SSiRC activity summary

Activity Leaders

Claudia Timmreck, Larry Thomason, Stefanie Kremser and Jean-Paul Vernier

Steering Committee

M Rex, C. Timmreck, L. Thomason, J-P. Vernier, S. Kremser, J.C. Antuna, J. Barnes, T. Deshler, M. Hermann, T. Peter, F. Prata, A. Robock, M. von Hobe, G. Mann, and S. Fadnavis.

The stratospheric aerosol layer is a key element of the climate system. It affects both, the radiative balance of the atmosphere and stratospheric chemistry, which in itself couples back to the radiative forcing of the atmosphere via its effect on ozone. However, because the stratospheric aerosol layer is prescribed in most Earth System Models, no allowance is made for how it might respond to climate change.

The stratospheric aerosol layer is highly variable and controlled (a) by the direct stratospheric input of major volcanic eruptions and of COS photolysis and (b) by the uplift of natural and anthropogenic emissions of sulfur containing substances, through the transport of air from the surface across the tropical tropopause into the stratosphere. In the lowermost stratosphere, isentropic quasi- horizontal transport from the upper tropical troposphere can also contribute to the sulfur budget.

Over the past decade, large progress in our understanding of the Tropical Tropopause Layer (TTL) has been achieved, leading to a much improved understanding of the transport, microphysical and chemical processes that control the composition of air that goes into the stratosphere.

Observations indicate that the stratospheric background aerosol layer had significantly increased over the last two decades possibly by an increased amount of moderate tropical volcanic eruptions or by increasing emissions of SO2 from South East Asia. Neglecting these changes in climate model predictions can lead to an overestimation of the global warming trend.

SSiRC as part of the Stratosphere-Troposphere Process and their Role in Climate Project (SPARC) coordinates the international research activities on (a) observing, understanding and modeling the processes that control the stratospheric sulfur and aerosol budget, (b) observing stratospheric sulfur and aerosol and reconciling the data sets from different instruments, (c) including an interactive aerosol layer in global chemistry climate modeling, and (d) modeling the climate feedback from the stratospheric aerosol layer. SPARC is a core project of the World Climate Research Program (WCRP).