The 12 November announcement by presidents Xi Jinping and Barack Obama of their countries' post-2020 targets for reducing greenhouse-gas emissions has been hailed as historic. It is important for a number of reasons. It is the first time China has made a formal, quantified commitment to emissions reduction (rather than merely slowing emissions growth). It reflects Obama's decision to use executive action to cope with important issues, so as to avoid the need for congressional approval (see Survival Editor Dana Allin's blog post from 18 November). Moreover, as a joint action between the world's two top emitters, it gives new impetus to the multilateral negotiations to reach a global agreement in Paris in December 2015.

But the most important thing is the numbers. The US has pledged to reduce its emissions (compared to 2005 levels) by 26–28% by 2025, and make best efforts to meet the higher number. China has pledged to reach peak emissions no later than 2030, make best efforts to do so earlier, and increase the share of non-fossil-fuel energy sources to 20% by 2030. How significant are these targets? Do they reflect real progress? Will they be enough, on their own or embodied in a global treaty, to prevent dangerous or even catastrophic climate change?