But only six months are left for completing a deal as fiendish in its complexity as it is unprecedented in ambition. Can it be done?

In the corridors of Bonn's MaritimHotel, where the 12-day round unfolded under the UN Framework Convention on Climate Change (UNFCCC), many delegates seemed to have quietly acknowledged the impossibility of sewing everything up in December in Copenhagen.

That goal is enshrined in the "BaliRoad Map," laid down at a global gathering in December 2007.

The vision is to set curbs on emissions of heat-trapping greenhouse gases beyond 2012, with intermediate targets for 2020 that would be ratcheted up all the way to 2050.

UNFCCC Executive Secretary Yvo de Boer insisted on Friday a "comprehensive" agreement could be reached in Copenhagen, and one "that can give a strong and definite answer to the (...) climate alarm that has been ringing loudly over the past few years."

European Union negotiator Artur Runge-Metzger said "a ratifiable treaty" was still in sight, and Jonathan Pershing, for the United States, likewise reiterated his endorsement of this aim. If so, a mountain of work lies ahead.

A 50-page draft negotiation blueprint has exploded to more than 200 pages after countries stuffed it with rival proposals, and may expand even further in informal talks in August.

There has been no progress on the biggest question, of how to share the burden of future emissions cuts -- and scientists say the proposals that are on the table fall dismally short of what is needed.

No agreement is in sight over helping poor countries to cope with the impacts of climate change and procure clean technology to avoid becoming the carbon culprits of the future.

"I don't see anyone coming forward with anything that could prepare the ground for a breakthrough," said Kim Carstensen of green group WWF. "What I see is the reverse, I see ground being prepared for a battle."

Just as worryingly, ideas are only now starting to be aired about an existential question -- the legal status of the future agreement -- which could revive friction between the United States and supporters of the Kyoto Protocol.

Nor has anyone broached the explosive problems of what teeth to build into the treaty for non-compliance, and how to punish Australia, Canada, Japan and other countries that are likely to overshoot their 2012 emissions targets under Kyoto.

Michael Zammit Cutajar, in charge of one of the two big negotiation groups, said he was unfazed that his draft text had ballooned, arguing breakthroughs traditionally come in the very final days or hours of haggling.

"This is like the evolutionary process in reverse. The Big Bang comes at the end," he quipped.

If past experience of climate negotiations is any guide, a breakthrough depends on movement at the very top.

There are some good opportunities to provide this before Copenhagen, with the G8 summit in Italy in July, which will also be attended by the heads of emerging giant economies, followed by an expected UN climate summit in September in New York.

Rumours abound, too, of preparations for accommodating President Barack Obama in Copenhagen, although whether this is in the role of deal-maker or deal-blesser is unclear. And past experience of climate negotiations also says that breakthroughs never dot 'i's or cross 't's.

The Kyoto Protocol, for instance, was born as a framework agreement in 1997 after exhausting talks.

But another four years were needed to complete its rulebook. Then footdragging byRussia over ratification meant the treaty eventually took effect in February 2005.

Karstensen and others said a likely scenario at Copenhagen would be a deal on core issues, followed by further negotiations to fill in the details.

Slippage from the "Bali Road Map" deadline would be acceptable, provided the core deal was strong and the follow-on talks wrapped up quickly, said Karstensen.

One major worry, though, is a gap between the end of 2012 and when the treaty would take effect, which could wreck the carbon markets created under Kyoto.

"A full and ratifiable treaty would have to emerge by the end of 2010. Later than that, I don't see it working," said Karstensen.