Major developing countries have dampened prospects for agreement on international carbon emissions reduction targets by insisting on distinguishing between the responsibilities of industrialised and emerging economies to act on climate change beyond 2020.
At a meeting in Durban, South Africa, last December, ministers agreed to negotiate a deal, for implementation from 2020, in which all countries participated, boosting prospects for agreement on medium-term climate targets.
Those UN-backed negotiations are the world's main multilateral process for agreeing carbon cuts and can help guide choices in global energy consumption and ambition in emissions markets. So far they have fallen short.
The Durban meeting seemed to overcome a division that has long threatened US participation.
The United States has complained that its main economic rival, China, as well as other major emerging economies, have escaped binding targets of any kind under the UN negotiations, after a Climate Convention in 1992 divided the responsibilities of Annex 1 (developed) and non-Annex 1 (developing) countries.
Last December, countries agreed a 2-page "Durban Platform for Enhanced Action" which appeared to sweep aside the dichotomy, making no reference to formally divided responsibilities and so appearing to launch a fresh era in negotiations.
However, on Friday, ministers from Brazil, South Africa, India and China (dubbed "BASIC" countries, in UN climate jargon) said any deal must explicitly distinguish action between industrialised and emerging economies and by implication between the United States and China, the world's top two emitters, appearing to put negotiations back at least one year.
The division between emerging and industrialised countries originates from Article 3 of the 1992 Climate Convention.
"The Parties should protect the climate system ... on the basis of equity and in accordance with their common but differentiated responsibilities and respective capabilities. Accordingly, the developed country Parties should take the lead," it said.
The Convention is the umbrella treaty for the subsequent 1997 Kyoto Protocol which required only Annex 1 (developed) countries to cut and report their emissions.
China, the world's top emitter, has not reported its annual output of greenhouse gases since 1994.
The exclusive focus on developed country action was the central reason why the US Senate did not ratify Kyoto.
The Durban agreement in December appeared to put an end to that division, when countries agreed "to launch a process to develop a protocol, another legal instrument or a legal outcome under the Convention applicable to all Parties".
The agreement made no mention of Annex 1 or non-Annex 1 countries, nor of "common but differentiated responsibilities".
"The Durban Platform for Enhanced Action calls for a comprehensive legal regime by 2020 that essentially eliminates the Annex I versus non-Annex I distinction," said Harvard University policy experts Joseph Aldy and Robert Stavins in an article published in the August 31 edition of the journal Science.
"This is a significant departure from the past. It is of vast potential importance, but only 'potential'."
That latter caution was justified. The 'BASIC' statement late on Friday said that a rigid separation should persist.
The statement summarised the outcome of a meeting in Brasilia involving Brazil's foreign minister, the vice minister of China's National Development and Reform Commission (NDRC), South Africa's minister of water and environmental affairs and India's ambassador to Brazil.
"They recognised that all countries should participate in an enhanced global effort to be implemented after 2020, under the UN framework convention on climate change, which would respect the principles of equity and common but differentiated responsibilities and differentiation between Annex I and non-Annex I Parties," the statement said.
"Ministers stressed that the institutional architecture built on the basis of (past) decisions ... will continue to function in the post-2020 period," it added.
It makes sense to calculate the ambition of countries in cutting carbon emissions according to wealth and greenhouse gas emissions, but not in rigid categories nearly three decades old by the time a deal takes effect.
China overtook the United States as the world's top emitter in 2006 or 2007, from less than half US emissions in 1992, the year of the original UN climate convention.
However, carbon dioxide is a long-lived greenhouse gas, remaining in the atmosphere for up to several centuries, meaning it makes sense to view cumulative rather than annual emissions.
Energy data suggest that cumulative US emissions of CO2 since 1900 are still more than double those of China.
But China is catching up: it now exceeds the United States in cumulative energy-related CO2 emissions over the past decade, from 2002-2011, at 64.5 billion tonnes compared with 62.9 billion, data from the oil company BP show.
Another measure for ambition to cut emissions is per capita national income (GDP).
World Bank data show that US per capita GDP in constant international dollars was 24 times that of China in 1992, narrowing to six times last year.
Such data suggest that the United States has an imperative to do more to cut carbon emissions under a multilateral agreement after 2020, but China is unhelpful clinging to Annex 1 / non-Annex 1 boxes.
The BASIC rhetoric will perpetuate suspicion between the world's top two emitters and make multilateral targets elusive.
The alternative is unilateral action such as the EU's plan to impose carbon tariffs on flights entering and leaving Europe, which has drawn general opposition from other major economies.
The next UN climate meeting in Doha, Qatar, from November 26 to December 7, will have a tough job to keep the peace.