Abstract: The IPCC has developed an approach to the treatment of uncertainties for the Fourth Assessment Report which is an evolution of the approach established for the Third Assessment Report. Several features of this evolution are discussed briefly here and this paper serves as an introduction to the "IPCC Uncertainty Guidance Note" for lead authors which is included as an appendix. Particular attention is given to use of the terms "likelihood" and "confidence" as alternative ways of expressing uncertainty. This distinction emerged in the Third Assessment Report and was the subject of some debate before finalising the Guidance Note, however, it is now recognized that each term provides a complementary aspect of describing uncertainties.

Key words: uncertainties, report, IPCC AR4