A draft copy of the report by the UN's Intergovernmental Panel for Climate Change (IPCC) also warns that if temperatures rise by more than 2C now expected before 2050, 20 per cent of the world's population will face a great risk of drought.
With that level of temperature rise, other parts of the world will face increased flood risk from rainfall and there will be a decrease in cereal harvest in some regions.
There will also be a rise of flooding, particularly around deltas in China and Bangladesh and low Pacific islands.
A UN panel of 2,500 climate change scientists, which won the Nobel peace prize this year with the former US vice-president Al Gore, compiled the report.
It says that most of the increase in global average temperatures since the mid-20th century is "very likely" to be the result of greenhouse gas emissions. Otherwise, global temperatures might have been expected to decrease.
The Synthesis Report, the fourth from the IPCC this year, is intended to inform negotiations on a new climate change treaty next month in Bali.
The scientists will say that it is possible to halt global warming if the world's greenhouse gas emissions start to decline before 2015. This, however, is highly unlikely. Emissions are projected to increase by up to 90 per cent by 2030 on present estimates, according to the report.
As scientists gather in Spain to agree to the summary of the report, environmentalists say they fear that it plays down the need for deep cuts in emissions.
The environmental group WWF, formerly the Worldwide Fund for Nature, says "vital facts" have been cut from the summary as a result of the influence of government officials from countries opposed to taking radical action.
These facts include a warning of more destructive hurricanes, the warming of the upper Pacific Ocean and the loss of glaciers in the Alps.
The report, to be published on Saturday in Valencia, will also not contain worrying evidence published in the past year that the Southern Ocean has started to take up less carbon dioxide from the atmosphere, accelerating the pace of global warming, or predictions that there could be other "positive feedbacks," such as the Amazon forest dying back and releasing carbon into the atmosphere.