Abstract
Abstract. Under the protocols of phase 3 of the Paleoclimate Modelling Intercomparison Project, a number of simulations were produced that provide a range of potential climate evolutions from the last millennium to the end of the current century. Here, we present the first simulation with the Community Earth System Model (CESM), which includes an interactive carbon cycle, that covers the last millennium. The simulation is continued to the end of the twenty-first century. Besides state-of-the-art forcing reconstructions, we apply a modified reconstruction of total solar irradiance to shed light on the issue of forcing uncertainty in the context of the last millennium. Nevertheless, we find that structural uncertainties between different models can still dominate over forcing uncertainty for quantities such as hemispheric temperatures or the land and ocean carbon cycle response. Compared to other model simulations, we find forced decadal-scale variability to occur mainly after volcanic eruptions, while during other periods internal variability masks potentially forced signals and calls for larger ensembles in paleoclimate modeling studies. At the same time, we were not able to attribute millennial temperature trends to orbital forcing, as has been suggested recently. The climate–carbon-cycle sensitivity in CESM during the last millennium is estimated to be between 1.0 and 2.1 ppm °C−1. However, the dependence of this sensitivity on the exact time period and scale illustrates the prevailing challenge of deriving robust constraints on this quantity from paleoclimate proxies. In particular, the response of the land carbon cycle to volcanic forcing shows fundamental differences between different models. In CESM the tropical land dictates the response to volcanoes, with a distinct behavior for large and moderate eruptions. Under anthropogenic emissions, global land and ocean carbon uptake rates emerge from the envelope of interannual natural variability by about year 1947 and 1877, respectively, as simulated for the last millennium.
Highlights
The last about 1000 years constitute the best opportunity previous to the instrumental period to study the transient interaction of external forcing and internal variability in climate, atmospheric CO2, and the carbon cycle on interannual to multi-decadal timescales
This study presents a simulation from 850 to 2100 CE with the fully coupled Community Earth System Model (CESM), including the carbon cycle, and provides an overview of the imprint of external forcing on different climate and carbon cycle diagnostics in the simulation
For comparison we draw on a number of PMIP3 simulations, simulations with CCSM4 and MPI-ESM
Summary
The last about 1000 years constitute the best opportunity previous to the instrumental period to study the transient interaction of external forcing and internal variability in climate, atmospheric CO2, and the carbon cycle on interannual to multi-decadal timescales. The instrumental record is often too short to draw strong conclusions on multi-decadal variability. The large and sometimes controversial body of literature on the magnitude and impact of solar and volcanic forcing on interannual to multi-decadal climate variability illustrates the challenges inherent in establishing a robust understanding of a period that is characterized by a small signal-to-noise ratio in many quantities and for which uncertainties in the external forcing remain (see, e.g., Wanner et al, 2008; PAGES 2k network, 2013; Schurer et al, 2013). A process-based quantitative explanation of the reconstructed preindustrial variability in atmospheric CO2 and carbon fluxes is largely missing.
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