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

sizes and magnetic data from the palaeosol section, were able to draw conclusions from their sample about changing monsoon strength 129,000 years ago, as the Eemian climate gradually warmed up. Because of how long it takes the oceans to absorb heat, it appeared, the dry winter monsoon responded much more rapidly than the summer one to the changing conditions. The result was a period of drought and continental-scale dust storms, before the summer monsoon pushed far enough inland to bring significant rainfall to the loess plateau.

      So could such a mechanism plausibly repeat in the two-degrees-warmer world? Studies of Pacific sea floor sediments suggest that the height of the Eemian interglacial saw global temperatures about 1°C higher than today's, making this period a potentially useful analogue for a warmer climate in the future-particularly as regional temperatures in a large continent like Asia would in turn have been a degree or so higher than the global average. And if it did take China's monsoon climate longer to make the transition from cool/dry to warm/wet 129,000 years ago, as some scientists believe, this does suggest a possible cause for the droughts and rising temperatures that have struck northern China in recent years. So whilst southern China can expect more flooding as the climate warms, the oceanic time lag means that it may take much longer for the rain-bearing summer monsoon to reach the drought-stricken north. With China split between two extremes, agriculture will inevitably suffer, and water-stressed cities like Beijing and Tianjin will continue to experience shortages-particularly as economic growth spirals upwards and underground aquifers are pumped dry. The Chinese government has begun construction of a massive water transfer project, which aims to take billions of cubic metres of water from the Yangtze River in the south to the thirsty cities in the north. However, even this mega-project-the largest ever constructed on the planet-will (if it works, which many doubt) have difficulty keeping the taps running. With a chronic shortage of water, China will not just struggle to develop a more affluent lifestyle, it will struggle to feed itself too.

       Acidic oceans

      Greenhouse gases released over the last hundred years or so have not only changed the climate; they have also begun to alter conditions in the largest planetary habitat of all-the oceans. At least half the carbon dioxide released every time you or I jump on a plane or turn up the air-conditioning ends up in the oceans. This may seem like a good place for nature to dump it, but ocean chemistry is a complex and delicate thing. The oceans are naturally slightly alkali, allowing many animals and plants which inhabit the seas to build calcium carbonate shells.

      However, carbon dioxide dissolves in water to form carbonic acid, the same weak acid that gives you a fizzy kick every time you swallow a mouthful of carbonated water. That's great for a glass of Perrier, but not so good if it's beginning to happen on a gigantic scale to the whole of the global ocean. And it is indeed beginning: humans have already managed to reduce the alkalinity of the seas by 0.1 pH units. As Professor Ken Caldeira of the Carnegie Institution's global ecology department says: ‘The current rate of carbon dioxide input is nearly 50 times higher than normal. In less than 100 years, the pH of the oceans could drop by as much as half a unit from its natural 8.2 to about 7.7.’ This may not sound like much, but this half point on the pH scale represents a fivefold increase in acidity. And because the oceans circulate only very slowly, even if atmospheric carbon dioxide levels are eventually stabilised-perhaps because humanity wakes up to their warming effect-these changes in ocean chemistry will persist for thousands of years.

      This fast-moving area of scientific research was the subject of a major report by the Royal Society in June 2005, which identified some of the main concerns that are increasingly keeping marine biologists awake at night. First and foremost is the possibility that even with relatively low future emissions during this century (equating to two degrees or less of temperature warming), large areas of the Southern Oceans and part of the Pacific will become effectively toxic to organisms with calcium carbonate shells after about 2050. With higher emissions, indeed, most of the entire global ocean will become eventually too acidic to support calcareous marine life.

      The most important life-forms to be affected are those that form the bedrock of the oceanic food chain: plankton. Although individually tiny (only a few thousandths of a millimetre across), photosynthesising plankton like coccolithophores are perhaps the most important plant resource on Earth. They comprise at least half the biosphere's entire primary production-that's equivalent to all the land plants put together-often forming blooms so extensive that they stain the ocean surface green and can easily be photographed from space. The places where phytoplankton thrive are the breadbaskets of the global oceans: all higher species from mackerel to humpbacked whales ultimately depend on them. Yet coccolithophores have a calcium carbonate structure, and this makes them especially vulnerable to ocean acidification. When scientists simulated the oceans of the future by pumping artificially high levels of dissolved CO₂ into sections of a Norwegian fjord, they watched in dismay as coccolithophore structures first corroded, and then began to disintegrate altogether.

      Acidification will also directly affect other ocean creatures. Crabs and sea urchins need their shells to survive, whilst fish gills are extremely sensitive to ocean chemistry-just as our lungs are to the air. Mussels and oysters, vitally important both as economic resources and as part of coastal ecosystems worldwide, will lose their ability to build strong shells by the end of the century-and will dissolve altogether if atmospheric CO₂ levels ever reach as high as 1,800 ppm (parts per million: this ppm measure means, very simply, that for every million litres of air there are 1,800 litres of carbon dioxide). Tropical corals, already badly hit by bleaching, will more and more be corroded by this increasing oceanic acid. Walk out to sea on a reef in 2090, and it may crumble beneath your feet. Ships, rather than being torn apart when they strike rocky coral, may find themselves ploughing through weakened reefs like sponge. Indeed, it's difficult to overstate just how dangerous an experiment we are now conducting with the world's oceans. As one marine biologist says: ‘We're taking a huge risk. Chemical ocean conditions 100 years from now will probably have no equivalent in the geological past, and key organisms may have no mechanisms to adapt to the change.’ Phytoplankton are also crucial to the global carbon cycle. Collectively they are the largest producer of calcium carbonate on Earth, removing billions of tonnes of carbon from circulation as their limestone shells rain down onto the ocean floor. There's nothing new about this process: the chalk in the cliffs and downs of southern England originally formed as the limey sludge from countless billions of dead coccolithophores back in the Cretaceous era. But as the oceans turn more and more acidic, this crucial component of the planetary carbon cycle could slowly grind to a halt. With fewer plankton to fix and remove it, more carbon will remain in the oceans and atmosphere, worsening the problem still further.

      Phytoplankton are also hit directly by rising temperatures, because warmer waters on the surface of the ocean shut off the supply of upwelling nutrients that these tiny plants need to grow. As with acidification, changes are already detectable today: in 2006 scientists reported a decline in plankton productivity of 190 mega-tonnes a year as a result of the current warming trend. Together these two factors, warming and acidification, represent a devastating double blow to ocean productivity. As Katherine Richardson, professor of biological oceanography at Aarhus University in Denmark, says: ‘These marine creatures do humanity a great service by absorbing half the carbon dioxide we create. If we wipe them out, that process will stop. We are altering the entire chemistry of the oceans without any idea of the consequences.’

      Wiping out phytoplankton by acidifying the oceans is rather like spraying weedkiller over most of the world's land vegetation, from rainforests to prairies to Arctic tundra, and will have equally disastrous effects. Just as deserts will spread on land as global warming accelerates, so marine deserts will spread in the oceans as warming and acidification take their unstoppable toll.

       The mercury rises in Europe

      Under normal circumstances, the human body is good at dealing with excess heat. Capillaries under the skin flush with blood, allowing the extra warmth to radiate into the air. Sweat glands pump out moisture, disposing of heat through evaporation. Heat can even be lost through panting, and the heart works overtime. During exercise, the normal body temperature of 37 degrees Celsius can rise to 38 or 39°C with no ill effects.

      But 2003 was not a normal summer, and the heatwave experienced in Europe during the three months of June,