“It would be difficult to overstate the gravity of this moment,” Ban said at the start of a four-day meeting of environment ministers.

“Without exaggeration, we can say: the future of our planet is at stake — people’s lives, the health of the global economy, the very survival of some nations.”

The 12-day marathon under the UN Framework Convention on Climate Change (UNFCCC) has seen feuds over the future of the Kyoto Protocol, the only legally-binding treaty for curbing dangerous greenhouses gases.

“I urge you to carefully consider a second commitment period under the Kyoto Protocol,” Ban said in his speech.

The UN chief admitted that economic problems and political discord meant the “ultimate goal” of a new legally-binding worldwide treaty “may be beyond our reach, for now.”

“In the absence of a global binding climate agreement, the Kyoto Protocol is the closest we have,” he said.

“While Kyoto alone will not solve today’s climate problems, it is a foundation to build on with important institutions. It provides the framework that markets sorely need… It is important that we do not create a vacuum.”

Ban’s appeal centered on the crux issue in Durban, where the outcome will be determined by a game of climate poker.

Kyoto’s death would leave a toxic political legacy among developing countries that see it as a totem of solidarity between rich and poor in fighting climate change.

It would also leave the UNFCCC with only voluntary pledges for carbon emissions — a format that scientists on Tuesday warned would barely brake the juggernaut of climate change.

Kyoto’s first round of emissions pledges expires next year.

But these promises apply only to rich countries, not developing ones, nor do they concern the United States, which boycotted Kyoto in 2001.

Rich Kyoto countries are refusing to sign on for a fresh round of commitments, saying this would be unfair if far bigger emitters get off the hook.

The European Union (EU) has offered to sign up for a second round of commitments, but only if it gets backing for a “roadmap” to a new, legally-binding pact encompassing the big carbon polluters, notably China and the United States.

Hopes of movement were raised on Sunday when China signalled willingness — linked with conditions — to embrace a future binding treaty after 2020.

But on Tuesday, positions were as far apart as ever.

“It’s not my impression that there has been any change at all in the Chinese position with respect to a legally-binding agreement,” said US chief delegate Todd Stern.

The United States had conditions of its own for any such a pact, said Stern.

“It would have to cover all major parties in a full way, so that it binds with equal force for everybody, unconditionally, (with) no escape hatches in the text,” he said.

In a show of unity, the world’s four emerging giants — Brazil, South Africa, India and China — said keeping Kyoto alive was essential.

“The Kyoto Protocol should be continued and a second commitment period is a must,” China’s top climate negotiator Xie Zhenhua, speaking in the name of the so-called BASIC group, told journalists.

“The most important issue for us in Durban is that a clear and ratifiable decision on a KP (Kyoto Protocol) second commitment period takes place. This must happen if KP parties are really committed to addressing climate change,” said India’s environment minister, Jayanthi Natarajan.

Stern pushed for UNFCCC parties to implement an agreement endorsed in Cancun, Mexico, last year.

This approach uses a roster of voluntary emissions curbs, by 2020, to tackle greenhouse gases and would set up a fund, worth 100 billion dollars a year by that date, to help poor countries.

But a study by German scientists released in Durban on Tuesday said that current carbon pledges would drive Earth to warming of 3.5 degrees Celsius (6.3 degrees Fahrenheit), compared to the 2.0 C (3.6 F) UN target.

Climate researchers say a 3.5 C (6.3 F) scenario would be very bleak, dooming tens of millions of people to worse droughts, floods, storms and rising seas.