Aerosols (liquid or solid particles suspended in the air) affect the climate directly by scattering and absorbing solar radiation and by scattering, absorbing and emitting thermal radiation, semi-derectly by absorbing solar radiation that leads to changes in cloud cover and liquid water path and indirectly by acting as cloud condensations nuclei (CCN). The aerosol indirect effect is usually split into two effects: the first indirect effect, whereby an increase in aerosols causes an increase in droplet concentration and a decrease in droplet size for fixed liquid water content (Twomey, 1974), and the second indirect effect, whereby the reduction in cloud droplet size affects the precipitation efficiency, tending to increase the liquid water content, the cloud lifetime (Albrecht, 1989), and the cloud thickness (Pincus and Baker, 1994).
It is widely recognized that anthropogenic activities may change the chemical composition and mass loadings of tropospheric aerosols and, in so doing, affect dynamics, reflective properties and lifetime of clouds on regional and global scale. It is also known, that aerosol effects on climate through direct, semi-direct, thermodynamical, indirect effects and associated feedbacks are large compared to other forcings (i.e., greenhouse gases, land-use, solar activity, etc). However, the uncertainties and unresolved issues surrounding tropospheric aerosols (i.e., complex interactions between trace gases, aerosols and clouds, and a wide range of spatiotemporal scales involved) inhibit them from being correctly addressed in global modeling studies and hinder the assessments of anthropogenically-induced climate change. The greatest uncertainty in the assessment of climate forcing by anthropogenic aerosols is their effect on clouds, referred to as the aerosol indirect effect (AIE). In the literature, there is a wide range of values given for AIE; therefore, evaluating the contributions from different parameters (i.e., meteorological fields, emission scenarios, cloud droplet activation parameterizations, aerosol microphysics) used in different Global Climate Models (GCM) in indirect forcing uncertainty is an important step toward constraining the AIE.
In our lab to address issues related to AIE the NASA Goddard Space Flight Center (GISS) ModelE2 (http://www.giss.nasa.gov/tools/modelE/) is used while the uncertainties related to AIE are addressed using the NASA Global Modeling Initiative (GMI) Chemistry and Transport Model (CTM) (http://gmi.gsfc.nasa.gov/)