Global warming is an "unequivocal" reality which will bring death and suffering to millions if it is left unchecked, say the world's leading experts on climate change.

There is at least a 90% chance human activities are the main cause of global warming since 1950, according to the draft of a report issued in Paris today by the Intergovernmental Panel on Climate Change (IPCC), which advises the United Nations.

The report, The Physical Science Basis of Climate Change, the first of four this year, draws on work by about 2500 specialists from more than 130 nations.

The experts predict the world will be 3C hotter by the year 2100, enough to leave four billion people short of water and to melt the Arctic ice cap, perhaps entirely.

The IPCC scientists are clear human activity is the driving force behind the changes.

"Concentrations of heat-trapping greenhouse gases in the atmosphere, largely from burning fossil fuels in power plants, factories and vehicles, have risen by more a third since before the Industrial Revolution," say the authors in a draft.

The IPCC's 90% probability is far higher than its last report, in 2001, which said the link was "likely", or at least a 66% chance.

It is likely greenhouse gases would have caused more warming in recent decades but their effects were partly offset by dust from volcanoes and pollution which reflects heat back into space, the authors say.

In Britain, which has just seen the warmest January since 1916, at 5.9C, the changes predicted by the IPCC would lead to warmer, wetter winters, more droughts during summer, and an increased risk of storms and freak weather.

Duncan McLaren, Friends of the Earth Scotland's chief executive, said: "This report will show with certainty we are to blame for the last 50 years of warming."