Last weekend, the leaders of the two largest emitters of greenhouse gases - China and the United States - formally joined the Paris Agreement, taking us 70 per cent of the way towards the critical mass of acceptance at which it will enter into force.

China's ratification is essential. It accounts for nearly a quarter of global emissions - as much as the US and European Union combined. On a matter like this, and especially in the country with the most mouths to feed, it is fair to assume that what Beijing says, goes.

With the United States it is not so simple. The elephant in the salon of the Paris Agreement is the one that is the cute little symbol of the US Republican Party.

It is not just that its candidate for the presidency, Donald Trump, is a climate change denier. It is a hoax, he has said, a Chinese ploy to undermine the competitiveness of American industry.


Trump has promised - if anything he says can be described that way - to "cancel" the Paris Agreement in his first 100 days in office and to rescind President Barack Obama's Climate Action Plan.

But even if we are spared a Trump presidency, that would not eliminate the risk that Obama has just written a cheque that might bounce.

The problem is the US constitution and its separation of powers.

The same Republican convention that confirmed Trump as its nominee agreed a platform which questions the scientific integrity of the United Nations' Intergovernmental Panel on Climate Change (IPCC), opposes any carbon tax and vows to forbid the Environmental Protection Agency (EPA) from regulating carbon dioxide.

And of the Paris Agreement, the Republican platform says "no such agreement can be binding upon the United States until it is submitted to and ratified by the Senate".

The Obama Administration, for its part, says the President has the authority to commit to the Paris Agreement, just as George Bush the elder did when he signed the original UN Framework Convention on Climate Change (UNFCCC).

The semantics of the US "joining" or "accepting" an international agreement seems to be about avoiding the Senate's constitutional right to advise and consent on the ratification of a treaty.

Regardless of such niceties, what matters in the end is whether the US, like other parties to the Paris Agreement, lives up to its undertaking. In the US case that is to reduce emissions by 27 per cent from 2005 levels by 2025.

China's commitment is that its annual emissions will peak before 2030. At what level and how steeply they would decline from the peak are matters on which it is non-committal.

But the fact that the two largest economies are on board, one of which is a lot richer than the other in per capita terms, has been crucial in overcoming the binary division between developed economies and the rest, which has bedevilled the geopolitics of climate change since the UNFCCC.

Would New Zealand deliver significantly different numbers?

A key question, however, is whether the US can achieve its target with policies delivered by administrative fiat, like using the EPA to regulate emissions from power stations.

If an effective US response requires legislation, as a carbon tax or nationwide cap-and-trade scheme would, then it would run into a roadblock manned by congressional Republicans. Even if Democrats regain a majority in the Senate in November, they are not expected to regain control of the House of Representatives.

Ultimately, in a democracy like the US, policy should reflect public opinion. Recently published polling by researchers at Yale and George Mason universities found 45 per cent of Americans either alarmed or concerned about climate change, while 29 per cent are dismissive, doubtful or disengaged. The rest are classified as cautious.

But when they were asked (this was last March) about how much difference this would make to their voting intentions in the forthcoming elections, the results were as follows. Among the 17 per cent alarmed about global warming, it counted as the second most important issue; among the 28 per cent classified as concerned, it ranked 12th; and among the other 55 per cent it was rated last among the 23 options provided.

If the Paris Agreement comes into force and is fully honoured by all its parties, it will be an historic achievement, serious progress towards bending the curve of global emissions.

Would New Zealand deliver significantly different numbers?

I doubt it.

Our Government has announced that it expects to ratify the Paris Agreement before the year is out. But we are still waiting for its plan to deliver on the commitment to cut emissions to 30 per cent below 2005 levels.

The Government has yet to report on, or release its response to, the review of the Emissions Trading Scheme.

Its policy seems to be to rely on the rest of the world to come up with an international carbon trading mechanism which will deliver carbon prices lower than the cost of meeting New Zealand's target through domestic action - which it regards as not remotely feasible - but not so low as to be, as Kyoto's became, environmentally worthless.

It also seems to be relying on the rest of the world to start mass-producing electric vehicles at prices Kiwis could afford.

That seems to be about it.

If the Paris Agreement comes into force and is fully honoured by all its parties, it will be an historic achievement, serious progress towards bending the curve of global emissions.

But it will not get us close to where we need to be to stave off calamitous climate change.

The UNFCCC secretariat's appraisal of the likely aggregate effect of the Paris commitments is sobering.

They remind us that according to the IPCC's most recent assessment report, the world has a "carbon budget" of around a trillion tonnes of carbon dioxide equivalent. That is as much as we can emit in total from 2011 on and still have a reasonable chance - odds of two to one - of keeping global warming below 2C above pre-industrial levels.

They estimate that the Paris Agreement's commitments mean we will have used up just over half of that budget by 2025, and three-quarters of it by 2030.

So then what?