Journal of Advances in Modeling Earth Systems (Dec 2019)

UKESM1: Description and Evaluation of the U.K. Earth System Model

  • Alistair A. Sellar,
  • Colin G. Jones,
  • Jane P. Mulcahy,
  • Yongming Tang,
  • Andrew Yool,
  • Andy Wiltshire,
  • Fiona M. O'Connor,
  • Marc Stringer,
  • Richard Hill,
  • Julien Palmieri,
  • Stephanie Woodward,
  • Lee deMora,
  • Till Kuhlbrodt,
  • Steven T. Rumbold,
  • Douglas I. Kelley,
  • Rich Ellis,
  • Colin E. Johnson,
  • Jeremy Walton,
  • Nathan Luke Abraham,
  • Martin B. Andrews,
  • Timothy Andrews,
  • Alex T. Archibald,
  • Ségolène Berthou,
  • Eleanor Burke,
  • Ed Blockley,
  • Ken Carslaw,
  • Mohit Dalvi,
  • John Edwards,
  • Gerd A. Folberth,
  • Nicola Gedney,
  • Paul T. Griffiths,
  • Anna B. Harper,
  • Maggie A. Hendry,
  • Alan J. Hewitt,
  • Ben Johnson,
  • Andy Jones,
  • Chris D. Jones,
  • James Keeble,
  • Spencer Liddicoat,
  • Olaf Morgenstern,
  • Robert J. Parker,
  • Valeriu Predoi,
  • Eddy Robertson,
  • Antony Siahaan,
  • Robin S. Smith,
  • Ranjini Swaminathan,
  • Matthew T. Woodhouse,
  • Guang Zeng,
  • Mohamed Zerroukat

DOI
https://doi.org/10.1029/2019MS001739
Journal volume & issue
Vol. 11, no. 12
pp. 4513 – 4558

Abstract

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Abstract We document the development of the first version of the U.K. Earth System Model UKESM1. The model represents a major advance on its predecessor HadGEM2‐ES, with enhancements to all component models and new feedback mechanisms. These include a new core physical model with a well‐resolved stratosphere; terrestrial biogeochemistry with coupled carbon and nitrogen cycles and enhanced land management; tropospheric‐stratospheric chemistry allowing the holistic simulation of radiative forcing from ozone, methane, and nitrous oxide; two‐moment, five‐species, modal aerosol; and ocean biogeochemistry with two‐way coupling to the carbon cycle and atmospheric aerosols. The complexity of coupling between the ocean, land, and atmosphere physical climate and biogeochemical cycles in UKESM1 is unprecedented for an Earth system model. We describe in detail the process by which the coupled model was developed and tuned to achieve acceptable performance in key physical and Earth system quantities and discuss the challenges involved in mitigating biases in a model with complex connections between its components. Overall, the model performs well, with a stable pre‐industrial state and good agreement with observations in the latter period of its historical simulations. However, global mean surface temperature exhibits stronger‐than‐observed cooling from 1950 to 1970, followed by rapid warming from 1980 to 2014. Metrics from idealized simulations show a high climate sensitivity relative to previous generations of models: Equilibrium climate sensitivity is 5.4 K, transient climate response ranges from 2.68 to 2.85 K, and transient climate response to cumulative emissions is 2.49 to 2.66 K TtC−1.