Communication by scientists with policy makers and attentive publics raises ethical issues. Scientists need to decide how to communicate knowledge effectively in a way that nonscientists can understand and use, while remaining honest scientists and presenting estimates of the uncertainty of their inferences. They need to understand their own ethical choices in using scientific information to communicate to audiences. These issues were salient in the Fourth Assessment of the Intergovernmental Panel on Climate Change with respect to possible sea level rise from disintegration of the Greenland and West Antarctic ice sheets. Due to uncertainty, the reported values of projected sea level rise were incomplete, potentially leading some relevant audiences to underestimate future risk. Such judgments should be made in a principled rather than an ad hoc manner. Five principles for scientific communication under such conditions are important: honesty, precision, audience relevance, process transparency, and specification of uncertainty about conclusions. Some of these principles are of intrinsic importance while others are merely instrumental and subject to trade-offs among them. Scientists engaged in assessments under uncertainty should understand these principles and which trade-offs are acceptable.