Variations of the global annual mean surface temperature during the past 2000 years: results from the CESM1

The influence of external forcings on surface temperature is not fully understood at decadal to multi-centennial timescales. This study uses simulation results from the Community Earth System Model (CESM) forced by total solar irradiation (TSI), greenhouse gases (GHGs) concentrations, volcanic eruptions (VOL), and land use and land covers (LULC) to investigate the possible impacts of external forcing on decadal to multi-centennial timescales over the past 2000 years. The simulated global annual mean surface temperature (GMST) variations show a good coherence with observations over the past 150 years and proxy-based reconstructions over the past 2000 years. Ensemble empirical mode decomposition (EEMD) is used to isolate the GMST variations on decadal to multi-centennial timescales. We found that the VOL forcing has a significant effect on GMST at decadal to multi-decadal timescales, but its effect weakens gradually as the timescale lengthens. The TSI forcing is a major factor of GMST on multi-decadal to sub-millennial timescales, and its effect strengthens as the timescale lengthens. The GHGs is the dominant driver of recent warming, but its effect is weak in the pre-industrial times. The LULC could lead to a slight cooling trend over the past 2000 years but has no large impact on GMST at decadal to multi-centennial timescales. This study reports a 2000-year length climate simulation and may contribute to better understanding external drivers of GMST at different timescales.