What roles can scientists play in public discourse?

What is a useful and proper role for scientists in the public arena? How can we best discriminate where the boundary lies between expert knowledge and values or political opinion, and how can we properly honor that line? What can we expect in the way of reception for our interventions, and how can we increase their efficacy? Involvement in public policy debates is a common and accepted role for scientists in many disciplines. In the sciences related to public health, it is taken for granted that experts will talk about the implications of their research for public policy, whether in regard to smoking, diet, or disease spread. There is also a remarkable track record of geoscientists taking a lead role in the public arena and actually affecting public policyF. Sherwood Rowland and Mario Molina collaborated on ozone depletion research at the Department of Chemistry at University of California, Irvine and then went on to make outstanding public contributions, as have James Hansen (at NASA's Goddard Institute for Space Studies); Robert Watson (first at NASA, then at the University of East Anglia); and, of course, the late Stephen Schneider (first at the U.S. National Center for Atmospheric Research, then Stanford) on climate. Some "public" geoscientists have restricted their activities to interpreting science for the wider public, while others have endorsed specific policy initiatives (see Figure 1). I firmly believe that the quality of public discourse and the information reaching policy makers were better for their interventions.