Life

by Tim Flannery

  Some idea of the power greenhouse gases have to influence temperature can be gained by examining other planets. The atmosphere of Venus is 98 per cent CO2, and its surface temperature is 477°C. Should CO2 ever reach even one per cent of Earth’s atmosphere, it would—all other things being equal—bring the surface temperature of the planet to boiling point.2

  If you want a visceral understanding of how greenhouse gases work, visit New York in August. It’s a time of year when the heat and humidity leave those who still trudge the streets in a lather. It feels like such an unhealthy heat—trapped in a crowded, built-up environment of concrete, hard edges, parched bitumen and sticky human bodies—that it is almost insupportable. And the worst of it comes at night, when humidity and a thick layer of cloud lock in the heat. I recall tossing and turning between sweat-soaked sheets in a room near the corner of 9th St and Avenue C. As my eyes became gritty and my skin began to crust up, I could smell the grime of the city’s eight million human bodies, along with their refuse and exuviae.

  Suddenly I longed to be in a desert—a dry, clear desert where no matter how hot the day, the clear desert skies of night bring blessed relief. The difference between a desert and New York City at night is a single greenhouse gas—the most powerful of them all—water vapour. Reflecting on the fact that water vapour retains two-thirds of all the heat trapped by all the greenhouse gases, I cursed the clouds overhead.3 But they have a saving grace too. Unlike the other greenhouse gases, water vapour in the form of clouds blocks part of the Sun’s radiation by day, keeping temperatures down.

  It’s testimony to human ignorance that as recently as thirty years ago less than half of the greenhouse gases had been identified and scientists were still divided about whether Earth was warming or cooling. Yet without these molecules our planet would be dead cold—a frigid sphere with an average surface temperature of −20°C. But we have known, and for some time, that these gases have been accumulating.

  CO2 is the most abundant of the ‘trace’ greenhouse gases and it’s produced whenever we burn something or when things decompose. In the 1950s, a climatologist named Charles Keeling climbed Mt Mauna Loa in Hawaii to record CO2 concentrations in the atmosphere. From this he created a graph, known as the Keeling curve, that is one of the most wonderful things I’ve ever seen, for in it you can see our planet breathing. Every northern spring as the sprouting greenery extracts CO2 from the great aerial ocean our Earth begins a great inspiration, which is recorded on Keeling’s graph as a fall in CO2 concentration. Then, in the northern autumn, as decomposition generates CO2, there is an exhalation that enriches the air with the gas. But Keeling’s work revealed another trend. He discovered that each exhalation ended with a little more CO2 in the atmosphere than the one before. This innocent perkiness in the Keeling curve was the first definitive sign that the great aerial ocean might prove to be the Achilles heel of our fossil-fuel-addicted civilisation. Looking back I see that graph as the Silent Spring of climate change, for one need do nothing more than trace its trajectory forward in time to realise that the twenty-first century would see a doubling of CO2 in the atmosphere—from the three parts per 10,000 that existed in the early twentieth century to six. And that has the potential to heat our planet by around 3°C, and perhaps as much as 6°C.

  Born in the Deep-Freeze

  2005

  When an icy mantle gradually crept over much of the northern hemisphere, the greater part of the animal life must have been driven southward, causing a struggle for existence which must have led to the extermination of many forms, and the migration of others into new areas. But these effects must have been greatly multiplied and intensified if, as there is good reason to believe, the glacial epoch itself…consisted of two or more alternations of warm and cold periods.

  ALFRED RUSSEL WALLACE, MAN’S PLACE IN THE UNIVERSE, 1903

  WE HUMAN BEINGS are, as our scientific name Homo sapiens suggests, the ‘thinking creatures’, and we are in the grand scheme of things very recent arrivals. The Period that gave birth to our species is called the Pleistocene, a word that means the most recent times. The ice age in which we evolved covers the last 2.4 million years, and because of its youth much of the evidence concerning it is still fresh. The first of our kind—moderns in every physical and mental respect—strode about Earth around 150,000 years ago in Africa, and there archaeologists have found bones, tools and the remains of ancient repasts. They had evolved from small-brained ancestors known as Homo erectus, who had been in existence for nearly two million years. Perhaps the driving force that changed some of ‘them’ into ‘us’ was the opportunity offered by the rich shorelines of the African rift lakes, or perhaps the bounty of the Agulhas Current that runs along the continent’s southern shores. In such places new foods and challenges may have favoured specialised tool use and selected for high intelligence. Whatever the case, the environment of these distant ancestors was very different from the one we inhabit today, for their world was dominated by an icehouse climate in which the fate of all living things was determined by Milankovich’s cycles. Whenever they conspired to expand the frozen world of the Poles, all over the planet chill winds blew and temperatures plummeted, lakes shrank or filled, bountiful sea currents flowed or slackened, and vegetation and animals alike undertook continent-long migrations.

  The genetic inheritance laid down in this world of ice is still with us. A great reduction in the diversity of our genes, for example, tells of a time around 100,000 years ago when humans were as rare as gorillas are today. We could then so easily have vanished, for 2000 fertile adults were all that stood between us and the eternal oblivion of extinction. But soon thereafter the great celestial cycles altered in ways that favoured our species, and by 60,000 years ago small bands of humans had wandered across the Sinai and out into Europe and Asia. By 46,000 years ago they had reached the island continent of Australia, and by 13,000 years ago, as the ice waned for a final time, they discovered the Americas. Now there were millions of us on the planet, and groups thrived from Tasmania to Alaska. Yet for thousands of years these intelligent people, who were like us in every physical and mental way, remained nothing but hunters and gatherers. In the light of our great accomplishments over the past 10,000 years this long period of stasis is an enigma. In order to understand it, we need to investigate the climate that minted our species. So let’s turn to the ice age and the work of those who have devoted their lives to unlocking its secrets.

  As we have seen, Earth’s sediments are full of climate-recording devices and, the closer we come to our own time, the more information they provide. At their best they yield an annual record of change that includes information on wind direction and speed, the chemistry of the atmosphere, the extent and type of Earth’s vegetative cover, the nature of the seasons and the composition and temperature of the oceans—in short, the state of Earth as it was, for example, 5120 years ago.

  One of the best sources of information about climate is, in its simplest form, evident to everyone. Look at a piece of timber and you can see, written in its fine texture and growth rings, a story of the way things were when that tree lived. Widely spaced rings tell of warm and bountiful growing seasons when the sun shone and rain fell at the right time. Compressed rings, recording little growth in the tree, tell of adversity when long, hard winters or drought-blighted summers tested life to the limits.

  The oldest living thing on our planet is a bristlecone pine growing more than 3000 metres up in the White Mountains of California. More than 4600 years old, it grows in Methuselah Grove alongside many other superannuated specimens. Its precise location is a closely guarded secret because, vulnerable to disturbance, it’s been slowly dying for the past 2000 years. Within its trunk this single tree holds a detailed, year-by-year record of climatic conditions in California. Match the pattern from the heart of the Methuselah tree with the rind of a dead stump nearby, and you may pierce time to a depth of 10,000 years. Tree-ring records of this length have now been obtained from both hemi
spheres, and there is even hope that the great kauri pines of New Zealand, whose timber can lie sound in swamps for millennia, will provide a record spanning 60,000 years of climatic change.

  For all its convenience and depth the climate record of the trees is relatively limited in what it can tell us. If you want a really detailed record you must turn to ice—but only in special places does it yield all its secrets. One such place is the Quelccaya ice cap in the high mountains of Peru. There the ice is laid down in a banded annual pattern, each year’s snowfall separated by a band of dark dust that is blown up from the deserts below during the winter dry season. Three metres of snow can fall on Quelccaya in a summer, and the falls of subsequent seasons compress it, first to firn (compacted snow) and then to ice. In the process, bubbles of air are trapped, which act as minute archives documenting the condition of the atmosphere. Australian scientists pioneered the techniques allowing methane, nitrous oxide and CO2 levels to be obtained from these bubbles, each of which reveals its own story about the past conditions of the biosphere. Even the dust is informative, for it tells of the strength and direction of the winds, and of conditions below the ice cap. And isotopes of oxygen in the ice can provide insights concerning the oceans and the distant polar ice caps.

  The ice sheets of Greenland and Antarctica yield Earth’s longest cores but because ice flows, the older ice is usually compressed and its annual bands disrupted. When the circumstances are right, however, truly spectacular records can be extracted. In the 1990s, European and American teams of researchers were sent to take ice-cores from the Greenland ice plateau. They couldn’t agree on a plan so they put down two corers, just far enough apart to ensure that any changes they detected were real and not a localised anomaly. The European team, drilling to the north, had a stroke of great luck, for their core was sunk atop granitic rocks whose radioactivity generated considerable heat. This melted the lowest layers of ice, which prevented the distortion of the layers above, thereby preserving a detailed climatic record extending back 123,000 years. Using this unique record the team was able to show that spectacular shifts in the North Atlantic climate occurred over just five annual ice-layers, and that 115,000 years ago Greenland experienced a hitherto unknown warm phase that was not mirrored in the Antarctic.1

  In June 2004, when an ice-core over three kilometres long was drawn from a region of the Antarctic known as Dome C (about 500 kilometres from the Russian Vostok base) even more spectacular results were obtained. The recovery of such a long ice-core must count as one of science’s greatest triumphs, for drilling through ice is more hazardous than you might imagine. The drill site was bitterly cold: −50°C at the beginning of the drilling season and −25°C in the middle of the Antarctic summer. The drill itself is just ten centimetres wide, and as it grinds its way downward a slender column of ice is separated and drawn to the surface. The first kilometre was especially difficult, for there the ice is packed with air bubbles, and as the core was drawn up these tended to depressurise, shattering the ice into useless shards. Worse, ice chips can clog the drill head, jamming it fast. In the summer of 1998–99 a drill head trapped over a kilometre below the surface forced the abandonment of the hole, leaving the team with no option but to start all over again. This time, as they drilled the three kilometres to the bottom, they stopped after each metre or two to bring the precious core to the surface.

  As the team passed the point reached by earlier drilling, the excitement was palpable.2 ‘You know you were getting stuff that had never been seen before,’ a team member said, and each kilometre advanced was celebrated with specially warmed champagne. Then, when they were almost at bedrock another problem emerged. Heat from the rocks below was melting the ice, threatening yet another jamming of the drill bit. The final hundred metres was drilled in late 2004, using as a makeshift bit a plastic bag filled with ethanol (to gently melt its way downwards).

  The core from Dome C takes us 740,000 years back in time, and as the final few hundred metres are yet to be dated there’s the chance that an even longer record will be obtained.3 This is an enormous development, for it allows us to glimpse how things stood around 430,000 years ago—the last time that the Milankovich cycles brought Earth into a position similar to that which it occupies today. Back then, the ice revealed, the warm (interglacial) period was exceptionally long, suggesting that our planet may have continued to experience mild conditions for a further 13,000 years.4

  Warm phases—even far briefer ones than the present—were, however, anomalies during the ice age. More typical are cold periods, including the so-called glacial maxima, when the grip of the ice is at its greatest. The last time this happened was between 35,000 and 20,000 years ago. Back then the sea level was more than 100 metres lower than it is today, altering the very shape of the continents; and North America and Europe’s most densely inhabited landscapes lay under kilometres of ice. Even regions south of the ice, such as central France, were treeless subarctic deserts, and their growing season of sixty days was an alternation of freezing northerly winds and a few still periods when a stifling haze of glacial dust filled the air.

  It’s often said that the priority of an agenda is determined by how big a thing is and how fast it’s moving, and by the end of the ice age changes were big and moving very fast indeed. So it’s no surprise that climatologists are especially interested in the period from around 20,000 to 10,000 years ago—as the glacial maximum began to wane—for over those ten millennia the overall surface temperature of Earth warmed by 5°C—the fastest rise recorded in recent Earth history.

  It is worth comparing the rate and scale of change during this period with what is predicted to happen this century if we do not reduce our emissions of greenhouse gases. If we pursue business as usual, an increase of 3°C (give or take 2°C) over the twenty-first century seems inevitable.5 While the scale of the change is less than that seen at the end of the last glacial maximum, the fastest warming recorded back then was a mere 1°C per thousand years.6 Today we face a rate of change thirty times faster—and because living things need time to adjust, speed is every bit as important as scale when it comes to climate change.

  Despite the keen focus of scientists on this period, details of how the world shifted from glacial maxima to warm interglacial have been slow coming. In 2000, analysis of a core from Bonaparte Gulf in Australia’s tropical north-west revealed that 19,000 years ago, over a period of just 100 to 500 years, sea levels rose abruptly by ten to fifteen metres, indicating that the thaw commenced far earlier than anyone had imagined.7 Because of difficulties dating the sediments this finding was first viewed with suspicion, but in 2004 a second study in the Irish Sea Basin showed a similar but better-dated rise.8 The fact that the world did not continue warming in consequence was puzzling, but when the immediate cause of the sea rise was identified the reason became clear. The water, it transpired, had come from the collapse of a Northern Hemisphere ice sheet, which poured somewhere between one-quarter and two Sverdrups’ worth of fresh water into the north Atlantic. The scale of ocean currents is measured in Sverdrups, named after the Norwegian oceanographer Hans Ulrich Sverdrup. A Sverdrup is a very large flow of water—1 million cubic metres of water per second per square kilometre—and by disrupting the Gulf Stream this influx had profound consequences.

  The Gulf Stream transports vast amounts of heat northward from near the equator—almost a third as much as the Sun brings to Western Europe, and that heat is borne in a stream of warm salty water. As it gives up its heat the water sinks because, being salty, it is heavier than the water around it, and this sinking draws more warm, salty water northwards. If the Gulf Stream’s saltiness is diluted with fresh water it does not sink as it cools, and no more warm water is drawn northward in its wake.

  The Gulf Stream has stopped flowing in the past. Without the heat it brings the melting glaciers begin to grow again and, as their white surface reflects the Sun’s heat back to space, the land cools. Animals and plants migrate or die and temperate regions
such as central France are plunged into a Siberian chill. The heat, however, does not vanish. Most of it pools around the equator and in the Southern Hemisphere, where it can cause the melting of glaciers in the south, so that the Sun’s rays fall on a dark sea surface instead of on ice, and are absorbed. This heats the world from the bottom up, so to speak, and with the Gulf Stream established once more courtesy of growing northern ice, the world enters another cycle of warming.

  Somewhere around two Sverdrups of fresh water is required to significantly slow the Gulf Stream, and the geological record confirms that this happened repeatedly between 20,000 and 8000 years ago. Thus the transition from the ice age to the warmth of today was no gentle segue, but instead the wildest of roller-coaster rides, whose high and low points had the sharpness of saw teeth.

  The most famous and well-studied of such spikes is the younger Dryas, named for an alpine flower whose pollen began showing up in unexpected places as a result of a well-documented chill. The sudden freeze began 12,700 years ago, after warming had caused the collapse of a massive ice-dammed lake of meltwater and the redirection of freshwater flow across the North American continent—from the Mississippi drainage to the St Lawrence. This big freeze lasted for 1000 years, and much of Europe was plunged into full ice-age conditions, leaving many parts of the continent uninhabitable. A further cooling event occurred 8200 years ago, and this one caused temperatures over Greenland to drop around 5°C for 200 years. As with the younger Dryas the breaching of an ice dam seems again to have been responsible, with the flow this time being directed into Hudson Bay.9

  As the crazy seesawing caused by alternating melting in the Northern and Southern Hemispheres progressed, it drew Earth jerkily yet inexorably towards its present state. And then this climatic madness gave way to the most serene calm. It was as if, says archaeologist Brian Fagan (Professor Emeritus at the University of California, Santa Barbara), a long summer had arrived whose warmth and stability the world had not seen for half a million years.10