CO2 Increase Experiments Using the CESM: Relationship to Climate Sensitivity and Comparison of CESM1 to CESM2

We examine the response of the Community Earth System Model Versions 1 and 2 (CESM1 and CESM2) to abrupt quadrupling of atmospheric CO2 concentrations (4xCO2) and to 1% annually increasing CO2 concentrations (1%CO2). Different estimates of equilibrium climate sensitivity (ECS) for CESM1 and CESM2 are presented. All estimates show that the sensitivity of CESM2 has increased by 1.5 K or more over that of CESM1. At the same time the transient climate response (TCR) of CESM1 and CESM2 derived from 1%CO2 experiments has not changed significantly—2.1 K in CESM1 and 2.0 K in CESM2. Increased initial forcing as well as stronger shortwave radiation feedbacks are responsible for the increase in ECS seen in CESM2. A decomposition of regional radiation feedbacks and their contribution to global feedbacks shows that the Southern Ocean plays a key role in the overall behavior of 4xCO2 experiments, accounting for about 50% of the total shortwave feedback in both CESM1 and CESM2. The Southern Ocean is also responsible for around half of the increase in shortwave feedback between CESM1 and CESM2, with a comparable contribution arising over tropical ocean. Experiments using a thermodynamic slab‐ocean model (SOM) yield estimates of ECS that are in remarkable agreement with those from fully coupled Earth system model (ESM) experiments for the same level of CO2 increase. Finally, we show that the similarity of TCR in CESM1 and CESM2 masks significant regional differences in warming that occur in the 1%CO2 experiments for each model.

[1]  M. Vizcaíno,et al.  Global Warming Threshold and Mechanisms for Accelerated Greenland Ice Sheet Surface Mass Loss , 2020, Journal of advances in modeling earth systems.

[2]  C. Hannay,et al.  Comparison of Equilibrium Climate Sensitivity Estimates From Slab Ocean, 150‐Year, and Longer Simulations , 2020, Geophysical Research Letters.

[3]  G. Meehl,et al.  Characteristics of Future Warmer Base States in CESM2 , 2020, Earth and Space Science.

[4]  K. Taylor,et al.  Context for interpreting equilibrium climate sensitivity and transient climate response from the CMIP6 Earth system models , 2020, Science Advances.

[5]  Axel Lauer,et al.  Earth System Model Evaluation Tool (ESMValTool) v2.0 – diagnostics for emergent constraints and future projections from Earth system models in CMIP , 2020, Geoscientific Model Development.

[6]  Klaus Zimmermann,et al.  Earth System Model Evaluation Tool (ESMValTool) v2.0 – technical overview , 2020, Geoscientific Model Development.

[7]  W. G. Strand,et al.  The Community Earth System Model Version 2 (CESM2) , 2020, Journal of Advances in Modeling Earth Systems.

[8]  K. Taylor,et al.  Causes of Higher Climate Sensitivity in CMIP6 Models , 2020, Geophysical Research Letters.

[9]  M. Mills,et al.  The Whole Atmosphere Community Climate Model Version 6 (WACCM6) , 2019, Journal of Geophysical Research: Atmospheres.

[10]  T. Andrews,et al.  LongRunMIP: Motivation and Design for a Large Collection of Millennial-Length AOGCM Simulations , 2019, Bulletin of the American Meteorological Society.

[11]  M. Mills,et al.  High Climate Sensitivity in the Community Earth System Model Version 2 (CESM2) , 2019, Geophysical Research Letters.

[12]  N. McFarlane,et al.  Sensitivity of Climate Simulations to the Parameterization of Cumulus Convection in the Canadian Climate Centre General Circulation Model , 1995, Data, Models and Analysis.

[13]  Andrew R. Bennett,et al.  Description and evaluation of the Community Ice Sheet Model (CISM) v2.1 , 2018, Geoscientific Model Development.

[14]  H. Chepfer,et al.  The Combined Influence of Observed Southern Ocean Clouds and Sea Ice on Top‐of‐Atmosphere Albedo , 2018 .

[15]  J. Kay,et al.  The influence of extratropical cloud phase and amount feedbacks on climate sensitivity , 2018, Climate Dynamics.

[16]  B. Santer,et al.  Comparing Tropospheric Warming in Climate Models and Satellite Data , 2017 .

[17]  Meng Li,et al.  Historical (1750–2014) anthropogenic emissions of reactive gases and aerosols from the Community Emissions Data System (CEDS) , 2017 .

[18]  G. Myhre,et al.  Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing , 2016 .

[19]  J. Fasullo,et al.  Climate Variability and Change since 850 CE: An Ensemble Approach with the Community Earth System Model , 2016 .

[20]  J. Fasullo,et al.  “El Niño Like” Hydroclimate Responses to Last Millennium Volcanic Eruptions , 2016 .

[21]  Veronika Eyring,et al.  Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization , 2015 .

[22]  T. Andrews,et al.  The inconstancy of the transient climate response parameter under increasing CO2 , 2015, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[23]  K.,et al.  The Community Earth System Model (CESM) large ensemble project: a community resource for studying climate change in the presence of internal climate variability , 2015 .

[24]  M. Webb,et al.  The Dependence of Radiative Forcing and Feedback on Evolving Patterns of Surface Temperature Change in Climate Models , 2015 .

[25]  Andrew Gettelman,et al.  Advanced two-moment bulk microphysics for global models. Part I: off-line tests and comparison with other schemes. , 2015 .

[26]  Andrew Gettelman,et al.  Advanced Two-Moment Bulk Microphysics for Global Models. Part II: Global Model Solutions and Aerosol–Cloud Interactions* , 2015 .

[27]  J. Kay,et al.  Processes controlling Southern Ocean shortwave climate feedbacks in CESM , 2014 .

[28]  D. P. Schanen,et al.  Higher-Order Turbulence Closure and Its Impact on Climate Simulations in the Community Atmosphere Model , 2013 .

[29]  W. Collins,et al.  The Community Earth System Model: A Framework for Collaborative Research , 2013 .

[30]  Chao Li,et al.  Deep-ocean heat uptake and equilibrium climate response , 2013, Climate Dynamics.

[31]  Cecilia M. Bitz,et al.  Time-Varying Climate Sensitivity from Regional Feedbacks , 2012 .

[32]  G. Danabasoglu,et al.  Climate Sensitivity of the Community Climate System Model, Version 4 , 2012 .

[33]  K. Taylor,et al.  Forcing, feedbacks and climate sensitivity in CMIP5 coupled atmosphere‐ocean climate models , 2012 .

[34]  Karl E. Taylor,et al.  An overview of CMIP5 and the experiment design , 2012 .

[35]  Andrew Gettelman,et al.  The Evolution of Climate Sensitivity and Climate Feedbacks in the Community Atmosphere Model , 2012 .

[36]  T. Delworth,et al.  Probing the Fast and Slow Components of Global Warming by Returning Abruptly to Preindustrial Forcing , 2010 .

[37]  G. Danabasoglu,et al.  Equilibrium Climate Sensitivity: Is It Accurate to Use a Slab Ocean Model? , 2009 .

[38]  Jonathan M. Gregory,et al.  Time Variation of Effective Climate Sensitivity in GCMs , 2008 .

[39]  Andrew Gettelman,et al.  A new two-moment bulk stratiform cloud microphysics scheme in the Community Atmosphere Model, version 3 (CAM3). Part I: Description and numerical tests , 2008 .

[40]  W. Collins,et al.  Radiative forcing by long‐lived greenhouse gases: Calculations with the AER radiative transfer models , 2008 .

[41]  K. Taylor,et al.  Estimating shortwave radiative forcing and response in climate models , 2007 .

[42]  Jonathan M. Gregory,et al.  A new method for diagnosing radiative forcing and climate sensitivity , 2004 .

[43]  G. Boer,et al.  Dynamical aspects of climate sensitivity , 2003 .

[44]  John F. B. Mitchell,et al.  The time‐dependence of climate sensitivity , 2000 .

[45]  S. Manabe,et al.  Response of a Coupled Ocean–Atmosphere Model to Increasing Atmospheric Carbon Dioxide: Sensitivity to the Rate of Increase , 1999 .

[46]  John F. B. Mitchell,et al.  Intercomparison and interpretation of climate feedback processes in 19 atmospheric general circulation models , 1990 .

[47]  R. Hill,et al.  Description and evaluation , 1976 .