Kitabı oku: «Six Degrees: Our Future on a Hotter Planet»

Table of Contents

Epigraph

Introduction

Part 1 - 1°

Chapter 1 - One Degree

Part 2 - 2°

Chapter 2 - Two Degrees

Part 3 - 3°

Chapter 3 - Three Degrees

Part 4 - 4°

Chapter 4 - Four Degrees

Part 5 - 5°

Chapter 5 - Five Degrees

Part 6 - 6°

Chapter 6 - Six Degrees

Part 7 - 7

Chapter 7 - Choosing Our Future

Notes

Index

Acknowledgements

Also by Mark Lynas

Copyright

About the Publisher

INTRODUCTION

The knock on the door came at night. In the darkness I could see two yellow jackets over black uniforms-the police. They were going door to door, the officers explained, to warn people in the area of the imminent risk of flooding. They handed over a photocopied leaflet, advising that we prepare to turn off the power and move all valuables upstairs, and were gone.

The rain had come two days earlier. It poured with torrential force for most of the day, accompanied by vivid flashes of lightning and intermittent claps of thunder. Roads were awash as flash floods swept off fields. Within hours, the rail link north was cut, and Oxford-like many other towns in the Midlands and southern England-was marooned. Four days later the waters were still rising, as a flood crest surged down the river Thames from more heavily inundated areas upstream. Turning on the television news I saw the pretty cathedral town of Tewkesbury turned into an island, both Cheltenham and Gloucester hit by power failures, and schools closed across the entire region. The rising flood swept over a water treatment plant, leaving a quarter of a million people with no drinking water for over a week. Though my own house was not flooded, whilst writing this I can still smell the stench of rotting waterweed left behind by the river on nearby Port Meadow.

The sheer intensity and violence of the rain reminded me of a tropical storm I rode out a few years earlier on the Outer Banks of North Carolina, whilst researching my first book High Tide. There was that same ominous dark quality to the sky, and the rainfall radar on the Meteorological Office website showed the same reds and whites of super-intense precipitation that I had previously witnessed whilst sheltering in the hurricane trackers' van near Cape Hatteras in 2002. Hurricanes generate some of the heaviest rainfall on Earth, and flooding during a hurricane strike is virtually a certainty. As the terrible drama that unfolded in New Orleans when Hurricane Katrina hit in 2005 showed, sometimes this flooding-combined with a monster storm surge-can be deadly.

All these events were windows into a changing world. Global warming is making the hydrological cycle more intense, causing heavier storms and more intense hurricanes to brew up out at sea. Yes, extreme weather has always been with us, but the fact that rising levels of greenhouse gases trap the sun's heat means that more energy is available in the system-so the worst is happening more and more often. The misery suffered in New Orleans three years ago felt to me like an insight into what the twenty-first century may have in store for many more of us, in a thousand locations across the world, as climate change accelerates.

The scenes lingered in my mind even as the city was emptied and the bedraggled survivors of New Orleans and the wider Gulf region were packed off to temporary shelters in Texas and elsewhere, where half a million still remain at the time of writing: arguably the first climate refugees, displaced permanently from their homes. I kept wondering: where next? What will happen as the world warms bit by bit? With up to six degrees Celsius of global warming on the cards over the next hundred years, according to the Intergovernmental Panel on Climate Change (IPCC), what will happen to our coasts, our towns, our forests, our rivers, our croplands and our mountains? Will we all, as some environmentalists suggest, be reduced to eking out a living from the shattered remains of civilisation in Arctic refuges, or will life go on much as before-if only a little warmer?

As I pondered these questions, I had already begun to sift through the latest scientific literature on global warming. I knew from earlier research for High Tide that scientists have now made hundreds of projections-mostly based on complex computer models-of how future global warming will affect everything from maize crops in Tanzania to snowfall in the Alps. Occasionally a particularly striking study makes headlines in the newspapers, but the vast majority of these forecasts are buried in obscure specialist journals, destined to be read only by other climatologists. Most of these journals are taken by Oxford University's Radcliffe Science Library, where they sit-undisturbed for weeks or even years on their dimly lit shelves-just a mile or so down the road from my own house. I realised that it was almost as if I had a Delphic Oracle in my back garden or Nostradamus living next door-except that these scientific prophecies were already coming true.

Earlier that year I had begun to make a daily pilgrimage down to the Radcliffe Science Library basement with my laptop, where as the weeks passed by I trawled through tens of thousands of scientific papers. Seasons came and went, and I barely noticed. Each relevant article, I slotted into a spreadsheet-papers about two degrees of global warming went into the two degrees slot, papers about five degrees of global warming went into the five degrees slot, and so on. Not all were computer model projections-some of the most interesting material came from palaeoclimate studies, investigations of how variations in temperatures have affected the planet during previous global warming events in prehistory. These records of past greenhouse episodes, I realised, could be analogues for the future: and they too slotted into my six degrees table according to the temperatures of the climatic periods they represented.

At the end, I found I had something truly unique: a degree-by-degree guide to our planet's future. And so, based on this raw material, the book gradually took shape: my first chapter included all the global warming impacts I could find associated with a one-degree rise in temperature, my second chapter covered two degrees, my third chapter covered three degrees … and on up the scale to six degrees-the worst-case scientific scenario. No scientist and no journalist has ever undertaken this work before with anything like this attention to detail, and never before has so much of this information been presented comprehensibly to a general audience in book form.

As the work emerged, I felt a nagging suspicion that maybe I should be keeping it all secret. Six Degrees was beginning to feel like a survival manual, full of indications about which parts of the globe might need to be abandoned, and which would be most likely to remain habitable. Maybe I should be sharing this information only with my family and friends, to give those people closest to me a quiet heads-up? Or perhaps I should get it out as widely as possible, as a sort of cautionary tale, to convince people to campaign for rapid emissions cuts and avoid the worst-case scenarios before it is too late?

Obviously I chose the second, more optimistic course. But a related question continued to bug me as I did early public presentations of Six Degrees material, particularly when I overheard a conversation in the toilets after one event in which an audience member apologised to another for dragging them out to something so depressing. I was truly shocked. Depressing? It had honestly never occurred to me that Six Degrees might be depressing. Yes, the impacts presented are terrifying-but they are also, in the main, still avoidable. Getting depressed about the situation now is like sitting inert in your living room and watching the kitchen catch fire, and then getting more and more miserable as the fire spreads throughout the house-rather than grabbing an extinguisher and dousing the flames.

It also dawned on me gradually when I tried to explain the book to non-specialists that most ordinary people have not got the slightest idea what two, four or six degrees of average warming actually means in reality. These still sound like very small changes when the mercury swings by fifteen degrees between night and day. To most of us, if Thursday is six degrees warmer than Wednesday, it doesn't mean the end of the world, it means we can leave the overcoat at home. Such are the vagaries of everyday weather. But six degrees of global average change is an entirely different prospect.

Consider this: 18,000 years ago, during the deepest freeze of the last ice age, global temperatures were about six degrees colder than today. In that frigid climate, ice sheets stretched across North America from sea to shining sea. As glacial grooves in the rocks in Central Park attest, New York was buried under a thick slab of ice, more than a mile deep as it stretched into the heart of the continent. Northern New Jersey was buried, as was all the Great Lakes area, and almost the entirety of Canada. Further south, the agricultural heartland of states like Missouri and Iowa would have been freezing tundra, blasted by dust-laden winds sweeping down from the ice cap, and underlain by layers of solid permafrost. During the ice age, humans were displaced far to the south, where places that are now subtropical, like Florida and California, maintained a temperate climate.

In addition, temperature swings were astonishingly rapid-several degrees in the space of a decade as the climate warmed and then cooled again. At one point, about 70,000 years ago, a huge supervolcano eruption in Indonesia blew thousands of cubic kilometres of dust and sulphur into the atmosphere, cutting off the Sun's heat and causing global temperatures to plummet. Humans were nearly wiped out in the ensuing ‘nuclear’ winter: the entire global human population crashed to somewhere between 15,000 and 40,000 individuals, a survival bottleneck which is still written in the genes of every human alive today. By implication, if six degrees of cooling was enough to nearly wipe us out in the past, might six degrees of warming have a similar effect in the future? That is the question this book seeks to answer.

Back in the summer of 2005, as I began my journey into humanity's likely future, I felt like Dante at the gates of the Inferno-privileged to see what few others have laid eyes upon, but also deeply worried by the horrors that seemed to lie ahead. Just as the poet Virgil was Dante's guide as he set forth into the Inferno, my guides are the many talented and passionate scientists who conducted the original research studies on which this book is based. I offer them my thanks, and hope they feel well represented by what follows.

‘Set out then, for one will prompts us both.

You are my leader, you my lord and master,’

I said to him, and when he moved ahead

I entered on the deep and savage way.

A technical note

As befits the task of any popular science writer, I have tried to make each case study come alive as much as possible without losing the rigour of the original document. Where the science itself has evolved through the years, I have tried to work this into the story. There were drawbacks of course: almost all the studies use different models, each model employing different underlying assumptions, so comparing them can sometimes be rather like comparing chalk and cheese. Each study also contains uncertainties, often expressed in quantitative terms-such is the nature of good science-and carefully weighed, thoughtful statements by the authors which cannot always be accurately reflected in a broad-brush, generalist approach such as this. I leave readers with queries about any of the information presented to follow up references and judge the original work for themselves. Do not complain to me either if you have doubts about the methodologies employed by the original studies: I am not a climatologist, I am merely the interpreter.

I might also add at this point, for the benefit of any readers who feel somewhat out of depth with the generally ‘scienticised’ nature of the climate change debate, a very general note of background on global warming. Essentially, this term (which I use interchangeably with ‘climate change’, although technically they do mean slightly different things) refers to the increase in global atmospheric temperatures as a result of increasing concentrations of greenhouse gases in the air around us. That greenhouse gases have a warming effect, rather like an extra blanket around the globe, is indisputable, and has been established physics for over a hundred years. These gases cause a ‘greenhouse effect’ because they are opaque to long-wave infrared radiation: heat coming in from the Sun is short-wave, and so passes straight through, but when this heat is re-radiated by the Earth, its wavelength is longer, and some is trapped by the gases-just as glass in a greenhouse also traps heat. If there were no greenhouse gases at all in the atmosphere, the Earth's average temperature would be about —18°C.

Since the beginning of the Industrial Revolution, concentrations of the principal greenhouse gas, carbon dioxide (CO₂), have risen by a third, whilst those of methane-another potent greenhouse gas-have doubled. Although there have been fluctuations between the decades, global temperatures have also risen in the last 150 years by about 0.8°C, and are expected to rise even faster over the next century as CO₂ levels rise further still. Partly these future temperature rises will be the result of emissions already in the past, and partly they will reflect rapid expected rises in greenhouse gas emissions from human activity. That we can avoid higher temperature increases by cutting back emissions is a key point that I seek to illustrate in this book.

Although I have done my best to ensure that the correct impact studies are presented in the correct chapters, there are occasions when the decision about what to put where is somewhat arbitrary. Many-most, in fact-papers do not state the precise global average temperature change that their study refers to, particularly if they are focusing on a regional change. A study on Arctic sea ice, for example, may be based on a range of different future carbon dioxide concentrations, none of which are interpreted as global temperature averages by the authors, leaving me with the difficult choice of estimating which chapter is the best fit. Different studies using the same future CO₂ concentrations do not necessarily share the same temperature projections, moreover: all models have different ‘sensitivities’ to atmospheric greenhouse gas increases, further complicating the procedure. It is important to emphasise, however, that all of the material in this book comes from the peer-reviewed scientific literature-at no point do I base predictions on less reliable sources like newspaper articles or campaign group press releases.

It is also important to note that the temperature scale of this book is based on the IPCC's landmark 1.4 to 5.8°C temperature range, published in its 2001 Third Assessment Report, which gives us predictions of up to six degrees. This is reflected in the structure of the chapters that follow. The three degree chapter, for example, covers global temperatures of 2.1°C to 3°C, whereas the six degree chapter covers 5.1°C to 5.8°C. In February 2007 the IPCC published its Fourth Assessment Report (AR4), which broadened the range of temperature projections for 2100. For the lowest emission scenario, where global greenhouse gas emissions dip sharply, warming by 2100 could be as low as 1.1°C, according to the AR4, whereas for the highest emissions scenario, global warming could reach 6.4°C. In other words, the range is broader, and the worst-case scenario is even more drastic than in the 2001 IPCC report-seven degrees on this book's scale.

The Fourth Assessment Report of the IPCC also surveys in detail the expected impacts of future climate change, covering much of the same territory as this book and referencing many of the same papers. The language is sufficiently non-technical for most laypeople to find it perfectly comprehensible-something of an improvement on previous reports. I would in particular direct interested readers to the Working Group II section of the AR4, in particular a table in the Summary for Policymakers which outlines in a simple degree-by-degree scale the expected impacts of warming from 1 to 5°C. (Why the table does not extend to six degrees, despite this being within the temperature scenario projections given by the IPCC, is not explained.) The full text of all IPCC reports is available on the web at www.ipcc.ch.

An admitted pitfall in choosing a temperature-based structure for this book is that it makes giving dates very hazardous. The world could become two degrees warmer by 2100, for instance, or it could already have hit that level as early as 2030. The speed of warming is crucial in determining the capacity of human civilisation and natural ecosystems to adapt to the changing climate, and readers are urged to bear this in mind. The other option of running through the twenty-first century decade by decade would, I feel, have been even more problematic given that the dates attached to different emissions scenarios and temperature changes are highly uncertain. This book only deals with what scientists call ‘transient’ climate change: because of the thermal inertia of the oceans it will take centuries for temperatures to stabilise at any given concentration of greenhouse gases into a so-called ‘equilibrium’ state.

I have also on occasion explored rather speculatively what the changes projected by today's scientists might mean for society in future. Might China invade Siberia to secure subarctic Lebensraum in a globe where only narrowing zones remain habitable? Might India and Pakistan's struggle over the diminishing headwaters of Himalayan rivers turn nuclear as their people go thirsty? Of course, I would be foolish to expect these predictions to come true in any literal sense-history teaches us that human events are too unpredictable to support such a deterministic approach. But of this I have no doubt: climate change is the canvas on which the history of the twenty-first century will be painted. Forewarned is forearmed.

Onward, then. Let us enter the Inferno together.

1°

1 ONE DEGREE

America's slumbering desert

It would be easy to walk right past them. Not many hikers pass this way, and those that do are unlikely to give a second thought to a few old stumps rooted in the river bed. In any case, this lonely spot, where the West Walker River canyon is at its narrowest as it plunges down the eastern flanks of California's Sierra Nevada, is not a place to linger-the area is notorious for sudden downpours and flash floods. The river runs almost the width of the entire gorge, and there's no place to climb to safety if the heavens open.

But these stumps have a story to tell. Dead trees can talk, in a way. An astute hiker or an observant angler would be puzzled: what are they doing in a river bed, a place now treeless because of the constant flowing water? Investigated by scientists in the early 1990s, the tree stumps were found to be Jeffrey pines-a common enough species for the area, but one that certainly doesn't normally root in rivers. What's more, these trees were old. Very old. Tissue samples revealed that the stumps dated from medieval times, and grew during two specific periods, centred on AD 1112 and 1350.

The mystery deepened when similar old stumps were revealed in Mono Lake, a large saltwater body a hundred kilometres to the south of Walker River, near the state border with Nevada. It's a spectacular location, famous for broad skies and sunsets, with little to interrupt the gently rolling arid landscape other than a few extinct volcanoes. The Mono Lake tree stumps belonged not just to pines, but also to other native species like cottonwoods and sagebrush, all rooted far below current-day natural lake levels and only revealed thanks to water diversion projects that supply far-away Los Angeles. Again, carbon dating revealed the same two time intervals as for the Walker River trees. Clearly, something significant had happened back in medieval times.

More evidence came from deeper in the mountains, hidden in two locations today famous for their giant sequoia groves-Yosemite and Giant Sequoia National Parks. These enormous trees, which in terms of total wood volume stand as the largest living organisms on Earth, are also among the oldest. Some living trees are up to 3,000 years old. And because each annual growth cycle leaves a clear ring, these monumental plants are also an excellent record of past climate. Over a decade ago, scientists sampling wood from dead giant sequoias noticed old fire scars on the edges of some of their rings. These scars were especially frequent during this same medieval period-between AD 1000 and 1300-as the old trees in West Walker River and Mono Lake were growing. Wildfires had raged in both national parks twice as frequently as before, and there can only be one plausible explanation-the woods were tinder-dry.

Raging wildfires, dry rivers and lakes-the pieces of the jigsaw were beginning to make sense. The area we now call California had in medieval times been hit by a mega-drought, lasting at different periods for several decades, and altering both landscape and ecosystems on a scale that dwarfs today's drought episodes. But just how geographically widespread was this event? Evidence from another lake, far away on the Great Plains of North Dakota, provides a partial answer. Moon Lake, like Mono Lake in California, is a closed basin, making it saline. Salinity fluctuates with the climate: in sequences of wet years, more fresh water ends up in the lake and salt levels go down. The converse is also true: in dry years, more water evaporates, leaving a more concentrated salty brine behind. Canadian scientists have now reconstructed long-term records of Moon Lake's saltiness by sampling the remains of tiny algae called diatoms-whose type and number fluctuate with salinity levels-from old lake sediments. Lo and behold, back before AD 1200, a series of epic droughts had swept the Great Plains, the return of which-the scientists agreed-‘would be devastating’.

An insight into the devastating nature of such a drought was gained by a team of biologists working in northern Yellowstone National Park, a good 1,500 kilometres to the south-west of Moon Lake, in Wyoming. They drilled into sediments spilled out by rivers, only to discover a peak in muddy debris flows-the product of flash floods-about 750 years ago. These flash floods had poured off mountainsides denuded of forest cover by frequent fires: so rather oddly, these flood debris flows are actually a classic sign of drought. It appeared that the whole of the western United States had been struck at the same time.

The effect on Native American populations in this pre-Columbian era was indeed devastating. Whole civilisations collapsed, beginning in the Chaco Canyon area of modern-day New Mexico. One of the most advanced societies on the continent at their peak, the Pueblo Indian inhabitants of Chaco Canyon erected the largest stone building on the North American continent before the European invasion, a ‘great house’ four storeys high, with over 600 individual rooms-much of it still standing today. Yet when the big drought came in AD 1130, they were vulnerable-population growth had already diminished the society's ecological base through the overuse of forests and agricultural land. Most people died, whilst the survivors went on to eke out a living in easily defended sites on the tops of steep cliffs. Several locations show evidence of violent conflict-including skulls with cut marks from scalping, skeletons with arrowheads inside the body cavity, and teeth marks from cannibalism.

Indeed, the whole world saw a changing climate in medieval times. The era is commonly termed the ‘Medieval Warm Period’, a time when-so the oft-told story goes-the Vikings colonised Greenland and vineyards flourished in the north of England. Temperatures in the North American interior may have been 1 to 2°C warmer than today, but the idea of a significantly warmer world in the Middle Ages is actually false. Recent research piecing together ‘proxy data’ evidence from corals, ice cores and tree rings across the northern hemisphere demonstrates a much more complicated picture, with the tropics even slightly cooler than now, and different regions warming and then cooling at different times. However small the global shift, the evidence is now overwhelming that what the western US suffered during this period was not a short-term rainfall deficit, but a full-scale mega-drought lasting many decades at least. As recently as 2007 US scientists reported tree-ring studies reconstructing medieval flows in the Colorado River at Lees Ferry, Arizona, showing that the river lost 15 per cent of its water during a major drought in the mid-1100s. For sixty years at a time, the river saw nothing but low flows-none of the floods that normally course down the Colorado arrived to break the dry spell. Indeed, the remarkable coincidence of these dates with evidence from New Mexico suggests that this was the very same drought that finished off the Chaco Canyon Indians.

To see the worst that even such a small change in climate can do, consider that most undramatic of places-Nebraska. This isn't a state that is high up on most tourists' ‘to do’ lists. ‘Hell, I thought I was dead too. Turns out I was just in Nebraska,’ says Gene Hackman in the film Unforgiven. A dreary expanse of impossibly flat plains, Nebraska's main claim to fame is that it is the only American state to have a unicameral legislature. Nebraska is also apparently where the old West begins-local legend in the state capital Lincoln insists that the West begins precisely at the intersection of 13th and O Streets, a spot marked by a red brick star.

But perhaps the most important Nebraska fact is that it sits in the middle of one of the most productive agricultural systems on Earth. Beef and corn dominate the economy, and the Sand Hills region in central Nebraska sports some of the most successful cattle ranching areas in the entire United States.

To the casual visitor, the Sand Hills look green and grassy, and in pre-European times they supported tremendous herds of bison-hence their high productivity for modern-day beef. But, as their name suggests, scratch down a few centimetres and the shallow soil quickly gives way to something rather more ominous: sand. These innocuous-looking hills were once a desert, part of an immense system of sand dunes that spread across thousands of kilometres of the Great Plains, from Texas and Oklahoma in the south, right through Kansas, Colorado, Wyoming, North and South Dakota, to as far north as the Canadian prairie states of Saskatchewan and Manitoba. These sand dune systems are currently ‘stabilised’: covered by a protective layer of vegetation, so not even the strongest winds can shift them. But during the Medieval Warm Period, when temperatures in the Great Plains region may only have been slightly warmer than now, these deserts came alive-and began to march across a fertile landscape which today is a crucial food basket for humanity. This historical evidence indeed suggests that even tiny changes in temperature could tip this whole region back into a hyper-arid state.

People who remember the 1930s Dust Bowl might think they have seen the worst drought nature can offer. In the toughest Dust Bowl years, between 1934 and 1940, millions of acres of Great Plains topsoil blew away in colossal dust storms. One, in May 1934, reached all the way to Chicago, dumping red snow on New England. Hundreds of thousands of people, including 85 per cent of Oklahoma's entire population, left the land and trekked west. All this took only an average 25 per cent reduction in rainfall-enough for ploughed farmland to blow away, but the giant dunes stayed put. What awoke the dunes from their long slumber nearly a thousand years ago was drought on an altogether different scale-with dramatically less rainfall, sustained over decades rather than just years.

In a world which is less than a degree warmer overall, the western United States could once again be plagued by perennial droughts-devastating agriculture and driving out human inhabitants on a scale far larger than the 1930s calamity. Although heavier irrigation might stave off the worst for a while, many of the largest aquifers of fossil water are already overexploited by industrialised agriculture and will not survive for long. As powerful dust and sandstorms turn day into night across thousands of miles of former prairie, farmsteads, roads and even entire towns will find themselves engulfed by blowing sand. New dunes will rise up in places where cattle once grazed and fields of corn once grew. For farmers, there may be little choice other than to abandon agriculture completely over millions of square kilometres of what was once highly productive agricultural land. Food prices internationally would rise, particularly if serious droughts hit other areas simultaneously. And although more southerly parts of the United States are expected to get wetter as the North American monsoon intensifies, residents here may not welcome an influx of several million new people.