Pleistocenes

Temp

Temp

Figure 8.4. The hypothesis that lack of Upper Montane Rain Forest in the Pleistocene may be explained by absence of a habitat with a suitable combination of mean annual temperature and UV-B insolation (diagram by G. Rapson, unpublished).

would then lead to a rise of UV in the tropics, but measurements show little or no change there (Stolarski et al., 1992; Madronich et al., 1995; Gleason, 2001).

The possible effect of global warming on UV insolation has also been studied. More greenhouse gases lead to a warmer atmosphere, but this leads to a cooler stratosphere (Austin et al., 1992; Shindell et al., 1998a). This might be expected to lead to more widespread conditions favoring the destruction of ozone. If this were to happen, it could result in a rise in UV levels in temperate regions, and possibly in the tropics also. Mathematical modeling shows, however, that this is unlikely. A model simulation did indeed suggest a decrease of ozone in middle latitudes, but a small increase (2-6%) in the tropics (Shindell et al., 1998b).

It thus appears that the tropical UV climate is rather stable. One possible exception to this could be the effect of volcanic eruptions. For example, the major eruption of Mount Pinatubo in the Philippines in 1991 resulted in the release of sulphur dioxide aerosols which destroyed stratospheric ozone and led to an observed small increase in ground-level UV for several years on a global scale (Gleason et al., 1993).

Upper limit of woody vegetation

Location of UMRF

Upper limit of woody vegetation

Location of UMRF

Loss of UMRF

Upper limit of ~ woody vegetation

Figure 8.4. The hypothesis that lack of Upper Montane Rain Forest in the Pleistocene may be explained by absence of a habitat with a suitable combination of mean annual temperature and UV-B insolation (diagram by G. Rapson, unpublished).

Loss of UMRF

Upper limit of ~ woody vegetation

Possible increases in tropical UV have been blamed for two major phenomena: the bleaching of tropical corals, and the decline and extinction of many Amphibia. Corals are indeed very sensitive to UV-A and UV-B, and many cases of coral bleaching and death have been reported. Corals have been shown to possess powerful UV-screening compounds, some of which are of commercial importance. It now appears, however, that a more likely cause of coral bleaching and death is a rise of water temperature. Even a rise of 1°C can be effective in this way (Gleason, 2001).

The Amphibia story is more complex, since the reported decline of Amphibia is worldwide. Many Amphibia are indeed sensitive to UV-B, which may significantly reduce the hatching of eggs, or may induce abnormalities (Blaustein et al., 2001,2003). 427 species of Amphibia (7.4% of known species) are critically endangered and 1,856 species (32.5%) are globally threatened. A high proportion of these (76.5%) are from the Neotropics, Afrotropics, or Australasia (Stuart et al., 2004). Nevertheless, the latest evidence suggests that increasing UV-B is unlikely to be the major single factor, and a fungal disease is now being investigated as the likely principal cause (Daszak et al., 2003). The complete answer may involve many interacting factors (Carey and Alexander, 2003; Collins and Storfer, 2003; Kats and Ferrer, 2003; Storter, 2003).

The possibility of investigating past ultraviolet radiation environments by the record of fossil pigments in lakes (Leavitt et al., 1997) is an interesting development which may prove to be of considerable value.

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