On the solar ultraviolet spectral irradiance during the Maunder Minimum

In the historical record of solar activity the period from 1645 to 1715 is a singular epoch during which the number of sunspots decreased markedly for a generation. Known as the Maunder Minimum, this solar epoch coincided with the coldest part of the Little Ice Age (circa 1450 to 1850). We estimate the change at this time in the output of solar ultraviolet (UV) radiation at wavelengths from 120 to 300 nm, relative to contemporary observations. Since this portion of the solar UV spectrum determines ozone composition in the stratosphere, our results bear on the historical variability of ozone and its potential climatic effects. Between the Maunder Minimum and 1986 (the present day solar activity minimum between cycles 21 and 22) we estimate reductions of 64% in the irradiance of the Lyman α line of neutral hydrogen (at 121.6 nm), 8% at 200 nm, and 3.5% in the wavelength range from 210 to 250 nm. The reduction in the solar output from the entire spectral band between 120 and 300 nm is estimated to be 0.17 W/m 2 , which is approximately 6% of the change in the total solar irradiance of 2.7 W/m 2 previously estimated by us (Lean et al., 1992a) over the same time span. Because of this diminished UV output due to very low solar activity the Maunder Minimum total ozone concentration may have been 4% below its 1980 level. While the climatic consequences of such a change have yet to be determined, recent work by Haigh (1994) on modulation of radiative climate forcing by stratospheric ozone emphasizes the need to understand the role of UV irradiance variability as one forcing mechanism.