Evidence of global warming is now "unequivocal" and unless action is taken the consequences will be "abrupt" and "irreversible", according to the fourth synthesis report of the Intergovernmental Panel on Climate Change (IPCC).
The report contains a 26-page summary on the risks of global warming that will be presented to environment ministers in Bali on December 3rd.
It says that it is 90 per cent certain that human activity is causing warming and some impacts are unavoidable even if immediate steps are taken to curb greenhouse gas emissions,.
It also presents a wide variety of measures to mitigate climate change and says implementing these solutions would only reduce global economic growth marginally.
"The science is clear, we have the solutions to deal with this issue affordably, and it's now up to politicians to put negotiations in place and come to grips with this," said Yvo de Boer, executive secretary of the UN Framework Convention on Climate Change, which is organising the upcoming climate change talks in Bali.
"There's been a lot of talk about a Bali roadmap, and this report is the compass to that roadmap."
The IPCC report says there has been an improvement in the understanding of climate change, particularly the relationship between impacts and vulnerabilities. This has enabled scientists to identify many of the risks associated with warming with a higher degree of confidence and to upgrade some of the risks identified in earlier reports. It contains a reassessment of five key "reasons for concern" already outlined in the third IPCC assessment report of 2001, concluding that there is "new and stronger" evidence of the impact of climate change on vulnerable communities and ecosystems.
Between 20 to 30 per cent of plant and animal species are likely to be at increased risk of extinction if increases in temperatures exceed 1.5-2.5 degrees over 1980-1999 levels.
The risk of big increases in sea levels due to the melting of the Greenland and possibly Antarctic ice sheets may be larger than previously expected, says the report, which indicates this could eventually push sea levels up by several metres.
Even without taking into consideration the melting ice sheets, the speed of sea level rises is already increasing due to thermal expansion or warming of the sea, currently at an average of 3mm per year up from 1.8mm per year between 1961 to 1993. The report also warns of projected increases in extreme weather events such as drought, heatwaves and cyclones.
Those parts of the world least able to cope, particularly poverty-stricken areas in Africa and Asia, are likely to bear the brunt of the impact of climate change. Between 75 to 250 million people in Africa are likely to suffer from water shortages by 2020 and in some African countries yields from rain-fed agriculture could be reduced by up to 50 per cent.
Increased rainfall and rising sea levels are likely to lead to more flooding in south and east Asia.