Some impacts

Global averages conceal an enormous variability in climate impacts across the regions of the world. As Schellnhuber said: 'If you have your head in the oven and your feet in the freezer, your average temperature may be unchanged but you are still pretty uncomfortable.' The larger part of the conference was given over to reports of what a four-degree world will be like to live in, with most emphasis on sea-level rise, water availability and changes to forests. The Paleoclimate record shows that small temperature changes are associated with large sea-level rises. As we saw, although the impact takes a long time to work its way through, a world 2—2.5 degrees warmer is a world with seas 25 metres higher. Pier Vellinga of Wageningen University in the Netherlands—a nation with more than its fair share of experts on sea-level rise— reminded us that during the last interglacial or warm period 122,000 years ago sea levels were ten metres higher than today. But then it was only 1.5—2 degrees warmer. Alarmingly, the rate of sea-level rise in that period was at times 2.5 centimetres a year, or 2.5 metres a century, so that quantum of sea-level rise is proven possible by the end of the twenty-first century.

The seas are rising due both to thermal expansion of the oceans and the melting of ice. (Interestingly, the world's oceans are three centimetres lower than they would otherwise be because humans have stored a lot of water behind dams over the last 50 years or so.) The record indicates that once the ice begins to melt it cannot be stopped; it becomes 'temperature independent', as Vellinga put it. 'I hesitate to say this,' he admitted, 'but it is a very important implication for today from what has happened in the past.' His assessment is that beyond two degrees the probability of the Greenland icesheet disintegrating is 50 per cent or more, which would mean an additional rise of seven metres in sea levels over the next 300—1000 years. Above two or three degrees the West Antarctic icesheet is also likely to disintegrate, adding another five metres to sea levels on top of this. Due to gravitational effects, sea-level rise is expected to be less than the average in the southern hemisphere and more than the average in the northern hemisphere. Although slowly rising seas are themselves a threat to low-lying areas, they are much more likely to be affected by big storm surges on top of higher seas.

Stefan Rahmstorf, professor of ocean physics at Potsdam University, noted that the best recent estimate is that by the end of the century sea levels will be 75—190 centimetres above 1990 levels (with the minimum level four times higher than the 18-59 centimetres projected in the 2007 Fourth Assessment Report of the IPCC), although under a four-degree scenario it is more likely to be in the range 98-130 centimetres. Of course, that would only be the level through which the seas pass on their way to something much higher because, even if we stabilise the temperature at some level, the seas will continue to rise for hundreds of years. Even cutting emissions to zero would not stop it.

Surprisingly, only 7-10 per cent of the global population lives within ten metres of the coast—that is, ten metres vertically— although the percentage is much higher in South and East Asia. Robert Nicholls of the University of Southampton estimates that this includes 136 port cities with populations of a million or more. (Rahmstorf observed that many nuclear power plants are located on coasts because they use sea water for cooling.) Nicholls gave the only up-beat talk of the conference, an optimistic account of how humans could adapt to rising seas if, by the end of the century, 'the whole world is like the Netherlands'. 'Is it mad,' he asked, 'to think we can put dikes around the world's coasts?' Some might think so. He believes it would be expensive but 'manageable'. Of course, most coastal wetlands would be lost, and countless living species along with them.

The ability of nations to defend against rising seas depends on their resources. Vellinga told us that the annual cost of coastal protection on the Netherlands is around 0.2 per cent of GDP. It's estimated that a one-metre rise in sea levels would double the cost as a percentage of GDP, although I could not help thinking that it is hard to imagine these figures have any relevance for Bangladesh. Naturally enough, thinking about how to respond to sea-level rise is most advanced in Holland, where there is now talk of establishing towns on top of raised dikes and of building floating cities, including floating greenhouses for food cultivation, a comment that stimulated discomforting memories of the post-apocalyptic film Waterworld. Another option being mooted, said Vellinga, is to raise the height of the entire country by taking vast quantities of sand from the North Sea, a suggestion greeted with some incredulity. Stefan Rahmstorf reminded participants that dikes will not save low-lying small island states from inundation. It is hard to imagine the citizens of the Pacific Island Tuvalu living on an atoll surrounded by a two- or three-metre high concrete wall.

Ultimately, with enough foresight and resources, we can defend against or retreat from rising seas. The availability of fresh water will be a more intractable determinant of survival in a four-degree world. It's expected that, overall, a warmer world will be more humid, with rainfall increasing by perhaps 25 per cent. But Professor Nigel Arnell from the University of Reading reminded us that changes in precipitation will be highly variable by region. Higher rainfall will be concentrated in northerly latitudes, with large parts of the world nearer the tropics suffering a severe decline. On the maps, areas shaded darker yellow (including Australia, southern Europe, western and central-southern United States) indicate precipitation declines of 10-30 per cent in a four-degree world. The red areas start to look like no-go zones with rainfall declines of 40-50 per cent expected in North Africa, southern Africa and great swathes of territory across the north of Latin America, including the Amazon. Run-off will decline by more than precipitation because of higher rates of evaporation before the water reaches streams and rivers. Arnell estimates that in a four-degree world around 1 billion people (a sixth of the current global population) will be exposed to increased water-resource stress. Cruelly, half of those already prone to flood will face an increased flood hazard.

The models indicate that 15 per cent of land currently suitable for cultivation will become unsuitable, while in cold regions the area suitable for cultivation increases by 20 per cent. This is reassuring perhaps for Siberia and Canada but disastrous for eastern and southern Africa, where land suitable for cultivation declines by around a third. As Schellnhuber drily observed: 'Two hundred million people will not move smoothly between a dry Sahel and a wet Siberia.' The relentless logic of the models proves over and over that the poor and vulnerable will be hardest hit by climate change, even though they are not responsible for causing it and are in the weakest position to defend themselves against it. Phillip Thornton, an agriculture expert from Nairobi, describes the prognosis for food supply in sub-Saharan Africa as 'appalling' because rain-fed agriculture in many areas would cease to be viable by the end of the century. Although the effects are way beyond the capacity for adaptation, we must nevertheless do all we can, including expanding investment in technology and strengthening institutions.

In one of the conference breaks, a scientist from Siberia told me that most people from where she comes from are looking forward to global warming. Arnell produced a chart showing estimated changes in heating and cooling requirements. In a four-degree world heating requirements in cold regions will decline by around 50 per cent; but in warm and hot areas the demand for cooling will double. When we remember that nearly 15,000 people, mainly elderly, died as a result of the August 2003 heatwave in France, cooling no longer seems to be a luxury.

Oxford University professor Yadvinder Malhi is an expert in tropical forests. In a fascinating talk he told us that organisms that have evolved in tropical regions are much less tolerant of temperature changes than those from temperate regions where temperatures show much larger daily and seasonal variation. High latitude species have a much greater thermal tolerance. Yet to accommodate a one-degree increase in temperature, tropical organisms have to travel three times as far horizontally as temperate ones. In the tropics this means moving 380 kilometres further from the equator. Currently, trees in the Andes are migrating at 25-35 metres a decade. Instead of moving horizontally, they can find cooler climates by migrating vertically. Moving a mere 180 metres up a mountain to accommodate a one-degree temperature rise is equivalent to a 380-kilometre trek across land. The problem is that mountains that begin as refugia become traps as warming continues and vertically migrating species discover that mountains have peaks.

Malhi posed the problem of global warming in an arresting way. Referring to the intricate patterns of interdependence of all living things and the systems in which they live, he explained that we are 'reweaving the web of life'. The way organisms respond to climate change will depend on the plasticity of their physiological thresholds (lower for tropical species), their capacity for rapid evolutionary adaptation, changes in behaviour and the opportunities for migration. Organisms have always been forced to adapt to changing climates or die, as they did when ice ages ended in the past. What is uniquely worrying now is that the rate of climatic change is too fast for many species to adapt to the new conditions.

The prospect of more fires in the Amazon is the object of great scientific concern. An average warming of four degrees across the globe would translate into five or six degrees in Amazonia. Deforested areas exposed to the sun become much hotter—an additional five degrees, making those areas a scorching ten degrees hotter. The figures powerfully reinforce the importance of ending deforestation. But it is not only tropical forests that will be affected by warming. Professor David Karoly of the University of Mel bourne described the effects of the freakish fires that swept across parts of Victoria in February 2009, the conditions for which were created by a prolonged drought and an extreme heatwave. On 7 February Melbourne recorded its highest-ever maximum of 46.4°C (115.5°F). Brush-tailed possums fell dead from the trees. Flying foxes, unable to cool their bodies, dropped from the sky. Australia's traditional fire danger warning scale, posted on large signs around the country, indicated risks ranging from 'low' to 'extreme'. The 2009 wildfires were off the scale, forcing the development of a new one. Now, above the old 'extreme' are two new categories, the top one being 'catastrophic' or 'code red'. The advice of fire authorities is that the only sensible response to a fire in those conditions is to run for your life. If this sort of devastating fire can occur when the Earth is only 0.8 degrees warmer than it was a century ago, observed Karoly, imagine what is possible when it is four degrees hotter. 'We are unleashing hell on Earth,' he said.

Britain is not expected to become hot and dry like Australia in a four-degree world, although it will undoubtedly be warmer and water shortages are expected in the south and east of England. Although forest fires in Britain are virtually unknown today, forest researcher Andy Moffat believes that planning should begin for their possibility. He suggested that changing conditions might increase interest in planting eucalypts in England, which would be seen by some as 'a crime'. Of course, eucalypts are notorious for their propensity for a hot burn.

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