There is a lot to play for. An early peak means a far greater impact on temperatures - but it will cost a lot more in funding to help poorer nations adapt. The hope is that by having an early phase down, emerging economies will not take intermediate steps but go for the most advanced and sustainable options.
Unusually, industry and environmental campaigners are fairly well aligned on the need for an early phase out. Governments and private donors are willing to step into the breach and last month offered $80m to speed the transition.