reflection of radiation back to space. Assumptions must be made regarding the relationship between the sulfate aerosol deposited at the surface and the extent and duration of a significant stratospheric dust veil. These assumptions are highly uncertain and can be only partially tested for a few recent eruptions (e.g., Stenchikov et al., 1998). Greater concentrations of trapped sulfates are typically indicative of larger eruptions, although the proximity of the source region to the ice core may be a complicating factor. Explosive tropical eruptions are more likely to impart a significant global radiative forcing because they provide an opportunity for the aerosol to spread throughout the global lower stratosphere. An eruption is assumed to have occurred in the tropics if its aerosols are recorded in ice cores at both poles.
Other indices of past volcanic activity have also been developed in past work. These include the Volcanic Explosivity Index, or VEI, which is based on qualitative volcanological information and should therefore be used with caution in studies seeking quantitative estimates of climate response (Robock and Free, 1995), and the Dust Veil Index (DVI; see, e.g., Robock, 2000). Some authors argue that the use of climate information in some DVI estimates leads to a potential circularity in using this index to diagnose climate response. Tree-ring reconstructions of continental summer temperature variations have also been used to estimate past volcanic forcing histories (Briffa et al., 1998), although a similar circularity obviously exists if the associated volcanic histories are used to diagnose the climate response to volcanic forcing. Zielinski (2000) and Robock (2000) provide excellent reviews and critiques of various indices of past explosive volcanic activity.
Ice core volcanic radiative forcing estimates have been developed for the past century to the past couple of millennia by numerous researchers (Robock and Free, 1995, 1996; Robertson et al., 1998; Robock, 2000; Crowley, 2000; Ammann et al., 2003; Crowley et al., 2003). The choice of ice cores used to define the volcanic forcing chronology leads to some significant differences among these different estimates. Some of the estimates assume that tropical eruptions dominate the annual global mean radiative forcing (e.g., Free and Robock, 1999; Crowley, 2000), whereas other reconstructions seek to take into account the influence of the latitudinal and seasonal characteristics of the eruptions (Robertson et al., 1998; Ammann et al., 2003).
Recognized biologically related surface forcings associated with the oceans include ocean color as well as biogenic aerosol emissions (e.g., dimethyl sulfide). Ocean color refers to the radiance backscattered at the air-sea interface. It is determined by water molecules (the blue wavelengths in particular), phytoplankton and detrital particles, and nonbiogenic sedi-