Long-term (250,000 BP to 50,000 AP) variations in ultraviolet and visible radiation (0.175-0.690 μm)

Recent biological studies demonstrating sensitivity to ultraviolet (UV) radiation coupled with research on the radiative impact of stratospheric ozone depletion pose a challenge to climate modelers and reconstructionists to determine past variations in UV radiation. In this study, solar radiative waveband totals are computed applying a numerical model linking the parameterization of the amount of radiation as a function of orbital parameters with the atmospheric solar scattering. Variations are computed for three continuous solar wavebands (0.225-0.285, 0.3-0.325 and 0.325-0.690 μm) and one non-continuous shortwave band (0.175-0.225/0.285-300 μm). With the assumption of invariant latitudinal ozone concentrations over the past 250,000 years, radiation in all wavebands displays significant variations from present levels due to changes produced by Milankovitch orbital mechanisms (obliquity, eccentricity and precession). Specifically: (a) orbital changes produced the following percent ranges from present-day values in the 0.3-0.325 μm waveband over the last 250,000 years: -19% to 30% (June Solstice), -26% to +32% (December Solstice) and -3% to +3% (equinoxes); (b) precessional influences control variations in the total daily radiation; and (c) orbital changes produce a decrease in the amount of visible and UV radiation over the next 10 kyr in the Northern Hemisphere for the spring and summer while increasing visible and UV for the fall and winter. These initial results for long-period radiative variations (a) provide a first comparative range of values in future UV radiative change study related to ozone depletion; (b) generate baseline values for researchers interested in the potential environmental and biological effects of past UV radiative variations; and (c) aid in analyses of the influence of solar radiative changes based upon orbital mechanisms on climatic change.