Growing bilateral cooperation between the United States and China to curb carbon emissions may lead to a shift in action towards voluntary national policy and away from international treaties imposing carbon caps.
That shift may be beneficial for climate action, by garnering wider support than that achieved by failed efforts in the past to corral all countries into a binding treaty.
The leaders of the world's two top carbon emitters both accept there is a growing problem of climate change.
Additional circumstances supporting action include falling emissions due to the shale gas revolution in the United States and a new commitment in China to curb pollution, which has stirred unrest.
The two countries can deeply influence the outcome of wider climate negotiations.
The United States and China forced a deal at the last major climate summit in Copenhagen four years ago, where they stitched a deal with South Africa, India and Brazil, side-lining the rest of the world.
Closer cooperation now would cement a shift in multilateral talks to non-binding action, as preferred by both countries, away from binding carbon caps as under Kyoto.
The next major summit is in Paris in 2015, for implementation from 2020, and may result in commitment to voluntary measures and policies, including efficiency standards and a gradual phase-out of particular greenhouse gases.
In formal talks backed by the United Nations, China and the United States appear poles apart.
Earlier this year they each submitted expectations for the next round of negotiations to the U.N. climate secretariat.
China insisted on emissions reductions targets for industrialised countries, while the United States preferred the vaguer notion of "contributions".
Posturing aside, however, they are on the same page.
As the top two emitters and therefore the chief targets of demands to make mitigation commitments, they each insist on a flexible agreement where they can define their own actions.
Neither refers to carbon caps for itself.
In its submission earlier this year, China said: "For developing countries, they will take diversified enhanced mitigation actions in the context of sustainable development, consistent with their national circumstances."
The United States said: "We consider that the agreement should provide for Parties to define their own mitigation contributions, taking into account national circumstances, capacity, and other factors that they consider relevant."
Leaders of both countries also appear to share a willingness to tackle the climate problem, as underlined during Secretary of State John Kerry's first visit to China in April.
"The two countries took special note of the overwhelming scientific consensus about anthropogenic climate change and its worsening impacts, including the sharp rise in global average temperatures over the past century, the alarming acidification of our oceans, the rapid loss of Arctic sea ice, and the striking incidence of extreme weather events occurring all over the world," they said in a joint statement.
"Both sides recognize that, given the latest scientific understanding of accelerating climate change and the urgent need to intensify global efforts to reduce greenhouse gas emissions, forceful, nationally appropriate action by the United States and China - including large-scale cooperative action - is more critical than ever."
Bilateral action has gathered steam since that April statement, both at presidential and ministerial levels.
In June, US President Barack Obama and China's Xi Jinping agreed to work together to phase down hydrofluorocarbons (HFCs), powerful greenhouse gases whose production has risen as a substitute for ozone-depleting chemicals.
The two foreign ministers have also forged cooperation.
Kerry's April visit to Beijing led to the formation of a new "U.S.-China Working Group on Climate Change", under an existing "Strategic and Economic Dialogue" established in 2009.
In July, under that new dialogue, the two countries agreed to develop initiatives to cut carbon emissions from heavy duty vehicles, buildings, manufacturing and coal-fired power, for example by advancing carbon capture and storage.
China and the United States forced a climate agreement on the rest of the world in 2009 in Copenhagen, separately brokering a deal to which other countries then had to agree.
Their accord limited action to voluntary, national carbon emissions targets, rather than a binding treaty as preferred by the European Union and least developed countries.
"The decision has been very difficult for me. We have made one step, we have hoped for several more," German Chancellor Angela Merkel said afterwards.
The more closely the United States and China cooperate now the more likely the next deal will be in the same mould.
That approach may succeed in attracting wide support, and perhaps not therefore sacrifice ambition, given that other major emitting countries including Japan, Russia, Canada and India will be cautious about agreeing internationally binding carbon caps rather than voluntary national measures.
And it would shift the role for the United Nations towards ensuring that global action is scientifically robust, and monitoring implementation, rather than brokering treaties.