But Nicholas Stern, a British economist who authored a 2006 report on climate costs, warned the energy clean-up lacked essential tools.
"We need sound policies, such as a strong price on carbon, and much more investments in technologies to reduce emissions, including electricity storage, renewables, nuclear power, and carbon capture and storage."
The summary said atmospheric greenhouse gas levels of about 500 ppm CO2eq by 2100, which yields a lesser chance of staying under 2C, would require emissions 25-50 percent lower by 2050 than in 2010.
From levels of 550 ppm CO2eq by 2100, the chances of reaching the 2C target become less than 50 percent, and worse.
The document -- which listed options but made no recommendations -- said emissions increased an unprecedented billion tonnes per year in the decade ending 2010, driven by rapid economic growth powered by fossil fuels.
On current trends, 2100 atmospheric levels could be almost double or even triple those of today, it added.
Options for action include phasing out fossil-fuel subsidies, investing in cleaner sources, and rolling out technology -- still in its infancy -- to capture and store carbon emissions from power plants that burn coal, oil and gas.
US Secretary of State John Kerry described the report as "a wake-up call" for entrepreneurs.
The list also includes cutting energy waste and halting deforestation, boosting low-carbon public transport systems and designing smarter cities that are less energy-hungry.
"The global energy market represents a $6 trillion (4.34-trillion-euro) opportunity, with six billion users around the world. By 2035, investment in the energy sector is expected to reach nearly $17 trillion," he said.
UN chief Ban Ki-moon said the report should prompt countries to act "swiftly and boldly on climate change."
European climate commissioner Connie Hedegaard threw down the gauntlet to other major emitters to "reduce emissions now".
"We in Europe will adopt an ambitious 2030 target later this year," she said. "Now the question is: when will YOU, the big emitters, do the same?"
The Summary for Policymakers was adopted on Saturday after a line-by-line scrutiny by government representatives and scientists.
The full report, over 2,000 pages, should be released within days.
In the first volume of its Fifth Assessment Report, released last September, the IPCC predicted temperatures could rise 0.3 to 4.8 degrees C this century and sea levels creep up by 26-82 centimetres (10-32 inches).
The second chapter, published last month, warned of the rising risk of conflict, hunger, floods and mass displacement from coastal erosion.