Homo Britannicus

by Chris Stringer

  We have been fortunate enough to live in one of the most stable periods of the last 500,000 years in terms of world climates, and this has allowed humans to spread and settle throughout most of the world, apart from the polar regions, and to enjoy an astonishing growth, from the few million people estimated for the end of the last ice age to our present figure of over 6 billion.

  If we look even further back in time, the Earth has suffered yet greater climatic insults. Our planet is over 4.5 billion years old, and after its molten surface had cooled, life appeared more than 3 billion years ago. During the following billion years, the world experienced its first ice age. Although the youthful sun was colder then, Earth’s atmosphere was initially high in greenhouse gases such as methane, which kept the surface and its primeval oceans warm. (A greenhouse effect is produced when some of the sun’s energy that would otherwise be reflected back from the surface into space is instead absorbed by gases like methane and carbon dioxide and continues to warm the planet.) However, by 2.3 billion years ago, plants making their food through the process called photosynthesis produced a gradual increase in oxygen at the expense of other gases, and temperatures plunged as the greenhouse effect weakened. Life clung on and the Earth recovered until about 750 million years ago when it seems that several even more severe cold events happened, and created a ‘snowball Earth’. The oceans froze and the planet’s surface was almost entirely covered with ice. While life must have survived in the seas under the ice, it was probably only the effect of volcanoes protruding through the ice and continuing to pour out enough of the greenhouse gases carbon dioxide and methane that broke the pattern. Otherwise Earth might never have emerged from its snowball, and would today look much like Europa, a moon of the planet Jupiter, whose briny oceans are covered by ice miles thick.

  The reasons for these astonishing ancient fluctuations are still unclear, partly because so much of the evidence for them is either buried deep or eroded away. The temperature of the young sun may have been less stable, there may have been much more interplanetary dust and debris to interrupt its warm rays, or the tilt of the Earth’s axis may have been different from today, making the Milankovitch effect (see p. 54) more extreme. Certainly the composition of the Earth’s atmosphere must have regularly changed with the constant interplay between cycles of volcanic activity, unstable weather patterns and the growing impact of life, while the shifting protocontinents constantly changed the geography of the Earth and its ocean circulations, with land masses sometimes at the poles, and sometimes straddling the equator.

  At the transition between the Permian and Triassic periods, about 250 million years ago, the largest known mass extinction occurred, with some 90 per cent of species dying out. Many scientists believe that this period had the warmest environment that life has ever experienced, and that this hot-house world was caused by a sudden release of previously frozen methane hydrates from the cold ocean depths. This huge injection of a powerful greenhouse gas imposed a 5°C rise on an already warming Earth, and in less than 100,000 years, most life had perished. During the Paleoeocene and Eocene, 60-45 million years ago, the continents were beginning to approach their modern configurations, although sea level was some 70 metres higher, and Earth was predominantly warm and humid, with alligators living near the North Pole, palm trees growing in Patagonia, and regions like Europe, North America and Australia covered with subtropical forests. Since then the Earth’s climate has deteriorated inexorably, with a mean fall of some 5°C and twice that amount in the Atlantic, and a build up of ice, first at the South Pole and later at the North. Various factors have been implicated in the cooling, two of the most important being a large land mass stuck at the South Pole and, as India crashed into Asia through continental drift, the inexorable rise of the Tibetan Plateau, which changed atmospheric circulation patterns.

  In the last million years, the Milankovitch cycles seem to have pushed Earth to ever-greater swings of climate, with the added superimposition of short-term fluctuations lasting a few millennia. The last of these really severe global climatic blips was the Younger Dryas of about 13,000 years ago. This briefly drove people out of Britain one last time, but also set the scene for the modern world by changing environments at a critical point in human history, catalysing the agricultural revolutions in south-western and eastern Asia with the domestication of cereals, animals and rice.

  After the upheavals of the Younger Dryas, the Earth’s climate settled into the relative stability of our present interglacial, the Holocene. In the more recent past, from about AD 800 to 1300 Europe experienced conditions generally slightly warmer than today, and this led Erik the Red to found colonies on the coast of Greenland, where Vikings fattened their cows in the summer and roamed to hunt seals and reindeer in the winter. Following their French kin, the British grew grapes and developed a thriving wine trade to rival that of their southerly neighbours. But in the 1400s, average temperatures dropped some 2 °C in many regions and this cold snap, known as the Little Ice Age, lasted about 500 years, give or take some short ameliorations. The Norse settlements in Greenland had vanished completely by 1500, while British vines soon withered and perished along with hundreds of thousands who starved to death across Europe as their crops failed. At times shipping could not move in the winter when the Thames and the canals of Holland froze over, as depicted in contemporary paintings. In many ways the Little Ice Age shaped Europe irreversibly, influencing social, political and agricultural changes that are still with us today. But during the 1800s, Europe started to warm again, heralding our present climatic era.

  Given the fact that the Earth has experienced such extremes of climatic fluctuations in its history, are the changing climates of today simply part of the natural cycle, or are we interfering with that cycle, for better or worse? Looking at climate changes during the last 200,000 years, and the ongoing Milankovitch cycles, Earth should have been entering a cooling phase now, with the expectation of major glaciation within the next 50,000 years. But many scientists believe that the effect of increasing greenhouse gases in the atmosphere will override any cooling trends in the immediate future. However, that news is not as good as it might seem, because there is a profound danger we will permanently force the Earth’s climate out of kilter.

  There are those who still doubt the reality of global warming and our role in it, yet the vast majority of scientific data are pointing in the same ominous direction, even if our incomplete understanding of all the parameters prevents precise predictions of how quickly things will change and how bad it will get. A thousand papers on climate issues published in the major science journals between 1993 and 2005 have agreed with the consensus position that global warming really is happening, and the seriousness is such that in 2006 the Chief Government Scientist of Britain, Sir David King, argued that global warming is now the single greatest threat facing humankind and our planet. He virtually accepted the inevitability of a 3°C rise, which could put a billion people at risk of starvation because of lost arable land and water shortages, as well as destroying large tracts of tropical forest and half the world’s wildlife reserves and corals. Temperatures could increase by a further 1.5° as a result of positive feedbacks in the climate resulting from the melting of sea ice, thawing permafrost and the acidification of the oceans.

  So what are the data leading to these worrying conclusions? According to weather records that go back 150 years, nineteen of the twenty warmest years have occurred since 1980, while 1998 was the warmest year since world temperature records were kept and 2003 provided Britain with a new record temperature of 38.5 °C. The exceptionally hot, dry summer of 2003 is estimated to have caused about 35,000 additional deaths in western Europe, and analyses suggest that global warming related to elevated carbon dioxide levels has made such summers four times more likely than in a world where the climate is stable. And it is not just Britain and Europe that have been affected, since Canada and Australia also had their hottest weather on record in 2005. Arctic sea ice dropped t
o its smallest ever extent, the Atlantic suffered a record hurricane season, and an unprecedented drought reduced the flow of the Amazon to its lowest known level. These developments are worrying enough, but there is evidence that we have not yet felt the full force of global warming because of a phenomenon called global dimming. Against expectations, and in seeming contrast to rising temperatures, the measured amount of sunlight reaching the Earth’s surface has fallen over the last fifty years. Research suggests that this is because of the high levels of polluting particles in the atmosphere from burning aviation fuel, coal, oil and wood. In clouds, these particles seed water droplets, increasing their reflectance and reducing the penetration of sunlight. Thus the warming effect of the increase in polluting greenhouse gases has so far been partly counteracted by the parallel increase in particles. As we reduce particle pollution, as we must, the brakes will be off for the full force of the greenhouse gases.

  And the proportion of greenhouse gases in the atmosphere, especially carbon dioxide, has risen relentlessly – CO2 levels are now over a third greater than before the Industrial Revolution and higher than any level recorded in gas bubbles trapped in Antarctic ice over the last 650,000 years. Already we are at a degree of global warmth approaching the maximum of any interglacial in the last 500,000 years, but if current trends in greenhouse gas emissions continue unabated CO2 is projected to be double that of pre-industrial levels by the end of this century, when global temperatures are likely to rise by about 3°C. Projections suggest that before then Britain will return to the level of Mediterranean warmth found at Pakefield 700,000 years ago. That may sound appealing. In the south British farmers could grow oranges, lemons, avocado and melon, our vineyards could rival those of France, and our beaches those of Spain. The landscapes of Britain would change dramatically as familiar forests and woodlands of oak and beech give way to Spanish chestnut, Turkish hazel, and olive trees, along with the plants and insects that accompany them. This may not seem so bad, but northern Britain could suffer increased rainfall, and warmer temperatures could lead to the spread of exotic diseases such as malaria, yellow fever and West Nile fever, as well as increased deaths from heat, skin cancer and food poisoning. Moreover, as the environmentalist James Lovelock put it, ‘our nation is now so urbanized as to be like a large city and we have only a small acreage of agriculture and forestry. We are dependent on the trading world for sustenance; climate change will deny us regular supplies of food and fuel from overseas.’ Worse still, after 2100 the climatic clock could start to be wound back 60 million more years to the subtropical levels of the Eocene, and beyond that, if the process cannot be stopped, a runaway greenhouse effect could return us to the scorching heat of the hot-house Earth of 250 million years ago, when most life on the planet was eliminated.

  Future climate change is not just about rising temperatures. Altering one critical parameter retunes the whole system in complex ways so that atmospheric circulation, ocean systems, wind, rain and snow patterns will all be altered. The thawing of permafrost in the north shows an example of the possible knock-on effects. Permafrost is ground that is frozen throughout the year, and it can run up to a mile deep in places like Siberia and Alaska – it makes up about a quarter of land area in the northern hemisphere. Already, land surfaces in parts of Alaska have become unstable through melting permafrost, causing roads and houses to subside into newly created morasses of mud and marshes, and producing ‘drunken forests’, where the trees lean at wild angles. Some permafrost is hundreds of thousands of years old, and it is estimated that the preservation and decay of past vegetation has accumulated about 450 billion metric tonnes of carbon in the Earth’s permafrosts – nearly a third of the total stored in soils globally. Should that carbon start to be liberated by melting and subsequent bacterial action, even larger quantities of CO2 and the much more powerful greenhouse gas methane would be added to the already warming atmosphere. The northern oceans would be seriously affected too, as melting ice and accelerated drainage would pour increasing quantities of fresh water into them; such runoff has already increased by some 7 per cent since the 1930s.

  The Arctic itself is heading towards a seasonally ice-free state for the first time in over a million years and the duration of seasonal snow cover is reducing noticeably, both of which reinforce global warming through the loss of an effect called albedo, the reflection back of solar radiation by snow and ice, keeping ground temperatures low in a feedback effect. This cycle is being disrupted as the degree of reflectance decreases, since newly exposed soils, vegetation and sea are darker than snow and ice, thus they absorb, store and then release the warmth. And a northward spread of trees and shrubs may itself add a seasonal rise of at least one degree to Arctic spring temperatures. The prospect of a radically different climate is a direct threat to the plants, animals and humans native to the Arctic. For example, polar bears live on ice throughout the year, making it their base to hunt, breed and raise their young. As the ice packs shrink and break up in the summer thaws, bears swim between ice floes to continue hunting. However, the Arctic ice caps have recently receded about 200 miles further north during thaws, forcing the bears to undertake far longer swims between floes of as much as sixty miles, swims through waters that may be stormier or with stronger currents. For the first time, significant numbers of adult carcasses are being found in the water in regions such as northern Alaska, bears that have either drowned or died of exhaustion; and the number of bears is declining right across their range. The Inuit, who are indigenous to the north American Arctic, have now petitioned the Inter-American Commission on Human Rights, accusing the US of destroying their traditional way of life and threatening their future through fuelling climate change, leading to the total degradation of their environment.

  Ironically, though, the place where global warming is showing its effects most strongly is at the other end of the Earth, the virtually uninhabited Antarctic. In the past forty years records show that this continent has warmed faster than any other. Sea ice has shrunk by 20 per cent since 1950, the west Antarctic ice sheet is dissolving at an accelerating rate, glaciers are thinning and retreating, and the Ross Sea is freshening through increased melting. In contrast, however, greater evaporation is leading to thicker snows in east Antarctica.

  We have seen how the past ebb and flow of the ice caused direct effects on sea level, with high stands in interglacials and falls of over 100 metres compared with present levels during the coldest stages. In addition, there were the effects of the growth or decay of heavy ice caps on the altitude of the land itself. If the most vulnerable ice sheets of today – those in Greenland and west Antarctica – start to melt with rising temperatures, this will herald a devastating increase in sea level. Many of the world’s largest cities such as London, New York, Kolkata, Cape Town, Hong Kong and Tokyo have been built on coasts, river estuaries or small islands. London was last seriously flooded in 1928, and the Thames Barrier began operation in 1982, built to cope with the risks known at that time. This barrier is a series of steel gates between concrete towers that can rapidly be closed if London is threatened by a tidal surge. It has protected the city very effectively for the past two decades, but it is now being raised with increasing frequency due to elevated sea levels, storms and tidal amplitude. It is likely to be ineffective after 2030, and to protect London from a possible £30 billion financial damage, not to mention the social, political and human costs, a new 10-mile-wide barrier across the Thames estuary between Essex and Kent is now under discussion.

  On the other side of the North Sea, the Dutch are adopting a very different approach. They have worked ceaselessly for centuries to protect their lands from flooding (there are over 10,000 miles of dykes, barriers and drainage channels), but the devastating floods of 1995 finally showed the futility of this constant battle, and forced what can really be called a sea change. Over the next half-century, the Dutch will allow an area more than twice the size of Greater London to flood as part of a strategy to live with and adapt
to the encroaching waters, and an increasing number of amphibious houses, shops and even greenhouses are being built, which can function equally well on the land or (anchored by metal posts), floating on water.

  Part of the present modest sea level rise is actually due to the thermal expansion of the oceans because of warming, but a predicted rise of 3°C would not only see further expansion, but probably also the beginning of the meltdown of the massive Greenland ice cap. Even a rise of one metre from a partial melt would threaten not only London, but Hull, Liverpool, the south coasts of England and Wales, and East Anglia, as well as the Netherlands, many cities on the US eastern seaboard, and a host of islands. For example, the Maldives in the Indian Ocean have 360,000 inhabitants living on specks of land that mostly lie less than a metre above sea level.

  If the whole ice cap disappeared, there would be an even more serious sea level rise of some seven metres, enough to engulf coastal and low-lying lands occupied by at least two billion people, including the deltas of the Nile, Niger, Ganges, Mekong, Amazon and Mississippi. The devastation of New Orleans by Hurricane Katrina in 2005 was predicted in detail long before it happened, and it was only the speed and scale of the evacuation programme that saved many thousands of lives. The disaster occurred, of course, without greatly elevated sea levels, but it may still be attributed to the effects of global warming on the Earth’s weather systems. Although the number of hurricanes forming in the mid-Atlantic during the last few years has been high, it is unclear whether this in itself is related to higher temperatures rather than natural variation. But the growing intensity of the hurricanes in terms of power and associated deluges fits perfectly with models that show how higher temperatures feed more energy into the weather systems that create and fuel the hurricanes. Thus as greenhouse gases build up in the atmosphere, the average strength of hurricanes should increase, and this will be part of a more general trend for our weather to become more extreme in terms of the levels of wind, rain, snow and, in the opposite direction, drought.