Climate change from increased CO2 and direct and indirect effects of sulfate aerosols

Abstract

A global coupled ocean‐atmosphere general circulation model without flux correction is integrated in a set of 75 year sensitivity experiments where the indirect forcing effect of sulfate aerosols is included for the first time in combination with transient greenhouse gas forcing and the direct effect of sulfate aerosols. Sulfate aerosol forcing increases from zero to present‐day estimates in the first 30 years of the integrations while equivalent CO2 forcing increases by 1% per year relative to the control experiment, similar to the rate of increase of observed greenhouse gas forcing over the period 1960–1990. Annual mean averages around year 30, analogous to present‐day conditions, indicate better agreement with recent observed geographic and zonal mean temperature anomaly patterns in the sulfate aerosol experiments and less warming in northern summer than winter. Sulfate aerosols then are increased following the IS92a scenario, while CO2 continues to increase at 1% per year. Averages around year 65, analogous to conditions roughly 35 years in the future, indicate warming almost everywhere in the troposphere over the globe as the CO2 forcing overwhelms the negative radiative forcing from the sulfate aerosols. There is also a general indication of weakening of the south Asian monsoon in the sulfate aerosol experiments. There is qualitative agreement in the patterns of the temperature changes, both geographic and zonal, between the different sulfate aerosol experiments, with the magnitude of the changes a function of the size of the forcing.