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

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.

      Further east, however, agriculture may actually benefit from warmer temperatures and higher rainfall. Rather as California offered sanctuary of a sort to displaced Okies' during the Dust Bowl, the Midwest and Great Lakes areas will need to provide jobs and sustenance to those who can no longer scratch a living from the sandy soils far out west, once the rains stop falling and the desert winds begin to blow.

       Already the day after tomorrow?

      Just as farmers on the High Plains of North America are watching their fields and grasslands blowing away in the relentless heat, their kinfolk across the Atlantic may be grappling with another problem: extreme cold. One of the most counter-intuitive projected impacts of global warming is the possible plunging of temperatures throughout north-west Europe as the warm Atlantic current popularly known as the Gulf Stream stutters and slows down. This is the scenario fictionalised in an exaggerated form by the Hollywood disaster epic The Day After Tomorrow, where a collapse in the Atlantic current triggers a new ice age, flash-freezing New York and London (although the good guy still gets the girl). Real-world scientists were quick to lambast the film for flouting the laws of thermodynamics, but they also acknowledged that the reality of a slowdown in the North Atlantic Ocean current may still be pretty scary, especially for those who live in a part of the world which is used to a mild maritime climate far out of keeping with its high northern latitude.

      A short technical aside is required here. Only a small part of the great current that delivers warm water into the North Atlantic is actually the real Gulf Stream: it, as its name suggests, is a stream of warm subtropical water heading north-east out of the Gulf of Mexico, which eventually becomes part of the much larger system of currents known to scientists as the Atlantic Meridional Overturning Circulation. The MOC is partly driven by the cooling and sinking of water at high latitudes off the coast of Greenland and Norway, where freezing Arctic air lowers its temperature and squeezes fresh water out as sea ice, leaving behind a heavy, salty brine which quickly sinks to the bottom of the ocean. From there it begins a return journey south-eventually surfacing (1,200 years later) in the Pacific. Scientists have long feared that a freshening and warming of the Norwegian and Greenland seas-due to higher rainfall, run-off from melting land glaciers and the disappearance of sea ice-could stop this water sinking, and shut down the great ocean conveyor. Hence the famous ‘Shutdown of the Gulf Stream’ scenarios familiar from newspaper headlines and the Hollywood movie.

      Far-fetched it may seem, but Atlantic circulation shutdown has always been more than just a theory. It has happened before. At the end of the last ice age, 12,000 years ago, just as the world was warming up, temperatures suddenly plunged for over a thousand years. Glaciers expanded again, and newly established forests gave way once more to chilly tundra. The period is named the ‘Younger Dryas’, after an arctic-alpine flowering herb, Dryas octopetala, whose pollen is ubiquitous in peaty sediment layers dating from the time. In Norway temperatures were 7-9°C lower than today, and even southern Europe suffered a reversal to near-glacial conditions. On the other side of the Atlantic, cooling also occurred, and there is evidence of rapid climate change from as far afield as South America and New Zealand.

      The culprit seems to be the sudden shutting-off of the Atlantic circulation due to the bursting of a natural dam holding back Lake Agassiz, a gigantic meltwater lake which had pooled up behind the retreating North American ice sheets. When the dam broke, an enormous surge of water (the lake's volume was equivalent to seven times today's Great Lakes) is thought to have poured through Hudson Bay and out into the Atlantic. This freshwater surge diluted