Recent variability of the solar spectral irradiance and its impact on climate modelling

Abstract

During periods of high solar activity, the Earth receives≈0.1 % higher total solar irra-diance (TSI) than during low activity periods. Variations of the solar spectral irradiance(SSI) however, can be larger, with relative changes of 1 to 20 % observed in the ul-traviolet (UV) band, and in excess of 100% in the soft X-ray range. SSI changes influence the Earth’s atmosphere, both directly, through changes in shortwave (SW) heatingand therefore, temperature and ozone distributions in the stratosphere, and indirectly,through dynamical feedbacks. Lack of long and reliable time series of SSI measure-ments makes the accurate quantification of solar contributions to recent climate changedifficult. In particular, the most recent SSI measurements show a larger variability in the10UV spectral range and anomalous changes in the visible and near-infrared (NIR) bandswith respect to those from earlier observations and from models. A number of recentstudies based on chemistry-climate model (CCM) simulations discuss the effects andimplications of these new SSI measurements on the Earth’s atmosphere, which maydepart from current expectations.This paper summarises our current knowledge of SSI variability and its impact onEarth’s climate. An interdisciplinary analysis of the topic is given. New comparisonsand discussions are presented on the SSI measurements and models available todate, and on the response of the Earth’s atmosphere and climate to SSI changes inCCM simulations. In particular, the solar induced differences in atmospheric radiative heating, temperature, ozone, mean zonal winds, and surface signals are investigated inrecent simulations using atmospheric models forced with the current lower and upperboundaries of SSI solar cycle estimated variations from the NRLSSI model data andfrom SORCE/SIM measurements, respectively. Additionally, the reliability of availabledata is discussed and additional coordinated CCM experiments are proposed.