As UN negotiators roll up their sleeves for the last push towards a universal climate deal, many fear their end-goal of halting global warming at two degrees Celsius is moving out of reach.

In the corridors of UN climate gatherings, negotiators concede the window of opportunity appears to be closing.

Few will admit this on the record, and none will even consider shifting the goal posts even as science points to a likely four-degree increase on current trends.

"The two-degree limit is perhaps symbolic, but if we exceed it, we take big risks," said climatologist Jean Jouzel -- pointing out the world is not geared for the climate extremes likely to result from a higher level of warming.

"On paper it (2 C) is still possible," Jouzel, vice president of the Inter-governmental Panel on Climate Change (IPCC), told AFP -- while admitting "it seems difficult".

The target of limiting planetary warming to 2 C (3.6 degrees Fahrenheit) over pre-industrial levels was formally adopted at a UN climate conference in Copenhagen in 2009, and remains the holy grail for negotiators drafting a new, global pact that must enter into force in 2020.

A 12-day session of talks that will close in Bonn on Friday forms part of the countdown to a much-anticipated UN summit in Paris in 2015 tasked with signing the most ambitious deal yet in the fight against global warming.

The pact will for the first time bind all the world's nations to measurable targets for curbing Earth-warming greenhouse gas emissions.

But while negotiators whittle away at geopolitical intricacies to agree on who must cut back how much, the level of pollutants being pumped into the atmosphere continues to rise.

Last month, the concentration of climate-altering carbon dioxide (CO2) in the atmosphere exceeded 400 particles per million for the first time since humans have walked the Earth.

The last report of the IPCC, which informs policy makers, said a 400 ppm ceiling was key to halting the thermometer rise at between 2 C and 2.4 C.

Last week, the International Energy Agency (IEA) said the world was on course for a temperature rise of between 3.6 C and 5.3 C.

Former UN climate chief Yvo de Boer said last year the two-degree objective was "out of reach".

And just this week, the German Institute for International and Security Affairs think tank wrote in The Guardian newspaper that "the two-degree target will have to be modified eventually."

"Nobody really wants to talk about the coming failure of the 2 C target. But from a political point of view it is pretty clear that a target that is considered to be unattainable cannot fulfil either a positive symbolic function or a productive governance function," it said.

The 20-year-old global climate negotiations have been hamstrung from the very beginning by nitpicking, procedural bickering and defence of national interests.

As the UN process hobbles along, national and bilateral efforts have started yielding results -- take a US-China deal reached last week to phase out potent "super greenhouse gases" -- hydrofluorocarbons found in refrigerators and air conditioners.

Yet the UN's 2 C target -- the only tangible goal for negotiators -- retains many staunch defenders, including EU climate commissioner Connie Hedegaard, who told a recent Paris conference the world couldn't just shift the target because it was "challenging".

The IEA said the 2 C target could still be met with political will to make a global move away from coal-fired power stations and phase out fossil fuel subsidies.

According to Jason Lowe of the Hadley Centre for Climate Prediction and Research in Exeter, sticking to a 2 C scenario would avoid one-fifth of the water stress problems associated with 4 C, and result in two-thirds fewer coastal flood victims.

"In terms of spring wheat productivity, it (2 C) avoids between a third and a half" of impacts, he said.

-- Flight of fancy? --

According to climatologist Jouzel, "we must cut our emissions by half at least from now to 2050, with a peak in 2020 at the latest" to stand any chance of keeping warming in check.

"We need political will and vision, and for the moment we don't have it," said one European negotiator in Bonn -- while stressing the 2 C target must not be changed.

"If we raise it, why would we stop at 3C? We cannot allow that kind of thinking."

For Ronald Jumeau, the negotiator for Seychelles -- one of the nations most at risk of climate change-induced sea level rise -- tampering with the target would amount to "sacrificing" the world's most vulnerable people.

In fact, Seychelles and other island states want it lowered to 1.5 C.

"If we were to abandon the 1.5-2 C target, the negotiators for the SIDS (Small Island Developing States), other low-lying countries and the LDCs (Least-Developed Countries) might as well pack our bags and go home and exchange them for life rafts," said Jumeau.