Hazardous effects of super-eruptions
The effects of a super-eruption on the areas in the immediate vicinity of the volcano are completely catastrophic. Explosive super-eruptions produce huge incandescent hurricanes known as pyroclastic flows, which can cover thousands to tens of thousands of square kilometres in thick deposits of hot ash. No living beings caught by a pyroclastic flow survive. However, these dramatic local effects are not of greatest worldwide concern. Globally, most repercussions will come from the effects of the volcanic ash and volcanic gases suddenly released into the atmosphere.
Volcanic ash fallout from a super-eruption will probably have severe effects over areas the size of a large continent. One centimetre thickness of volcanic ash is easily enough to disrupt most forms of agriculture, and lesser amounts (a few millimetres) can destroy many kinds of crops. A super-eruption can cover tens of millions of square kilometres in several centimetres of ash.
The most vulnerable areas are North and South America and Asia, when account is taken of locations of such volcanoes. Europe has at least one supervolcano (the Phlegrean Fields). It is possible that the area around Kos and Nisyros in the Aegean might be characterised as a super-volcano. If such an eruption were to take place at any of these sites, then a substantial part of the global economy would inevitably be devastated and many parts severely incapacitated. Any technologically advanced city would be very vulnerable to the effects of ash, including pollution of water supplies, disruption of transport systems, and failure of electronic equipment. There would also be severe disruption of aviation.
Volcanic ash fallout from a super-eruption will probably have severe effects over areas the size of a large continent. One centimetre thickness of volcanic ash is easily enough to disrupt most forms of agriculture, and lesser amounts (a few millimetres) can destroy many kinds of crops. A super-eruption can cover tens of millions of square kilometres in several centimetres of ash.
The most vulnerable areas are North and South America and Asia, when account is taken of locations of such volcanoes. Europe has at least one supervolcano (the Phlegrean Fields). It is possible that the area around Kos and Nisyros in the Aegean might be characterised as a super-volcano. If such an eruption were to take place at any of these sites, then a substantial part of the global economy would inevitably be devastated and many parts severely incapacitated. Any technologically advanced city would be very vulnerable to the effects of ash, including pollution of water supplies, disruption of transport systems, and failure of electronic equipment. There would also be severe disruption of aviation.
Ash from super-eruptions has covered almost all of North America in the past - from the small volume 1980 Mount St. Helens fallout, to huge ones from Yellowstone about 2 million and 600,000 years ago. The two ash deposits from the Jemez Mountains (Valles caldera) volcanic centre at 1.6 and 1.1 million years ago are so far-flung that they have not yet been recognized. The whole of the USA has been covered by ash in the geologically recent past. [Source: http://volcanoes.usgs.gov]
The most significant global threat from super-eruptions is, however, to global climate and weather. Large explosive volcanic eruptions eject huge amounts of volcanic dust and gas into the stratosphere. The gases are dominated by water, but also commonly include significant amounts of sulphur dioxide, carbon dioxide and chlorine. A great deal has been learnt over the last few decades on the effects of volcanic dusts and gases on climate, from careful examination of climate records and observations on eruptions. The eruption of Mount Pinatubo (Philippines, 1991) allowed some of these ideas to be tested in detail.
Dust and gases injected by an eruption into the stratosphere reflect solar radiation back to space or themselves absorb heat, cooling the lower atmosphere. This fact has led to the concept of 'volcanic winter'. Silicate dust (tiny ash particles) is thought to be less important, because its residence time in the stratosphere is quite short (only a few weeks to months at most). The main thing causing global cooling after a major eruption is sulphur dioxide gas, which reacts with water to form tiny droplets of sulphuric acid, which remain in the stratosphere for two or three years as an aerosol.
Measurements of the Pinatubo sulphur dioxide injection by satellite-borne instruments showed that the aerosol plume encircled the globe in only three weeks, and then slowly dispersed to cover much of the Earth in the following two years. Eventually the circulation pattern of the upper atmosphere deposits the aerosol particles back to the surface in the polar regions. Stratospheric aerosols absorb heat so that, on average, the upper atmosphere is heated and the lower atmosphere is cooled significantly. However, the response of the atmosphere is complex, so that there are areas of both highly anomalous heating and cooling in the few years following an eruption.
Measurements of the Pinatubo sulphur dioxide injection by satellite-borne instruments showed that the aerosol plume encircled the globe in only three weeks, and then slowly dispersed to cover much of the Earth in the following two years. Eventually the circulation pattern of the upper atmosphere deposits the aerosol particles back to the surface in the polar regions. Stratospheric aerosols absorb heat so that, on average, the upper atmosphere is heated and the lower atmosphere is cooled significantly. However, the response of the atmosphere is complex, so that there are areas of both highly anomalous heating and cooling in the few years following an eruption.
Left: Visible wavelength view of top of the Mount Pinatubo eruption cloud as it was spreading out in the stratosphere about 1.5 hours after the start of the climactic eruption on 15 June 1991 (top) and 2 hours later (bottom). By this stage it had already reached over 700 kilometres across, and it eventually reached over 1200 km across! This is the view that you would see if you were in a satellite circling or hovering way above the Earth. – Both images were collected by the Japanese GMS satellite.There is compelling evidence from meteorological records and tree-rings, that eruptions like Tambora (1815), Krakatoa (1883) and Pinatubo (1991) caused substantial and measurable cooling of the Northern Hemisphere. In these cases the lower atmosphere cooled by an average of 0.5 - 1°C in the following two years. This averaged cooling masks significant anomalies, such as frosts in the middle of the summer of 1816 in New England (the so-called "year without a summer" that followed).
With an eruption at low latitudes, like Pinatubo, these fluctuations are caused by the increased temperature difference in the stratosphere between high and low latitudes. The results of detailed atmospheric studies after Pinatubo indicate quite complex patterns of pronounced summer cooling in many parts of the Northern Hemisphere but also pronounced winter heating in continental interiors. For example there were a few degrees of summer cooling over the US and Europe, and winter warming over Northern Europe and Siberia. The change in climate, however, is a short-term phenomenon. For moderate sized eruptions climate returns to the pre-eruption situation once the global aerosol has disappeared (after about three to four years).
It is not clear how the understanding and observations of relatively small historic eruptions, like Krakatoa and Pinatubo, can be extrapolated to super-eruptions. It can safely be assumed that their effects will be more severe, but the Earth's climate system is not well enough understood to be very confident in detailed predictions. In principle, putting twice as much aerosol in the stratosphere should double the predicted climatic effect. But climate systems are complex, with important feedback processes. Thus the consequences of very much larger injections cannot be forecast with much confidence.
Image above: Satellite data from NASA’s SAGE sensor map the atmospheric aerosol cloud that developed around the globe following the climactic eruption on June 15. The four panels show the amount of aerosols (blue – low; red- highest concentration) in the period before the eruption (top left), 2 weeks after (top right), 2 months after (bottom left) and 6 months after (bottom right). Note that after 6 months the entire atmosphere is covered by an aerosol veil. The aerosols reduce incoming solar radiation and cause temperature changes on Earth. (Figure from McCormick, M.P. et al., Nature 272, p. 399-404, 1995).
It is also possible that other components (dusts and other gases) may have a much more significant role when injected into the stratosphere in much larger amounts. As in many situations with global climate there are forcing factors that might inhibit and forcing factors that might magnify the effects of a supervolcano eruption on climate. Understanding can best be advanced by investigating the effects of large volcanic injections in global climate models. Encouragingly, such models showed good agreement with observations made of the effects of Pinatubo.
Recently, observations from ice cores have been made on the possible effects of the Toba super-eruption, 75,000 years ago. If these data do reflect the Toba event, they suggest that aerosol formation and fallout lasted for six years. The volcanic winter would not only be more severe than for a Pinatubo-scale eruption, but would last much longer. Models suggest that a Toba-sized super-eruption would inject so much sulphur gas into the atmosphere that the stratosphere chemistry would be substantially perturbed - allowing for more prolonged climate-forcing. Some models suggest super-eruptions can cause cooling of 3°C to 5°C, which in global climate terms represents a catastrophic change.
It may not sound like much, but a mere 4°C cooling, sustained over a long period, is enough to cause a new Ice Age. However, great caution is needed in attributing causes and effects in a system as complex as global climate, and more detailed modelling research is required. Initial computer climate-model runs by scientists at the UK Meteorological Office’s Hadley Centre for a Toba-sized eruption suggest Northern Hemisphere temperature drops of 10oC. This would freeze and kill the equatorial rainforests.
These important issues are still in the hypothesis stage, and are as yet unproven, because our detailed knowledge of the timing of events 75,000 years ago, of the complex physical and chemical processes involved, and of the applicability of current climate models to such scenarios, needs to be significantly improved.
Other effects of a large eruption were detected after Mount Pinatubo erupted in 1991. The presence of volcanic aerosols can accelerate ozone-destroying reactions and affect changes in nitrogen in the stratosphere and tropospheric carbon monoxide concentrations. Ozone is destroyed by anthropogenically-released chemicals, but it appears that volcanic aerosols can act to catalyse these reactions. While the ozone layer remains vulnerable, a super-eruption could further adversely affect the deterioration of global ozone. Ozone is essential for life on Earth by protecting us from severe ultraviolet radiation.
Major changes in methane production were also observed after the Mt Pinatubo eruption. These changes are not yet fully explained but show that eruptions can affect the biosphere and the carbon cycle on a global scale.
Continue to next section, Comparisons; public perception and risk
Recently, observations from ice cores have been made on the possible effects of the Toba super-eruption, 75,000 years ago. If these data do reflect the Toba event, they suggest that aerosol formation and fallout lasted for six years. The volcanic winter would not only be more severe than for a Pinatubo-scale eruption, but would last much longer. Models suggest that a Toba-sized super-eruption would inject so much sulphur gas into the atmosphere that the stratosphere chemistry would be substantially perturbed - allowing for more prolonged climate-forcing. Some models suggest super-eruptions can cause cooling of 3°C to 5°C, which in global climate terms represents a catastrophic change.
It may not sound like much, but a mere 4°C cooling, sustained over a long period, is enough to cause a new Ice Age. However, great caution is needed in attributing causes and effects in a system as complex as global climate, and more detailed modelling research is required. Initial computer climate-model runs by scientists at the UK Meteorological Office’s Hadley Centre for a Toba-sized eruption suggest Northern Hemisphere temperature drops of 10oC. This would freeze and kill the equatorial rainforests.
These important issues are still in the hypothesis stage, and are as yet unproven, because our detailed knowledge of the timing of events 75,000 years ago, of the complex physical and chemical processes involved, and of the applicability of current climate models to such scenarios, needs to be significantly improved.
Other effects of a large eruption were detected after Mount Pinatubo erupted in 1991. The presence of volcanic aerosols can accelerate ozone-destroying reactions and affect changes in nitrogen in the stratosphere and tropospheric carbon monoxide concentrations. Ozone is destroyed by anthropogenically-released chemicals, but it appears that volcanic aerosols can act to catalyse these reactions. While the ozone layer remains vulnerable, a super-eruption could further adversely affect the deterioration of global ozone. Ozone is essential for life on Earth by protecting us from severe ultraviolet radiation.
Major changes in methane production were also observed after the Mt Pinatubo eruption. These changes are not yet fully explained but show that eruptions can affect the biosphere and the carbon cycle on a global scale.
Continue to next section, Comparisons; public perception and risk