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ESD Reviews: Climate Feedbacks in the Earth System and Prospects for Their Evaluation

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ESD Reviews: Climate Feedbacks in the Earth System and Prospects for Their Evaluation Earth Syst. Dynam., 10, 379–452, 2019 https://doi.org/10.5194/esd-10-379-2019 © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License. ESD Reviews: Climate feedbacks in the Earth system and prospects for their evaluation Christoph Heinze1,2, Veronika Eyring3,4, Pierre Friedlingstein5, Colin Jones6, Yves Balkanski7, William Collins8, Thierry Fichefet9, Shuang Gao1,a, Alex Hall10, Detelina Ivanova11,b, Wolfgang Knorr12, Reto Knutti13, Alexander Löwc,†, Michael Ponater3, Martin G. Schultz14, Michael Schulz15, Pier Siebesma16,17, Joao Teixeira18, George Tselioudis19, and Martin Vancoppenolle20 1Geophysical Institute and Bjerknes Centre for Climate Research, University of Bergen, Postboks 7803, 5020 Bergen, Norway 2NORCE Norwegian Research Centre, Bergen, Norway 3Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany 4Institute of Environmental Physics (IUP), University of Bremen, Bremen, Germany 5College of Engineering, Mathematics and Physical Sciences, University of Exeter, Exeter, UK 6National Centre for Atmospheric Science (NCAS), University of Leeds, Leeds, UK 77Laboratoire des Sciences du Climat et de l’Environnement, CEA-CNRS-UVSQ-UPSaclay, Gif-sur-Yvette, France 8Department of Meteorology, University of Reading, Reading, UK 9Université catholique de Louvain, Earth and Life Institute, Georges Lemaître Centre for Earth and Climate Research, Louvain-la-Neuve, Belgium 10Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, USA 11Nansen Environmental and Remote Sensing Center (NERSC), Bergen, Norway 12Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden 13Institute for Atmospheric and Climate Science, ETH Zürich, Zurich, Switzerland 14Forschungszentrum Jülich, Jülich, Germany 15Norwegian Meteorological Institute, Oslo, Norway 16Royal Netherlands Meteorological Institute, De Bilt, the Netherlands 17Department of Geoscience & Remote Sensing, Delft University of Technology, Delft, the Netherlands 18Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA 19NASA Goddard Institute for Space Studies, New York City, USA 20Sorbonne Université, Laboratoire d’Océanographie et du Climat, Institut Pierre-Simon Laplace, CNRS/IRD/MNHN, Paris, France acurrent address: Institute of Marine Research, Bergen, Norway bcurrent address: Scripps Institution of Oceanography, La Jolla, USA cformerly at: Department for Geography, Ludwig Maximilian University, Munich, Germany †deceased, 2 July 2017 Correspondence: Christoph Heinze ([email protected]) Received: 21 November 2018 – Discussion started: 5 December 2018 Revised: 10 May 2019 – Accepted: 10 May 2019 – Published: 10 July 2019 Abstract. Earth system models (ESMs) are key tools for providing climate projections under different sce- narios of human-induced forcing. ESMs include a large number of additional processes and feedbacks such as biogeochemical cycles that traditional physical climate models do not consider. Yet, some processes such as cloud dynamics and ecosystem functional response still have fairly high uncertainties. In this article, we present an overview of climate feedbacks for Earth system components currently included in state-of-the-art ESMs and Published by Copernicus Publications on behalf of the European Geosciences Union. 380 C. Heinze et al.: Climate feedbacks in the Earth system and prospects for their evaluation discuss the challenges to evaluate and quantify them. Uncertainties in feedback quantification arise from the in- terdependencies of biogeochemical matter fluxes and physical properties, the spatial and temporal heterogeneity of processes, and the lack of long-term continuous observational data to constrain them. We present an outlook for promising approaches that can help to quantify and to constrain the large number of feedbacks in ESMs in the future. The target group for this article includes generalists with a background in natural sciences and an interest in climate change as well as experts working in interdisciplinary climate research (researchers, lecturers, and students). This study updates and significantly expands upon the last comprehensive overview of climate feedbacks in ESMs, which was produced 15 years ago (NRC, 2003). 1 Introduction: the Earth system model dilemma – mate system interactively, including the carbon cycle and at- complexity vs. uncertainty mospheric trace species. Such complex model simulations reveal prevailing deficiencies in our capability to project the Anthropogenic emissions of greenhouse gases (GHGs) and evolution of the full Earth system. These deficiencies need to aerosols (as well as respective precursor tracers) have altered be overcome. How can we assess the quality of the ESM sim- the radiative balance of the Earth and induce changes in the ulations and how might we eventually reduce uncertainties? climate on top of natural variations (IPCC, 2013). Interna- From observations, we have identified many of the physi- tional negotiations agreed on keeping the maximum global cal and biogeochemical processes operating within the Earth increase in global mean surface temperatures below C2 K system, yet our understanding of these processes and their relative to pre-industrial levels through reductions in GHG interactions on a global scale is still emerging. Observational emissions (see discussions in Randalls, 2010, and Knutti et data are often sparse, and observational time series rarely ex- al., 2015), while the signatory countries pledged to make tend over climate timescales. Many important processes or efforts to keep warming below C1.5 K (UNFCCC, 2015). mechanisms in the Earth system cannot be well constrained In recent years, many atmosphere–ocean general circulation through measured parameters. Furthermore, many parame- models (AOGCMs; see glossary entry on general circulation ters of known relevance in the Earth system cannot be ob- models) have been extended to Earth system models (ESMs) served directly. The situation is particularly challenging for that are used to project the extent, characteristics, and tim- feedbacks acting on timescales longer than a decade due to ing of climate change under given future scenarios (ENES, sparse data coverage or a lack of high-quality measurements 2012). ESMs also contribute to the design of feasible miti- from the instrumental record. The lack of observational con- gation pathways (e.g. through the computation of allowable straints underlines the need for employing models in order to emissions in order to achieve a certain climate target; Ciais make any useful statement about the future evolution of the et al., 2013; Collins et al., 2013). ESMs are advanced cli- climate system at all. This presents challenges concerning the mate models which in addition to physical processes, also methods and strategies used in assessing ESM performance simulate a range of relevant biogeochemical cycles (land– with respect to the real world. biosphere, ocean biogeochemistry, atmospheric chemistry, This article summarizes the major climate-relevant feed- and aerosols). Special attention in current ESMs is given to backs to be considered for such an analysis and provides an the carbon cycle (Bretherton, 1985; Flato, 2011; Jones et al., outlook for constraining feedback in Earth system models. 2016) (Fig. 1). Compared to conventional, purely physical, We focus on climatic changes occurring over typical “sce- coupled AOGCMs, ESMs include more process representa- nario timescales” (i.e. several hundred years from the pre- tions, variables, and also climate-relevant feedbacks on both industrial period). We thus do not consider, for instance, the short (instantaneous to a few years) and long (decades to cen- long-term effects of ice sheet variations and changes in the turies to millennia) timescales. ESMs are being continuously land–sea distribution due to sea-level change and tectonics. expanded to include additional processes. For example, the The goal is to familiarize the reader with the various known ESMs which form part of the Coupled Model Intercompari- major climate feedbacks and to show that there are strategies son Project Phase 6 (CMIP6) (Eyring et al., 2016a) will for and tools available for understanding and constraining those the first time include interactive ice sheets (Nowicki et al., feedbacks. 2016), and several models will have interactive chemistry The last major review of climate feedbacks covering sev- and aerosols (Collins et al., 2017). Multi-model ensembles eral Earth system reservoirs was carried out in 2003 (NRC, of ESM simulations driven by GHG emissions show a larger 2003). In addition, Bony et al. (2006) provided a review on spread in projections of climate variables (such as surface how well we understand and evaluate climate change feed- temperature; see Meehl et al., 2007a) than do physics-only back processes but focusing on physical feedbacks. Since simulations driven by GHG concentrations. This increase in 2006, considerable progress has been made in Earth sys- uncertainty is a result of simulating a bigger part of the cli- tem modelling. Similarly to the National Research Council Earth Syst. Dynam., 10, 379–452, 2019 www.earth-syst-dynam.net/10/379/2019/ C. Heinze et al.: Climate feedbacks in the Earth system and prospects for their evaluation 381 Figure 1. The Earth system as an extension of the physical climate system (“Bretherton diagram” drawn anew with modifications, extensions,
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