"It's a low premium to pay to reduce the risk of major climate damage," Bill Hare, a Greenpeace adviser who co-authored the report, said after four days of meetings and talks in Bangkok, the Thai capital.
The report said that even an increase of 2C could subject two billion people to water shortages by 2050 and threaten extinction for 20 to 30 per cent of the world's species.
To keep within the two-degree increase, emissions of carbon dioxide need to drop between 50 and 85 per cent by 2050, the report said.
However, technological advances - particularly in producing and using energy more efficiently - meant that such targets were within reach, the report said.
It highlighted the use of nuclear, solar and wind power, more energy-efficient buildings and lighting, as well as capturing and storing carbon dioxide produced by coal-fired power stations and oil and gas rigs.
The report, agreed by scientists and officials from more than 100 countries, does not set out policies.
It reviews the latest science and costings of ways to curb emissions growth and is designed as a blueprint for governments. Current policies were inadequate, and action was needed now, it said.
"The need for immediate short-term action in order to make any significant impact in the longer term has become apparent," it said.
Delegates said the onus was now on governments to put the report into action.
"This is a good report to guide governments," Stephan Singer, of the environmental group WWF, said.
In some cases, technology could lead to substantial benefits, such as cutting health costs by tackling pollution.
Even changing planting times for rice paddies or managing cattle herds and sheep flocks better could cut emissions of methane, another powerful greenhouse gas, the report said.
At talks, China and Europe argued about the costs and levels of greenhouse gas emissions which ought to be allowed. Delegates also debated the role of nuclear power.
China, the world's second-largest emitter of greenhouse gases after the United States, wanted the IPCC report to exclude language that would promote stabilising emissions near current levels - partly because of the limited economic studies available.