Billions of people face shortages of food and water and increased risk of flooding, experts at a major climate change conference have warned.
The bleak conclusion came ahead of the publication of a key report by hundreds of international environmental experts.
Agreement on the final wording of the report was reached after a marathon debate through the night in Brussels.
People living in poverty would be worst affected by the effects of climate change, the gathered experts said.
"It's the poorest of the poor in the world, and this includes poor people even in prosperous societies, who are going to be the worst hit," said Rajendra Pachauri, chairman of the Intergovernmental Panel on Climate Change (IPCC).
Mr Pachauri said those people were also the least equipped to deal with the effects of such changes.
Outlining the report's findings, Martin Parry, co-chairman of IPCC Working Group II, said evidence showed climate change was having a direct effect on animals, plants and water.
"For the first time, we are no longer arm-waving with models; this is empirical data, we can actually measure it," he told a news conference.
Key findings of the report include:
75-250 million people across Africa could face water shortages by 2020
Crop yields could increase by 20% in East and Southeast Asia, but decrease by up to 30% in Central and South Asia
Agriculture fed by rainfall could drop by 50% in some African countries by 2020
20-30% of all plant and animal species at increased risk of extinction if temperatures rise between 1.5-2.5C
Glaciers and snow cover expected to decline, reducing water availability in countries supplied by melt water
The report states that the observed increase in the global average temperature was "very likely" due to man-made greenhouse gas emissions.
The scientific work reviewed by IPCC scientists includes more than 29,000 pieces of data on observed changes in physical and biological aspects of the natural world.
Eighty-nine percent of these, it believes, are consistent with a warming world.
Year of reports
Scientists and politicians have welcomed the report.
"This further underlines both how urgent it is to reach global agreement on reducing greenhouse gas emissions and how important it is for us all to adapt to the climate change that is already under way," European Environment Commissioner Stavros Dimas told the Reuters news agency.
"This is another wake up call for governments, industry and individuals. We now have a clearer indication of the potential impact of global warming, some of which is already inevitable," said Martin Rees, president of the Royal Society.
"The challenge is now to support those people living in the most vulnerable areas so that they are able to adapt and improve their ways of life."
The wording of the summary of the report, which will be sent to world leaders in time for a G8 summit of industrialised nations in June, was finally decided after scientists and government officials from more than 100 countries worked through the night.
Several delegations, including the US, Saudi Arabia, China and India, had asked for the final version to reflect less certainty than the draft.
It is the second in a series of IPCC reports coming out this year, together making up its fourth global climate assessment.
The first element, on the science of climate change, released in February, concluded it was at least 90% likely that human activities are principally responsible for the warming observed since 1950.
The third part, due in May, will focus on ways of curbing the rise in greenhouse gas concentrations and temperature.
A fourth report in November will sum up all the findings.