Breathing new life into the Kyoto Protocol is a "tall order" and the toughest challenge facing global talks on climate change, the UN climate chief said Sunday on the eve of the conference.

Christiana Figueres, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), pointed to the enormity of the task amid escalating demands from developing countries and green campaigners to save the only legally-binding treaty to cut greenhouse gases.

Taking place against an ever-grimmer backdrop, the talks drew appeals on Sunday from the Catholic and Anglican churches to cut carbon emissions and help poor countries facing worsening drought, flood, storms and rising seas.

Kyoto's first round of emissions pledges by rich countries expires next year, but only the European Union (EU) -- which accounts for barely 11 percent of global CO2 emissions -- has said it might renew its vows.

Defectors such as Japan and Russia, along with the United States, which never ratified Kyoto, are eyeing a parallel forum in the 194-nation UNFCCC that focuses on voluntary emissions curbs.

Determining Kyoto's survival while at the same time pursuing this second track "is a tall order for governments", UNFCCC Executive Secretary Christiana Figueres said at a press conference.

"This conversation [between the two tracks], from a political level, is the most difficult issue facing this conference."

A breakthrough -- if there is one -- will come in the last days of the November 28-December 9 talks when ministers arrive in Durban to try to push through a final compromise, she said.

Developing countries are lobbying hard for Kyoto to be kept alive, and some campaigners have warned of angry protests.

The good news coming into negotiations is a "growing momentum for action," Figueres added, pointing to recent actions in nearly 20 countries to reduce carbon pollution.

The bad news is a wave of new data showing that carbon dioxide levels have hit record highs and are causing more intense and frequent extreme weather.

As a result, the window for lower-cost mitigation is fast closing.

"These reports are sounding alarm bells for urgent action," she said.

The UNFCCC process remains deeply shaken by the near-collapse of the Copenhagen summit in 2009.

At Durban, a bottom-line outcome would be some sort of lifeline for Kyoto, and progress on parallel issues ranging from technology transfer and the build-up of a 100-billion-dollar a year Green Climate Fund for poorer countries, Figueres said.

At a rally in Durban, Nobel Peace laureate Desmond Tutu branded climate change a "huge, huge enemy" that threatened the common home of humanity, imperilling rich and poor alike.

"No one, no country can fight that enemy on his own... an enemy called global warming, climate change," he said.

"We have only one home. This is the only home we have. And whether you are rich or poor, this is your only home... you are members of one family, the human race."

In Europe, the leaders of the Roman Catholic and Anglican churches urged the climate talks to forge an outcome.

"I hope all members of the international community will agree a responsible, credible and united response to this worrying and complex phenomenon," Pope Benedict XVI told pilgrims in St Peter's Square.

The pope, who frequently speaks out on environmental issues and will be sending a delegation to Durban, said the agreement should also "take into account the needs of the poorest populations and of future generations".

The Archbishop of Canterbury, Rowan Williams, the leader of the world's Anglicans, called on the talks to show "real moral leadership".

In a video message, he urged rich nations to detail how they will fulfil their pledges for the Green Climate Fund.

"We need to see some security, some guarantees, about emissions cuts. We need to see some clarity about a real integrated response to questions around clean energy, food security, clean water and bio-diversity," he said.

What outcome for Durban climate talks?
Durban, South Africa (AFP) Nov 27, 2011 - Veteran watchers of the UN climate process differ in their predictions for talks opening in Durban on Monday, but all foresee an outcome falling short of a major breakthrough.

A cascade of alarming news from scientists underscores the urgent need to slash CO2 emissions if humanity is to have a fighting chance of capping the rise in global temperature at two degrees Celsius (3.6 degrees Fahrenheit), a goal enshrined at last year's climate forum in Cancun, Mexico.

But negotiators in Durban -- still rattled by the near-collapse of the over-reaching 2009 Copenhagen Summit -- have set their sites lower, analysts say.

The talks, under the 194-nation UN Framework Convention on Climate Change (UNFCCC), run through December 9.

"There are three scenarios that people are talking about for Durban," said Alden Myer of the Washington-based Union for Concerned Scientists, outlining results ranging from modest progress to complete deadlock.

The key to all of them lies in the fate of the UNFCCC's Kyoto Protocol, the world's only legally-binding agreement to curb greenhouse gases, Meyer and other experts say.

While the treaty itself is not threatened, its first five-year roster of commitments -- under which rich nations must cut carbon emissions by about five percent, compared to a 1990 benchmark -- closes at the end of 2012.

The developing world, exempt from such constraints, wants advanced economies to renew their Kyoto vows.

But many -- including Japan, Canada and Russia -- have bluntly refused to do so as long as the world's top polluters remain unconstrained by international law.

After playing a major role in drafting Kyoto, the United States opted out in 2001.

Number One emitter China has set its own goals for improved energy efficiency, but refuses to take on carbon-cutting targets under a global regime, as do emerging giants India and Brazil.

Rich countries under the Protocol, adopted in 1997, accounted for 64 percent of carbon emissions in 1990. Today they emit less than a third.

Only the European Union (EU) -- responsible for barely 11 percent of global CO2 output -- is ready to commit again, and only under one condition.

It wants all major emitters to back the completion of a legally-binding global climate pact, perhaps by 2015, into which the treaty could be subsumed.

"That is what a roadmap should do: describe some principles, the process and the timetable for what should come next," said EU climate chief Connie Hedegaard. "Without a roadmap, no second commitment period."

Here, then, are three possible scenarios:

1. BEST CASE

A rump group of rich Kyoto nations led by the EU takes on new carbon-cutting obligations, provided the United States and China endorse a "roadmap".

"A political second commitment period would establish a parallel process under Kyoto, thereby keeping it alive so that it can -- potentially -- fight another day," said Daniel Bodansky, a law professor at Arizona State University.

The talks would also make headway on a "Green Climate Fund", sketched in Cancun, slated to provide at least 100 billion dollars annually by 2020 to help poorer nations prevent and cope with climate change.

2. MIDDLE OF THE ROAD

Durban fails to muster a soft rollover of Kyoto pledges because Washington refuses to sign on to a future comprehensive climate deal -- a hard sell in Washington for the Obama administration during an election year.

But progress is made towards rendering the climate fund operational, and fleshing out schemes for forests, technology transfer, adaptation along with new rules on monitoring and verifying emissions reduction claims.

This package would at least show that the UNFCCC process is able to deliver incremental practical solutions.

3. WORST CASE

"The worst case is that the anger over the perceived death of Kyoto, and the failure of leadership by developing countries, lead to a blockage of even the Cancun decisions," said Meyer.

"That puts you back into a kind of crisis mode where people are questioning what the UN process is really bringing to the table."

For some countries at the talks, even the "best case" scenario outlined here may seem like too little too late, but the chances of a more robust outcome seem dim to nil, experts say.