the period between 20,000 and 30,000 years ago. “On balance, we have been slightly lucky not to experience any super-eruptions since then,” Rougier said in a statement. “But it is important to appreciate that the absence of super-eruptions in the last 20,000 years does not imply that one is overdue. Nature is not that regular.”
He also told IFLScience that geologists now "have a much better database [of big eruptions] than we did a decade ago."
Since humanity survived this pair, and the even larger explosion at Toba, Sumatra, 75,000 years ago, it's likely we'll make it through a future event on the same scale. But that doesn't mean we would remain unscathed. Just how devastating such an event would be depends on its size, location, and timing, but Rougier told IFLScience; "The direct effect of a super-eruption would be to sterilize the land for thousands of miles... The indirect effect (about which there is more uncertainty) would change global weather patterns for possibly decades."
Rougier said the work is also important for applying the techniques his team has developed to other rare events that might not be well recorded. Even though these events are infrequent, some of them, such as very big earthquakes and smaller, locally damaging, eruptions are more common, and therefore more threatening than super-eruptions. To Rougier's mind, these are much more worth worrying about, along with the disasters that are becoming more frequent due to our own behavior.
Unfortunately, this hasn't stopped headlines misrepresenting Rougier's statements by suggesting disaster is imminent.