A retired climate scientist with decades of leadership in climate modeling and policy. Formerly led climate impact modeling at Lawrence Livermore National Laboratory (1968–93), including studies on volcanic eruptions, nuclear war, and greenhouse gases, and headed its atmospheric sciences division. Served in Washington, DC (1993–2002) as senior scientist and founding executive director roles within the U.S. Global Change Research Program and its National Assessment.

Since retirement, has worked pro bono as Chief Scientist for Climate Change Programs at the Climate Institute, served as President of the International Association of Meteorology and Atmospheric Sciences, contributed to IPCC and Arctic climate assessments, and authored legal declarations on climate science, including for Massachusetts v. EPA. Current work focuses on climate intervention, energy system transformation (HVDC grids), decision-making frameworks for climate risk, and renewable energy via river, tidal, and ocean-current turbines.

Holds a BS in Engineering from Princeton (1964) and a PhD in Applied Science from UC Davis (1968), with early pioneering work on climate models of intermediate complexity and glacial–interglacial cycles.