The cost of catastrophes

Five Global Catastrophes That Could Happen Tomorrow

How would you like to die?

If supporters of the Taurid Complex model are to be believed, and I should say now that their views remain very much in the minority amongst advocates of the impact threat, then we may have only another thousand years or so before a series of blinding flashes and crashing sonic booms heralds the arrival of the next batch of fragmented comet. Alternatively, we could face oblivion tomorrow or have to wait 100,000 or more years before a city is obliterated or a thousand millennia before the world plunges into cosmic winter beneath a cloud of pulverized rock. But whenever the skies next fall, how will it affect us? This will depend on three things: (i) the size of the object, (ii) how quickly it is travelling, and (iii) whether it hits the land or the ocean. Everything else being equal, the larger the impactor the more devastating and widespread will be its effects. To reiterate, a body in the 50-100-metre size range carries enough destructive power to wipe out a major city or a small European country or US state. The level and extent of associated devastation will increase progressively with larger impactors until the critical 2-kilometre size is reached. In addition to causing appalling destruction on a regional or sub-continental scale, the arrival of an object of this size will affect the entire planet through engendering a period of dramatic cooling and reduced plant growth. For impactors larger than 2 kilometres the effects on the planet’s ecosystems become progressively more severe until mass extinctions wipe out a significant percentage of all species. The 10-kilometre object that struck the Earth off the Mexican coast at the end of the Cretaceous period, 65 million years ago, not only finished off the dinosaurs but also two-thirds of all species living at the time. Even more disturbingly, there is evidence of a major impact event at the end of the Permian period some 250 million years ago that left fewer than 10 percent of species alive. In all, at least 7 out of 25 major extinctions in the geological record have been linked with evidence for large impacts, although as I mentioned in the previous chapter there is a school of thought that plays down the environmental effects of impact events and prefers to implicate huge outpourings of basalt lava in the great extinctions of the past.

The destructive potential of a chunk of rock hurtling into the Earth is directly related to the kinetic energy it carries, and this reflects not only the size of the object but also the velocity of the collision. Because they travel substantially faster, therefore, impacts by so-called long-period comets, whose orbits carry them far out into interstellar space, cause more destruction than either NEAs or local comets that follow orbits confined to the heart of the solar system. Both the nature and scale of devastation also depend upon whether the impactor hits the land or the sea. Two-thirds of our planet’s surface is covered by water, so statistically, this is where the majority of asteroids and comets strike. In such cases, the amount of pulverized rock hurled into the atmosphere might be reduced, compared to a land collision. However, this small benefit is likely to be at least partly countered by the formation of giant tsunamis capable of wreaking havoc across an entire ocean basin. Furthermore, the gigantic quantities of water and salt injected into the atmosphere may severely affect the climate and even temporarily wipe out our protective ozone shield. Most of the evidence for the environmental effects of impacts comes from studies of just two events, one small and the other enormous.

At the low end of the scale, in 1908 a small asteroid, estimated at around 50 metres across, penetrated the Earth’s atmosphere and exploded less than 10 kilometres above the surface of Siberia in a region known as Tunguska. This huge blast, which expended roughly the energy equivalent of 800 Hiroshima atomic bombs, was heard over an area four times the size of the UK and flattened over 2,000 square kilometres of full-grown forest. The blast registered on seismographs thousands of kilometres distant and the atmospheric shock wave was picked up by barographs time and again as it travelled three times around the planet before dissipating. The gas and dust generated by the explosion led to exceptionally bright night skies over Europe, sufficient – according to one contemporary report – to allow cricket to be played in London after midnight. Because of its inaccessibility, the first Russian expedition did not reach Tunguska until a quarter of a century later, when Leonid Kulik and his team were perplexed by the absence of the huge crater they were expecting. Instead, they found a circular patch of badly charred and flattened trees 60 kilometres across, formed by the airburst as the rock disintegrated explosively due to the huge stresses caused by entry into the atmosphere. As the region was sparsely inhabited, casualties due to the impact were small, with perhaps a few killed and up to 20 injured, although reports are understandably sketchy. Four hours later, however, and the Earth would have rotated sufficiently to bring the great city of St Petersburg into the asteroid’s range and the result would have been catastrophic.

The Tunguska event pales into insignificance when compared to what happened off the coast of Mexico’s Yucatan Peninsula 65 million years earlier. Here a 10-kilometre asteroid or comet – its exact nature is uncertain – crashed into the sea and changed our world forever. Within microseconds, an unimaginable explosion released as much energy as billions of Hiroshima bombs detonated simultaneously, creating a titanic fireball hotter than the Sun that vaporized the ocean and excavated a crater 180 kilometres across in the crust beneath. Shock waves blasted upwards, tearing the atmosphere apart and expelling over a hundred trillion tonnes of molten rock into space, later to fall across the globe. Almost immediately an area bigger than Europe would have been flattened and scoured for virtually all life, while massive earthquakes rocked the planet. The atmosphere would have howled and screamed as hypercanes five times more powerful than the strongest hurricane ripped the landscape apart, joining forces with huge tsunamis to batter coastlines many thousands of kilometres distant.

Even worse was to follow. As the rock blasted into space began to rain down across the entire planet, so the heat generated by its re-entry into the atmosphere irradiated the surface, roasting animals alive as effectively as an oven grill, and starting great conflagrations that laid waste the world’s forests and grasslands and turned fully a quarter of all living material to ashes. Even once the atmosphere and oceans had settled down, the crust had stopped shuddering, and the bombardment of debris from space had ceased, more was to come. In the following weeks, smoke and dust in the atmosphere blotted out the Sun and brought temperatures plunging by as much as 15 degrees Celsius. In the growing gloom and bitter cold, the surviving plant life wilted and died while those herbivorous dinosaurs that remained slowly starved. Life in the oceans fared little better as poisons from the global wildfires and acid rain from the huge quantities of sulphur injected into the atmosphere from rocks at the site of the impact poured into the oceans, wiping out three-quarters of all marine life. After years of freezing conditions the gloom following the so-called Chicxulub impact would eventually have lifted, only to reveal a terrible Sun blazing through the tatters of an ozone layer torn apart by the chemical action of nitrous oxides concocted in the impact fireball: an ultraviolet spring – hard on the heels of the cosmic winter – that fried many of the remaining species struggling precariously to hang on to life. So enormously was the natural balance of the Earth upset that according to some it might have taken hundreds of thousands of years for the post-Chicxulub Earth to return to what passes for normal. When it did the age of the great reptiles was finally over, leaving the field to the primitive mammals – our distant ancestors – and opening an evolutionary trail that culminated in the rise and rise of the human race. But could we go the same way? To assess the chances, let me look a little more closely at the destructive power of an impact event.

At Tunguska, destruction of the forests resulted partly from the great heat generated by the explosion, but mainly from the blast wave that literally pushed the trees over and flattened them against the ground. The strength of this blast wave depends upon what is called the peak overpressure, that is the difference between ambient pressure and the pressure of the blast wave. In order to cause severe destruction, this needs to exceed 4 pounds per square inch, an overpressure that results in wind speeds that are over twice the force of those found in a typical hurricane. Even though tiny compared with, say, the land area of London, the enormous overpressures generated by a 50-metre object exploding low overhead would cause damage comparable with the detonation of a very large nuclear device, obliterating almost everything within the city’s orbital motorway. Increase the size of the impactor and things get very much worse. An asteroid just 250 metres across would be sufficiently massive to penetrate the atmosphere; blasting a crater 5 kilometres across and devastating an area of around 10,000 square kilometres – that is about the size of the English county of Kent.

Raise the size of the asteroid again, to 650 metres, and the area of devastation increases to 100,000 square kilometres – about the size of the US state of South Carolina. Terrible as this all sounds, however, even this would be insufficient to affect the entire planet. In order to do this, an impactor has to be at least 1.5 kilometres across, if it is one of the speedier comets, or 2 kilometres in diameter if it is one of the slower asteroids. A collision with one of these objects would generate a blast equivalent to 100,000 million tonnes of TNT, which would obliterate an area  500 kilometres across – say the size of England – and immediately kill perhaps tens of millions of people, depending upon the location of the impact.

The real problems for the rest of the world would start soon after as dust in the atmosphere began to darken the skies and reduce the level of sunlight reaching the Earth’s surface. By comparison with the huge Chicxulub impact, it is certain that this would result in a dramatic lowering of global temperatures but there is no consensus on just how bad this would be. The chances are, however, that an impact of this size would result in appalling weather conditions and crop failures at least as severe as those of the ‘Year Without a Summer’, which followed the 1815 eruption of Indonesia’s Tambora volcano. As mentioned in the last chapter, with even developed countries holding sufficient food to feed their populations for only a month or so, large-scale crop failures across the planet would undoubtedly have serious implications. Rationing, at the very least, is likely to be the result, with a worst case scenario seeing widespread disruption of the social and economic fabric of developed nations. In the developing world, where subsistence farming remains very much the norm, widespread failure of the harvests could be expected to translate rapidly into famine on a biblical scale. Some researchers forecast that as many as a quarter of the world’s population could succumb to a deteriorating climate following an impact of a 2-kilometre object. Anything much bigger and photosynthesis stops completely. Once this happens the issue is not how many people will die but whether the human race will survive. One estimate proposes that the impact of an object just 4 kilometres across will inject sufficient quantities of dust and debris into the atmosphere to reduce light levels below those required for photosynthesis.

Because we still don’t know how many threatening objects there are out there nor whether they come in bursts, it is almost impossible to say when the Earth will be struck by an asteroid or comet that will bring to an end the world as we know it. Impact events on the scale of the Chicxulub dinosaur-killer only happen every several tens of millions of years, so in any single year, the chances of such an impact are tiny. Any optimism is, however, tempered by the fact that – should the Shiva hypothesis be true – the next swarm of Oort Cloud comets could even now be speeding towards the inner solar system.

Failing this, we may have only another thousand years to wait until the return of the dense part of the Taurid Complex and another asteroidal assault. Even if it turns out that there is no coherence in the timing of impact events, there is statistically no reason why we cannot be hit next year by an undiscovered NEA or by a long-period comet that has never before visited the inner solar system. Small impactors on the Tunguska scale pose less of a threat because their destructive footprints are tiny compared to the surface area of the Earth. It would be very bad luck if one of these struck an urban area, and most will fall into the sea. Although this might seem a good thing, a larger object striking the ocean would be very bad news indeed. A 500-metre rock landing in the Pacific Basin, for example, would generate gigantic tsunamis that would cause massive damage to every coastal city in the hemisphere within 20 hours or so. The chances of this happening are actually quite high – about 1 percent in the next 100 years – and the death toll could be in the tens of millions if not higher.

The most recent estimate of the frequency of 1-kilometre impacts is 600,000 years, but the youngest impact crater produced by an object of this size is almost a million years old. Of course, there could have been several large impacts since, which either occurred in the sea or have not yet been located on land. Fair enough, you might say, the threat is clearly out there, but is there anything on the horizon? Actually, there is. A dozen or so asteroids – mostly quite small – could feasibly collide with the Earth before 2100. Realistically, however, this is not very likely as the probabilities involved are not much greater than 1 in 10,000 – although bear in mind that these are pretty good odds. If this was the probability of winning the lottery then my local agent would be getting considerably more of my business. Most worrying is the 320-metre Near Earth Asteroid, MN4, discovered late in 2004 and recently named Apophis, the Greek name for the Egyptian God Apep – the destroyer. At one point, the probability of Apophis striking the Earth on 13 April 2029 was thought to be as high as 1 in 37. Now, to everyone’s relief, those odds have increased to 1 in 8,000. Again, these may sound very long odds, but they are actually only 80 times greater than those offered during summer 2001 for England beating Germany 5–1 at football. A few years ago, scientists came up with an index – known as the Torino Scale – to measure the impact threat, and so far Apophis is the first object to register and sustain a value greater than zero. At present, it scores a 1 on the scale – defined as ‘an event meriting careful monitoring’. The object is the focus of considerable attention as efforts continue to better constrain its orbit, and it is perfectly possible – as we find out more – than it could rise to 1 on the Torino Scale, becoming an ‘event meriting concern’. It is very unlikely, however, to go any higher, and let’s hope that many years elapse before we encounter the first category 10 event – defined as ‘a certain collision with global consequences’. Given sufficient warning, we might be able to nudge an asteroid out of the Earth’s way but due to its size, high velocity, and sudden appearance, we could do little about a new comet heading in our direction.