"Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.
"Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions."
The document published on Friday is a summary for ministers and policymakers of a weightier report which will be released in full on Monday.
Scientists and politicians from 195 countries spent the past week debating every word in the summary, often into the early hours of the morning, including a 5am finish yesterday.
The report warns that global temperatures have risen by 0.85C since 1880, with the fastest increases since 1950.
Greenhouse gas concentrations have increased to their highest level in at least 800,000 years, including a 40 per cent rise in carbon dioxide since pre-industrial times driven largely by the burning of fossil fuels.
Some 90 per cent of the extra heat accumulated since 1971 has been absorbed by the oceans, and sea levels have risen by 19cm (7.5in) in a century.
The Arctic is losing ice at a rate of half a million square kilometres per decade, although it is very likely that Antarctic sea ice is increasing, the report said.
In future it is "virtually certain" that most land areas will experience more heatwaves and warmer weather, although occasional extremely cold winters will still occur.
The contrast between wet and dry areas will increase, while the area covered by monsoons is likely to expand.
Britain will most likely see wetter winters and drier summers, while a slowing of the ocean current which causes the Gulf Stream will probably weaken, offsetting some of the warming.
The report also predicts that:
• Surface temperatures will rise by between 0.3C and 4.8C above pre-industrial levels by 2100, with most models predicting an increase of at least 1.5C and little chance of a rise below 2C.
• Arctic sea ice will continue to shrink and thin, potentially disappearing completely in late summer by 2050.
• Global sea levels will rise by a further 26cm to 82cm by 2100, most likely at a faster rate than in the past 30 years.
• The oceans will absorb more carbon, leading to waters becoming more acidic and posing a threat to marine life.
Even if emissions are cut quickly, most of these effects will be felt for several centuries due to carbon dioxide already in the atmosphere, the scientists said.
"The heat is on. We must act," said Ban Ki-moon, the secretary-general of the UN, while John Kerry, the US secretary of state, said the figures were "yet another wake-up call", adding that "those who deny the science or choose excuses over action are playing with fire".
The report also addressed the so-called "pause", or the comparative lack of atmospheric warming, seen since 1998, which scientists said was far from unusual.
The report makes clear that each of the past three decades has been successively warmer at the Earth's surface than any preceding decade since 1850, and that 1983-2012 was likely to have been the warmest 30-year period for 1,400 years.
William Hague, the Foreign Secretary, said the report "confirms that climate change is already happening, as a result of human activity".
He added: "Governments, businesses and individuals all have a responsibility to tackle climate change. The longer we delay, the higher the risks and the greater the costs to present and future generations."
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