The US and China agreement on carbon emissions

've often said that any progress on greenhouse gas emissions has to include the ten largest emitters, and especially China and the who are responsible for 40% of global emissions.

Hence the recent agreement between Obama and Xi is significant. Obama has said the US will reduce its emissions to 28% below its 2005 level. Xi has said China will peak its emissions by 2030.

The agreement is not legally binding, and Obama has only two years left in office. But it provides an opportunity for a global agreement, which will include all the major emitters.

However setting a date for China to peak is not the same as setting a level at which they may peak. The US has agreed to a cut of 2 gigatons by 2025, while China is forecast to increase by 8 gigatons. And China's per capita emissions are already higher than the .