There are only two natural hazards that could cause a global catastrophe, namely asteroid impact and a gigantic volcanic eruption. The former has generally received more attention but the latter poses a higher risk because they happen more often. Most people are aware of super-volcanoes like Yellowstone in Idaho, which have had gigantic eruptions in the past, but will not know how likely such an event might be or what are the possible consequences for humanity.
Written records of Earth’s volcanism only extend back a couple of thousand years, but this is a very tiny fraction of the billions of years of Earth history. Geological Investigations, however, show that many eruptions have occurred that are a hundred times greater than the largest historic eruption. Such super-eruptions will happen again, so the most obvious questions are when, where and how will humanity be affected.
Before discussing the answer to these questions, it is first necessary to explain what large means for volcanoes. We have two measures of eruption size: how much magma (molten rock) is erupted and how fast the magma is erupted. In general, as magnitude (the amount erupted) increases, so does the power of the eruption (the rate of eruption). The famous eruption of Krakatoa (Indonesia) in 1883 is a useful yardstick. Here, an amazing two cubic miles of magma were erupted and the eruption was so powerful that its sound could be heard thousands of miles away. Deadly flows of hot volcanic ash reached tens of miles away across the sea and the whole atmosphere was so polluted that there were spectacular sunsets for the next two years. About two or three eruptions the size of Krakatoa happen every century. A super-eruption is a one with a magnitude that is at least 50 times Krakatoa, an astonishing volume of 100 cubic miles.
In the last several years I have been working with colleagues around the world to make a database of the size of volcanic eruptions and their age. This database has allowed colleagues and I at Bristol University to estimate how frequently super-eruptions happen. Our best estimate is about every 17,000 years and the last known one was around 26,000 years ago in New Zealand. This does not mean, however, that one is imminent as the 17,000 years figure is just the average, so gaps between super-eruptions can be much greater than 20,000 years.
Where will the next one be? In the last few million years super-eruptions have happened in many countries, including the USA, Mexico, Chile, Bolivia, Japan, Indonesia, Turkey and New Zealand.
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Around the volcano the effects of a super-eruption will be completely devastating. Typically the hot flows of ash can reach 50 to 70 miles in all directions and can flow over mountains thousands of feet high. The volcanic ash can cover continents; it is estimated that the eruption of the Toba super-volcano in Sumatra covered the whole of India in a several inches of ash and ash may take decades to erode away in arid environments. It seems likely that an area of about a million square miles or more will become almost uninhabitable with catastrophic crop failure and pollution of water resources. Economic activity of every kind will cease to function in the affected area. Famine and huge migrations of people seem inevitable.
The most worrying concern though is the effects of an eruption on climate. Large volcanic eruptions produce global pollution of the atmosphere in the form of tiny droplets of sulphuric acid known as aerosols. These droplets circulate through the stratosphere and solar energy is reflected back into space so overall the climate cools. The circulation in the atmosphere also changes dramatically because the aerosols also absorb heat. These effects are now well understood. In 1815, the eruption of Tambora (about five times bigger than Karkatoa) caused 1.5oC of cooling in 1816 and led to the crop failure and famine in New England and Europe in the year without a summer. Models of the atmosphere predict these observed effects well, giving some confidence that the same models can be used to predict what would happen in a super-eruption. The results are alarming with many continental interiors cooling by 10 to 15oC and the deep freeze lasting for over a decade. Together with changing patterns of wet and dry conditions, huge food shortages and global economic collapse seem inevitable. It is hard to see more than a small fraction of the current world’s population surviving such hostile conditions.
Civilisation is threatened but a super-eruption is very unlikely to cause human species extinction. Indeed, archaeological investigations indicate that human communities survive and then thrive following super-eruptions. The climate models also indicate that not all areas are so drastically affected. Oddly the northern polar regions become much warmer in winter. The climate of coastal areas and tropical archipelagos like Indonesia are moderated by being near the ocean, which takes a long time to heat up or cool down (hundreds of years) so it seems likely that there will be many surviving communities around the world especially on islands and at coasts.
My analysis based on the current scientific understanding must be regarded as very tentative. The topic of the impacts of a super-eruption has been little researched and attempts to identify volcanoes capable of future super-eruptions have been limited. The chances of a super-eruption in the lifetime of a single generation are about 1 in 300 so there is an argument that we have enough more immediate concerns to worry much about super-eruptions. However, a smaller but nonetheless huge eruption in the wrong place could be very disruptive on a global scale. An eruption the size of Tambora in 1815 happens every 300 years. Further, another super-eruption is inevitable and our crowded world already seems very vulnerable to what could well be catastrophic.
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