China completes the climate circle
A fair bit of the doubt and confusion surrounding next month's UN climate summit has suddenly cleared, with the world's two biggest greenhouse gas emitters - the US and China - announcing pledges on curbing greenhouse gas emissions.
As is set out in the Bali Action Plan - the agreement made at the UN summit two years ago - the US pledge takes the form of an actual cut in emissions.
China - whose per-capita emissions are far lower - vows to reduce carbon intensity, the amount of CO2 emitted per unit of GDP, by 40-45% from 2005 levels by 2020.
This is ambitious - more ambitious than many observers had expected.
But it doesn't mean China's emissions will fall - in fact they are still likely to rise, with the rate at which economic growth rises outstripping the rate at which carbon intensity falls.
In fact, the target could be met in a number of ways.
One would be to use all energy more efficiently. Another would be to increase the proportion of energy deriving from low-carbon sources such as wind turbines and nuclear reactors.
A third would be to produce goods of higher value without changing the nature of energy production and use, raising GDP while leaving emissions unchanged.
In practice, the Chinese plan will probably include a mixture of those three elements. As my colleague Roger Harrabin reported from his recent trip to China, energy efficiency is being targeted - certainly in new developments - while investment in renewables is forging ahead.
The possibility had been raised - not least on this blog - that the recent bilateral talks between Presidents Barack Obama and Hu Jintao might prove crucial in allowing these pledges to be put forward, because the politics of the two countries on climate change are wrapped up in several important ways.
Firstly, as the two countries produce about two-fifths of the world's greenhouse gas emissions, they clearly hold the key more than any others to a deal that really will curb human-induced climate change; everyone else knows that these two governments have to be fully on board.
Secondly, as the world's largest historical emitter and one of the largest per-capita emitters, the US is the country that developing nations have most in their sights when they talk about the duty of the rich to lead.
Conversely, China is the country that US senators have most in their sights when they talk about the need for all major emitters to take action.
A related point is that at some point in the future, China will become the main US rival for the title of the world's biggest economy, which brings issues of competitiveness into the mix.
Although details of the talks that Mr Obama and Mr Hu had during the former's recent visit to Beijing remain under wraps, one logical conclusion would be that the two leaders were able to agree on a formulation that would be mutually acceptable - and that here, we are seeing the fruits of that agreement.
China now becomes the latest major developing nation after Indonesia, Brazil and South Korea to pledge a target; and as we approach within touching distance of the Copenhagen summit, virtually all of the major cards are on the table on curbing emissions - though not on other issues such as finance and technology transfer.
Without emission pledges from the US and China, negotiations in Copenhagen would have lacked a large part of the underpinning vital if any kind of deal is to be struck.
But whether developing countries are impressed by the size of the US commitment is another matter.
China itself says it wants developed nations to cut carbon by 40% from 1990 levels by 2020 - and Mr Obama's pledge, at about 3%, is a lot less than that.