Two hundred years ago, on April 10, 1815, the volcano Mount Tambora in Indonesia had a climactic explosion in a sequence of explosive eruptions that lasted several months and ejected about 150 km3 of tephra into the atmosphere. The explosion was heard in a large area spanning more than a thousand kilometers. The ash cloud eventually covered an area on the northern hemisphere reaching from Indonesia over Europe to the East coast of the United States ( Figures 1-2).
At the time of the eruption, people in Europe had no idea that they were heading for what became known as ìthe year without summerî and a hard time with cold and severe weather, food shortages, and epidemics. Many communities on the Northern hemisphere turned out unprepared for the severe impacts of the eruption and the climate signal it produced. Today, the news of such an eruption, no matter how remote, would spread around the globe more or less immediately. But is the modern, globally connected society better prepared for such an event than communities were 200 years ago? How likely is it that we will experience an eruption as severe in this century? Do we know the risk that these eruptions pose and can we afford not to consider this risk in our efforts for disaster risk reduction (DRR)? Could large volcanic eruptions cause global disasters or even a global catastrophe? The European Science Foundation (ESF) just published a science position paper that studied these questions in detail.1 The following is a summary of the main findings of this position paper.
On the scale defined by the Volcanic Explosivity Index (VEI)2, which uses a combination of the volume of the erupted tephra and the eruption plume height to assess the severity of an eruption, the Mount Tambora eruption measured VEI 7. During the Holocene, the most recent geological epoch that began 11,700 years ago, there were seven known VEI 7 eruptions (Figure 3); and possibly one or two more that are not known yet. All but one of these eruptions took place at a time when global population was far below 1 billion. At the time of the Mount Tambora eruption, population had just reached 1 billion. With a population above 7 billion and heading for 12 billion 3 , a recurrence of a VEI 7 eruption could have extreme consequences, potentially causing a global disaster. The probability of such an event occurring in the 21th century is 5-10%. Consequently, VEI 7 and larger eruptions are a severe threat to our modern society.
Humanity is exposed to a broad ensemble of natural and anthropogenic hazards that could cause global disasters and catastrophes 4,5,6,7. Geohazards such as earthquakes, landslides, volcanic eruptions, tsunamis, and floods cause significant loss of lives and properties. Most of these losses occur during high-impact events and they are increasing, as more and more people live in areas exposed to such hazards.
Recent events such as Hurricanes Katrina in 2005 and Sandy in 2012, the 2004 Indian Ocean tsunami, the 2011 Tohoku earthquake and tsunami, and the 2013 Typhoon Haiyan illustrate the destruction extreme hazards can inflict on a modern society, particularly through cascading effects and chains of failure. They also show that the risks associated with extreme natural hazards are still difficult to estimate and that procedures for reducing the disaster risk and mitigating the resulting losses are inadequate. This is even more so for more extreme events that could occur any time.
The recent major geohazards are dwarfed by the largest geohazards that occurred several times during the Holocene (e.g., Figure 4). If such a mega hazard were to occur today, the resulting disaster impacts would be unparalleled. Efforts in DRR are challenged by the nature of such extreme events: they are rare, occur as surprises, and tend to have high impacts. Because they are rare and we lack direct experience, the serious threat posed by extreme events tends to be underrated. The increasingly complex built environment and global economic dependencies can lead to domino effects amplifying the direct impacts of the hazards.
The sensitivity of our modern communities was exemplified by the widespread impacts of the minor Eyjafjallajökull eruption VEI 4; 0.25 km3) in Iceland in 2010. Global disasters caused by extreme hazards have the potential to severely impact global economy, food security and stability. Floods and droughts are major threats that potentially could reach planetary extent through secondary economic and social impacts. With mega cities and crucial industries in areas exposed to natural hazards, earthquakes, tsunamis, and volcanic eruptions might cause disasters that could exceed the coping capacity of the global economy. Unfortunately, the more we learn to cope with the relatively frequent hazards that we experienced during the last 50-100 years, the less we are worried about the low-probability, high-impact events, which do not occur every century, but which might occur in the near future.
As a consequence, threats from the less frequent extreme floods, droughts, volcanic eruptions, asteroid impacts, solar storms, etc. often are not appropriately accounted for in DRR discussions.
Addressing the challenges the rare, high-impact events pose to human life and property is essential for long-term sustainability of civilization. Given the nature of the extreme hazards, most ideas about them are based on indirect evidence, and particularly the impacts of the hazards on environment and society are difficult to assess with certainty. Risk as conventionally defined ñ the product of hazard probability, value of assets exposed to the hazard, and the vulnerability of the assets ñ is hard to assess. The hazard probability goes to zero, and we lack the knowledge to reliably estimate the vulnerabilities, especially from indirect and cascading effects, both in the near and far-field of the hazardous event.
Increasing global resilience and reducing the disasters induced by the occurrence of extreme hazards at an acceptable economic cost requires a solid scientific understanding of the impacts these hazards could have on modern society. While the probabilities of most natural hazards do not change much over time, the sensitivity of the built environment and the embedded socio-economic fabric has changed. Exposure to geohazards has increased dramatically in recent decades and continues to do so. Most of the increasing losses occur during less frequent high-impact events at the upper end of the hazard spectrum. The increasing complexity of societies allows even moderate hazardous events to cause regional and global disasters. Understanding the disaster risk therefore requires distinguishing between the event (the occurrence of a hazard) and the processes that are triggered by this event and that determine its consequences.
For risk assessments, it is crucial to understand the processes triggered by the event in the complex coupled human-natural system that lead, or do not lead, to so-called X-events 7. X-events are rare, surprising, and have potentially huge impact on human life. These X-events are outliers outside of the “normal” region that could lead to “the collapse of everything.” Increasingly, the complexity of modern life amplifies the impacts of natural hazards. Although we understand the “how” and “why” for most of the natural hazards events (although not necessarily the “when”), how such hazards lead to X-events is less studied and understood. For many natural hazards, the unfolding time is short, but the impact time can be much longer. Events that have a short unfolding time but large total impacts over very long impact times are those that are surprising and difficult to prepare for. Extreme geohazards fall into this class of events.
The extreme earthquakes that occurred during the last 2000 years have illustrated the destruction they can inflict, both directly and indirectly through tsunamis. The resulting disasters are amplified in areas with poor building infrastructure. As a consequence, the earthquakes with the largest magnitude are not necessarily those that turn out to cause the most fatalities or greatest damage. In general, poor countries that are exposed to the same level of hazards as more developed countries experience a disproportionate number of disasters. Poverty, often paired with corruption, is the basis for processes that can turn hazards into disasters, and the means to increase preparedness and resilience are not sufficiently available in areas with high degrees of poverty. The very recent earthquake in Nepal underlines the causal link between poverty, corruption, a poorly built environment, and the extent of the damage caused by a natural hazard.
Categories of Disasters
X-events differ in terms of the disasters they cause. We distinguish four categories:
a. Extinction-Level Events are so devastating that more than a quarter of all life on Earth is killed and major species extinction takes place.
b. Global Catastrophes are events in which more than a quarter of the world human population dies and that place civilization in serious risk.
c. Global Disasters are global scale events in which a few percent of the population die.
d. Major Disasters are events exceeding $100 Billion in damage and/or causing more than 10,000 fatalities.
Volcanic eruptions experienced in the last few decades often have a high ratio of fatalities to the immediately impacted population. All but one of these eruptions were relatively minor and direct impacts were local. For larger volcanic eruptions, volcanic ash and gases can induce large indirect effects often exceeding the direct impacts in the near-field of the volcano. This is illustrated by a number of eruptions that took place in the last few hundred years.
Extreme geohazards that occurred throughout the last few thousand years rarely caused major disasters because population density was low, the built environment was not sprawling into hazardous areas to the same extent as today, and the complexity of human societies was much lower than today. Similar extreme events today could cause unparalleled damage on a global scale and worsen the sustainability crisis. Simulation of these extreme hazards under present conditions can help to assess the disaster risk and underline the fact that we have been lucky during the last century.
The intercomparison of natural hazards indicates that large volcanic eruptions are the low-probability geohazards with potentially the highest impact on our civilization. Large volcanic eruptions can have more severe impacts through atmospheric and climate effects and can lead to severe problems in food and water security, as emphasized by the widespread famine and diseases that were rampant after the Laki 1783 and Tambora 1815 eruptions. Hence extreme volcanic eruptions pose a higher associated risk than all other natural hazards with similar recurrence periods, including asteroid impacts.
So far, modern civilization has not been exposed to an eruption comparable to the most extreme events during the Holocene. However, under the present conditions of a globally connected civilization facing food, water and energy scarcity, the largest eruptions during the Holocene would have had major global consequences.
Events like the Toba eruption 74,000 years ago could return humanity to a pre-civilization state.
In terms of energy release per event, extreme volcanic eruptions are the largest high-intensity terrestrial phenomena known 8. Considering the long-term, time-averaged mass eruption, volcanic M7 eruptions are associated with a 10-100 times larger contribution than M8 and M9 eruptions. At recurrence periods of up to 100,000 years, explosive volcanic eruptions are more frequent than asteroid impacts with similar energy releases. Although energy release of the 1 in 1 Ma volcanic eruption is comparable to that of equally frequent impactors, volcanic eruptions may be far more impactful since they are more likely to occur on land, are associated with large amount of ash and gas emissions, and are likely to impact climate and food security more severely. This has important implications for risk assessments of extreme events and DRR. The impacts on our modern society could result in a global disaster, and it is timely to take measures to reduce this risk.
It needs to be mentioned here that the anthropogenic climate change expected for the 21st century may be associated with a higher risk than any other geohazard at the 500-year to several thousand-year event scale, with the upper limit of this risk being very uncertain. The probability of severe impacts is very high, and it keeps increasing with every year passing without a significant effort to mitigate climate change. March 2015 was the first month in which the globally average atmospheric carbon dioxide content was above 400 ppm for the whole month, marking a significant milestone on our journey to a much warmer planet, and there are few signs that humanity will manage to get together in a significant effort to curb the current trends. Increasingly, communities are impacted and forced to migrate. Adding a major volcanic eruptions to the pre-stressed global community could easily cascade into major food scarcity, famine, epidemics, large-scale migration, economic instability, and social unrest.
With the prospect of the global population reaching 12 billion by 2100 3, humanity faces the crucial challenge of developing in a very limited time an effective program to reduce the risk of global disasters and catastrophes caused by natural hazards. Considering risk as the product of hazard probability, sensitivity to the hazard, and the value of the exposed assets, it is obvious that risk mainly can be reduced by reducing sensitivity and exposure. Adaptation and mitigation efforts to reduce sensitivity and exposure are insurance against the risk. Willingness to engage in adaptation and mitigation depends on risk perception. The challenge of extreme geohazards is that they are infrequent and risk awareness is generally low. Therefore, the costs for adaptation and mitigation are often postponed.
Risk awareness and monitoring, as well as the capabilities and means to mitigate risk, are highly uneven across the world. As a result, potential hazards are much more closely monitored in wealthy countries than in the developing world. Low risk awareness combined with poverty and corruption turns hazardous events more easily into disasters throughout the developing world. However, the largest hazards are global in nature, and efforts need to be made to have a well-developed global monitoring system for geohazards in support of early warnings. An international governance structure is needed to coordinate global risk assessments and responses.
Research focusing on community disaster resilience is at its beginning. Simulation of selected extreme hazards under present conditions can help to identify weaknesses in the global socio-economic system that could lead to cascading effects. Essential variables to be observed by a human observatory need to be identified. Research on the response of our global community to a warning that an extreme hazard is developing is limited and efforts need to be made to understand the impacts of such warning on global stability and preparedness.
Although significant efforts have been made to coordinate global Earth observations (e.g., through the efforts of the Group on Earth Observations, GEO), a comprehensive monitoring system of systems that could give timely warning for an impending extreme volcanic eruption is not in place. A monitoring system should combine surface displacements, gravity changes, seismicity, chemical variables, and infrasound to detect emerging volcanic eruptions and assess their potential magnitude ahead of the main eruption.
In conclusion, it has to be acknowledged that humanity is poorly prepared to meet the challenge of extreme geohazards. In particular, a large volcanic eruption (VEI 7 or larger) would challenge modern society to the core. Reasons for not being prepared include low perceived likelihood, a low political sensitivity, a disconnect between the scientific communities and decision-makers, the lack of socially acceptable strategies including the cost of preparing, and the common belief that consequences are so extreme that preparedness is futile. To overcome these issues, a better process for understanding the available scientific knowledge and using it in proactive decision making needs to be developed.
If we want to reduce the risk associated with extreme geohazards, particularly severe volcanic eruptions, the global community needs to facilitate the development of several elements in science, monitoring, and governance:A global scientific framework for strategic extreme geohazards science in support of warnings, preparedness, mitigation and response to be implemented by governments, communities, and the private sector on global scales in order to minimize the impacts of extreme geohazards; Scenario contingency planning to better understand the threats and reduce the risk particularly by reducing systemic weaknesses that could lead to cascading effects; Improved risk awareness through dissemination of information on the risk associated with extreme geohazards; A global monitoring system to provide early warning for emerging extreme volcanic eruptions; An informed global governance capable to respond to emerging global threats and coordinating measures to increase preparedness and general resilience with the goal to reduce the global disaster risk.
As an immediate step, the existing International Charter on Space and Major Disasters should be extended to also cover actions increasing preparedness and cases of emerging threats for early warning purposes.
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