The document, a draft of which has been seen by UK daily The Independent, is to be issued in Paris on Friday by the UN's Inter-governmental Panel on Climate Change (IPCC).
Rising temperatures may hamper Earth's natural ability to absorb carbon dioxide
The report says that temperature increases of between 2 and 4.5C are highly likely this century, but gains of 6C "or more" cannot be ruled out.
The document, to be published at the end of a four-day meeting, is agreed by consensus by around 500 scientists and government representatives.
Human role
The draft makes a strong link between increases in man-made carbon dioxide emissions and climate change.
The document says that since the last such report, released in 2001, "confidence in the assessment of the human contributions to recent climate change has increased considerably.
Herve le Treut,
Director,
Dynamic Meteorology Laboratory,
Pierre-Simon Laplace Institute,
France
"It is highly likely that the warming observed during the past half century cannot be explained without external forcing (human activity)."
In the 2001 report, scientists forecast a global atmospheric temperature increase of between 1.4 and 5.8C by 2100.
The draft also warns of "positive" feedbacks, which could play a role in accelerating temperature increases.
The report says that as the world warms it causes increased evaporation, leading to higher concentrations of water vapour, thereby exacerbating the greenhouse effect.
The document notes that water vapour concentrations over oceans have increased by four per cent since 1970.
The IPCC's climate models also warn that rising temperatures will hamper the planet's natural ability to absorb carbon dioxide.
Heat waves are likely to be more intense, more frequent and longer-lasting, and tropical storms and hurricanes will probably be stronger.
Tags: UN report climate