Second marriages have been called the triumph of hope over experience, and the same might be said of the annual conferences of the parties to the United Nations Framework Convention on Climate Change.
Expectations of the global gathering now under way in Cancun, Mexico, are so low that it will be deemed a success if it manages to avoid the acrimonious shambles of the last one in Copenhagen a year ago.
The geopolitics of climate change remain extraordinarily difficult. Inevitably so, because serious measures to rein in emissions would raise the cost of energy.
No one likes that idea and it is especially onerous for countries which are undergoing, or have yet to undergo, an energy-intensive growth spurt in their economic development.
The world's largest emitter, China, bridles at demands from developed countries for more transparency around its pledge under the Copenhagen Accord to curb the growth in its emissions.
The second-largest emitter, the United States, has a credibility problem of its own.
The Obama Administration was unable to get a national emissions trading scheme passed by Congress even before the "shellacking" of the recent mid-term elections.
It is now off the agenda for at least two years, raising questions about whether US pledges internationally to reduce emissions are worth the paper they are written on.
Those it made at Kyoto in 1997 were not. This time it depends on whether enough can be achieved through judicious use of the Clean Air Act to live up its undertaking.
China and the US represent more than 40 per cent of global emissions.
Meanwhile uncertainty swirls around the future of the Kyoto Protocol, the treaty committing every developed country (bar the US) to reduce emissions. Its first commitment period runs to the end of 2012.
Doubts surround the willingness of the larger non-European members of the Kyoto club to sign up for a second commitment period if - as seems inevitable - it continues to exclude the world's largest and fastest-growing emitters.
Japan's Vice-Minister for Global Environmental Affairs, Hideki Minamikawa, said a week ago that: "Even if the Kyoto Protocol's extension becomes a major item on the agenda at Cancun, and Japan finds itself isolated over it, Japan will not agree to it."
The Europeans, who have been in the vanguard of action on climate change, remain divided about whether their 2020 emissions should be a 20 or a 30 per cent cut from 1990 levels. The smaller number would be only a modest improvement from what they have already achieved.
Where to draw the line - at which doing their fair share ends and futile self-sacrifice begins - depends on what the rest of the world is willing to do. The same, of course, is true of New Zealand and it forms the backdrop to next year's review of the ETS.
Adrian Macey, who until earlier this year was New Zealand's climate change ambassador, is the vice-chairman of the Kyoto Protocol part of the global talks.
He says that while no one expects a new climate treaty to emerge at Cancun it can constitute a stepping stone to such an agreement, though not necessarily by the next meeting in Durban, South Africa, a year from now. Copenhagen, for all its shortcomings, provided at least a first glimpse of what such a deal could look like: from developed countries, big money for a climate fund and a reaffirmation of commitments to reduce emissions, in exchange for developing countries' meaningful and transparent actions to restrain the growth in theirs.
The future of Kyoto is a delicate issue, Macey says. Developing countries, in particular, see Kyoto and the commitments under it as the only concrete actions so far by the international community to address climate change and want it to continue.
Regardless of how post-2012 commitments are framed, it is important to preserve the strengths and achievements of the Kyoto system, Macey argues.
At its heart is an attachment to trading in emission permits, which in theory should minimise the costs of reducing emissions and tend to equalise those costs across sectors and countries.
It requires a robust system of inventories and carbon accounting. Kyoto provides some efficiency-enhancing connectivity among the emerging archipelago of carbon markets. It would be a shame to lose that.
Kyoto's Clean Development Mechanism (CDM) needs overhaul but provides a framework under which money and technology can flow from developed countries to climate-friendly projects in developing ones.
"The future of CDM is absolutely one of the top issues for developing countries," Macey says. "Not just its continuing existence, but how to broaden it out and make it more readily available to some of the smaller countries."
They will also be looking for progress towards a global fund to finance mitigation and adaptation efforts.
Copenhagen talked of something that would eventually be worth US$100 billion ($135 billion) a year. But that could be a hard sell in a world where rich countries are laden with debt and fiscal belts are being tightened all over the place.
About 85 countries have pledged under the Copenhagen Accord to reduce their emissions, or to constrain their growth, up to 2020. But the UN estimates those collective pledges will still see global emissions in 2020 exceed the level thought "likely" to be consistent with limiting global warming to 2C, by anything from 5 billion to 9 billion tonnes - or 11 to 20 per cent.
One thing is certain: whatever is decided, or left undecided, in Cancun will make no difference to the laws of nature or the laws of mathematics.
Because emissions accumulate in the atmosphere, the later and higher the peak in global emissions is the steeper and more costly the subsequent decline will have to be, because it is the area under that curve that matters.
And that assumes that we will not cross some threshold for runaway, catastrophic global warming if, for instance, the vast amounts of methane trapped in the permafrost of the high northern latitudes is released. No one can be dogmatic about where that threshold is.
No doubt the ministers and officials assembled at Cancun understand all that perfectly well. They have to do their best in the world as they find it.
But will their best be good enough? It is looking increasingly unlikely.