UN climate talks resume in Warsaw on Monday amid a slew of warnings about a potentially disastrous rise in greenhouse-gas emissions.

Though the stakes are high, no specific targets have been set for this round, hosted by one of the world's biggest coal polluters just two years before the tortuous global process must deliver a new deal.

"There is no doubt that we have to act and that we have to act NOW," UN climate chief Christiana Figueres said ahead of the talks, which wrap on November 22 at ministerial level.

She urged nations to set aside differences and focus on limiting warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

"We can still put policy in place to get our vital 2 C goal," she told the Chatham House think tank in London last month.

Experts say the 2 C objective, set in 2009, may not be a safe haven and warn it will be badly overshot on current emissions trends.

This week, the UN Environment Programme (UNEP) said the chances of meeting 2 C were "swiftly diminishing", while the World Meteorological Organisation (WMO) reported atmospheric levels of greenhouse gases hit a new record high in 2012.

In September, the UN's Intergovernmental Panel on Climate Change (IPCC) predicted global surface temperatures could climb on average by as much as 4.8 C (8.6 F) this century -- a recipe for catastrophic heatwaves, floods, droughts and sea-level rise.

"We are heading to a world whereby temperatures will rise by maybe three, possible even up to five degrees," said Andrew Steer, head of the US research group, the World Resources Institute (WRI).

"This is very, very scary stuff and evidence is accumulating weekly, monthly, as to how dangerous this will be."

"Science has told us that emissions need to peak well within this decade," said Tasneem Essop of green group WWF.

"If we are not going to address that particular challenge... then we certainly are going to be missing the opportunity to stop catastrophic climate change."

The problem lies in invisible, heat-trapping gases from burning fossil fuels, which provide the backbone of the world's energy supply today.

Reducing this pollution requires costly efforts in efficiency and a switch to cleaner energy, which helps to explain why the UN negotiations have been such a battlefield.

Eyes on 2015 deal

The Warsaw meet of the UN Framework Convention on Climate Change (UNFCCC) has no measurable targets.

But observers hope it will at least do some legwork for a much-trumpeted deal, due to be signed in Paris in 2015 for implementation five years later.

"They need to come out of there (Warsaw) with a shared vision of the process and the timeframe leading up to Paris, and what they are trying to achieve," said Alden Meyer of the US environmental group the Union of Concerned Scientists.

"There is not even agreement now on whether Paris is the deadline for the final agreement. Some countries are saying, 'Maybe Paris ought to be the place where we get the framework and the rules, and then the numbers (for emissions cuts) come later'."

Other nations, though, want draft country pledges on the table when UN chief Ban Ki-moon hosts a climate summit in New York next September.

The gloves may also come off over how to help poorer nations cope with climate change.

Rich economies have yet to show how they intend to meet a pledge, made back in 2009, to muster $100 billion per year from 2020.

"The developed world -- they have capacity, they have means, they have money to handle this - but you do not," Ban said in Ouagadougou, capital of the impoverished Sahel state of Burkina Faso, on Thursday.

"It is only natural that the developed world provide the necessary funding, necessary technology so that you can address [this] impact, mitigate and adapt to these changing situations."

Then there is a new, and contentious, issue of additional "loss and damage" the West is expected to cover for poor countries' climate-related harm -- a point that nearly scuppered last year's talks in Doha.

"You could have a blowup of the whole process over loss and damage... or over the money -- anything could catalyse a real fight," said Meyer of the Warsaw talks.