presently increasing the scope and possibilities of inverse methods (Arellano et al., 2004; Palmer et al., 2003; Heald et al., 2004). Variational data assimilation methods are now being developed to improve the detail in the characterization of sources enabled by large observational datasets (e.g., Kaminski et al., 2002). Future inverse model studies should make use of available observations of aerosol surface concentrations and optical depths, as well as the information contained in the observed correlations between species concentrations, for example, between CO2 and CO (Suntharalingam et al., 2004) or methane and ethane (Xiao et al., 2004). These correlations can improve the top-down constraints on the sources and also reduce the errors associated with CTM transport.
A comprehensive database of radiative forcings and effects exists primarily for the past 25 years because many of the relevant observations require space-based observations. Present in this epoch are two major volcanic eruptions (El Chichon and Mt. Pinatubo), a few significant El Niños (1983, 1997), and two solar irradiance cycles. The reconstruction of much longer-term records of forcings and effects is crucial for a broader perspective.
Empirical analyses of correlations between adopted radiative forcing histories and climate reconstructions provide exploratory but limited insights into the relative roles of radiative forcings of climate change in the recent past (e.g., Lean et al., 1995; Mann et al., 1998; Waple et al., 2002). Correlations of various proxies of climate change and radiative forcings during the Holocene suggest the influences of solar variability and orbital motions on a range of climate phenomena including drought (Hodell et al., 2001), rainfall (Neff et al., 2001), and North Atlantic winds and surface hydrography (Bond et al., 2001). Other studies characterize the evolution of variability modes as sources of historical climate change, including the Arctic Oscillation (Noren et al., 2002) and the El Niño/Southern Oscillation (ENSO; Moy et al., 2002). Another type of forcing response investigation is the effect of ice sheet changes during the last glacial maximum (e.g., Manabe and Broccoli, 1985).
Detailed physical insight into the role of past natural radiative forcing requires that documented climate reconstructions be compared with model simulations driven by the actual geophysical forcings. However, some current limitations hamper our ability to draw precise conclusions from such comparisons, even in the recent past. Moderate differences exist, for example, between various alternative reconstructions of past hemispheric temperature trends (e.g., Folland et al., 2001; Jones et al., 2001; Mann et al., 2003; see Jones and Mann, 2004, for a comparison of multiple reconstruc-