Abstract
Abstract. We present the Alfred Wegener Institute's contribution to the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) wherein we employ the Community Earth System Models (COSMOS) that include a dynamic vegetation scheme. This work builds on our contribution to Phase 1 of the Pliocene Model Intercomparison Project (PlioMIP1) wherein we employed the same model without dynamic vegetation. Our input to the PlioMIP2 special issue of Climate of the Past is twofold. In an accompanying paper we compare results derived with COSMOS in the framework of PlioMIP2 and PlioMIP1. With this paper we present details of our contribution with COSMOS to PlioMIP2. We provide a description of the model and of methods employed to transfer reconstructed mid-Pliocene geography, as provided by the Pliocene Reconstruction and Synoptic Mapping Initiative Phase 4 (PRISM4), to model boundary conditions. We describe the spin-up procedure for creating the COSMOS PlioMIP2 simulation ensemble and present large-scale climate patterns of the COSMOS PlioMIP2 mid-Pliocene core simulation. Furthermore, we quantify the contribution of individual components of PRISM4 boundary conditions to characteristics of simulated mid-Pliocene climate and discuss implications for anthropogenic warming. When exposed to PRISM4 boundary conditions, COSMOS provides insight into a mid-Pliocene climate that is characterised by increased rainfall (+0.17 mm d−1) and elevated surface temperature (+3.37 ∘C) in comparison to the pre-industrial (PI). About two-thirds of the mid-Pliocene core temperature anomaly can be directly attributed to carbon dioxide that is elevated with respect to PI. The contribution of topography and ice sheets to mid-Pliocene warmth is much smaller in contrast – about one-quarter and one-eighth, respectively, and nonlinearities are negligible. The simulated mid-Pliocene climate comprises pronounced polar amplification, a reduced meridional temperature gradient, a northwards-shifted tropical rain belt, an Arctic Ocean that is nearly free of sea ice during boreal summer, and muted seasonality at Northern Hemisphere high latitudes. Simulated mid-Pliocene precipitation patterns are defined by both carbon dioxide and PRISM4 paleogeography. Our COSMOS simulations confirm long-standing characteristics of the mid-Pliocene Earth system, among these increased meridional volume transport in the Atlantic Ocean, an extended and intensified equatorial warm pool, and pronounced poleward expansion of vegetation cover. By means of a comparison of our results to a reconstruction of the sea surface temperature (SST) of the mid-Pliocene we find that COSMOS reproduces reconstructed SST best if exposed to a carbon dioxide concentration of 400 ppmv. In the Atlantic to Arctic Ocean the simulated mid-Pliocene core climate state is too cold in comparison to the SST reconstruction. The discord can be mitigated to some extent by increasing carbon dioxide that causes increased mismatch between the model and reconstruction in other regions.
Highlights
Climate projections provide policy makers with a range of possible future climates (Collins et al, 2013)
In order to illustrate the response of Community Earth System Models (COSMOS) to Climate Model Intercomparison Project Phase 6 (CMIP6) model forcing, we show results of selected CMIP6-related simulations towards the documentation of COSMOS as a non-CMIP6 model toolbox
We suggest that the inference of potentially reduced ties between simulated mid-Pliocene temperature and model equilibrium climate sensitivity (ECS) in some PlioMIP2 models is of particular relevance to the following question: to what extent could the mid-Pliocene serve as an analogue to future climate? In the short to medium term, climate projections are mostly controlled by increased levels of greenhouse gases in the atmosphere
Summary
Climate projections provide policy makers with a range of possible future climates (Collins et al, 2013). C. Stepanek et al.: PlioMIP2 simulations with COSMOS vironmental conditions under the impact of elevated carbon dioxide of anthropogenic origin – or, in other terms, to climate change. Stepanek et al.: PlioMIP2 simulations with COSMOS vironmental conditions under the impact of elevated carbon dioxide of anthropogenic origin – or, in other terms, to climate change This issue is urgent and not academic in nature. When directly studying future climate by means of modelling, we have to rely on the precision and accuracy of climate models These are continuously improved (Flato et al, 2013) but per the definition of a model will always only provide an idealised representation of processes and mechanisms that drive the Earth’s climate system. Model dependency of simulated climate is to be expected and has been shown (e.g. Haywood et al, 2009a)
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