Climate change may have "serious, pervasive and irreversible" impacts on human society and nature, according to a draft report.
The United Nations is seeking approval for the document but warns in it that governments still have time to avert the worst.
The warning comes as more than 100 governments and top scientists meet in Copenhagen today for a meeting to edit the report.
It will form the main reference point for nations meeting in Paris in late 2015 and working on a deal to fight climate change. The report will be published on November 2.
Last week, EU leaders agreed to cut - before 2030 - emissions by 40 per cent below 1990 levels in a shift from fossil fuels towards renewable energies, and urged other major emitters, led by China and the US, to follow.
Peru's environment minister Manuel Pulgar-Vidal, who will host a UN meeting to lay the groundwork for the Paris summit, said the report by the Intergovernmental Panel, which draws on three mammoth scientific documents, will show the need for urgent and ambitious action.
The Scottish government has pledged that by 2020 the country will be 100 per cent reliant on producing its energy needs from renewable sources.
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