UN climate talks in Mexico are deadlocked over a modest package to fight global warming that is a test of the UN's ability to tackle the issue.
Following are scenarios for success or and failure at the Nov 29 to Dec 10 meeting of 192 countries in Cancun, Mexico.
Controversy over the Kyoto Protocol -- at its heart a dispute about how rich and poor nations share the burden of curbing greenhouse gas emissions -- has dominated Cancun.
The 1997 protocol requires emissions cuts by almost 40 industrialized countries from 2008-2012. Developing countries want rich nations to set deeper cuts under Kyoto until 2020, while emerging nations sign up for a separate accord.
At least three developed countries, Japan, Canada and Russia, instead want a single new binding agreement that lists pledges by all nations.
The talks leave open whether the Kyoto Protocol should be the basis for future greenhouse gas emissions cuts.
That unblocks a modest deal that would set up a "global climate fund" to help poor nations, create a mechanism to share clean technologies, protect tropical forests and help the poor adapt to impacts ranging from storms to rising sea levels.
That deal is far short of the goals set at the 2009 Copenhagen summit, which agreed to a non-binding deal to limit climate change but failed to deliver an all-encompassing treaty that many nations had wanted.
The measures are less important than proving that the unwieldy talks can come up with an agreement as China and India becoming more assertive and rich nations struggle with weak growth.
A Cancun deal would put talks back on track and revive hopes that a wider, legally binding treaty is possible in the future. Few are pinning hopes on a treaty at the next meeting in Durban, South Africa, in 2011.
The first real deadline for decisions is June 30, 2012. The Kyoto Protocol is meant to be renewed six months before its first period runs out at the end of 2012.
Failure in Cancun could squelch any hope of solving climate change any time soon via the United Nations, which demands unanimity from all.
Climate change could become a lower priority for governments, even though the UN panel of climate scientists said world emissions need to peak by 2015 to avoid the worst of more droughts, floods, mudslides and rising seas.
The annual talks would continue but governments might invest less effort, abandoning hopes that the UN is the right place to oversee a shift toward cleaner energy.
Failure could drives alternative approaches to tackle the problem, perhaps through the G20 or other major economy groups, parallel to, or instead of, the United Nations.
Connie Hedegaard, EU climate commissioner
"It's absolutely crucial that this process, which is the only one we have...that it can prove that it can deliver results. We know we will not get everything done, but we must get something done. To come out of here with nothing is not a political option as I see it, and I also see it as a very dangerous option, actually, for the whole of multilateralism."
Rob Stavins, Harvard University
"The UN process has real problems, potentially fatal. Anything under the United Nations tends to polarize developing and industrialized countries. Most likely going forward we'll have a mix of institutions. This (the United Nations) could still be the coordinating body. Now, it's putting all your eggs in one basket."
Elliot Diringer, PEW Center on Global Climate Change
"I think that (no deal) would be damaging, if not fatal, to the (UN) process. Parties recognize that and that's why I am reasonably optimistic. Hopefully Cancun will mark a departure from the mindset of (agreeing) binding targets or nothing."
Swedish environment minister Andreas Carlgren
"The future strength of multilateral cooperation is at stake."
Tuvalu deputy prime minister Enele Sosene Sopoaga
"We cannot afford to have to have endless meetings...most importantly we cannot afford to be held hostage by countries fingerpointing on climate change. This is life and death, a survival issue for Tuvalu."
Yvo de Boer, climate adviser KPMG, former UN climate chief
"A failure here would be damaging to the credibility of the (UN) process, and I think that realization is driving people to come to a result. People are here with a general recognition that something needs to come out of this."