GCM SIMULATIONS OF VOLCANIC AEROSOL FORCING .1. CLIMATE CHANGES INDUCED BY STEADY-STATE PERTURBATIONS

The authors have used the Goddard Institute for Space Studies Climate Model II to simulate the response of the climate system to a spatially and temporally constant forcing by volcanic aerosols having an optica l depth of 0.15. The climatic changes produced by long-term volcanic a erosol forcing are obtained by differencing this simulation and one ma de for the present climate with no volcanic aerosol forcing. These cli matic changes are compared with those obtained with the same climate m odel when the CO2 content of the atmosphere was doubled (2 x CO2) and when the boundary conditions associated with the peak of the last ice age were used (18 K). In all three cases, the absolute magnitude of th e change in the globally averaged air temperature at the surface is ap proximately the same, approximately 5 K. The simulations imply that a significant cooling of the troposphere and surface can occur at times of closely spaced, multiple, sulfur-rich volcanic explosions that span time scales of decades to centuries, such as occurred at the end of t he nineteenth and beginning of the twentieth centuries. The steady-sta te climate response to volcanic forcing includes a large expansion of sea ice, especially in the Southern Hemisphere; a resultant large incr ease in surface and planetary albedo at high latitudes; and sizable ch anges in the annually and zonally averaged air temperature, DELTAT; DE LTAT at the surface (DELTAT(s)) does not sharply increase with increas ing latitude, while DELTAT in the lower stratosphere is positive at lo w latitudes and negative at high latitudes. In certain ways, the clima te response to the three different forcings is similar. Direct radiati ve forcing accounts for 30% and 25% of the total DELTAT(s) in the volc ano and 2 x CO2 runs, respectively. Changes in atmospheric water vapor act as the most important feedback, and are positive in all three cas es. Albedo feedback is a significant, positive feedback at high latitu des in all three simulations, although the land ice feedback is promin ent only in the 18 K run. In other ways, the climate response to the t hree forcings is quite different. The latitudinal profiles of DELTAT(s ) for the three runs differ considerably, reflecting significant varia tions in the latitudinal profiles of the primary radiative forcing. Pa rtially as a result of this difference in the DELTAT(s) profiles, chan ges in eddy kinetic energy, heat transport by atmospheric eddies, and total atmospheric heat transport are quite different in the three case s. In fact, atmospheric heat transport acts as a positive feedback at high latitudes in the volcano run and as a negative feedback in the ot her two runs. These results raise questions about the ease with which atmospheric heat transport can be parameterized in a simple way in ene rgy balance climate models.