"Future generations will rue the years of inaction," Steve Sawyer, a veteran observer who heads the Global Wind Energy Council (GWEC), a Brussels green industry association, says grimly.

"Some generations will rue it very much -- those that survive."

In 2000, the world placed its faith in the UN Framework Convention on Climate Change (UNFCCC), the creation of the famous Rio summit.

But the following year, the vehicle started to shake and its wheels began to rattle when US President George W. Bush abandoned the Kyoto Protocol, the sole treaty to set down targets for curbing carbon gases.

Crippled by the walkout of the world's wealthiest economy and biggest carbon emitter, Kyoto limped along, failing to brake a relentless surge in heat-trapping gases.

In 2007, in its landmark Fourth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) issued a blunt warning.

Without swift action to slow, halt and reverse the growth in emissions, the world was on course for between 1.8 and 4.0 degrees Celsius (3.2-7.2 degrees Fahrenheit) of warming, the UN's top climate scientists said.

By century's end, hundreds of millions could be at threat from drought, flood, storms, rising seas, disease, malnutrition and homelessness.

The shock report reduced the lobby of climate skeptics to a rump, galvanised public opinion and nailed climate change to the top of the political agenda.

On December 18 2009 in Copenhagen, leaders' rhetoric -- and the UN format itself -- were put to the test.

The day had been billed as the moment when humankind would unite, each nation pledging a sacrifice towards a global pact that, from 2013, would shrink climate change from mortal peril to manageable risk.

Instead, it became a finger-pointing fiasco.

Terrified the talks would collapse, a couple of dozen leaders from the most powerful countries -- including the United States, the European Union, Japan, China, India and Brazil -- huddled over a so-called "Copenhagen Accord."

They gathered around a table, frenziedly adding or crossing out text on the planet's future before eventually settling on the lowest common denominator.

They set a goal of limiting warming to 2 C (3.6 F) -- but did not say when carbon emissions should peak, which scientists say has to be around the middle of the next decade.

Nor did they identify key staging points in the medium or long term, in 2020 and 2050. And national pledges on carbon emissions were voluntary, carrying no penalty if breached.

Within hours, the document was savaged when it was put to the wider community of nations. The most outspoken lashed it as a "coup d'etat" against the UN system, a stitchup by an elite, a betrayal of the poor and a slap to expert opinion.

In the end, the critics were sidelined. The conference gavelled the Accord through without even putting it to a vote. UN credibility lay in ruins and the blame game began.

Why did things go so wrong in Copenhagen? And what can be salvaged?

Some say the fudge was inevitable and even argue the outcome is not so bad.

They note that Barack Obama, scrapping Bush's climate legacy and gingerly steering an emissions bill through Congress, had scant room to offer deeper US concessions in Copenhagen.

Then there are China, India and Brazil, hostile to anything resembling a binding emissions target, arguing they have the right to exploit fossil fuels to rise out of poverty.

Yet these high-population countries have become mega-polluters in their own right, accounting for the bulk of the 29-percent surge in carbon emissions between 2000 and 2008, the latest year for which figures are available.

Seeking an agreement in Copenhagen was like trying to square a circle, says WWF's Kim Carstensen, who blames "the lost years" of the Bush era for turning climate debate into trench warfare.

For UN chief Ban Ki-moon and others, the deal at least is the first to combine rich and poor nations in a single framework for emissions pledges.

They argue it provides a matrix for work that could lead to a fully-fledged treaty next December in Mexico City.

Other commentators are gloomier.

They blame the complex UNFCCC process, a spider's web of a thousand interlinked strands, where decision-making is driven by consensus among 194 nations. This offers plenty of room for delaying tactics or sabotage.

"In Copenhagen, we saw greater political will than ever, yet a handful of ideologically-driven countries nearly thwarted a deal," says Elliot Diringer, vice president for international strategies at the Pew Center on Global Climate Change, a US think tank.

"It's time to assess whose interests the process really serves. If the process can't be reformed, it may be time to forge coalitions of the willing."

If so, the toughest question of all emerges: Can nation-states, built by definition to defend national interests, rise to the challenge of serving the global good?

Carstensen agrees the future for cutting deals on carbon emissions could lie with a smaller, nimbler arena. It could be bilateral or a gathering of major emitters -- a format ironically proposed by Bush, a devil to green campaigners.

Even so, says Carstensen, "we will still need a global framework around climate action -- and we don't have anything better than the UN to do this. So I think the UNFCCC will continue to have a role."

Joris den Blanken of Greenpeace brands Copenhagen "a historic failure" by political leaders.

Yet he contends the setback should not mask a more positive, underlying change.

"This decade must be seen as the decade when people woke up to the challenge of climate change and began creating a climate movement," says den Blanken.

"Over the next few years, as awareness grows and the effects of climate change unfortunately become more apparent, politicians will have no other choice but to fall in line."

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