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

produce normal circumstances. In Switzerland the mercury climbed above 30°C as early as 4 June, rising to a maximum of 41.1°C in the south-east of the country on 2 August-the sort of searing temperature more often associated with the Arabian Desert than temperate central Europe. Across the continent, records tumbled: in Britain temperatures reached 100° Fahrenheit for the first time. Beaches were packed as holidaymakers enjoyed the summer heat, but in big cities like Paris, a hidden disaster was unfolding.

      The first symptoms of heat stress may be minor. An affected individual will feel slightly nauseous and dizzy, and perhaps get irritable with those around. This needn't yet be an emergency: an hour or so lying down in a cooler area, sipping water, will cure early heat exhaustion with no longer-lasting symptoms. But in Paris, August 2003 there were no cooler areas, especially for elderly people cooped up in their airless apartments. It wasn't so much the high temperatures of the day, but the fact that things didn't cool down enough at night to give the body time to recover. The effects were cumulative, and the most dangerous-and often fatal-form of heat stress then became much more likely: hyperthermia or heatstroke.

      Once human body temperature reaches 41°C (104°F) its ther-moregulatory system begins to break down. Sweating ceases, and breathing becomes shallow and rapid. The pulse quickens, and the victim may rapidly lapse into a coma. Unless drastic measures are taken to reduce the body's core temperature, the brain is starved of oxygen and vital organs begin to fail. Death will be only minutes away unless the emergency services can quickly get the victim into intensive care.

      These emergency services failed to save over 10,000 Parisian heatstroke victims in the summer of 2003. Mortuaries quickly ran out of space as hundreds of dead bodies, mainly of elderly and marginalised people, were brought in each night. The crisis caused a political furore as people accused politicians and municipal administrators of being more concerned with their long August holidays than with saving lives in the capital. Estimates vary, but across Europe as a whole, between 22,000 and 35,000 people are thought to have died.

      The heatwave and drought also devastated the agricultural sector: crop losses totalled around $12 billion, whilst forest fires in Portugal caused another $1.5 billion of damage. Major rivers such as the Po in Italy, the Rhine in Germany and the Loire in France ran at record low levels, grounding barge traffic and causing water shortages for irrigation and hydroelectric production. Toxic algal blooms proliferated in the denuded rivers and lakes. Melt rates on mountain glaciers in the Alps were double the previous record set in 1998, and some glaciers lost 10 per cent of their entire mass during the heat of that one summer. Meanwhile-as described in chapter 1-melting permafrost caused rockfalls in mountain areas like the Matterhorn.

      It wasn't long before questions were asked about global warming's possible contribution to the disaster. Meteorologists who investigated past hot spells found that the 2003 heatwave was off the statistical scale-a one-in-several-thousand-year event. According to an analysis by UK-based climatologists, twentieth-century global warming has already doubled the risk of such a heatwave occurring. Right across Europe, according to research published in 2007, the frequency of extremely hot days has tripled over the last century, and the length of heatwaves on the Continent has doubled. The conclusion is stark: the 2003 summer hot spell was not a natural disaster.

      The intensity of the heatwave also tells us something about the future. Averaged across the whole continent, temperatures were 2.3°C above the norm. So does that mean that in the two-degree world, summers like 2003 will be annual events? It seems so: in the UK-based study mentioned above, scientists used the Met Office's Hadley Centre computer model to project future climate change with increasing greenhouse gas emissions, and concluded that by the 2040s—when temperatures globally in their model are still below two degrees-more than half the summers will actually be warmer than 2003.

      That means that extreme summers in 2040 will be much hotter than 2003-and the death toll will rise in consequence-perhaps reaching the hundreds of thousands. Elderly people may have to be evacuated for months at a time to air-conditioned shelters, and outside movement during the hottest part of the day will become increasingly dangerous. Temperatures may soar to highs commonly experienced today only in North Africa, as rivers and lakes dry up and vegetation withers across the entire continent. Crops which require summer rainfall will bake in the fields, and forests which are more accustomed to cooler climes will die off and burn. As a result, catastrophic wildfires may penetrate north into new areas, torching broadleaved forests from Germany to Estonia.

      Here again the summer of 2003 gives us a glimpse of things to come. Europe-wide monitoring systems showed a 30 per cent drop in plant growth across the continent, as photosynthesis began to shut down in response to the twin stresses of high temperatures and crippling drought. From the deciduous beech forests of northern Europe to the evergreen pines and oaks of the Mediterranean rim, plant growth across the whole landmass slowed and then stopped. Instead of absorbing carbon dioxide from the air, the stressed plants instead began to emit it; around half a billion tonnes of carbon was added to the atmosphere from European plants, equivalent to a twelfth of total global emissions from fossil fuels. This is a positive feedback of critical importance, because it suggests that as temperatures rise-particularly during extreme heatwave events-carbon emissions from forests and soils will also rise, giving a further boost to global warming. And if these land-based emissions are sustained over long time periods and large areas of the Earth's surface, global warming could begin to spiral out of control, as the next chapter shows.

      We may have come dangerously close to that point during the 1998-2002 mid-latitudinal drought in the northern hemisphere, which left plants withering through regions as far afield as the western US, southern Europe and eastern Asia. One study showed that carbon emissions which would normally have been taken up by plants instead accumulated in the atmosphere, explaining the abnormally large jumps in the atmospheric CO₂ concentration in following years. (Jumps which caused jitters among many climate change watchers about whether runaway positive feedbacks might have already begun.) Over a billion tonnes of extra carbon poured out of plants and soils in response to the drought and heat.

      At the time of writing, the heatwave of 2003 has already begun to fade in people's memories, and the ‘normal’ summers of the following two years will have begun to soak up some of the extra carbon that entered the atmosphere during that deadly hot spell.

      But we forget at our peril. The summer of 2003 was a ‘natural experiment’ whose conclusions should be taken very seriously. This wasn't just some output from a computer model, whose assumptions and projections can be legitimately challenged. It actually happened. Moreover, the near-repeat of the 2003 heatwave in the summer of 2006 suggests that if anything the models are underestimating the likely frequency and severity of future heatwaves.

      We have been warned.

       Mediterranean sunburn

      Perhaps the most striking images from 2003's hot summer came from Portugal, where gigantic forest fires swept through the tinder-dry landscape, destroying orchards, torching houses and killing eighteen people. In total an area almost the size of Luxembourg was devastated. The conflagrations were so huge that they cast palls of smoke right over the North Atlantic, with both fires and smoke easily visible from space. The fires must have been particularly shocking for tourists, many of whom flock to southern Portugal from northern Europe-more in search of the sun than several days of smoke inhalation.

      However, one study shows that such wildfires are going to be an increasingly common sight for holidaymakers to southern Europe and the Mediterranean. Climate change simulations show the region getting drier and hotter as the subtropical arid belt moves northward from the Sahara. In the two-degree world, two to six weeks of additional fire risk can be expected in all countries around the Mediterranean rim, with the worst-hit regions being inland from the coast where the temperatures are highest. In North Africa and the Middle East virtually the whole year will be classified as ‘fire risk’.

      These fires will be driven on by scorchingly hot temperatures.

      The number of days when the mercury climbs over 30°C is expected to increase by five to six weeks in inland Spain, southern France, Turkey, northern Africa and the Balkans. The number of ‘tropical nights’, when temperatures don't cool off past 20°C, will increase by a month, and