Reductions in shortwave radiation with increased atmospheric aerosols and other particulates are well established. This reduction of total shortwave radiation also coincides with an increase in the diffuse portion of the shortwave radiation (Black et al., 2006; Roderick, 2006) as aerosols and particulates increase the scatter of direct beam radiation (Fig. 13.1). Surface ultraviolet radiation (UV; < 400 nm) levels are also influenced by atmospheric aerosols and other pollutants, both causing varying degrees of attenuation and scattering of the radiation. While increased levels of UV-B radiation due to ozone depletion during the latter part of the 20th century are unquestioned (McKenzie et al., 2007), reductions in surface-level UV radiation due to pollution are ubiquitous over some surprisingly large areas of the Northern Hemisphere (McKenzie et al., 2001) and can cancel the increased
UV-B radiation from ozone depletion (Ma and Guicherit, 1997). At present, since stratospheric ozone levels are already showing signs of recovery, UV levels would be expected to decrease in the near future (McKenzie et al., 2007), even without global dimming.
Much is known about the effects of ambient and elevated UV-B (290 nm - 320 nm) on plants (see reviews by Barnes et al., 2005; Caldwell et al., 2007; and others), but the direct effects of global dimming on plants and those mediated by associated changes in UV-B radiation have not been effectively assessed. In particular, experiments specifically designed to assess changes in UV-B under global dimming have not been conducted. Interactions between quantities of photosynthetically active radiation (PAR; 400 nm - 700 nm) and UV on plant performance are known (e.g., Krizek, 2004), but are further complicated by the nature of PAR and UV radiation. Because solar UV radiation has a greater diffuse component than the longer wavelengths of sunlight, the penetration of PAR and UV into plant canopies is different (Flint and Caldwell, 1998). In particular, it has been shown that the ratio of PAR to UV in a canopy differs dramatically depending on whether solar radiation is measured in a sunfleck or the shade (Flint and Caldwell, 1998). Not surprisingly, therefore, generalizations on how total solar radiation changes, as occurs with global dimming and brightening, interact with recovery from ozone depletion are lacking.
Research previously conducted that examined plant responses to changes in total solar radiation, as might be expected under global dimming, involved both modeling and experimental efforts. Modeling studies have generally shown increases in canopy photosynthesis with global dimming as a result of increased penetration of diffuse PAR into the canopy. For example, a substantial increase in the proportion of diffuse radiation following the eruption of Mt. Pinatubo appears to have increased photosynthesis to the same extent that atmospheric CO2 showed an unexpected decline (Roderick et al., 2001). Experimental studies to assess plant effects resulting from changes in total solar radiation have typically involved manipulation of total sunlight. However, these studies often use substantial amounts of shade (e.g., 30% and 60% shade, Raveh et. al., 2003), which is a much greater change in solar radiation than occurs with global dimming. Therefore, results from these studies make extrapolation to global dimming scenarios tenuous. Results from a limited number of experiments using minimal shading (e.g., 13% used by Fischer, 1975) suggest that some effects can be seen due to even limited shade levels, and may be more affected in plant communities that are light-limited (Black et al., 2006).
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