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

Sellar, Alistair A. and Jones, Colin G. and Mulcahy, Jane P. and Tang, Yongming and Yool, Andrew and Wiltshire, Andy and O'Connor, Fiona M. and Stringer, Marc and Hill, Richard and Palmieri, Julien and Woodward, Stephanie and de Mora, Lee and Kuhlbrodt, Till and Rumbold, Steven T. and Kelley, Douglas I. and Ellis, Rich and Johnson, Colin E. and Walton, Jeremy and Abraham, Nathan Luke and Andrews, Martin B. and Andrews, Timothy and Archibald, Alex T. and Berthou, Ségolène and Burke, Eleanor and Blockley, Ed and Carslaw, Ken and Dalvi, Mohit and Edwards, John and Folberth, Gerd A. and Gedney, Nicola and Griffiths, Paul T. and Harper, Anna B. and Hendry, Maggie A. and Hewitt, Alan J. and Johnson, Ben and Jones, Andy and Jones, Chris D. and Keeble, James and Liddicoat, Spencer and Morgenstern, Olaf and Parker, Robert J. and Predoi, Valeriu and Robertson, Eddy and Siahaan, Antony and Smith, Robin S. and Swaminathan, Ranjini and Woodhouse, Matthew T. and Zeng, Guang and Zerroukat, Mohamed (2019) UKESM1 : Description and Evaluation of the U.K. Earth System Model. Journal of Advances in Modeling Earth Systems, 11 (12). pp. 4513-4558. ISSN 1942-2466

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.