The first is the role that ocean biogeochemistry plays in modulating the oceanic uptake of anthropogenic CO 2. The scientific focus of CMIP6, and its role within the IPCC, puts two facets of marine biogeochemical dynamics into sharp focus. While CMIP6 supports a broad array of scientific activities, its emphasis on understanding past, present, and future climate has led to numerous contributions to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC) that have shaped global climate policies. This contribution describes and evaluates the ocean biogeochemical component of simulations with the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL-ESM4.1) that were contributed to the 6th Coupled Model Intercomparison Project (CMIP6 Eyring et al., 2016). Projections suggest that continued CO 2 increases could significantly decrease ocean productivity and the ocean's capacity to sequester atmospheric carbon.
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. 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. 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.
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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. This paper describes and evaluates the ocean biogeochemical component of the Geophysical Fluid Dynamics Laboratory's Earth System Model 4.1 (GFDL-ESM4.1). We conclude with a discussion of model limitations and priority developments. 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 time-scales and, in contrast to previous GFDL ESMs, (c) a moderate reduction in global net primary production that was amplified at higher trophic levels. 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. Simulations robustly captured large-scale observed nutrient distributions, plankton dynamics, and characteristics of the biological pump. 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). 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.
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.
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