Seminar Abstract

Robert Charlson

Tuesday, April 20, 2010

Uncertainties in Climate Projections Caused by Aerosol Effects: Do we know enough now to move ahead with control of greenhouse gas emissions?

Global climate forcing by aerosols (i.e., the change in Earth's energy balance imposed by anthropogenic particles in the atmosphere) is estimated to range from about -0.5 to -1.5 W/m^2, which is the dominant factor in the uncertainty in total climate forcing (the sum of greenhouse gas plus all other forcings). In turn, this uncertainty in total forcing precludes accurate quantification of climate sensitivity from the temperature record over the 20th century. This uncertainty has allowed various climate models to fit the temperature record despite large differences in climate sensitivity. This uncertainty in aerosol forcing has not been significantly reduced over the past 20 years. This seminar traces the early history of aerosol/climate research, in particular the first estimates of aerosol forcing made in the early 1990's which were largely based on observations of atmospheric aerosol properties. Subsequent efforts have been dominated by models of increasing complexity while attempts at improving the global observational basis via satellites is still in development. Simultaneous cloud and aerosol observations from instruments aboard multiple satellites, coordinated with in-situ observations in key regions, could provide key multivariate observations. Finally, the total forcing will become less uncertain as the greenhouse gases continue to accumulate in the atmosphere, such that the need for control of those gases will become increasingly urgent, even if the present uncertainty in aerosol forcing is not reduced. Meanwhile, reducing the uncertainty of the aerosol forcing is necessary if we are to be able to understand the causes of the observed warming over the 20th century and to refine the projections of climate change in the future.