Leading article: The world's biggest polluters can no longer ignore the evidence

Two months ago the United Nations Inter-governmental Panel on Climate Change gave us chapter and verse on the science of global warming. And now the new IPCC report spells out the effect this heating of the earth will have on the planet's inhabitants.

This synthesis of the work of 2,500 scientists tells us that climate change is already having a significant effect on our environment. Animals, plants and water systems are under pressure now. The IPCC also highlights a discernible impact on human societies. But it is in its predictions that the report is most worrying. It argues that about a third of the earth's species face a greater risk of vanishing if global temperatures rise 2C above the average of the 1990s. Ecosystems in areas of coral reef, sea ice, tundra and boreal forests are under serious threat.

The IPCC also argues that desertification, drought and rising sea levels will affect billions of people. Africa, home to the poorest people on the planet, will be hardest hit. Some 75 million to 200 million more people there will be exposed to water shortages and crop failure. It estimates that some African nations might have to spend 5 to 10 per cent of their gross domestic product on adapting to climate change. Small island communities will also be at severe risk. A sea-level rise is expected to exacerbate flooding, storm surges, coastal erosion and other hazards faced by such communities.

But all human societies will be affected. A runaway thaw of the Himalayan glaciers that feed rivers throughout Asia is likely to cause massive flooding and avalanches. Higher temperatures will mean heatwaves, more severe storms and droughts in North America. Europe will suffer the same. And there will also be a greater risk of flooding as the Alpine glaciers disappear. In Latin America, eastern parts of the Amazon rainforest will turn to savannah. In Australasia, the Great Barrier Reef is predicted to lose much of its coral by as early as 2020.

The politics of the IPCC is complicated because every nation has to sign off on the report. Negotiations in Brussels on the wording of the document have been fraught. Scientists have accused certain governments of watering down some of their findings. Russia, China, Saudi Arabia and the United States have been the principal objectors. Their complaints resulted in the removal of parts of a chart from the report highlighting the effects of climate change that kick in with every temperature rise of 1C. China tried to eliminate a note saying there is "very high confidence" that climate change is already affecting all continents. And US delegates rejected the suggested wording that areas of North America will suffer "severe economic change" from global warming.

China's objections are particularly worrying. If that country continues to grow at the present rate it will be the world's biggest economy - and polluter - by the middle of the century. It is vital that China recognises the seriousness of the threat of climate change. Yet it is remarkable that despite such political meddling, the final report is so unequivocal. The IPCC finds that climate change presents one of the most serious threats ever faced by human life on the planet.

The third part of IPCC report part will be released next month. This will concentrate on ways human societies can curb the rise in greenhouse gas concentrations. All this evidence will be presented to the G8 summit in Berlin in June. The world's governments have endorsed a report that makes the scale of the problem of climate change abundantly clear. It is now their responsibility to do what is necessary to alleviate this crisis.

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