Clear and quantitative discussion of uncertainties is critical for public policy making on climate change. The recently completed report of the Intergovernmental Panel on Climate Change assessed the uncertainty in its findings and forecasts. The uncertainty assessment process of the IPCC should be improved in the future by using a consistent approach to quantifying uncertainty, focusing the quantification on the few key results most important for policy making. The uncertainty quantification procedure should be fully documented, and if expert judgment is used, a specific list of the experts consulted should be included.