The clearest evidence of growing hopes for a successful climate conference in Copenhagen over the next two of weeks is the long list of heads of government, now including Prime Minister John Key, who plan to attend.

Only a few weeks ago it appeared to be heading for a train wreck.

What has averted that is Danish Prime Minister Lars Lokke Rasmussen's acknowledgment that Copenhagen is not going to be another Kyoto. It will not yield a treaty which governments take home and ratify, or not.

In the new climate of realism, key governments have responded to Rasmussen's call to "put numbers on the table". This is an area where numbers speak louder than words.

President Barack Obama said the United States will offer to cut greenhouse gas emissions "in the range of 17 per cent" by 2020, provided the US Senate agrees.

The US House of Representatives has already passed a cap-and-trade bill aimed at delivering a 17 per cent cut. But, as in Australia, Senate approval is expected to be a higher hurdle to clear.

The catch is that the Americans are talking about 17 per cent below what they emitted in 2005. It cannot therefore compare with the European Union's offer of a 20 to 30 per cent cut from 1990 levels, or New Zealand's offer of 10 to 20 per cent, also from 1990. The US offer represents a 3 to 4 per cent cut from 1990 levels.

It was followed by an offer from China, which is now the world's largest emitter, to cut by 40 to 45 per cent the carbon intensity of the Chinese economy from 2005 levels by 2020.

Carbon intensity is emissions per unit of gross domestic product. China's emissions would continue to grow, but more slowly than its economy.

The problem is that it is not clear how much China emitted in 2005 and even less clear how much its economy will expand over the next 10 years, so it is hard to say how much its emissions could increase and still be consistent with this target. Nor is it clear how much difference there would be between this and business as usual.

In the early stages of industrialisation, carbon intensity climbs as production of electricity, steel, cement and other emissions-intensive products expand. But as economies develop, their manufacturing moves up the value chain and services sectors become more important, so carbon intensity declines again.

Very rough preliminary estimates suggest China's offer would reduce its emissions, relative to business as usual, by anything from 12 per cent to not at all.

A leading US think tank, the Pew Centre for Global Climate Change, estimates that the targets on offer from developed countries represent a collective reduction in emissions of between 13 and 19 per cent from 1990 levels by 2020. This falls well short of 25 to 40 per cent which the United Nations' Intergovernmental panel on Climate Change has said is needed.

Proposals from the major emerging economies are estimated to be 10 to 18 per cent below business as usual overall, also well short of the 15 to 30 per cent thought to be necessary.

Climate Change Negotiations Minister Tim Groser emphasises the importance of what developing countries undertake. Collectively they represent a majority of global emissions now and at least three-quarters of the expected growth in emissions.

They remain resolute in their refusal to take on the kind of absolute emission caps expected of developed countries and their insistence that there will be no deal in Copenhagen without money up front from the developed world.

The European Union and Commonwealth have talked in terms of US$10 billion ($13.83b) by 2012, of which New Zealand's share would be some fraction of one per cent.

Against this background, what might emerge from Copenhagen as the first step of a two-step process towards a binding treaty?

It needs to be more than pious words of the kind heard at the Apec and Commonwealth summits. It should put a ratchet under the 2020 emissions target numbers developed countries have tabled and be "politically binding", in the sense that to renege on them later would carry an unacceptable cost in global opprobrium.

It may also embody some global objectives stabilising atmospheric greenhouse gas concentrations at 450 parts per million of CO2 equivalent or limiting warming to two degrees, and perhaps a global emissions reduction target for 2050.

"In the end," says Pew Centre president Eileen Claussen, "the ultimate test of Copenhagen's success will be if it puts countries on a clear path to concluding a legally binding treaty in 2010."

A central unresolved problem is the "architecture" of the international climate change regime post-2012, or how a treaty would relate to different parts of the world community.

In addition to the distinction between developed and developing countries, and the differing expectations for each, there is a spilt within the developed world between the United States and those countries with obligations under the Kyoto Protocol - that is, everyone else.

The US is not expected to join Kyoto. Some argue this does not matter, that what counts is the quality of the internal climate change policy Congress adopts. For the Obama Administration to promise more than Congress is willing to deliver (as Al Gore did at Kyoto in 1997) would be not only dishonest but counter-productive.

That leaves an open question whether the Kyoto system should continue beyond 2012, with tougher targets, or be replaced by a looser arrangement involving the US.

Groser will not estimate when a treaty might be concluded. He says years of international negotiations have left him deeply sceptical of deadlines.

But Claussen says the presence of so many heads of state in Copenhagen shows an unprecedented level of political engagement.

"This could be putting us on the verge of a critical turning point in the global climate effort."