U.K. Community Earth System Modeling for CMIP6

Abstract

We describe the approach taken to develop the United Kingdom's first community Earth system model, UKESM1. This is a joint effort involving the Met Office and the Natural Environment Research Council (NERC), representing the U.K. academic community. We document our model development procedure and the subsequent U.K. submission to CMIP6, based on a traceable hierarchy of coupled physical and Earth system models. UKESM1 builds on the well‐established, world‐leading HadGEM models of the physical climate system and incorporates cutting‐edge new representations of aerosols, atmospheric chemistry, terrestrial carbon, and nitrogen cycles and an advanced model of ocean biogeochemistry. A high‐level metric of overall performance shows that both models, HadGEM3‐GC3.1 and UKESM1, perform better than most other CMIP6 models so far submitted for a broad range of variables. We point to much more extensive evaluation performed in other papers in this special issue. The merits of not using any forced climate change simulations within our model development process are discussed. First results from HadGEM3‐GC3.1 and UKESM1 include the emergent climate sensitivity (5.5 and 5.4 K, respectively) which is high relative to the current range of CMIP5 models. The role of cloud microphysics and cloud‐aerosol interactions in driving the climate sensitivity, and the systematic approach taken to understand this role, is highlighted in other papers in this special issue. We place our findings within the broader modeling landscape indicating how our understanding of key processes driving higher sensitivity in the two U.K. models seems to align with results from a number of other CMIP6 models.

The Tuning Strategy of IPSL‐CM6A‐LR

“…The word tuning, rather vague and specific to the climate modeling community, designates here the full phase of debugging and calibrating parameters, targeting some metrics or model behavior once the model's physical content has been fixed. Tuning of free parameters in particular is now recognized as a key step in the development of a climate model, in particular with the purpose of stabilizing the global mean temperature at a reasonable level (e.g., Hourdin et al., 2017; Mauritsen et al., 2012; Schmidt et al., 2017; Senior et al., 2020). Yet, it is particularly important to document aspects of the simulated climate which were explicitly targeted by the tuning from those which correspond to emerging properties of the simulated climate.…”

“…Yet, it is particularly important to document aspects of the simulated climate which were explicitly targeted by the tuning from those which correspond to emerging properties of the simulated climate. In this respect, and in the framework of climate change simulations, one may clearly distinguish model groups that claim to tune the model ECS and/or the historical trajectory of global mean temperatures (e.g., Danabasoglu et al., 2020; Mauritsen & Roeckner, 2020) and those who claim not to (e.g., Dunne et al., 2020; Senior et al., 2020). The IPSL strategy should be classified in the second category, although it relies on the use of a present‐day equilibrium setup.…”

ACCESS datasets for CMIP6: methodology and idealised experiments

“…By contrast, HadGEM3-GC31-LL uses the same atmosphere but a different ocean (NEMO ver. 3.6;Storkey et al 2018), and simulated a significantly higher ECS (ECS = 5.5°C; Senior et al 2020). This suggests that the ocean and sea-ice components may play a significant role in ECS; however, further investigation is required to understand this complexity.…”

Comparing CMIP6 climate model simulations of annual global mean temperatures to data

“…We are using 318 runs from 58 models using the CMIP6 historical forcings (see Supplementary Material for a list). For each of the models we use all i1p1f1 runs of the models chosen (except for the two HadGEM3 models for which we use i1p1f3 runs [10], the UKESM and CNRM models where we use i1p1f2 runs [11,12] and the i1p2f1 run for CanESM5-CanOE [13]) for the years 1880-2014. The grey curves in Figure 1 shows the ensemble of global annual average temperature, anomalized relative to the entire period.…”