Catastrophe watch

Super-eruptions, asteroid impacts and cosmic winters—such cataclysmic events, known as gee-gees, are no longer science fiction. The tsunami has helped focus minds on the potential dangers. We must act now
June 18, 2005
London, 30th July 2030. The sky is a menacing iron grey, and has been so since soon after the cataclysmic super-eruption in the US five months earlier. Snow lies half a metre deep on Oxford Street, and on the frozen Thames crowds of men, women and children jostle at stalls to barter for dubious meat scraps to supplement their meagre state rations. Across the planet, millions of people have already died from the cold, while hundreds of millions starve as harvests continue to fail. A combination of freezing conditions and civil strife has triggered the breakdown of society in many countries, and the global village has fragmented into a million isolated hamlets, each faced with a daily battle for survival.

Drawing a line between science fiction and science fact, or between scaremongering and informing, can be notoriously difficult, and never more so than when dealing with those rare but inevitable cataclysmic events capable of tearing our comfortable world apart. The horror of Boxing day 2004, when more than a third of a million lives were lost in the space of a few hours, provided a glimpse of the reality. A few months later, the BBC television drama Supervolcano presented us with another taste of what we may face in the future. Reactions to the two events, one factual, one fictional, were contradictory. On the one hand, the Asian tsunami was lamented as impossible to predict or prepare for, a bolt from the blue. On the other hand, the BBC was charged by some with scaremongering for highlighting the terrible consequences of a future volcanic super-eruption in Wyoming's Yellowstone national park. Yet it is hardly surprising that we are caught napping by extreme geophysical events if attempts to educate and inform about the threat they pose attract such hostility.

Whether we ignore them or not, geophysical phenomena far more lethal and destructive than the Asian tsunami are on their way. Volcanic super-eruptions, asteroid and comet impacts and ocean-wide tsunamis large enough to dwarf the Boxing day waves have left their imprint on our planet's surface during its 4.6bn-year history, and they are not going to hold back simply because we have arrived on the scene. Furthermore, the hazardous events associated with human-induced climate change could make matters worse. These include a dramatic slowdown or shutdown of the Gulf stream and associated ocean currents, leading to bitter winters in Europe and eastern North America; and a rapid rise in sea levels in response to the catastrophic melting of one or both of the Greenland and west Antarctic ice sheets.

The legacy of the Asian tsunami is not all bad. It is contributing to the establishment of tsunami early-warning systems, not only in the Indian ocean itself, but also in the Atlantic basin and Caribbean, thereby dramatically raising likely survival rates in the future. Equally importantly, the catastrophe has focused attention upon other geophysical hazards capable of having a severe regional or global impact. Notwithstanding some scepticism, primarily in the press, global geophysical events (GGEs) or gee-gees—such as the BBC's Yellowstone eruption—have successfully made the transition from science fiction to science fact. Broadly speaking, they are now recognised for what they are: extreme natural events with probabilities of occurrence far below 1 per cent in any single year, but approaching 100 per cent in the long term. By forcing individuals, the media, governments, international agencies and disaster managers to recognise that the advent of a natural disaster affecting the entire planet is only a matter of time, the Asian tsunami has helped to raise awareness of the gee-gee menace. This, however, is only the beginning. We need to know far more about the nature of threats and how often we can expect them to occur. Most importantly, we need to know if we can act to prevent or avoid a future global catastrophe, or, at the least, mitigate or manage the worst consequences.

With thoughts and emotions stirred up by the Asian tsunami still fresh in many minds, perhaps an appropriate place to start in any rummage through the gee-gee portfolio is with a closer examination of the mega-tsunami threat. The Boxing day tsunami was not a "mega-tsunami," but the statistics are nonetheless truly astounding: 305,276 dead or missing, 500,000 injured, 410,000 buildings destroyed and close to 8m people displaced, impoverished and/or unemployed. How can mere waves have such a catastrophic impact? The explanation lies in the unique behaviour of tsunamis. Most are generated by the near-instantaneous uplift of a huge area of seabed in response to an earthquake. The movement imparts a jolt to the ocean above, causing it to oscillate and send waves hurtling away from the source of the displacement. Unlike wind-driven waves, tsunamis involve the entire water column from surface to sea floor, and are capable of travelling at speeds of 800-900km per hour in deep water—about as fast as a jumbo jet. Their wavelengths are measured in hundreds of kilometres (compared to a few tens of metres for the storm waves that crash on to the British coast). This means that a tsunami floods in as a wall of water, rather like a giant Severn Bore, which keeps coming for several minutes before taking just as long to retreat. Earthquake-triggered tsunamis may be more than 40 metres high, and estimates from Indonesia's Aceh province, which bore the brunt of the December tsunami, suggest that the waves may have been higher than 30 metres.

Mega-tsunamis, however, may be ten times higher than this when they strike land. Rather than being formed by earthquakes, they result from a comet or asteroid impact in the ocean or, more commonly, a giant underwater landslide or the collapse of an island volcano. About 7,000 years ago, waves inundated parts of Scotland, Iceland and Greenland following the collapse on to the floor of the north Atlantic of a mass of sediment from the Norwegian continental shelf that was bigger than the Isle of Wight. Further back in time, tsunami deposits are found 400 metres above sea level on the flanks of Hawaii's Kohala volcano—evidence of a massive landslide from neighbouring Mauna Loa.

It seems that such giant collapses occur in one ocean basin or another every 10,000 years, or possibly less, and we may not have to wait long, geologically speaking, for the next one. During an eruption in 1949, the western flank of the Cumbre Vieja volcano on the Canary island of La Palma detached itself from the rest of the edifice and dropped four metres seaward. This mass of rock up to a third the size of the Norwegian landslide remains poised like a Damoclean sword over the north Atlantic, and is expected at some time in the future—perhaps next year, maybe several thousand years from now—to collapse into the ocean. A worst case scenario envisages tsunamis in excess of 100 metres devastating the Canary Islands and waves of 20 metres or more in height closing in on the Caribbean and the east coast of North America between six and 12 hours later. Even the south coast of Britain could expect to experience a tsunami on a scale similar to that generated in the Indian ocean on Boxing day. Without prior evacuation, the resulting death toll could be measured in millions, with massive physical damage to the coastal cities of the US, severely disrupting the global economy.

While the economic consequences would be felt across the world, the collapse of the Cumbre Vieja volcano would not physically affect the whole planet. The same cannot be said, however, for the biggest volcanic blasts of all—the so-called super-eruptions that punctuate the earth's history every 50,000 years or so. The 1883 explosion of Indonesia's Krakatoa volcano is probably the best known eruption of all, thanks in part to Simon Winchester's recent account. Compared to a super-eruption, however, this event, which killed more than 36,000 people, was little more than a firecracker. The last super-eruption tore the crust apart in New Zealand's North Island around 26,000 years ago, ejecting over 1,000 cubic kilometres of ash and debris—more than 50 times that blasted out at Krakatoa. One of the greatest explosive eruptions in earth's history, at Toba in Sumatra 50 millennia before North Island, ejected close to 3,000 cubic km of volcanic debris—sufficient to bury Britain in four metres of ash.

Such gargantuan eruptions are regionally devastating, with ground-hugging pyroclastic flows of magma and gas, and metres-thick piles of falling ash covering hundreds of thousands of square kilometres. The last super-eruption at Yellowstone, around 640,000 years ago, buried half the US. Even 1,500km away from the eruption, ash lay a third of a metre deep and has been found by geologists as far afield as Los Angeles in California and El Paso in Texas. But it is not debris or ash that qualify a super-eruption as a gee-gee; it is the huge cloud of gas that is released.

Sulphur-rich gases are generated in all volcanic eruptions. In the largest, enormous volumes combine with water vapour high in the atmosphere to form tiny droplets—or aerosols—of sulphuric acid. Stratospheric winds transport the aerosols across the planet, forming a veil that reflects and absorbs incoming solar radiation. The result is a rapid and dramatic cooling of the troposphere (the lowest 10km or so of the atmosphere)—a so-called volcanic winter. After Toba, temperatures fell rapidly to near or below freezing across much of the planet, and remained there for perhaps five or six years. Conditions for our ancestors must have been appalling, so much so that some anthropologists have linked the event to a human population crash that may have reduced our species to a few thousand individuals.

Volcanic super-eruptions are not the only geophysical events capable of triggering dramatic falls in global temperatures. By hurling enormous quantities of fine dust into the stratosphere, collisions with large bodies from space can have the same effect, this time leading to a so-called cosmic, rather than volcanic, winter. The threat from impacting comets and asteroids has been recognised for some years, and long-standing plans to find out more were accelerated in 1994 by observations of the collision between Jupiter and 21 fragments of the Shoemaker-Levy comet. Spectacular images of impact scars larger than the earth in Jupiter's gaseous envelope focused minds on the effect such an event would have on our own planet, and funding suddenly became available for sky surveys to identify objects that might one day threaten our world.

During the course of its annual passage around the sun, the earth has many close encounters with asteroids; chunks of rock ranging in size from a few tens of metres to several kilometres. Collisions, however, are rare. The last confirmed impact occurred at Tunguska, Siberia, in 1908, when an asteroid 40-50 metres across broke up and exploded 10km above the Taiga. Despite being barely half the length of a football pitch, the asteroid caused a blast that flattened more than 2,000 square kilometres of forest. Were such an object to strike central London rather than the Siberian wastes, it would obliterate everything within the M25. Collisions with objects of this size probably occur every several centuries, and while they cause local devastation, the planet as a whole remains unaffected. With greater size, however, the effects become more widespread. A 1km object, for example, would obliterate an area the size of England, or generate a mega-tsunami should it land in the ocean. A 2-3km object—the jury is out on the critical diameter—would load the atmosphere with sufficient dust to cause a plunge in global temperatures and the onset of a cosmic winter that could last for several years. A billion people could die as a result of the global breakdown in agriculture before the planet started to warm up again. However, there has been good news in the last couple of years: the frequency of collisions with 1km-sized objects has been downgraded from 100,000 to 600,000 years, while 2km asteroids are now expected to strike earth only every few million years.

One of the most imminent gee-gees is also one of the least addressed: the next major earthquake to strike at the heart of Tokyo. Home to more than 30m people—a quarter of Japan's population—and the headquarters of two thirds of the country's industrial giants, a major quake is expected to cause widespread mayhem some time in the next century or so. While the physical effects will be confined to the Tokyo-Yokohama region, the cost of rebuilding could bring the global economy to its knees. The last time the Japanese capital was struck by a big earthquake was in 1923, when building collapse and post-quake conflagrations claimed up to 140,000 lives and reduced the city to rubble. Next time, owing to higher building standards, the death toll is forecast to be lower—around 60,000. Estimates of the economic cost, however, range from $3.3-4.4 trillion, up to 44 times greater than the cost of the 1995 Kobe earthquake—the costliest natural catastrophe the world has ever experienced. Such a loss could cause a worldwide economic collapse comparable to the Wall Street crash.

But it is accelerating climate change that is the single most likely trigger of the next gee-gee. Top of the list is a slowdown or shutdown of the Gulf stream and the associated currents that ameliorate the western European climate and prevent British winters being as cold as those of Labrador. Just a few years ago, climate change models assigned a small probability to such an event taking place a few centuries down the line. Recent observations indicate, however, that the pattern of circulation in the north Atlantic is already changing, while new climate models predict a very good chance of a weakening of the Gulf stream before 2100. One model forecasts a 45 per cent chance of weakening with a 3 degree Celsius temperature rise, while another predicts this may occur with a rise of just 2 degrees. As new research suggests that the global temperature rise by the end of the century may well be substantially higher than 2 or 3 degrees, the chances seem pretty much even that we will lose the warming influence of the Gulf stream before the new century chimes in.

How does global warming lead to regional cooling? The underlying mechanism is quite straightforward. The Gulf stream and associated currents carry warm, salty water from the tropics northwards to the margins of Britain and Europe, where they keep temperatures several degrees warmer than they would otherwise be. As the warm waters reach the Arctic seas, they cool, and because they are saltier than the surrounding ocean—and so more dense—they sink and return to the tropics along the sea floor. Accelerated melting of Arctic ice, elevated levels of precipitation and increased flow from Siberian rivers into the Arctic ocean are already, however, leading to dilution of the salty tropical waters. If this continues, they will eventually be reduced in density to the extent that they will not sink. The return flow of water to the tropics will close down and the circulation will stall. A recent study by the British meteorological office has shown that without the warming effect of the Gulf stream, the entire northern hemisphere will cool. Within six years, winters would return to those of the late medieval "little ice age," with sea-ice clogging the English channel and North sea and temperatures falling as low as minus 20 degrees. Furthermore, as the north Atlantic currents form just one element of a global system of ocean circulation known as the ocean conveyor, the effects may stretch much further afield, leading—for example—to failure of the Asian monsoon and widespread drought.

So far, so bad. Is there nothing we can do to avoid such global catastrophes, or at the very least mitigate the worst of their effects? With respect to any major weakening of the Gulf stream, prevention is not now an option, however much we cut back on greenhouse gas emissions. If it happens, we must adapt as best we can, modifying our energy, transport, agriculture and health policies to cope with bitter winters and shorter growing seasons.

Surprisingly, combating the asteroid impact threat may prove easier, and given advance warning of perhaps a decade or two, we already have the technology to give an asteroid with our name on it the small nudge required to turn a possible collision into a certain miss. Before the Shoemaker-Levy collision with Jupiter in 1994, the number of scientists involved in locating and tracking potential earth "impactors" was described by one as "smaller than the workforce of a McDonald's restaurant." But boosted by higher funding, mainly from Nasa, a number of sky surveys have now identified more than 3,000 so-called near-earth asteroids (NEAs), including more than 750 with diameters over 1km. By 2008, Nasa expects 90 per cent or so of all NEAs more than 1km across to have been identified and their orbits projected far into the future. A group established by Apollo 9 astronaut Rusty Schweickart is planning to test a system for nudging threatening asteroids off course, and hopes to demonstrate its capability in the next 15 years.

Coping with volcanic super-eruptions and ocean-wide mega-tsunamis is more problematic. Despite suggestions to the contrary by enthusiastic amateurs, we cannot burrow into a volcano primed for a super-eruption to allow it to "let off steam." The energies stored within are simply too great. Neither can we remove the unstable flank of La Palma's volcano bit by bit to prevent it collapsing into the north Atlantic. I once calculated, in response to just such a suggestion, that assuming an open truck could carry away 10 cubic metres of rock at a time, it would require 15-50bn journeys to remove the unstable mass. Even with a loaded truck leaving every minute of every day, this would take between 10m and 35m years, and doesn't even take into account the fact that a good percentage of the slide is underwater.

Managing the next great volcanic blast or ocean-wide mega-tsunami clearly requires some advance warning. Although there are around 3,000 active and potentially active volcanoes, we are only monitoring 100 or so. The unstable flank of the Cumbre Vieje volcano has no surveillance at all. If we are to cope with the worst excesses of a volcanic winter, we will need years to prepare: stockpiling food and establishing frameworks for maintaining social order during the period of disruption. We would not be able to prevent the massive damage to coastal communities caused by a future collapse of the Cumbre Vieja, but monitoring of the slide may provide the vital warning needed for mass evacuation.

In broader terms, we need to recognise that global catastrophes are certain to feature in our future. At the very least we need to be aware of the nature of the threats and their potential ramifications, and we need to plan for how—both as individual states and as a global community—we are going to cope. We need to look at ways of making the planetary economy more resilient to the financial mayhem that would arise from a Tokyo quake, and we need to develop a framework for developed countries to help low and medium-income states through the worst of any future global catastrophe. The Asian tsunami has provided us with a taste of what we might have to face when the next gee-gee occurs. It has also opened a window of opportunity for national governments and international agencies to think about how they will handle something far bigger. Memories are short, however. Other priorities are crowding in and the window is already starting to close. We must make the most of it before it is too late.