9.2.2.2 Aerosol Scattering and Cloud Feedback in Models and Observations

One line of observational evidence that reflective aerosol forcing has been changing over time comes from satellite observations of changes in top-of-atmosphere outgoing shortwave radiation flux. Increases in the outgoing shortwave radiation flux can be caused by increases in reflecting aerosols, increases in clouds or a change in the vertical distribution of clouds and water vapour, or increases in surface albedo. Increases in aerosols and clouds can cause decreases in surface radiation fluxes and decreases in surface warming. There has been continuing interest in this possibility (Gilgen et al., 1998; Stanhill and Cohen, 2001; Liepert, 2002). Sometimes called ‘global dimming’, this phenomena has reversed since about 1990 (Pinker et al., 2005; Wielicki et al., 2005; Wild et al., 2005; Section 3.4.3), but over the entire period from 1984 to 2001, surface solar radiation has increased by about 0.16 W m^–2 yr^–1 on average (Pinker et al., 2005). Figure 9.3 shows the top-of-atmosphere outgoing shortwave radiation flux anomalies from the MMD at PCMDI, compared to that measured by the Earth Radiation Budget Satellite (ERBS; Wong et al., 2006) and inferred from International Satellite Cloud Climatology Project (ISCCP) flux data (FD) (Zhang et al., 2004). The downward trend in outgoing solar radiation is consistent with the long-term upward trend in surface radiation found by Pinker et al. (2005). The effect of the eruption of Mt. Pinatubo in 1991 results in an increase in the outgoing shortwave radiation flux (and a corresponding dimming at the surface) and its effect has been included in most (but not all) models in the MMD. The ISCCP flux anomaly for the Mt. Pinatubo signal is almost 2 W m^–2 larger than that for ERBS, possibly due to the aliasing of the stratospheric aerosol signal into the ISCCP cloud properties. Overall, the trends from the ISCCP FD (–0.18 with 95% confidence limits of ±0.11 W m^–2 yr^–1) and the ERBS data (–0.13 ± 0.08 W m^–2 yr^–1) from 1984 to 1999 are not significantly different from each other at the 5% significance level, and are in even better agreement if only tropical latitudes are considered (Wong et al., 2006). These observations suggest an overall decrease in aerosols and/or clouds, while estimates of changes in cloudiness are uncertain (see Section 3.4.3). The model-predicted trends are also negative over this time period, but are smaller in most models than in the ERBS observations (which are considered more accurate than the ISCCP FD). Wielicki et al. (2002) explain the observed downward trend by decreases in cloudiness, which are not well represented in the models on these decadal time scales (Chen et al., 2002; Wielicki et al., 2002).

Figure 9.3. Comparison of outgoing shortwave radiation flux anomalies (in W m^–2, calculated relative to the entire time period) from several models in the MMD archive at PCMDI (coloured curves) with ERBS satellite data (black with stars; Wong et al., 2006) and with the ISCCP flux data set (black with squares; Zhang et al., 2004). Models shown are CCSM3, CGCM3.1(T47), CGCM3.1(T63), CNRM-CM3, CSIRO-MK3.0, FGOALS-g1.0, GFDL-CM2.0, GFDL-CM2.1, GISS-AOM, GISS-EH, GISS-ER, INM-CM3.0, IPSL-CM4, and MRI-CGCM2.3.2 (see Table 8.1 for model details). The comparison is restricted to 60°S to 60°N because the ERBS data are considered more accurate in this region. Note that not all models included the volcanic forcing from Mt. Pinatubo (1991–1993) and so do not predict the observed increase in outgoing solar radiation. See Supplementary Material, Appendix 9.C for additional information.