Preprints
https://doi.org/10.5194/gmd-2020-365
https://doi.org/10.5194/gmd-2020-365

Submitted as: development and technical paper 14 Dec 2020

Submitted as: development and technical paper | 14 Dec 2020

Review status: a revised version of this preprint is currently under review for the journal GMD.

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

Leah Birch1, Christopher R. Schwalm1, Sue Natali1, Danica Lombardozzi2, Gretchen Keppel-Aleks3, Jennifer Watts1, Xin Lin3, Donatella Zona4, Walter Oechel4, Torsten Sachs5, Thomas Andrew Black6, and Brendan M. Rogers1 Leah Birch et al.
  • 1Woodwell Climate Research Center
  • 2National Center for Atmospheric Research
  • 3University of Michigan
  • 4San Diego State University
  • 5GFZ German Research Centre for Geosciences
  • 6University of BC

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.

Leah Birch et al.

 
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Leah Birch et al.

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Short summary
The high latitude landscape or Arctic-Boreal Zone has been warming rapidly, impacting the carbon balance, both regionally and globally. Given the possible global effects of climate change, it is important to have accurate climate model simulations. We assess the simulation of the Arctic-Boreal carbon cycle in the Community Land Model (CLM 5.0). We find biases in both the timing and magnitude photosynthesis. We then use observational data to improve the simulation of the carbon cycle.