In the brief history of environmentalism -- and, some would argue, in the longer sweep of human history itself -- the stakes at the 12-day conference in Copenhagen have never been higher.

The goal: to roll back the peril of hunger, disease, drought, flood, storm and rising seas created by mankind's unwitting impact on the weather system.

To achieve this aim, the 192 members of the UN's Framework Convention on Climate Change (UNFCCC) must show solidarity and sacrifice on an unprecedented scale.

More than 100 leaders are set to attend the finale on December 18. They are under ratcheting pressure to seize the day rather than a photo opportunity, to seal a deal rather than preside over a fiasco.

Trillions of dollars, powerful economic and national interests and the livelihoods of millions underpin the Copenhagen moment.

Countries must agree to curb their use of coal, oil and gas, the fossil fuels hewn from the ground or drilled from beneath the earth that have powered our prosperity -- and helped create the carbon monster.

And they must set up a financial safety net for poor countries least to blame for global warming but most exposed to its wrath, and provide them with technology to avoid becoming big polluters in turn.

"The aim is nothing less than to slice through the Gordian knot intertwining climate change and development," says Jean-Charles Hourcade of the International Centre on Environment and Development (CIRED), a French thinktank.

Some thinkers, like British economist Nicholas Stern, liken the December 7-18 conference in importance to Bretton Woods, the 1944 conference that reshaped the world's monetary system.

Others see it as a do-or-die moment for the United Nations, for it raises core questions about the ability of nation states to cooperate.

"It is a very crucial test of the UN system," Rajendra Pachauri, head of the UN panel of climate scientists, told AFP. "It is an extremely important test of the ability of nation states to get together and manage the global commons."

The road to Copenhagen began two years ago, at UNFCCC talks in Bali, Indonesia.

There, after arduous wrangling, countries set their eyes on a global pact that would take effect from 2013, after current pledges expire under the Kyoto Protocol, the world's first emissions-curbing accord.

Bit by bit, hopes that Copenhagen would yield a soup-to-nuts treaty have vanished.

The lesser objective now is a strong outline accord, one that can be fleshed out by further negotiations in 2010.

Yet there is no guarantee that even this skeletal agreement can be reached.

Mistrust is entrenched among -- and within -- the three main negotiation groups.

Poor countries are angry that rich countries, as a bloc, have not come nearly far enough on their emissions and funding proposals.

Developing nations, they say, will not sign up to any targeted, binding emissions of their own, arguing they too have the right to use cheap, plentiful fossil fuels to haul themselves out of poverty.

The European Union (EU), meanwhile, is looking to the United States, the world's No. 2 polluter, to dig deep into its pocket and its carbon pollution.

The US, meanwhile, is turning to the emerging giants -- China, No. 1 emitter, as well as India and Brazil -- for proof that their emissions measures, while voluntary, will be tough, transparent and verifiable.

Even if this negotiation triangle can be smoothed out into a consensus, another problem lurks: what kind of legal form should this agreement take?

Poorer countries are clamouring for a second round of pledges under the Kyoto Protocol, yet this seems out of the question so long as the United States remains outside that treaty.

The negotiations are likely to start low-paced, building to a crescendo in the middle of the second week with the arrival of environment ministers, followed by the heads of state or government, including the leaders of the United States, China, Germany, France and Britain.

The likelihood beckons of frenzied all-night climate poker in the back rooms.

Green activists have scheduled demonstrations on Saturday, December 12, while a hard-left group has threatened to interrupt the talks at Copenhagen's Bella Center on December 16.