While the Earth's climate has always changed naturally, for the first time human activity is now a major force affecting the process, with potentially drastic consequences.

Huge volumes of fossil fuels in the form of gasoline, oil, coal and natural gas are used everyday, releasing carbon dioxide. This, together with other emissions generated by human activity, such as methane and nitrous oxide, accentuate the natural 'greenhouse effect' that makes the Earth habitable. Carbon dioxide is the most important anthropogenic greenhouse gas, with annual emissions growing 80 per cent in 1970–2004.

The unprecedented speed of change is threatening social and environmental systems that cannot adjust at the same pace. More extreme weather events are occurring worldwide, sometimes in unexpected ways — such as the flooding in the dry region of Ethiopia in 2007. The UN Intergovernmental Panel on Climate Change (IPCC) says some changes may be sudden and irreversible. A rise in sea levels and extensive losses in biodiversity are just two of a range of likely impacts. Developing countries, with less technical, economic and institutional capacity to adapt, are likely to find it hardest to cope.

Given the legacy of greenhouse gas emissions and rising global energy consumption, further warming is unavoidable.

Mitigation measures to reduce the extent of global warming are crucial. The Kyoto Protocol — the first international agreement on tackling climate change — stipulates that industrialised countries, historically responsible for most emissions to date, must act first to curb emissions, giving time for developing countries to grow their economies and raise their peoples' living standards. But it also recognised that sooner or later developing countries will have to act. The IPCC predicts that emissions from developing countries — led by rapidly industrialising economies such as those of Brazil, China and India — will exceed those from developed countries during the first half of this century.

Energy and climate policies must plan for a world with a smaller carbon footprint, as well as for each nation's economic path.

Scores of measures are under discussion, such as compensation for countries that forgo forest clearance, tax incentives for low-emission technologies, the use of nuclear power and the development of new energy sources.

But the emergence of biofuels as a possible alternative to oil in transport shows how complex the issues are and underlines the need for much more research. Initially, many policymakers championed biofuels as indubitably beneficial. Gradually, however, researchers began to look more closely at individual biofuels and to make lifecycle assessments of their costs and benefits, in terms not only of carbon emissions, but also their impact on other areas of life, such as food prices and water consumption.

Adaptation to climate change is as important as mitigation. This applies particularly to developing countries, short of capital and skills, already struggling to generate economic growth and to deliver services such as healthcare, and vulnerable to the extreme weather events forecast by the IPCC. A recent IPCC report named areas particularly likely to be affected by climate change as Africa, small islands, Asian and African megadeltas and the Arctic.

Whatever actions are taken, the IPCC says that all the scenarios for climate stabilisation indicate that 60–80 per cent of reductions in emissions will come from energy supply and use, and industrial processes, "with energy efficiency playing a key role in many scenarios".

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