The joint announcement on climate change contributions by the United States and China marks a step-change in diplomacy in the run up to a potential global deal in Paris next year.

The timing of the announcement will inject momentum into the international negotiations, coming at an important moment before the next round in Lima in early December, and ahead of all countries submitting their intended contributions in the first quarter of 2015. Other countries, especially developed and emerging economies like Australia, Canada, Japan, India, Brazil and South Africa, will be recalibrating their offers in light of the US-China statement.

In the statement, the United States says it intends to achieve economy-wide targets of reducing emissions by 26-28% below the 2005 level in 2025; while China intends to achieve a peaking of CO2 emissions around 2030 and to increase the share of non-fossil fuels in primary energy consumption to around 20% by 2030. This builds on the recent deal in October by the European Union to reduce its emissions by at least 40% in 2030 from a 1990 baseline.

This is the first time that China has put a date on peak emissions, and it is highly symbolic that it made the pledge alongside the United States. China deserves credit from the international community for stepping up to the plate and showing leadership on this issue. It is equally important that President Obama has signaled continuing commitment to act on climate change. Indeed, whether Obama is able to make this deal stick despite resistance from the new Republican-controlled Congress is now a critical question for his legacy, as well as for future US-China cooperation.

The substance of both countries' announcements falls short of what scientists say is needed to avoid dangerous climate change. The US goal of 26-28% in 2025 is less than what US legislation proposed at the time of the Copenhagen Summit in 2009 was supposed to achieve - this implied a 30% reduction in 2025. To put this in context, the Intergovernmental Panel on Climate Change projects that current efforts put us on a pathway for 3.7 to 4.8 degrees celsius of global warming – far above the 2 degrees target that governments have agreed to work towards.

The level of ambition in the Chinese offer is not yet clear, because the volume of peak CO2 has not been announced and there is some flexibility on the peak year -'around 2030’ but as soon as possible. Some experts had hoped for an earlier peak in 2025, but a near-2030 peak is not necessarily incompatible with a global pathway to 2 degrees this century. The 'shape of the emissions curve' – when emissions start to plateau, and how sharply they fall after the peak – is just as important.

For the complete article, please see Chatham House.