Because of their specific knowledge, scientists are well positioned to identify environmental threats to humankind, sound the alarm, and propose and comment, at least on a general level, on potential responses. However, many policy makers and scientists believe that scientists should have no more to say about public issues than anyone else and that science can only tell us how the world is, not how it ought to be. Although there are deep differences between science and policy, the line between policy-relevant and policy-prescriptive science is under continual negotiation, and there is no uniquely "objective" way of characterizing facts. In the authors’ view, scientists should generally refrain from making recommendations in areas far from their expertise and from making categorical policy declarations, but when their expertise is relevant scientists should not be excluded from the policy process whether by external forces or by self-censorship. Scientists’ input and influence has played a key role in the past, and remains essential in shaping responses to important policy questions such as what to do about anthropogenic climate change.