Fingerprints of Solar Irradiance Changes During the Last Millennium: Impact of Different Background Trends on the Detection in Transient Climate Simulations

Authors: Caspar.M. Ammann (1), H.-S. Oh (2), and P. Naveau (3)

Affiliations: 1) National Center for Atmospheric Research, Boulder, Colorado,

2) University of Alberta, Canada,

3) University of Colorado

Solar irradiance changes have been accurately measured only over the last few decades. Assumptions on additional century-scale trends, although commonly applied in climate studies, have recently been questioned.  We present results from multi-century transient simulations with a coupled Ocean-Atmosphere General Circulation Model (GCM) forced by a range of forcing estimates: previously considered best-guess values (~0.25% of Total Solar Irradiance), very large solar changes (0.65% TSI) as well as essentially absent low-frequency trends (0.1% TSI). The temporal evolution of the forcing was based on 10-Beryllium fluxes measured in polar ice cores.

First, we extract the solar signals in the coupled GCM simulations using non-decimated discrete wavelet transforms. This method provides a flexible, multi-scale tool to separate solar signals from internal noise and helps to characterize them both spatially and over time. Identified solar fingerprints are then compared between the differently forced experiments and signal to noise issues are discussed. Several proxy based climate reconstructions are also decomposed and compared with the simulated climate signals. Focusing at the strongest solar signals, detection of solar variations in the climate reconstructions is achieved at different spatial scales. Finally, the contribution of solar variability to the total climate variability is estimated over time. This includes periods with either very strong or weak solar forcing as well as the late 20th century when anthropogenic forcings become dominant.