Physics Colloquium, March 27, 2012
Global Warming: The basis in physics Raymond T. Pierrehumbert

Louis Block Professor in Geophysical Sciences The University of Chicago

All good scientists are skeptics to one extent or another, but when it comes to global warming, most of what generally tries to pass as "skepticism" is grounded in profound (and perhaps willful) ignorance of the subject matter. In this talk, I will review the physical basis leading to the conclusion that global warming caused by industrial society's emission of carbon dioxide poses a substantial threat to the welfare of human societies and natural ecosystems. I divide the argument up into three "baskets." In the first basket go arguments such as the attribution of carbon dioxide increases to anthropogenic emissions, the persistence of carbon dioxide in the atmosphere, ocean acidification, and the effect of increased carbon dioxide on the Earth's energy budget and hence its temperature. These arguments rely not at all on complex general circulation models of climate, and rest on physical and chemical principles so unassailable that to doubt them makes no more sense than to doubt special relativity or Newton's laws of motion (each within their appropriate spheres of application). In the second basket go emergent feedback phenomena which do depend intrinsically on modeling the fluid dynamics of the atmosphere, but which are nonetheless at present well understood both theoretically and observationally. Chief among these is the water vapor feedback. The third basket (largely filled by cloud effects) is also emergent and dependent on fluid modeling, but is still subject to very considerable uncertainties. Though clouds could, in principle, reduce sensitivity of climate to carbon dioxide, they have an even greater potential to increase climate sensitivity to truly alarming levels. I will explain this potential in terms of simple physical principles governing cloud optical properties. The current inability to provide sharp upper bounds on cloud-induced climate sensitivity implies a greater (rather than lesser) need to reduce carbon dioxide emissions. Claims that climate sensitivity is definitely too low to constitute a threat deserve to be treated with the utmost skepticism.

Dr. Pierrehumbert's Website

4:00 p.m., Physics Research Building (PRB), Room 1080

Reception at 3:45 p.m., Atrium, PRB