The Economist presents A lack-of-progress report on the Intergovernmental Panel on Climate Change (the IPCC), which

relied, first, on measuring gaps between incomes in poor countries and incomes in rich countries, and, second, on supposing that those gaps would be substantially narrowed, or entirely closed, by the end of this century. Contrary to standard practice, the IPCC measured the initial gaps using market-based exchange rates rather than rates adjusted for differences in purchasing power. This error makes the initial income gaps seem far larger than they really are, so the subsequent catching-up is correspondingly faster. The developing-country growth rates yielded by this method are historically implausible, to put it mildly. The emissions forecasts based on those implausibly high growth rates are accordingly unsound.

...even the scenarios that give the lowest cumulative emissions assume that incomes in the developing countries will increase at a much faster rate over the course of the century than they have ever done before.

Even though they're professionals, they don't know everything:

The problem is that this horde of authorities is drawn from a narrow professional milieu. Economic and statistical expertise is not among their strengths. Making matters worse, the panel's approach lays great emphasis on peer review of submissions. When the peers in question are drawn from a restricted professional domain--whereas the issues under consideration make demands upon a wide range of professional skills--peer review is not a way to assure the highest standards of work by exposing research to scepticism. It is just the opposite: a kind of intellectual restrictive practice, which allows flawed or downright shoddy work to acquire a standing it does not deserve.