Climate and carbon-cycle variability over the last millennium

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 lowamplitude 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 CO 2 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 CO 2 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 disCorrespondence to: J. H. Jungclaus (johann.jungclaus@zmaw.de) tinct 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.

Solar forcing as an important trigger for West Greenland sea-ice variability over the last millennium

“…The ocean component MPI-OM (Marsland et al, 2003), including the dynamics of sea ice formulated using viscous-plastic rheology (Hibler, 1979), has an average horizontal resolution of 3 Â 1.8 with 40 uneven vertical layers. The climate model was previously utilized to analyse the climate of the last millennium (Jungclaus et al, 2010), the last glacial (Zhang et al, , 2014 and the last interglacial period (Varma et al, 2012;, as well as warm climates of the Miocene (Knorr et al, 2011;Knorr and Lohmann, 2014) and the Pliocene (Stepanek and Lohmann, 2012;Dowsett et al, 2013). To evaluate the corresponding changes in SIC, two sensitivity experiments with high and low solar radiation anomalies (1367 ± 3 W/m 2 ) were performed under pre-industrial (PI) conditions.…”

Last interglacial temperature seasonality reconstructed from tropical Atlantic corals

“…COSMOS (Jungclaus et al, 2010) is comprised of an atmospheric model ECHAM5, an ocean model MPIOM, and a dynamical vegetation model JSBACH. Further details underpinning the models incorporated into COSMOS can be found in Stepanek and Lohmann (2012) and Felis et al (2015).…”

Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium

“…The relative abundance of high resolution proxy data for this period with respect to earlier ones has facilitated reconstructions of relevant climate variables-most notably temperature and hydroclimate parameters-on different spatial scales (e.g., Mann et al 2008;Cook et al 2010;Pages 2k Consortium 2013). Estimations of past forcing factors have additionally allowed simulating the climate evolution of this period with models of different complexity (e.g., Crowley 2000;Bauer et al 2003;González-Rouco et al 2003b;Servonnat et al 2010;Jungclaus et al 2010), thus making systematic comparisons between simulations Abstract We assess the use of the meridional thermalwind transport estimated from zonal density gradients to reconstruct the oceanic circulation variability during the last millennium in a forced simulation with the ECHO-G coupled climate model. Following a perfect-model approach, model-based pseudo-reconstructions of the Atlantic meridional overturning circulation (AMOC) and the Florida Current volume transport (FCT) are evaluated against their true simulated variability.…”