http://www.bbc.co.uk/news/science-environment-19193146

Comments by the US climate envoy last week discussing the value of the 2C targetin international climate change negotiations have provoked quite a response.

Todd Stern, who leads the US negotiating team in the UN climate convention (UNFCCC) and performed the same role at the recent Rio+20 summit, told an audience at Dartmouth College that insisting on the target in negotiations would lead to "deadlock".

The approach needed more "flexibility", he said.

The negotiations he's referring to concern the Durban Platform - an oddly-chosen name for a process agreed at last year's UN talks in South Africa.

Governments agreed to conclude negotiations by 2015 on a new global deal that would include to different extents every nation, to come into effect in 2020.

As I reported earlier this week, the comment went down very badly with the blocs pushing for faster action on climate change - the Alliance of Small Island States (Aosis) and the EU - with Marshall Islands minister Tony de Brum describing this flexibility as "a death sentence".

Later, the African Group of countries weighed in, spokesman Seyni Nafo saying: "This is not a game with numbers; its a question of people's lives, and so I am not sure there is much space for the 'flexibility' Mr Stern has spoken of."

But Ed King of the Responding to Climate Change website asked whether Mr Stern didn't have a point, given the difficult internal politics of the world's two biggest emitters.

"The aim is to avoid a 2C rise not just for 2015 or 2020 but stretching into the next 100 years," he writes.

"For that to be achieved the USA (and China) has to be on board."

In the middle of this comment storm, Mr Stern's office issued a statement designed to be a clarification.

"Of course, the US continues to support this [2C] goal; we have not changed our policy.

"My point in the speech was that insisting on an approach that would purport to guarantee such a goal - essentially by dividing up carbon rights to the atmosphere - will only lead to stalemate given the very different views countries would have on how such apportionment should be made.

"My view is that a more flexible approach will give us a better chance to actually conclude an effective new agreement and meet the goal we all share."

I say the statement is "designed to be a clarification" because actually, I'm not sure it is.

Certainly, countries have very different views on how "carbon rights to the atmosphere" should be divided up.

We've seen that in abundance at successive UN climate talks dating back at least to 1997 and the agreement of the Kyoto Protocol.

So to that extent, Mr Stern's suggestion of not worrying too much about trying to build a 2C guarantee into the 2015 deal but instead starting "with a regime that can get us going in the right direction and that is built in a way maximally conducive to raising ambition, spurring innovation, and building political will" makes some sense.

However, as he acknowledges: "This kind of flexible, evolving legal agreement cannot guarantee that we meet a 2C goal."

Which begs the question; what use is such an agreement if it doesn't?

It's worth returning at this point to the basic point of the UN climate convention: "The ultimate objective is to achieve... stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."

In some ways it's a badly phrased objective, because greenhouse gas concentrations at a level that are "dangerous" for inhabitants of drought-prone East Africa or low-lying Tuvalu may be absolutely fine in Paris and indeed beneficial in Yakutsk.

But there it is. And the importance of the 2C figure is that it's come to represent a kind of general, averaged-out notion of what "dangerous" means.

Many countries argue it's too lenient. A majority favour 1.5C; a few hold out for 1C.

No government - in public, at least - argues it's too strict.

So as far as there is a general consensus about these things, when Mr Stern advocates a negotiating process that "cannot guarantee" a 2C goal, one way of seeing that is as an acknowledgement that governments shouldn't aim to fulfil the basic objective of the UN climate convention; which is obviously political dynamite.

And yet elsewhere in his speech, he is extremely forthright in arguing that "dangerous anthropogenic climate interference with the climate system" has to be avoided.

For all the protestations of "sceptics", he says: "The atmosphere doesn't care. Its temperature will continue its implacable rise, with all the consequences that entails, unless we act to stop it."

On the evidence of warming - the succession of hot years, the Arctic sea ice melt, ocean acidification, and so on: "They warn of droughts and floods and extreme storms.

"They warn of water shortages, food shortages and national security risk. They warn of what 11 retired generals and admirals wrote about in 2007 - climate change becoming a 'force multiplier for instability in some of the most volatile regions of the world'.

"And they introduce the threat of catastrophic, non-linear change."

... all of which is exactly why most governments are not only supporting the 2C target (if not a smaller figure), but remain determined to get a deal in 2015 that ensures it's achieved.

The most intriguing question - and despite making some enquiries, I've turned up nothing definitive on this - is whether the US is alone in proposing an approach to negotiations that doesn't explicitly aim for 2C.

Since the Copenhagen UN summit of 2009, the US has formed part of what's been dubbed a "coalition of the unwilling", which also, to varying degrees, has included China, India, Russia, Canada, Japan and some of the Gulf states.

Submissions that the US and China sent to the UNFCCC in March, outlining ideas for moving forward on the Durban Platform, don't mention the 2C target at all.

An omission - or something more meaningful?

Meanwhile, submissions from India and Saudi Arabia claim that in order to meet 2C, it's necessary only for the traditional developed countries to restrain emissions - even though the science makes clear that at some stage, all countries will have to take a hit.

The uncomfortable reality is that India, Saudi Arabia and other fast-developing countries can still point at the US and its fellow early industrialisers - the UK, Germany, Japan - and legitimately argue that none has yet been willing to make emission cuts that their historical responsibilities justify.

And without that leadership, they won't follow.

To Mr Stern, the implication of all this is that a process based on slicing up "rights to the atmosphere" can't work.

Politically, he may be right. But it's hard to see any path to a world free of dangerous anthropogenic interference with the climate system unless governments do find a way of apportioning these rights.