Abstract
Abstract. A long-standing task in climate research has been to distinguish between anthropogenic climate change and natural climate variability. A prerequisite for fulfilling this task is the understanding of the relative roles of external drivers and internal variability of climate and the carbon cycle. Here, we present the first ensemble simulations over the last 1200 years with a comprehensive Earth system model including a fully interactive carbon cycle. Applying up-to-date reconstructions of external forcing including the recent low-amplitude estimates of solar variations, the ensemble simulations reproduce temperature evolutions consistent with the range of reconstructions. The 20th-century warming trend stands out against all pre-industrial trends within the ensemble. Volcanic eruptions are necessary to explain variations in pre-industrial climate such as the Little Ice Age; yet only the strongest, repeated eruptions lead to cooling trends that differ significantly from the internal variability across all ensemble members. The simulated atmospheric CO2 concentrations exhibit a stable carbon cycle over the pre-industrial era with multi-centennial variations somewhat smaller than in the observational records. Early land-cover changes have modulated atmospheric CO2 concentrations only slightly. We provide a model-based quantification of the sensitivity (termed γ) of the global carbon cycle to temperature for a variety of climate and forcing conditions. We diagnose a distinct dependence of γ on the forcing strength and time-scales involved, thus providing a possible explanation for the systematic difference in the observational estimates for different segments of the last millennium.
Highlights
We present the first such simulations for the last millennium, with a comprehensive Earth System Model (ESM) including a fully interactive carbon cycle. This is a significant advance over previous efforts, which have been restricted to Energy Balance Models (Crowley, 2000) and ESMs of Intermediate Complexity (e.g., Gerber et al, 2003; Goosse et al, 2005), to single realisations of coupled models without a carbon cycle (e.g., Gonzales-Rouco et al, 2003; Ammann et al, 2007, Servonnat et al, 2010), or to simulation that did not span the entire last millennium (Stendel et al, 2006; Tett et al, 2006; Spangehl et al, 2010)
MPIOM applies a conformal mapping grid with a horizontal resolution ranging from 22 km to 350 km. This grid set-up is a low-resolution version of the model used for the scenario simulations (Jungclaus et al, 2006) for the Intergovernmental Panel of Climate Change (IPCC) and the Coupled Carbon Cycle Climate Modelling Intercomparison Project (C4MIP, Friedlingstein et al, 2006) simulations
As a first step we demonstrate the ability of the model to reproduce important aspects of the recent period of global climate change
Summary
Northern Hemisphere (NH) temperature reconstructions (Jansen et al, 2007; Mann et al, 2008; Mann et al, 2009) for the last millennium differ substantially among each other, raising questions (Trouet et al, 2009; Mann et al, 2009) about the spatial and temporal extent of climatic epochs such as the Medieval Warm Period (MWP, ca. 900–1250 AD) and the Little Ice Age (LIA, ca. 1500-1850 AD). Climate models’ ability to reproduce the observed records can help understand the mechanisms behind the climate variability of the last millennium and provides context for current and future climate change. A model system that simulates both the carbon-cycle and the spatially resolved internal climate variability allows us to exploit the fullness of the observational record in order to quantify both the forced response of the system and the carbon-cycle climate feedback. We present the first such simulations for the last millennium, with a comprehensive Earth System Model (ESM) including a fully interactive carbon cycle. In this paper we concentrate on the analysis of the relative role of internal variability and external forcing for shaping NH temperature and on the evolution of the carbon cycle over the last 1200 years.
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