But how is this event, two centuries old, instructive today? It turns out that a similar catastrophe has a one in six risk of recurring during this century, according to analyzes of volcanic deposits over the last 60,000 years. And it is precisely the existence of this major risk that an article published in the journal recalls Nature from November 14. In this comment widely relayed in the international press, Markus Stoffel, professor at the Section of Earth and Environmental Sciences (Faculty of Sciences) and at the Institute of Environmental Sciences (ISE), and two of his colleagues point out that with a human population 8 times larger to feed, a highly interconnected economy and global warming in full swing, the repercussions of such an eruption, if it were to occur in the next five years, would be unprecedented measure with those of 1815. The authors therefore call on the scientific community to take up the subject in order to prepare the world as best as possible for an event which we know is, by nature, inevitable.
Potentially huge damage
“The question is not whether such an eruption will occur but rather when,” insists Markus Stoffel. The potential damage is enormous and we are not ready. The economic impact of such an event was recently estimated by the insurer Lloyd’s at several trillion dollars per year. But these figures and the consequences in general are marred by significant uncertainties. We certainly know how volcanism influences the climate: the eruption sends sulfur dioxide into the stratosphere where it forms sulfate aerosols which reflect the sun’s rays and cool the earth’s surface. But we lack knowledge of the details, where the devil hides. The extent of the phenomenon depends on the quantity, vertical distribution and size of these particles, all parameters which vary from one case to another. The effects on precipitation are even more difficult to predict, as are those on agriculture and economic markets. Added to this is global warming which further complicates the situation. So there is work to be done.”
Learn more about the past
In their article, the Geneva scientist and his colleagues Christophe Corona, of the CNRS, and Scott St. George, of the insurance broker WTW, present a three-step plan. They first recommend better understanding the events of the past in order to derive the most reliable models possible for those of the future. Sulfur dioxide emissions have only been able to be precisely measured by satellite since the eruption of Pinatubo in the Philippines in 1991. For those before, the data are rarer, buried in the ice of Greenland or Antarctica, and are only strong for very large eruptions. These gaps make modeling the evolution of the plume from each of these events and the cooling it induces particularly difficult.
The life cycle of aerosols is also poorly understood. Counterintuitively, it is possible that large eruptions produce larger aerosols that reflect the sun less effectively and fall more quickly from the stratosphere, causing less cooling than smaller eruptions. Furthermore, little is known about the influence of these sulfur emissions on climatic phenomena such as El Niño or the monsoons.
To help fill these gaps, the authors suggest linking geological data and models from past climates to historical volcanic records, already existing and yet to be collected, preserved in ice and in tree rings.
Interaction with warming
The second step, according to the authors, is to study how volcanic eruption-induced cooling may interact with current human-caused warming. In a warmer world, many chemical and physical processes that take place in the atmosphere, oceans or on land are changed. Current climate changes result, for example, in an acceleration of ascending air flows from the tropics to high latitudes which will limit the coagulation of aerosols. These finer particles reside longer in the stratosphere and cause more lasting cooling.
Global warming also increases ocean stratification, which disrupts the mixing of surface waters with deeper ones. In the event of a major volcanic eruption, cooling will be particularly significant in the upper layers of the water column and the air masses above the ocean surface. As for the increasing number of extreme climatic phenomena, such as torrential rains, the melting of ice caps or the rise in sea levels, they also add uncertainty about the consequences of a cataclysmic eruption which would occur today. .
“None of this is taken into account in current climate models,” notes Markus Stoffel. Moreover, the latter, to predict the importance of a future eruption, are currently based on a list of events which occurred between 1850 and 2014. A list which therefore does not include the Tambora disaster.
Mitigate impacts
Finally, the authors suggest that scientists, in collaboration with economic and financial analysts as well as policy makers, combine these improved climate models with those simulating changes in agriculture and food shocks to design strategies to mitigate effects of a catastrophic eruption.
“The eruption of Pinatubo in 1991 caused a reduction of 9% in corn harvests and 5% in those of wheat, rice and soya,” recalls Markus Stoffel. But this is only a medium-sized eruption which does not allow us to assess the danger. We recommend building a realistic vision of future risks by considering an eruption the size of Tambora which would occur in a climate similar to that of today, by adding a climatic phenomenon like The Childjust to make things worse. This would allow insurers and reinsurers to better estimate the financial costs of such an event.”
