UKESM1: Description and Evaluation of the U.K. Earth System Model
Observation and Context
Reliable future climate projections require understanding how the global biosphere, atmospheric composition, and biogeochemical cycles interact with physical warming and anthropogenic emissions. While its predecessor, HadGEM2-ES, provided foundational insights, modern Earth system research demands process-complete modeling that couples physical climate dynamics with interactive carbon-nitrogen cycles, dynamic vegetation, land use management, unified stratospheric-tropospheric chemistry, and modal aerosols. Building such a process-complete model creates complex inter-component dependencies and the risk of cascading biases.
Hypothesis
If the Met Office and NERC couple the HadGEM3-GC3.1 physical core with advanced terrestrial biogeochemistry, ocean biology, and interactive tropospheric-stratospheric chemistry, the resulting UK Earth System Model (UKESM1) will maintain a stable pre-industrial state, accurately simulate present-day observations, and reliably capture critical Earth system feedbacks despite complex internal coupling.
Experiment and Methodology
Researchers developed and tested UKESM1 using core CMIP6 experimental frameworks:
- Physical Core: Built upon the HadGEM3-GC3.1 model, incorporating the NEMO ocean model, CICE sea ice, the Met Office Unified Model atmosphere, and the JULES land surface simulator.
- Terrestrial Biogeochemistry: Upgraded JULES with coupled carbon and nitrogen cycles, advanced plant functional types, explicit crop/pasture land management, and harvest carbon fluxes.
- Ocean Biogeochemistry: Incorporated the MEDUSA-2 intermediate-complexity plankton ecosystem model to simulate carbon, nitrogen, silicon, iron, and oxygen cycles, coupled with an interactive surface dimethyl sulfide (DMS) submodel.
- Atmospheric Chemistry and Aerosols: Integrated the UKCA model featuring unified stratospheric-tropospheric chemistry, Fast-JX interactive photolysis, biogenic volatile organic compound (BVOC) emissions, and the GLOMAP modal aerosol scheme.
- Tuning and Evaluation: Fine-tuned critical parameters (such as snow-vegetation albedo interactions, bare soil fractions, terrestrial productivity, and ocean DMS minimums) to eliminate pre-industrial long-term drifts before executing multi-century pre-industrial controls, abrupt 4×CO2, 1%/yr CO2 increases, and a 9-member historical ensemble (1850–2014).
Results and Data
- Pre-Industrial Stability: The pre-industrial control demonstrated excellent long-term stability with minimal drift; cumulative carbon pool changes remained well below standard C4MIP thresholds (±0.1 GtC/yr), and net top-of-atmosphere (TOA) radiation stayed close to zero.
- Surface Climate and Vegetation: UKESM1 accurately captured global vegetation distribution, biome structures, and surface air temperature patterns, though it exhibited a cold winter bias over North America and Europe and a dry precipitation bias over the Amazon.
- Atmosphere and Chemistry: Tropospheric ozone in 2014 reached 314 Tg, matching observational estimates (300±12 Tg). Methane lifetime settled at 8.4 years.
- Climate Sensitivity: Idealized experiments revealed high climate sensitivity relative to prior model generations: equilibrium climate sensitivity (ECS) was 5.4 K, transient climate response (TCR) ranged from 2.68 to 2.85 K, and transient climate response to cumulative emissions (TCRE) was 2.49 to 2.66 K TtC−1.
- Historical Temperature Anomaly: The historical ensemble showed stronger-than-observed Northern Hemisphere cooling between 1950 and 1970 (driven by aerosol and land use forcing), followed by rapid warming through 2014.
Conclusion and Climate Impact
The hypothesis was supported: UKESM1 successfully operates as a highly sophisticated, process-complete Earth System Model that maintains long-term pre-industrial stability while simulating complex biogeochemical feedbacks. Although its high climate sensitivity and mid-century cooling bias highlight ongoing challenges in managing complex component interactions, UKESM1 provides a state-of-the-art framework for exploring future climate pathways, carbon emission budgets, and environmental mitigation strategies for CMIP6.
Full Citation
Sellar, A. A., Jones, C. G., Mulcahy, J. P., Tang, Y., Yool, A., Wiltshire, A., O’Connor, F. M., Stringer, M., Hill, R., Palmieri, J., Woodward, S., de Mora, L., Kuhlbrodt, T., Rumbold, S. T., Kelley, D. I., Ellis, R., Johnson, C. E., Walton, J., Abraham, N. L., … Zerroukat, M. (2019). UKESM1: Description and evaluation of the U.K. Earth System Model. Journal of Advances in Modeling Earth Systems, 11(12), 4513–4558. https://doi.org/10.1029/2019MS001739