A statistical volcanic forcing scenario generator for climate simulations

The climate system is continuously affected by forcings that add to its inherent variability. Recently, the dominant influence shifted from mostly natural factors to the rapidly increasing anthropogenic greenhouse gas and aerosol forcing. Climate change simulations for the 21st and 22nd centuries then employ possible story lines of human socioeconomic development with associated radiative forcing that exclusively explore the potential human influence on climate. None of the scenarios, however, include natural factors that dominated climate variations prior to the large anthropogenic emissions. This leads to a discontinuity at the transition between the historical and the future projection period. Similarly, studies of transient climate variations before the last 1-2 millennia generally use only the well-known, slowly varying forcings such as orbital or greenhouse forcing derived from ice cores. While past solar irradiance variations can be reasonably estimated from cosmogenic isotope data, no well-dated, high-resolution information exists before about A. D. 500 that would allow for an implementation of forcing from explosive volcanism. Here, we present a statistical approach to generate statistically (and geophysically) realistic scenarios of volcanic forcing that are based on the properties of the longest available volcanic forcing series derived from ice cores. The resulting scenarios do not carry direct temporally predictive or hindcast capabilities, but they allow for an appropriate evaluation of natural uncertainty on various timescales. These series can be applied to ensure a seamless integration of an important natural forcing factor for climate change simulations of periods where such forcing is not available.