There has been a tremendous improvement in the understanding of atmospheric aerosols and their climate effect over the last decades, with some important observational and modelling breakthroughs. Long-term measurements of aerosols (e.g., Putaud et al. 2010, Andrews et al. 2011), observational campaigns (e.g., Quinn & Bates 2005), and remote sensing from space and ground (Holben et al. 1998, Remer et al. 2008) have remarkably increased knowledge about the composition and characteristics of atmospheric aerosols. However, an understanding of the greater complexity of atmospheric aerosols has at the same time limited more robust quantification of their climate effect. The first estimate of the direct aerosol effect in the early 1990s was limited to sulphate aerosols (Charlson et al. 1991), with estimates for BC coming a few years later (Haywood & Shine 1995). Observations have shown that OC is an important aerosol component (Novakov et al. 1997, Ramanathan et al. 2001), and substantial investigations have later explored the complex composition and optical characteristics of this compound (e.g., Kanakidou et al. 2005, Graber & Rudich 2006). Global aerosol models today provide RF estimates for a large set of aerosol components, such as sulphate, BC (from fossil fuel and biomass burning), OC (primary and secondary from fossil fuel and biomass burning), and nitrate (Jacobson 2001, Liao & Seinfeld 2005, Koch et al. 2009). In addition, multi-model studies are performed to understand and reduce uncertainties due to model differences (Schulz et al. 2006).
An example of recent progress is reduced uncertainty in the estimate of the total direct aerosol effect. This estimate was made possible by advances that have occurred on both the modelling and the observational side, and was based on a combination of global aerosol models and observation based methods (mostly remotely sensed data). Initially, observational estimates of RF were up to three times stronger than model based calculations (Forster et al. 2007). Consistency between these two different approaches has subsequently been reached, and was found to arise from necessary and simplified assumptions of the pre-industrial aerosol composition in the observation-based method (Myhre 2009). Although the uncertainty in the total direct aerosol effect is reduced, it is still substantial compared to uncertainties associated with greenhouse gases. In addition the uncertainty in individual RF for several of the aerosol components, such as BC, OC, and nitrate, is large.
Similar to the early estimates of the direct aerosol effect, many of the first model estimates of the aerosol indirect effect only accounted for the effect of sulphate particles acting as CCN ( Kaufman & Chou 1993, Jones et al. 1994). Furthermore, they only included the influence of sulphate aerosols on cloud albedo, disregarding any effects on cloud lifetime and extent. With the realization that other aerosol species of anthropogenic origin could also form cloud droplets and that effects on cloud lifetime and extent were also possible, global climate models estimated the aerosol indirect effect to be stronger (e.g., Lohmann & Feichter 1997, Menon et al. 2002). Some even predicted this cooling effect to be comparable in magnitude to the warming greenhouse effect. Recent publications have later pointed to oversimplifications in model representation of clouds and how their lifetimes are affected by aerosols (e.g., Stevens & Feingold 2009). It is now acknowledged that aerosol effects on cloud lifetime will vary with the cloud type in question, and that complex feedback processes can sometimes complicate the ultimate cloud response to aerosol perturbations. Recent model studies have found that by forming ice in super-cooled liquid clouds, aerosols may in fact shorten cloud lifetime, because of the more efficient precipitation formation when cloud ice is present (e.g., Lohmann & Hoose 2009, Storelvmo et al. 2011). In summary, whether aerosols are acting as CCN or IN or are simply modifying atmospheric stability by absorbing solar radiation, there is still high uncertainty associated with their effect on cloud lifetime. This uncertainty reflects how challenging it is to represent aerosol-and-cloud processes that occur on microscopic scales in models that have resolutions of tens to hundreds of kilometres. Although much uncertainty remains, model and satellite estimates of the cloud albedo effect seem to converge on a negative RF that has about half the magnitude of the positive RF attributed to increasing CO2 concentrations.