Validation of terrestrial biogeochemistry in CMIP6 Earth system models: a review

Abstract

Abstract. The vital role of terrestrial biogeochemical cycles in influencing global climate change is explored by modelling groups internationally through land surface models (LSMs) coupled to atmospheric and oceanic components within Earth system models (ESMs). The sixth phase of the Coupled Model Intercomparison Project (CMIP6) provided an opportunity to compare ESM output by providing common forcings and experimental protocols. Despite these common experimental protocols, a variety of terrestrial biogeochemical cycle validation approaches were adopted by CMIP6 participants, leading to ambiguous model performance assessment and uncertainty attribution across ESMs. In this review we summarize current methods of terrestrial biogeochemical cycle validation utilized by CMIP6 participants and concurrent community model comparison studies. We focus on variables including the dimensions of evaluations, observation-based reference datasets, and metrics of model performance. To ensure objective and thorough validations for the seventh phase of CMIP (CMIP7), we recommend the use of a standard validation protocol employing a broad suite of certainty-weighted observation-based reference datasets, targeted model performance metrics, and comparisons across a range of spatiotemporal scales.

Matrix Approach to Accelerate Spin‐Up of CLM5

“…With the growing of complexity and comprehensiveness of land models, land carbon dynamics simulated by earth system models are highly variable and fit poorly with observations (Luo et al., 2015; Spafford & MacDougall, 2021; Tian et al., 2015). It is crucial to understand sources of uncertainties.…”

Comparison of C3 Photosynthetic Responses to Light and CO2 Predicted by the Leaf Photosynthesis Models of Farquhar et al. (1980) and Goudriaan et al. (1985)

“…Considering the enormous uncertainty that still exists in the magnitude of carbon-climate feedbacks in Earth system models (Spafford & MacDougall, 2021) we argue that the availability of fundamentally different model approaches capable of predicting near-similar responses in leaf-level photosynthesis can facilitate testing and model development via intercomparison. Differences between FvCB and G85-based photosynthesis schemes in future model intercomparison should be acknowledged while photosynthesis parameters between vegetation types can be aligned to minimize model variance due to inconsistent model input parameters.…”

A modern-day Mars climate in the Met Office Unified Model: dry simulations

“…This scheme uses a two-stream correlated-k method as described by Walters et al (2019) and references therein. This scheme has been used extensively for studies of Earth (Spafford and Macdougall, 2021), in addition to hot Jupiters (Mayne et al, 2014), sub-Neptunes (Drummond et al, 2018) and rocky exoplanets (Boutle et al, 2020;Eager et al, 2020).…”