Addressing Biases in Arctic-Boreal Carbon Cycling in the Community Land Model Version 5

Abstract

Abstract. The Arctic-boreal zone (ABZ) is experiencing amplified warming, actively changing biogeochemical cycling of vegetation and soils. The land-to-atmosphere fluxes of CO2 in the ABZ have the potential to increase in magnitude and feedback to the climate causing additional large scale warming. The ability to model and predict this vulnerability is critical to preparation for a warming world, but Earth system models have biases that may hinder understanding the rapidly changing ABZ carbon fluxes. Here we investigate circumpolar carbon cycling represented by the Community Land Model 5 (CLM5.0) with a focus on seasonal gross primary productivity (GPP) in plant functional types (PFTs). We benchmark model results using data from satellite remote sensing products and eddy covariance towers. We find consistent biases in CLM5.0 relative to observational constraints: (1) the onset of deciduous plant productivity to be late, (2) the offset of productivity to lag and remain abnormally high for all PFTs in fall, (3) a high bias of grass, shrub, and needleleaf evergreen tree productivity, and (4) an underestimation of productivity of deciduous trees. Based on these biases, we focus model development of alternate phenology, photosynthesis schemes, and carbon allocation parameters at eddy covariance tower sites. Although our improvements are focused on productivity, our final Model Recommendation results in other component CO2 fluxes, e.g. Net Ecosystem Exchange (NEE) and Terrestrial Ecosystem Respiration (TER), that are more consistent with observations. Results suggest that algorithms developed for lower latitudes and more temperate environments can be inaccurate when extrapolated to the ABZ, and that many land surface models may not accurately represent carbon cycling and its recent rapid changes in high latitude ecosystems, especially when analyzed by individual PFTs.

A boreal forest model benchmarking dataset for North America: a case study with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC)

“…The high summer RMSE provided a major contribution to the overall model performance. This could be explained by the fact that model-observation discrepancies for CO 2 and energy fluxes tend to be higher with higher fluxes in summer (Kuppel et al 2012, Birch et al 2021. The monthly biases indicate that the model overestimated the seasonal amplitudes in NEP among sites except for US-Prr.…”

“…Terrestrial ecosystem models (TEMs), including those integrated as the land component in Earth system models, are crucial tools to estimate boreal forests' responses to climate warming (Fisher et al 2018, Birch et al 2021. Recent decades have seen a rapid development in TEM skill to simulate carbon, water and energy fluxes and balances of terrestrial ecosystems (Fisher et al 2014, Bonan andDoney 2018).…”

Impact of Snow Thermal Conductivity Schemes on pan-Arctic Permafrost Dynamics in CLM5.0

“…The domain for this study is between latitudes 57-90°N and consists of 204086 grid points with a triangular resolution which varies between 116.3 and 179.4 km 2 , giving a rectangular resolution of around 12 km 2 . This is a similar domain to Birch et al (2020), who used a coarser resolution.…”

Review for Birch et al. gmd-2020-365

“…Such "feedbacks" between different parts of the model complicate model development and the identification of root causes for model bias. The chosen formulation of spring phenology is all the more surprising since this complication is avoided by the use of growing-degreeday-based models that are standard and well-established (see e.g., Richardson et al, 2018;Hufkens et al., 2018) for robust simulations of spring phenology (with some modifications like chilling requirement).…”