Sir

I read with interest your News article (Nature 409, 445; 2001) covering the approval of the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) Working Group I in Shanghai in January. As a coordinating lead author of one of the chapters, lead author of the technical summary and a member of the drafting team of the summary for policy-makers, I would like to clarify two points.

First, although climate modelling has advanced during the past five years, this is not the main reason for the revised range of temperature projections. The higher estimates of maximum warming by the year 2100 stem from a more realistic view of sulphate aerosol emissions. The new scenarios assume emissions will be reduced substantially in the coming decades, as this becomes technically and economically feasible, to avoid acid rain. Sulphate emissions have a cooling effect, so reducing them leads to higher estimates of warming.

Second, it is incorrect to say that the report does not consider new research on a possible change, or even shut-off, of the North Atlantic thermohaline circulation. This important new insight is addressed and assessed in detail in chapters seven and nine, amplified in the technical summary of Working Group I and explicitly carried over to the summary for policy-makers. All relevant and recent publications are cited.