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Christopher S. Bretherton and Peter M. Caldwell

1. Introduction Climate change is a defining problem of our time. It is hard to plan for future warming without knowing its magnitude, but our ±1 σ “likely” confidence range for equilibrium climate sensitivity (ECS; the global-average surface warming due to doubling CO 2 and letting the climate re-equilibrate) is currently 1.5–4.5 K ( IPCC 2013 )—which is disturbingly large. This uncertainty has persisted for decades despite large advances in our understanding of the climate system ( Knutti

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Timothy M. Merlis, Isaac M. Held, Georgiy L. Stenchikov, Fanrong Zeng, and Larry W. Horowitz

1. Introduction Constraining the magnitude of the response of surface temperature to changes in the concentrations of greenhouse gases and other radiative forcing agents is a central goal of climate research. A substantial amount of research has addressed the magnitude of the equilibrium response of the global-mean surface temperature to doubled carbon dioxide concentration, the equilibrium climate sensitivity (ECS). However, the climate system, and the oceans in particular, will be out of

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Kyle C. Armour, Cecilia M. Bitz, and Gerard H. Roe

1. Introduction The response of the earth's climate to changes in forcing is often characterized in terms of the equilibrium climate sensitivity , the global equilibrium surface warming under a doubling of atmospheric CO 2 . This definition has facilitated direct comparison of different estimates of climate change, be they instrumental, proxy, or model derived (e.g., Hegerl et al. 2007 ; Allen et al. 2007 ; Edwards et al. 2007 ; Knutti and Hegerl 2008 , and references therein). A closely

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Markus Huber, Irina Mahlstein, Martin Wild, John Fasullo, and Reto Knutti

temperature patterns, such as the global-mean, the land–ocean contrast, the annual cycle (AC), and the interhemispheric difference (NS), were used in previous studies to describe global climate variability and change ( Karoly and Braganza 2001 ; Braganza et al. 2003 , 2004 ). The observed temperature, radiation fields, and trends have been extensively used to constrain climate sensitivity and future temperature projections ( Forest et al. 2002 ; Harvey and Kaufmann 2002 ; Knutti et al. 2002 ; Gregory

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Andrew E. Dessler

1. Introduction Equilibrium climate sensitivity (ECS; i.e., the equilibrium warming in response to a doubling of CO 2 ) is one of the quantities that controls how much future warming we will experience in response to greenhouse gas emissions from anthropogenic activities. As such, it is frequently viewed as one of the most important numbers in climate science and much effort has been expended over decades attempting to constrain its value. ECS can be calculated from observations or models as (1

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Peter M. Caldwell, Mark D. Zelinka, and Stephen A. Klein

1. Introduction How much will our greenhouse gas emissions warm our planet? This is a defining question of our time. The magnitude of this warming is usually characterized in terms of the equilibrium climate sensitivity (ECS), which is the global-average surface temperature response to doubling CO 2 from preindustrial conditions and letting the planet return to equilibrium. Because the planetary response to future changes in atmospheric composition is difficult to determine based on

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Xin Qu, Alex Hall, Anthony M. DeAngelis, Mark D. Zelinka, Stephen A. Klein, Hui Su, Baijun Tian, and Chengxing Zhai

1. Introduction The equilibrium climate sensitivity (ECS), defined as the equilibrium global and annual-mean surface air temperature response to a doubling of atmospheric CO 2 in general circulation models (GCMs), has consistently exhibited a large spread (ranging from 2 to 5 K) since the first assessment of the Intergovernmental Panel for Climate Change in the 1990s ( Mitchell et al. 1990 ; Kattenberg et al. 1996 ; Cubasch et al. 2001 ; Meehl et al. 2007 ; Collins et al. 2013 ). The

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Simon F. B. Tett, Daniel J. Rowlands, Michael J. Mineter, and Coralia Cartis

1. Introduction Considerable uncertainty exists about how sensitive the climate is to changes in CO 2 . This is often summarized as “equilibrium climate sensitivity” ( S ): the equilibrium global-average temperature change in response to a doubling of CO 2 . The fourth assessment report from the Intergovernmental Panel on Climate Change (IPCC) reported that S was likely (more than 66% chance) to be in the range 2.0–4.5 K and values greater than 4.5 K could not be ruled out ( Meehl et al

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Olivier Geoffroy and David Saint-Martin

, b , hereafter G13a and G13b , respectively). The inconstancy of the radiative properties have been suggested to explain most of this difference ( Gregory et al. 2015 ). Energy balance models or similar models such as impulse response functions (e.g., Good et al. 2012 ) have been widely used to constrain climate sensitivity from the observed present warming (e.g., Gregory et al. 2002 ; Otto et al. 2013 ). These constraints generally lead to sensitivity estimates lower than those projected

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Ken Caldeira and Ivana Cvijanovic

, manuscript submitted to Climate Dyn. ). Furthermore, sea ice decline has been shown to affect midlatitude circulation patterns ( Francis et al. 2009 ; Overland and Wang 2010 ; Deser et al. 2010 ; Screen and Simmonds 2013 ). The overall influence of sea ice loss on global climate sensitivity and global temperature increase in CO 2 doubling simulations has been previously addressed by Ingram et al. (1989) and Rind et al. (1995 , 1997) . Climate sensitivity to a change in radiative forcing is

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