Future projections of surface UV-B in a changing climate

Abstract

[1] Results of comprehensive long-term simulations of surface all-sky and clear-sky ultraviolet (UV) radiation through 1960-2100 are presented. A new earth system model, MIROC-ESM-CHEM, is used for the simulation, which considers key processes that change the surface UV radiation: atmospheric dynamics and chemistry affecting ozone in the stratosphere and troposphere, aerosols and clouds in the troposphere, and changes in surface albedo with sea ice and snow cover. In contrast to previous assessments considering only the effect of long-term change in stratospheric ozone, the simulated long-term behavior of UV radiation in this study is strongly affected by other processes. In one of two simulations, all-sky UV radiation in the northern midlatitudes is projected to increase in the 21st century despite the expected recovery of the stratospheric ozone layer. Reductions in aerosols and clouds are expected to overcompensate for the effect of ozone recovery. The results are sensitive to the future socioeconomic scenario, describing GHG concentrations and emissions of aerosol and ozone precursors in the troposphere. The interannual variability of UV radiation associated with the 11 year solar cycle and local processes is also discussed.

Anthropogenic changes in the surface all-sky UV-B radiation through 1850–2005 simulated by an Earth system model

“…The ozone hole recovery (surface albedo reduction) is obvious in the SH extratropics (Arctic) in the first half of this century, while ozone increases due to transport changes in the lower stratosphere dominate in the second half of this century (see Watanabe et al, 2011a for details of the RCP4.5 and RCP8.5 simulations). The higher radiative forcing scenario with higher GHG concentrations generally results in less UV-B radiation.…”

“…In addition to the historical simulations, four future projection simulations for the 2006-2100 period have been performed based on the Representative Concentration Pathway (RCP) scenarios (Moss et al, 2010;Meinshausen et al, 2011). Since results of the RCP4.5 and RCP8.5 simulations have already been presented and discussed in Watanabe et al (2011a), the Appendix of the present paper briefly discusses the long-term behaviors of the surface UV-B radiations in all four scenarios, RCP2.6, RCP4.5, RCP6.0 and RCP8.5, using 1850-1859 as a reference period. Figure 1 shows the long-term evolution of the annual mean surface all-sky UV-B radiation averaged over five latitudinal bands, which is shown as a relative change to the 1850 level.…”

“…Detailed model descriptions and experimental settings of the historical and future simulations are given in Watanabe et al (2011a) and Watanabe et al (2011b). MIROC-ESM-CHEM is a comprehensive ESM consisting of coupled atmosphere, ocean, sea-ice, river, land-surface, aerosol, atmospheric chemistry, and ocean and terrestrial carbon cycle models (Watanabe et al, 2011b).…”

“…Bais et al (2011) further considered changes in climatological cloud forcing on erythemal UV. More recently, Watanabe et al (2011a) and Watanabe and Yokohata (2012) further considered changes in cloud forcing, tropospheric aerosols, tropospheric ozone, and surface albedo. They analyzed future projection simulations through 1960-2100 performed using a comprehensive Earth system model (ESM) including these processes, and highlighted the importance of changes in tropospheric ozone and aerosols that had not been considered in previous studies.…”

“…The main goal of the present study is to evaluate the longterm anthropogenic changes in the surface all-sky UV-B radiation through 1850-2005 by analyzing transient historical simulations of the ESM used by Watanabe et al (2011a). In particular, responses of the surface all-sky UV-B radiation to anthropogenic changes in ozone and aerosols will be separately evaluated using a series of historical simulations including/excluding these changes.…”