This graphic shows a comparison of eruption sizes using the volume of magma erupted from several volcanoes.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mike Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

The Yellowstone Volcano Observatory gets many questions about the potential for Yellowstone, or some other caldera system, to end all life on Earth. So we'll answer that question right away — a large explosive eruption at Yellowstone will not lead to the end of humanity. (Most Yellowstone eruptions do not fit this worst-case scenario anyhow, but rather are lava flows.) The aftermath of such an explosion wouldn't be pleasant, certainly, but we won't go extinct. How do we know? Because this "super eruption" experiment has already been run. Twice!

The area around large explosive eruptions is devastated by hot flows of rock and ash. Ashfall out to distances of hundreds of kilometers can be many inches thick, according to simulations carried out by USGS scientists. Besides these local implications, volcanic eruptions that expel massive amounts of ash and gas into the atmosphere can have global impacts.

In the atmosphere, sulfur dioxide in volcanic gas mixes with water to form sulfuric acid, which condenses to form fine sulfate aerosols. These aerosols reflect the heat of the sun back out into space, which can cause cooling of up to several degrees worldwide if the eruption is large enough.

This may not sound like much, but it can be devastating to agriculture. In Europe and North America, 1816 was known as the "year without a summer" because of global cooling caused by the 1815 eruption of Tambora, in Indonesia. A large number of crops failed as a result of the cooling due to the eruption. The more recent 1991 eruption of Pinatubo, in the Philippines, also caused global cooling, perhaps by an average of about 1 degree during the following year.

Huge volcanic eruptions like the most recent big Yellowstone explosion, which occurred 631,000 years ago, are about 10 times bigger than that of Tambora in 1815, and perhaps 100 times bigger than Pinatubo's 1991 blast. So it's easy to see that such huge eruptions probably have a significant impact on global climate that might last for years. But will this lead to the extinction of humanity?

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No. We can be confident of this because there have been two massive explosions while humans were present on Earth, and both of these were actually larger than Yellowstone's most recent cataclysmic eruption. These eruptions were from Toba, Indonesia, about 74,000 years ago and from Taupo, New Zealand, about 26,500 years ago.

For many years the Toba eruption was suspected of causing the near extinction of humanity — Homo sapiens only appeared in Africa about 200,000 years ago, so at the time of the Toba eruption, our species was still not particularly advanced or widespread. Genetic evidence suggests that at the time our ancestors migrated away from Africa, shortly after the time of the Toba eruption, there were only a few thousand individuals. This has led some scientists to speculate that the Toba eruption nearly wiped out humanity.

Evidence from archaeology, however, suggests that humanity did not suffer greatly from the effects of the Toba eruption. For example, studies from hominid (but not necessarily Homo sapiens) sites in India show that there was little change in activities before and after the eruption, which is preserved by a widespread ash layer in southern Asia. Further away, lake cores in east Africa suggest that the environmental impact of the eruption was not catastrophic. And archaeological evidence from southern Africa shows that Homo sapiens were thriving during and following the eruption.

This is not to say that life was easy. Global climate was undoubtedly affected, and the ash deposits in some areas demonstrate that the direct impact was widespread. But humans are an adaptable species. In fact, there is no evidence of any extinction of any species due to the Toba or Taupo eruptions.

This is not meant to make light of the impacts of future large explosive eruptions. In fact, a Toba-sized eruption is not needed to cause changes in climate, as the smaller (but still large) Tambora eruption demonstrated. Rather, we hope to explain why claims that Yellowstone will cause an end to humanity are wrong. Such eruptions will cause major changes to the environment and will require humans to cope with extreme conditions. Our dependency on global trade, electricity, and other aspects of modern life will be affected and create challenges that our stone-age ancestors did not have to deal with, but we are an adaptable species. Humans would not go extinct.

So the next time you hear someone say that Yellowstone is going to wipe out humanity you can tell them that they are incorrect. The proof? Events larger than Yellowstone's most recent big explosion have already happened while humans have been on the planet. Twice. And we're still here to write about it.

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