Your Special Report “The costs of global warming” (Nature 439, 374–375; 2006) gives an unbalanced picture of the emissions scenarios developed by the Intergovernmental Panel on Climate Change (IPCC).

In contrast to the claim that these scenarios are outdated, a recent peer-reviewed assessment has concluded that, with a few notable exceptions, they compare reasonably well to recent data and projections for gross domestic product, population and emissions (D.v.V. and B.O'N. Clim. Change, in the press. doi: 10.1007/s10584-005-9031-0; see

Although we believe that progress on scenario development can and will be made, the elements of ‘up-to-date’ economic theory identified as overlooked — “how future societies will operate, how fast the population will grow, and how technological progress will change things” — are either too vague to be meaningful, or are issues the community has been dealing with for years. The Energy Modeling Forum has a 30-year history of model comparisons, exploring the implications for climate policy of a range of rates of economic growth and technological change (D. W. Gaskins and J. P. Weyant Am. Econ. Rev. 83, 318–323; 1993, and J. P. Weyant Energy Econ. 26, 501–515; 2004).

It is not correct to imply that the scenarios only use market exchange rates, or that they all assume that “the economies of poor countries will quickly catch up with those of rich nations”. Some scenarios are also reported in terms of purchasing-power parity exchange rates in the original 2000 IPCC Special Report. The debate on the emissions impacts of alternative exchange rates in economic modelling is not conclusive, but such impacts are likely to be small compared with the influence of technology, lifestyle and climate policies. And in no scenario do developing countries become as affluent as industrialized ones.

The assumed degree of catching up in the scenarios covers a wide range of possibilities. Focusing on a small number of most-likely futures ignores lessons from history: if the world always worked according to best-guess projections, we would now be living with nuclear power too cheap to meter and no ozone hole.