Six Degrees: Our Future on a Hotter Planet. Mark Lynas

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