Ministers from around the planet launched a five-day scramble in Paris Monday to answer "the call of history" with a deal to spare mankind from climate disaster.

The 195-nation UN talks have been billed as the last chance to avert the worst consequences of global warming: deadly drought, floods and storms, and rising seas that will engulf islands and densely-populated coastlines.

"The opportunity to rise to the call of history is not given to everyone or every day," UN climate chief Christiana Figueres told the conference on the northern outskirts of the French capital.

"History has chosen you here, now."

UN Secretary General Ban Ki-moon also warned negotiators "the clock is ticking towards a climate catastrophe", and implored them to put aside rows and soft compromises that have cursed previous UN climate campaigns.

"The world is expecting more from you than half-measures and incremental approaches. It is calling for a transformative agreement," Ban said.

"The decisions you make here will reverberate down the ages."

Taking effect from 2020, the Paris accord would seek to limit emissions of heat-trapping greenhouse gases driven by coal, oil and gas -- the backbone of the world's energy supply today.

The goal is to limit global warming to under two degrees Celsius (3.6 degrees Fahrenheit) from pre-Industrial Revolution levels.

Scientists say the planet is already nearly halfway to 2C, which means the rise in fossil-fuel emissions must peak soon, and go quickly into reverse, to meet the precious objective.

- Ray of hope -

Offering a rare glimmer of optimism, a new study said fossil-fuel emissions of the major global warming culprit, carbon dioxide, were set to decline in 2015.

It would be the first such planetary-wide fall in a period of economic growth, said the report in the journal Nature Climate Change, explaining that decreased coal use in China was the main factor.

But it cautioned 2015 was unlikely to be the year in which CO2 peaks -- a glittering objective for climate campaigners -- and said emissions would take years to decline "substantively".

The Paris talks opened November 30 with a record-breaking gathering of 150 world leaders issuing a chorus of warnings about mankind's fate if planet warming went unchecked.

After a week of talks, negotiators met a Saturday deadline to produce a broad blueprint but left unresolved many of the deep and complex divisions that condemned previous UN efforts to failure.

But environment and foreign ministers, including US Secretary of State John Kerry, now have to rip out hundreds of bracketed words or sentences in the blueprint that denote disagreement.

- 'Difficult week' -

The ministers face a Friday deadline to reach an accord, seeking to resolve the arguments that primarily pit rich nations against the poor.

"We have a difficult week ahead of us... with all the major issues still unsolved," Luxembourg Environment Minister Carole Dieschbourg told reporters.

Among the slew of fundamentals in dispute are how far and how fast to limit global warming, and how to review national commitments to curb greenhouse gas emissions.

The UN's Ban urged nations to be bold, pressing ministers to agree to five-year reviews of national carbon-curbing pledges, and to initiate the scrutiny even before the planned deal would come into effect in 2020.

Small island states at risk of being swamped in a warmer world are also part of a big coalition of vulnerable nations pressing for a more ambitious accord of limiting planetary warming to less than 1.5C.

"Any further temperature increase beyond 1.5 degrees Celsius will spell the total demise of Tuvalu and other low-lying island nations," Tuvalu Prime Minister Enele Sosene Sopoaga told the conference.

"If we save Tuvalu we save the world."

- Money fight -

Another potential deal-buster is money.

Rich countries promised in 2009 to muster $100 billion (92 billion euros) a year from 2020 to help developing nations make the costly shift to clean energy, and cope with the impacts of climate change.

But how the promised funds will be raised remains unclear -- and developing countries are pushing for a promise that the amount will be ramped up beyond 2020.

Meanwhile, rich nations are insisting developing giants work harder to tackle their greenhouse gases, noting much of the world's future emissions growth will come from their fast-growing economies.

"It is time to get rid of this rigid differentiation between developed and developing in a way that prevents us from maximising our progress going forward," Kerry said after flying in from the United States.

The burden-sharing row dates back to 1992, when the UN Framework Convention on Climate Change (UNFCCC) was set up at the Rio Earth Summit.

For poorer countries, the division is a touchstone of solidarity, but among rich economies, it is becoming an anachronism.

European Climate Commissioner Miguel Arias Canete told journalists that developed countries now account for under 35 percent of global emissions.

"Those responsible for the remaining 65 percent of the global emissions must also take urgent action," Canete said.

Offering a gloomy but timely reminder of a future in which emissions go unchecked, a "red alert" for air pollution was issued in China's capital, Beijing, on Monday.

As winter coal burning for heating escalated in Beijing, authorities warned people to stay indoors and ordered many factories to close. Half of Beijing's private cars will also be ordered off the roads from Tuesday.