The December Copenhagen climate summit – foolishly sold as the last-chance saloon for solving global warming – looms ever larger. Estimates of the money needed to help countries (particularly developing ones) mitigate and adapt to climate change are piling up faster than negotiators' airmiles.

Earlier this year, China, India and South Africa demanded rich countries spend 0.5-1 per cent of GDP to help them fight climate change. Yesterday a British report argued UN adaptation estimates of $49bn-$171bn a year (by 2030) were two to three times too low. Next week, African leaders will consider plans to ask rich countries for $67bn a year from 2020.

Three things connect these numbers. They are large, commensurate with the scale of the problem. They are speculative, trying to preview an uncertain future. Most importantly, while quantification hones our understanding of the challenge and specific numbers are useful negotiating weapons, they obscure a wider point. The world does not need to agree now on just how much money flows between countries as a result of climate change. It needs agreement on carbon targets and, down the road, mechanisms and incentives to achieve them, with equity as one consideration. Transparent pricing for carbon is more important than fantasy climate payments.