Roughly 44% of the carbon dioxide that humans release into the atmosphere each year comes from China and the United States. These countries are the big fish at the United Nations climate negotiations, and for years they have been at loggerheads, each deflecting calls to curb emissions by pointing to the other. As talks languished and emissions increased, the rest of the world’s major emitters often seemed content to sit back and point the finger at them. Now that these two powerhouse polluters have brokered an unexpected deal on emissions, can the world hope that those days are in the past? All eyes are on the UN climate meeting in Lima next week (see page 473).

The chances of forming a meaningful climate agreement at the follow-up UN summit in Paris next year have clearly improved, but a dose of scepticism is warranted. Under the US–China agreement, inked in Beijing on 12 November by presidents Barack Obama and Xi Jinping, the United States would curb its emissions by at least 26% by 2025, and China would hasten the development of low-carbon energy to ensure that its own soaring emissions peak around 2030. These are not insignificant targets, but both nations could do more if they were really serious about addressing climate change. Moreover, both pledges come shrouded in their own particular doubt.

From the US perspective, Obama has two years left in his presidential term to get the ball rolling and even if he succeeds, that ball will roll into an uncertain future. His primary weapon is a proposed set of regulations for existing power plants, which the US Environmental Protection Agency says would reduce emissions by 30% from 2005 levels by 2030. Assuming that they clear the inevitable court challenges, these regulations and the action already taken on vehicle fuel efficiency will go a long way, but more will be needed.

The stand-off between the United States and China is emblematic of a larger rift.

As it stands, many Republicans are lined up against the president’s climate policies. They have already been critical of Obama’s agreement with China, and as of January they will control both houses of Congress. Some observers predict that policies on climate could be among the major issues in the presidential election two years from now. That would be a welcome first, because much will ride on the outcome.

As for China, the headline promise is maddeningly vague: ‘around 2030’ does not tell us precisely when emissions will peak, and Xi did not specify how high emissions will climb before then. China is already on track to meet its existing goal of producing 15% of its power from low-carbon sources by 2020. So its promise to extend that to 20% by 2030 makes the latest commitment less than revolutionary. And although some energy researchers have suggested that China could level off its emissions by 2025, most baseline scenarios suggest that without active engagement, the country’s emissions would continue to rise until 2050, albeit slowing down once more Chinese citizens have finished filling their homes with energy-hungry appliances.

This agreement has as much to do with political momentum as commitments. The stand-off between the United States and China is emblematic of a larger rift in the negotiations and has its roots in both morality and science. Developing countries rightly expect those who have profited from polluting the atmosphere to lead the way in curbing emissions; industrialized countries rightly counter that they cannot do it alone, given that most of the growth in greenhouse-gas emissions is in the developing world, where more than 1 billion people still live without electricity. Unfortunately, the climate does not care about such questions.

Five years ago, at the most recent headline climate summit in Copenhagen, world leaders took their first step in breaking down the legal wall between developed and developing countries. Until then, under the 1997 Kyoto Protocol, only developed countries — notably minus the United States, which ducked out — had obligations to reduce emissions. In Copenhagen, many developing countries stepped forward with climate pledges, but the negotiations nearly collapsed. Significant battles remain over commitments, financial aid and how to structure an agreement, but most countries now accept that this must be a collective effort.

In theory, the US–China agreement is the last major piece of this puzzle. If it translates into cooperation on a new climate accord, other countries may be encouraged to engage seriously. At a minimum, those who have been pointing the finger at the United States and China would need to come up with another excuse.

All involved will get the first glimpse of how this changes the international dynamic when negotiators gather in Lima. Fingers crossed.