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

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 . Plain Language SummaryWe describe the development and behavior of UKESM1, a novel climate model that includes improved representations of processes in the atmosphere, ocean, and on land. These processes are inter-related: For example, dust is produced on the land and blown up into the atmosphere where it affects the amount of sunlight falling on Earth. Dust can also be dissolved in the ocean, where it affects marine life. This in turn changes both the amount of carbon dioxide absorbed by the ocean and the material emitted from the surface into the atmosphere, which has an affect on the formation of clouds. UKESM1 includes many processes and interactions such as these, giving it a high level of complexity. Ensuring realistic process behavior is a major challenge in the development of our model, and we have carefully tested this. UKESM1 performs well, correctly exhibiting stable results from a continuous pre-industrial simulation (used to provide a reference for future experiments) and showing good agreement

Forcings, Feedbacks, and Climate Sensitivity in HadGEM3‐GC3.1 and UKESM1

“…A potential issue with this vegetation‐snow BGC response in UKESM1 is that it co‐locates with a region of model bias. Comparison of UKESM1 against satellite observations shows that this region is too bright in the present day and is likely to be too bright in the piControl also (Sellar et al, ). This bias is related to model parametrization of the partial burying of vegetation by snow, and Sellar et al () conclude that this effect may be too strong in the model, for grasses in particular.…”

“…Hardiman et al () describe an ozone redistribution scheme that is included in the climate change simulations used here. UKESM1 is the latest state‐of‐the‐art U.K. Earth System Model. It builds on the low‐resolution “physical climate” model HadGEM3‐GC3.1‐LL with the inclusion of, amongst other things, terrestrial carbon and nitrogen cycles—including an interactive vegetation model—ocean biochemistry and a unified tropospheric‐stratospheric chemistry scheme (Sellar et al, ). Details of the model components, couplings, performance, and evaluation are given in Sellar et al ().…”

“…However, we do not believe this parameter change has a significant impact on our results (see section 4). Similarly, in the construction of UKESM1, no retuning of the HadGEM3‐GC3.1‐LL physical parameters was required (Sellar et al, ). Hence, we interpret the differences in results between HadGEM3‐GC3.1‐LL and UKESM1 as due to the inclusion of Earth system processes.…”

Past warming trend constrains future warming in CMIP6 models

“…This probably leads to future warming projections being biased high. Thus, the raw ensemble median and spread of future warming in CMIP6 (and therefore most other variables that scale to first order with global mean temperature) are not representative of a distribution constrained by observed trends, even if some of those models show a more realistic representation of processes in individual components than their CMIP5 predecessors (20)(21)(22). Conversely, CMIP6 models with climate sensitivity values that are within the IPCC AR5 likely range show warming trends much more consistent with the observations.…”

“…High ECS models (defined here as ECS > 4.5°C; shown in dark red color) have difficulties reproducing the observed warming trend (Fig. 2, A to C) (20)(21)(22). The observationally constrained likely ranges of TCR estimates based on CMIP6, CMIP5, or both combined ( Fig.…”