(7) Global Catastrophes

Will a volcanic eruption destroy humanity? Scientists warn that world must begin preparing for explosive global catastrophe



Charles Dickens, white Christmases, and the Little Ice Age

It seems likely that almost everyone who has read this far is familiar with the Ice Age, but what about the Little Ice Age? This is the term used by climatologists to describe a cold period that lasted from at least 1450 - and possibly 1200 - until between 1850 and the start of the twentieth century. Over this period, glaciers advanced rapidly, engulfing alpine villages, and sea ice in the North Atlantic severely disrupted the fishing industries of Iceland and Scandinavia. Eskimos are alleged to have paddled as far south as Scotland, while the once thriving Viking community in Greenland was cut off and never heard from again.

Annual mean temperatures in England during the late seventeenth century were almost one degree Celsius lower than for the period 1920-1960, leading to bitter, icy winters in which ‘frost fair’ carnivals were held regularly on a frozen River Thames and snowfall was common. The snowy winters described in many of the works of Charles Dickens may well be a reflection of this colder climatic phase, and they have certainly done much to nurture our constant expectation of - and wish for - an old-fashioned ‘white Christmas’.

Just what was the cause of the Little Ice Age remains a matter of intense debate. Clearly, however, as most of the cold snap occurred prior to the industrial revolution there can be no question of human activities having played a role. Despite this, it is vital that we understand the Little Ice Age in the context of global warming, if for no other reason than if we don’t appreciate the natural variations of our planet’s recent climate it is well nigh impossible to unravel the effects arising from human activities. In fact, the Little Ice Age was not the only significant departure from the climatic norm – if there is such a thing – in historical times. Immediately prior to this cold snap, Europe, at least, was reveling in the so-called Medieval Warm Period. This time of climate amelioration, between about 1000 and 1300 ad, saw grapes grown in north of England - as they are again in today’s warming climate – while the Norse settlers of Greenland were able to graze their livestock in areas that until recently were buried beneath ice.

The emergence of the world from the Little Ice

Age towards the end of the nineteenth century, coincident with the acceleration of industrialization on a global scale, has contributed in no small way to current arguments on the causes of contemporary warming. As I noted in the last chapter, the overwhelming scientific consensus views global warming as being anthropogenic in nature, but some still hold out for an entirely natural cause; seeing the current warming in terms of the planet coming out of the Little Ice Age and entering another warm period analogous to the Medieval Warm Period. Although the available evidence irresistibly supports a human cause rather than a purely natural warming, there can be no doubt that the impact of human activities is superimposed upon a natural variability that in the recent past has resulted in significant climate change. But what is the cause? One of the most likely culprits is the Sun, whose output continues to vary on time scales ranging from 100 to 10,000 years. For example, the two coldest phases within the Little Ice Age corresponded closely with two periods of apparently reduced solar activity; the Spörer Minimum between 1400 and 1510 ad and the Maunder Minimum from 1645 to 1715. During these times, virtually no sunspots were visible and auroras were almost non-existent, suggesting a fall in the rate of bombardment of the Earth by solar radiation. While solar physicists estimate that the Sun during the Maunder Minimum may have been just a quarter of one per cent dimmer than it is today, this might have been sufficient to cause the observed cooling. Other factors may also have made a contribution, however, and a recent theory has given elevated levels of explosive volcanic activity at the time – including the great 1815 eruption of Indonesia’s Tambora volcano - at least a supporting role in the Little Ice Age cooling. Large volcanic explosions are particularly effective at injecting substantial volumes of sulphur dioxide and other sulphur gases into the stratosphere – that part of the atmosphere above 10 kilometers or so. Here they mix with atmospheric water vapor to form a fine mist of sulphuric acid that cuts out a proportion of incoming solar radiation and leads to a cooling of the troposphere (the lower atmosphere) and surface.

A very British ice age

The more we learn about past climate change, the more it becomes apparent that dramatic variations can occur with extraordinary rapidity. The return - possibly within a few decades - from increasingly clement conditions to the bitter cold of the Younger Dryas, 12,800 years ago, demonstrates this, as does the similarly rapid transition from the Medieval Warm Period to the Little Ice Age. Equally disturbing is the tendency for the climate to flip suddenly from one extreme to another when it is under particular stress, as it is at the moment from anthropogenic warming. Is there any way that current global warming can actually bring a return to colder conditions? While this would seem to be counter-intuitive, there is increasing evidence that this may well happen - at least as far as the UK and northwest Europe, and perhaps the entire North Atlantic region, are concerned. The only reason why it is possible for tropical palms to thrive in western Ireland and southwest England is because the Gulf Stream carries northwards warm water from the Caribbean. As a result, the UK and Ireland are substantially warmer than comparable latitudes in eastern Canada, which have to put up with sub-Arctic conditions.

But what would happen if the supply of warm water from the south were shut down? It is highly likely that the British climate – and perhaps that of much of northwest Europe - would become bitterly cold, and some have suggested it could even rival that of Svalbard (formerly Spitsbergen), the ice-shrouded islands off east Greenland where the polar bear is king. In a recent study, the UK Met Office simulated just what might happen if the Gulf Stream were to shut down. In the following decade, the entire northern hemisphere would cool, with the effect strongest around the North Atlantic. In the UK, bitter winters arrive within a few years of shutdown, with temperatures plunging to -10 degrees Celsius and below. One of the ways of weakening or shutting down the Gulf Stream is by short-circuiting it through releasing huge quantities of cold fresh water into the North Atlantic, and this is just what is predicted by a number of different climate models developed to look at the impact of global warming in this century and beyond. The most recent forecasts suggest that a 2-3 degrees Celsius temperature rise, which is almost certain by 2100 if not well before, will result in a 45 per cent probability of a dramatic slowdown or shutdown of the Gulf Stream. In little more than half a century, then, the seas around the UK could be significantly cooler, altering prevailing weather patterns and bringing colder conditions to the region. While the rest of the world roasts, the North Atlantic region could conceivably start to slide into a freeze very much bitterer than the Little Ice Age. And this might be just the start. The knock-on effects of changes to the ocean circulation in the North Atlantic may spread, overwhelming the current warming and bringing a return of the ice across the northern hemisphere. In conclusion, then, let’s take a look at prospects for the return of the Ice Age and the role mankind may already be playing in its reappearance.

Out of the frying pan into the fridge

In terms of the Milankovitch Cycles, our planet is already primed for the end of the current interglacial period and a return to full Ice Age conditions. Some believe that all that is needed is a trigger; a sudden shock to the system that will knock the climate out of equilibrium and set it wobbling before it collapses into an altogether less friendly state. It is questionable whether global warming can provide a shock of the appropriate magnitude, but new research is leading to increasing concern that the legacy of warming today may be freezing tomorrow. Once again, the key seems to lie in the ocean circulation system of the North Atlantic, which appears to be closely bound up with past switches from warm to cold episodes and vice versa. The Gulf Stream that most people are familiar with is actually only one part of a system of currents known by a variety of names, of which the Atlantic Overturning Circulation is probably the most revealing. As the warm, salty waters of the Gulf Stream head northwards they cool and consequently become denser. As a result, by the time they have reached the Arctic Ocean they have sunk to form a cold, deep-ocean current that heads south once more to join the wider system of ocean currents known as the Global Conveyor.

It now looks as if the operation of the Atlantic Overturning Circulation is seriously disrupted whenever cold conditions grip the northern hemisphere. During the Younger Dryas, for example, the circulation appears to have been severely reduced, lowering north European temperatures by as much as 10 degrees Celsius. Recent evidence on ocean temperatures and salinities, gleaned from studies of the shells of tiny marine organisms known as foraminifera, also points to a much weaker Gulf Stream at the height of the last Ice Age some 20,000 years ago. Then, it seems, the Gulf Stream had only two-thirds of its current strength, suggesting that the entire circulation system was comparably weakened. The question is, did this weakening have a role to play in the triggering of the last Ice Age, or was it merely a consequence? No one really knows, but there is a general feeling that a weakening of the circulation results in much colder conditions in the northern hemisphere and that such a weakening appears to be associated with large influxes of cold water into the North Atlantic. Due to melting of Arctic sea ice and the Greenland Ice Cap, this is just what is predicted to happen in the next few centuries.

During the Younger Dryas, 12,800 years ago, the release of huge quantities of water from glacial lakes resulted only in a short-lived cold snap of a thousand years or so. Then, however, the Earth was at a point in the pattern of Milankovitch Cycles when temperatures were on the way up. Now, we are poised at the transition between the present interglacial and the next Ice Age, and without the polluting effects of human activities temperatures could be expected to be on the way down. It is not unreasonable to at least consider, then, that the influx of cold, fresh water into the Arctic Ocean may trigger not just a brief period of cold in northwest Europe, but a new Ice Age affecting the entire northern hemisphere. And we may not have too long to wait. In the 1990s, US climate modelers Ronald Stouffer and Alex Hall ran a comprehensive computer model of the Earth’s climate system for almost a decade to find out what it had in store for us in the next few millennia. What they discovered was seriously disturbing. The model predicts that, in around 3,000 years’ time, intense westerly winds over Greenland will help to push large quantities of fresh Arctic water into the North Atlantic. Because of its low density, this bitterly cold water will remain at the surface, cooling the air above, and creating a low-pressure weather system that will reinforce the westward gales through a positive feedback mechanism. The effect is forecast to cool the North Atlantic by up to 3 degrees Celsius and also to weaken the Atlantic Overturning Circulation, bringing colder conditions to northwest Europe. In the model, the chilly scenario only persists for 40 years or so, but the authors are concerned that if global warming promotes the melting of Greenland ice on a grand scale, this added input of cold water might amplify a brief regional cooling into a widespread and persistent freeze. Even more worryingly, the first signs of the coming chill may already have been detected, with recent measurements revealing that an important current running south between Scotland and the Faeroe Islands has slowed by around 20 per cent in the past 50 years. Could this be the first evidence of the breakdown of the Atlantic Overturning Circulation and the slow but steady deterioration of the climate into bitter cold?

One of the best means of illustrating just what a bad time this is for us to be experimenting with the global climate is by comparing the temperature profile of this interglacial period with that of the last. It is rather sobering to see that the natural temperature trend is already downwards, and in fact this fall has been going on for several thousand years. At the moment, it looks as if the downward trend is being reversed by anthropogenic warming, and without greenhouse gas emissions the world would be around 3 degrees colder in around 8,000 years’ time - well on its way to the next Ice Age. Although fending off the chill at the moment, however, the impact of global warming on the Atlantic Overturning Circulation might well ultimately accelerate the arrival of the next Ice Age. By now I hope to have convinced you that it is at least feasible for the current global warming to trigger colder conditions, and that this may be the result of the continued and unmitigated emission of greenhouse gases. So what happens if the world comes to its senses and we cut back significantly on the amount of carbon dioxide and other gases that we pump into the atmosphere? Well, you have seen the graphs – the ice will get us anyway. It is simply just a matter of whether we want to take the icy plunge on its own or spend some time baking in the sauna first. Whichever choice we make, there is no denying that life for our descendants will become increasingly hard, should the ice return. Life in Europe, North America, Russia, and central and eastern Asia will be pretty much impossible, engendering mass migrations southwards accompanied undoubtedly by bloody wars fought over living space and resources. The climate of Ice Age Earth is simply not suited to sustaining a population totaling 8–10 billion, or thereabouts, and widespread famine alongside civil strife is certain to lead to a severe culling of the human population. There is no question that our race will survive, as it did the last time that the ice left its polar fastnesses, but it is likely to be but a pale shadow of its former self.

Facts to fret over

• Between 800 and 600 million years ago, the Earth was a frozen snowball covered with ice a kilometer or more thick.

• Just 600 human generations have passed since the end of the last Ice Age.

• At the height of the last Ice Age, temperatures in the UK were 15-20 degrees Celsius lower than they are now, and over much of North America, more than 25 degrees Celsius lower.

• Sea levels have risen by over 120 meters since the ice started to retreat around 18,000 years ago.

• A temperature rise of just 2-3 degrees Celsius – which is virtually certain before 2100 - could result in a 45 per cent probability of a dramatic slowdown or shutdown of the Gulf Stream.

• An Atlantic current flowing between Scotland and the Faeroe Islands has weakened by 20 per cent in the last 50 years.

• Without greenhouse gas emissions, the world could be 3 degrees Celsius colder in 8,000 years.

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