So who is right? Is climate change a real problem, or a natural, cyclical phenomenon? And why do an increasing number of people - the British Prime Minister Tony Blair among them - describe it as the 'most important issue ever to face humanity' and a danger 'greater than terrorism or weapons of mass destruction'?
Hot air: the basics
Hardly anyone still disputes the basic science. Just like the glass on a greenhouse, a blanket of water vapour and other 'greenhouse gases' - notably carbon dioxide, methane and nitrous oxide - trap some of the sun's radiation as it bounces off the surface of the earth. This natural greenhouse effect has enabled life to develop in all its complexity - without it the planet would be frozen solid - but since the industrial revolution the amount of greenhouse gas has risen. As a result more heat has been trapped, causing a warming of the atmosphere.
During the twentieth century, global atmospheric temperatures have risen by about 0.7°C. However, since the mid-1970s the rate of warming has tripled. The last decade has seen nine of the warmest ten years since records began in 1861. 2004 was the fourth-warmest ever. In descending order of heat the five warmest-ever years were: 1998, 2003, 2002, 2004, 2001. Moreover, new evidence - compiled from 'proxy data' like ice cores and tree rings - suggests that temperatures are now higher than they have been for over a thousand years.
Another highly significant greenhouse gas is methane, which traps heat 30 times more efficiently than carbon dioxide. Over the past two centuries methane concentrations in the atmosphere have more than doubled. A fifth of methane emissions today come from the cultivation of rice. Pipeline leaks, the flatulence of cattle and termites and forest fires also contribute to rising methane levels.
Significant synthetic chemicals which contribute to global warming include halons like the chlorofluorocarbons (CFCs) and long-lived gases like sulphur hexafluoride. Some of these synthetic gases are several thousand times more powerful as greenhouse gases than CO2 - and last almost indefinitely.
There is no doubt that humans are to blame for the increase in atmospheric greenhouse gas concentrations, but does that explain the rise in temperature? Very likely, says a UN worldwide body - the Intergovernmental Panel on Climate Change (IPCC) - which brings together climate scientists and other experts. In its last landmark report in 2001 the IPCC stated that "most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations".
The IPCC also made some scary predictions - that by 2100 temperatures could rise by between 1.4°C to 5.8°C. Whilst the low end of this spectrum would be survivable, the high end would lead to almost unimaginable changes in the planetary system. For comparison, during the last ice age, when glaciers reached Manhattan and southern England, global temperatures averaged only 5°C cooler than now. Even a moderate temperature rise would cause rising sea levels, shifting weather patterns and an increase in the frequency of extreme weather events resulting in massive traumas both for human populations and for nature.
Some sceptics have suggested that the recent warming is just part of a natural cycle. But previous changes in the earth's climate - such as ice ages - have been driven by changing output from the sun, or changes in the planet's orbit. None of these are sufficient to explain current warming, whilst greenhouse gases provide a clear match.
But uncertainties do remain. No-one knows for sure exactly how sensitive the earth's climate system is to elevated greenhouse gas levels. Most computer models suggest that a doubling of CO2 (from a pre-industrial level of around 270 parts per million (ppm) to 550 ppm) would lead to a temperature rise of about 3°C. So why is the IPCC's figure - of 1.4°C to 5.8°C by 2100 - so broad? The reason is that the biggest uncertainty of all is future human use of fossil fuels. A business-as-usual path would lead to atmospheric carbon dioxide concentrations nearing 1000 ppm by the end of this century, pushing the planet closer to the latter figure of extreme warming. A cleaner development path - with renewables substituting for oil, coal and gas - would constrain warming at a much lower level.
Where will all this pollution come from? Currently the United States, with less than 5 per cent of the world's population, is responsible for emitting almost a quarter of global CO2. China, with over a billion people, comes second in the CO2 emission stakes, but by 2020 it will probably be emitting more CO2 than the United States (see Figure 1). China and India both have enormous coal reserves, and plan to burn huge amounts to generate power for their surging economic growth.
(See chart on 'Average Global Temperature and Atmospheric Carbon Dioxide Concentrations, 1950-2001')
Hot air: current impacts
Perhaps the clearest evidence of current global warming comes from the world's mountain ranges. Glaciers are melting everywhere from the Alps to the Himalayas. In the United States, the original 150 glaciers within Glacier National Park are now down to a mere fifty. The Alps have lost a half of their ice over the last century. Africa's Kilimanjaro lost 80 per cent of its ice cap between 1912 and 2000. The only glaciers currently growing in size are a few in Scandinavia, where the rising temperatures have been matched by an increase in snowfall.
Also melting fast are some parts of the earth's polar regions. The Antarctic Peninsula is the most rapidly warming area of the planet, and the Larsen B ice shelf on the peninsula's eastern edge - a floating shelf the size of Luxembourg - collapsed dramatically in 2002. On the other side of the world, the Arctic is - according to a 2004 landmark report called the Arctic Climate Impact Assessment (ACIA) - experiencing "some of the most rapid and severe climate change on earth." Snow cover has declined by 10 per cent over the past 30 years, and melting permafrost is destroying buildings and forests alike. Summer sea ice extent has declined by a fifth in the past three decades, and the 2002 ice melt on Greenland broke all records.
Melting land ice combines with thermal expansion of the oceans to cause sea level rise, which is now having a visible impact on vulnerable Pacific atolls like Tuvalu, whose people now suffer annual bouts of flooding. Coastal erosion is also an increasing problem, and 70 per cent of the world's sandy shorelines are now retreating. Saltwater intrusion into low-lying areas has been documented as far afield as China's Yangtze delta and Australia's Mary river. According to satellite monitoring systems, the rate of sea level rise has recently begun to accelerate, and has now reached 2.8mm per year.
The expected result of an increase in global temperature is an increase in extreme weather as the planetary hydrological cycle speeds up. More rainfall is now recorded in higher latitudes, whilst droughts are more profound in the arid sub-tropics. Heatwaves have killed increasing numbers of people: upwards of 30,000 perished in Europe during the blistering summer of 2003. Although caution needs to be exercised in linking specific events to climate change, a statistical analysis of the 2003 summer heatwave by Peter Stott of the Hadley Centre for Climate Prediction, published in Nature, showed that human greenhouse gas emissions made it twice as likely (see: Human contribution to the European heatwave of 2003). 2004 also saw a very active Atlantic hurricane season, with a record four storms ploughing into Florida - whilst a tropical cyclone also formed for the very first time in the south Atlantic, hitting Brazil. However, the eastern Pacific saw a below-average number of storms.
Already climatic change has begun to alter the distribution of certain species. For example, it has enabled birds like cattle egret to extend their range northwards. Two worldwide studies published in 2003 found evidence of changes in wildlife behaviour ranging from earlier frog breeding and the arrival of migrant birds to a general move towards the poles for butterflies, fish and hundreds of other plant and animal species. In most cases the changes have been damaging: hundreds of thousands of Scottish seabirds failed to raise young during the 2004 breeding season due to a collapse in the numbers of sand eels, the birds' main food source - itself related to a shift north of cold-water plankton.
Hot air: future impacts
The IPCC predicts that as a result of global warming sea levels will rise by between 0.09 and 0.88 metres by 2100. This may not sound like much, but a half-metre rise in sea level would put paid to many low lying islands, especially in the Pacific, and cause vast saline inundations in countries like Bangladesh. Many cities - among them Mumbai (Bombay) and Bangkok - could be threatened too. Millions of people will be forced to move as 'environmental refugees' to higher ground.
Future global warming will affect different regions in different ways. Take, for example, Europe. A report by the European Acacia Project predicts that parts of southern Europe will become so hot that the tourist industry could suffer, and alpine glaciers could melt entirely by the end of the century. In northern Europe snowfall and rain may increase. In the north cropland productivity is likely to increase, but with the warmer weather will come a greater risk of pests and diseases spreading.
The IPCC scientists predict that a decrease in rainfall in some parts of Africa, Latin America and Asia - another likely consequence of global warming - could lead to a decline in grain yield of 10-15 per cent over the next 50 years. Indeed, the IPCC suggests that global warming could trigger more extreme weather events - floods, droughts and the like - which will have a serious impact on food production and human populations. Areas of the world which depend on runoff from glaciated mountains would see a catastrophic drop in water supplies as the final icefields melt.
The costs of global warming, in financial terms, could be astronomic. Already the cost of property damage is rising at around 10 per cent a year, and between 1970 and 2000 economic losses caused by natural disasters doubled. Some insurance experts warn that the industry could be in danger of running out of money if it is to meet claims stemming from climate-related disasters in the not too distant future (see graph). Warmer ocean waters are likely spark more intense hurricanes, with more damaging winds and much heavier rainfall.
Overall, global warming is likely to be a disaster for biodiversity, unravelling complex ecosystems as species migrate to stay within their climate zone. And those that benefit tend to be invasive competitors, adding to the non-climate extinction crisis. A report published in the scientific journal Nature in 2004 suggested that up to a third of species could be 'committed to extinction' because of climate change by 2050. Some scientists have speculated that global warming could lead to the complete wiping out of the Amazon rainforest, causing a biological catastrophe unparalleled in modern times. This is likely to be accompanied by the widespread loss of coral reefs due to more frequent 'bleaching' in warmer waters.
For a decade now scientists and policy makers have been wrangling over how to tackle global warming. At Kyoto in 1997 industrialised nations agreed to reduce their greenhouse gas emissions by 5.2 per cent relative to the 1990 levels by the period 2008-2012. The targets varied: a 20 per cent reduction was pledged by Germany; 12.5 per cent by the UK; 7 per cent by the US; while some countries, like Australia, negotiated large increases.
As far as most environmentalists and many climate scientists are concerned, the cuts agreed upon are too small to make much difference. However, the Kyoto Protocol was significant in that rich nations pledged to cut their emissions without requiring poor nations to do the same - yet. Inevitably greenhouse gas emissions in the developing world will rise over the coming years. For one thing their populations will continue to rise; for another, the better off people become, the more likely they are to become significant consumers of energy, and thus emitters of carbon and other greenhouse gases.
While some countries are on course to meet their targets - the scaling down of the coal industry has helped the United Kingdom, for example, to do so - others are not. To meet its targets by the end of this decade, the United States would have had to slash present emissions by 20 per cent. This is very unlikely to happen, not least because the United States, along with Australia, withdrew from the process when George W. Bush became president in 2000.
The Bush Administration declared Kyoto 'dead', but the rest of the world - led particularly by the European Union - begged to differ. With Russian ratification in December 2004, the Kyoto Protocol will finally come into legal force in February 2005. Although developing nations are not required to make cuts at this stage, Kyoto remains a truly global treaty: at the last count 132 countries, including 38 industrialised ones, have now ratified the Protocol (see: The Kyoto Protocol).
The European Union did not wait for full ratification before taking measures to help it comply with the Kyoto provisions. In January 2005 it launched an emissions trading scheme, which allows individual companies to buy and sell 'pollution permits' in order to reduce their costs - providing they do not exceed the 'cap' or absolute limit on emissions from highly polluting industries.
Leaving the politics aside, how can we reduce emissions of greenhouse gases? Most obviously, by reducing the use of fossil fuels, by using energy more efficiently, and by switching from fossil fuels to alternative sources of energy such as solar and wind power. The biggest short-term move which the United States could take, according to the Sierra Club, is to improve the fuel efficiency of cars (see: New experiment slashes vehicle pollution).
Significantly, many industries which just a few years ago rubbished the whole notion of global warming now accept that it is a reality. Companies like BP Amoco and Shell are not only reducing their emissions voluntarily, they are also investing in alternative sources of energy - though they still spend much less than on their continuing search for oil.
Ultimately, tackling global warming will be a collaborative venture, with governments, businesses and individuals, in both the North and the South, participating in a whole range of emission-reducing activities. As for international action, the Kyoto Protocol is only a first step. It would merely stabilise or reduce by a few percentage points the emissions of greenhouse gases compared to 1990 levels.
If severe global climate warming damage is to be prevented, industrialised countries will have to cut those levels by about 80 per cent by 2050. Developing countries will also have to make a transition to cleaner energy in the coming decades in order to avoid a climate disaster.
Mark Lynas is an environmental journalist and author of High Tide, published by Flamingo/HarperCollins, London, 2004, £16.99 (hb)
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