Ocean Biogeochemistry in GFDL's Earth System Model 4.1 and Its Response to Increasing Atmospheric CO₂

Abstract

This contribution describes the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL-ESM4.1), assesses GFDL-ESM4.1's capacity to capture observed ocean biogeochemical patterns, and documents its response to increasing atmospheric CO 2. Notable differences relative to the previous generation of GFDL ESM's include enhanced resolution of plankton food web dynamics, refined particle remineralization, and a larger number of exchanges of nutrients across Earth system components. During model spin-up, the carbon drift rapidly fell below the 10 Pg C per century equilibration criterion established by the Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP). Simulations robustly captured large-scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. The model overexpressed phosphate limitation and open ocean hypoxia in some areas but still yielded realistic surface and deep carbon system properties, including cumulative carbon uptake since preindustrial times and over the last decades that is consistent with observation-based estimates. The model's response to the direct and radiative effects of a 200% atmospheric CO 2 increase from preindustrial conditions (i.e., years 101-120 of a 1% CO 2 yr −1 simulation) included (a) a weakened, shoaling organic carbon pump leading to a 38% reduction in the sinking flux at 2,000 m; (b) a two-thirds reduction in the calcium carbonate pump that nonetheless generated only weak calcite compensation on century timescales ; and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. We conclude with a discussion of model limitations and priority developments. Plain Language Summary This paper describes and evaluates the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL-ESM4.1). GFDL-ESM4.1 was developed to study the past, present, and future evolution of the Earth system under scenarios for natural and anthropogenic drivers of Earth system change, including greenhouse gases and aerosols. The response of the ocean's vast carbon and heat reservoirs to accumulating greenhouse gases greatly reduces their atmospheric and terrestrial impacts, but also puts ocean environments and the marine resources they support at risk. Relative to previous models, GFDL-ESM4.1 improves the representation of (a) ocean food webs connecting plankton and fish; (b) biological processes influencing the sequestration of carbon in the deep ocean; and (c) land-atmosphere-ocean nutrient exchanges. While simulations have biases, they capture many critical aspects of the global ocean carbon cycle and ocean ecosystem, including the observed uptake of anthropogenic carbon over the last~150 yr. Projections suggest that continued CO 2 increases could significantly decrease ocean productivity and the ocean's capacity to sequester atmospheric carbon.

Ocean Ammonia Outgassing: Modulation by CO2 and Anthropogenic Nitrogen Deposition

“…The main characteristics of GFDL-ESM4.1 are described in detail in Dunne et al (2020) and in Adcroft et al (2019), Zhao et al (2018), Stock et al (2020), and Horowitz et al (2020) for the overall coupled, ocean dynamics, atmospheric dynamics, ocean biogeochemistry, and atmospheric chemistry components, respectively. The nominal horizontal resolution of the atmosphere and ocean models are ≃100 km and 0.5°, respectively.…”

“…Excretion also occurs in COBALTv2 when net phytoplankton growth becomes negative (i.e., under low light conditions). More details can be found in Stock et al (2020).…”

“…The uptake of NH x by phytoplankton is controlled by the N required to support growth rates achieved under prevailing nutrient and light conditions (Geider et al., 1997; Stock et al, 2020) and their relative preference for NH x relative to (Dortch, 1990; Glibert et al., 2015; Syrett, 1981). COBALTv2 allows for the inhibition of the uptake of and NH x in the presence of abundant NH x and , respectively, following O'Neill et al.…”

The GFDL Earth System Model Version 4.1 (GFDL‐ESM 4.1): Overall Coupled Model Description and Simulation Characteristics

“…Ocean (Stock et al, 2020;Adcroft et al, 2019) Horizontal resolution 1/4°Mercator with enhanced polar resolution (1,440 × 1,080) burden. The BVOC emissions are calculated online in the AM4.1 using the Model of Emissions of Gases and Aerosols from Nature (MEGAN;Guenther et al, 2006Guenther et al, , 2012, as a function of simulated air temperature and shortwave radiative fluxes.…”

Simple Global Ocean Biogeochemistry With Light, Iron, Nutrients and Gas Version 2 (BLINGv2): Model Description and Simulation Characteristics in GFDL's CM4.0

“…This BLINGv2 model is applied within Geophysical Fluid Dynamics Laboratory's (GFDL) new CM4.0 (Held et al, 2019) model with the 1° horizontal and 33 hybrid layer vertical resolution “fast chemistry” AM4.0 atmosphere (Zhao et al, 2018) and ¼° horizontal and 75 hybrid layer vertical resolution OM4 ocean using Modular Ocean Model version 6 (MOM6; Adcroft et al, 2019). GFDL's fully comprehensive coupled carbon‐chemistry‐climate Earth system model, ESM 4.1, that includes state‐of‐the‐art ocean biogeochemistry based on COBALTv2 (Stock et al, 2020) is described elsewhere (Dunne et al, 2020).…”