A while back, I had an interesting debate with Ian Castles about the significance or otherwise of the choice between market and purchashing-power parity (PPP) exchange rates in the IPCC projections of global warming. You can read a number of contributions from Castles and David Henderson at the Lavoisier Institute website and their main article is published in Energy & Environment”, vol. 14, nos. 2 & 3: 159-85. They argue that the use of market exchange rates understates the income of poor countries and therefore overstates the growth in income and energy use that will occur as they catch up with rich ones.
My criticism is directed at the second part of this claim. I say that if the demand for energy is modelled using market exchange rates, the choice of income aggregate doesn’t matter much. If the income estimate is biased downwards, so will be the estimate of the rate at which energy use grows with income. I make this point in my submission to the Stern Review in the UK.
These debates are inevitably complicated, but someone with the right skills can make them a lot clearer. I can think of no one better than Erwin Diewert, who has been the leading researcher* in the theory of index numbers for the last thirty years (and a big name in other fields). So I asked Erwin for comments and was both surprised and pleased to receive not just comments but a whole paper, not quite by return mail, but after only a few days.
I think it’s fair to summarise by saying that Diewert agrees with my main point, but also agrees with Castles and Henderson that the IPCC should change its modelling approach.
I agree with everything in the comment, but there a couple of points of emphasis I would place differently, as noted in my response . In particular, I stress the importance of consistency. Using PPP numbers for income, then plugging in income elasticities derived from studies using market exchange rates, is a recipe for disaster.
* I should also mention Sidney Afriat who is famous for being both brilliant and esoteric. Afriat has made fundamental contributions to the field.