The world can still meet the UN goal to limit global warming provided it cuts annual greenhouse gas emissions by 40-70 percent by 2050, a top expert panel said Sunday.

The cost will not be crippling, but the longer it takes to switch to cleaner energy sources, the harder and more expensive it will become to halt warming at the UN's targeted two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

The Intergovernmental Panel on Climate Change (IPCC) warned starkly that on present trends the planet would be 3.7-4.8 C warmer by 2100 -- a level many scientists say could be catastrophic.

"There is a clear message from science: to avoid dangerous interference with the climate system, we need to move away from business as usual," said Ottmar Edenhofer, co-chairman of the UN expert group that compiled the report.

While most scenarios for meeting the target "require substantial investments", he said, this "does not mean that the world... needs to sacrifice economic growth."

UN climate chief Christiana Figueres said the findings challenged governments to make the world carbon-neutral in the second half of the century.

"We cannot play a waiting game where we bet on future technological miracles to emerge and save the day," she said in a statement.

The report is the third and final chapter of a mammoth overview by the Nobel Prize-winning panel -- its first assessment since 2007.

Compiled by hundreds of experts over four years, it collates the available science on climate change, seeking to inform national policies and the faltering global effort, which Figueres oversees, to formulate a pact by 2015 on curbing emissions.

A summary of the document, issued Sunday, warns that delaying further cuts to 2030 would "substantially increase the difficulty" of reaching the 2C goal.

For a 66 percent or "likely" chance of meeting it, the concentration of greenhouse gases in the atmosphere must be contained at about 450 particles per million of CO2 equivalent (ppm CO2eq) in 2100 -- compared to 430 ppm CO2eq in 2011.

This would entail a 40-70 percent emissions reduction from 2010 to 2050, nearing zero by 2100.

- Unprecedented emissions rise -

There would also be a "tripling to nearly a quadrupling" in the share of energy from renewable and nuclear sources and from traditional fossil or new biofuel sources whose emissions are captured.

This will come at a cost -- clipping about 0.06 percentage points annually off growth in global consumption, which would otherwise have been about 1.6-3.0 percent per year over the century, according to the estimates.

The calculation did not factor in potential savings from tackling emissions, including healthier humans and ecosystems and energy security.

"The IPCC is clear that acting on climate change is possible, beneficial and affordable," said WWF climate representative Samantha Smith.

"If we act now, costs will be only a very small fraction of global economies."

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.

- 'Wake-up call' -

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.

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.

US Secretary of State John Kerry described the report as "a wake-up call" for entrepreneurs.

"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.