Biogeochemical Change

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Human activities are impacting local, regional, and even global biogeochemical cycles in fundamental ways, both directly, for example through pollution or land modification, and indirectly through modification of Earth’s climate. These changes include climate change, agricultural land use, ocean acidification, aquatic eutrophication, and many others. Understanding the biogeochemical changes and feedbacks involved is central to projecting the full scope of these impacts.

GFDL Research

A primary focus of GFDL’s Earth system modeling effort in the Earth System Processes and Interactions Division is to represent global biogeochemical cycles of elements within and between the atmosphere, ocean and land, components, their sensitivity to climate and human activities, and the associated feedbacks. These efforts include representation of Future Carbon Uptake, coupled carbon-climate interactions (e.g. Arora et al., 2020), as well as nitrogen, phosphorus, methane, iron, and calcium carbonate cycling, among others.

These efforts play critical roles in fulfilling NOAA’s mandates to understand and project both future climate and the influence of climate and biogeochemical change on humans, and to provide stewardship of the atmosphere and oceans, particularly in the context of ocean acidification. In addition, this work supports interagency and intergovernmental research on land biogeochemistry. GFDL is involved in a variety of community efforts including CMIP6 and the previous generation CMIP5, MAREMIP US Clivar Ocean Carbon Uptake Working Group, EMBRACE, and CMIP6.

Representation of global ocean biogeochemistry

For over two decades, GFDL has worked collaboratively with researchers at Princeton University to develop GFDL’s capacity in ocean biogeochemical modeling. This work had its origins in the Ocean Carbon Model Intercomparison Project (Najjar et al., 2007) based on simple surface nutrient restoring (Sarmiento and Le Quere, 1996; Sarmiento et al., 1998), and slowly evolved to include multiple nutrient restoring (Gnanadesikan, 1999; Gnanadesikan et al., 2004; Jin et al., 2005), and eventually fully prognostic ocean biogeochemical models (Dunne et al., 2010; 2013). At the same time, efforts to synthesize the observed biogeochemical patterns and mechanisms for coupling between organic carbon cycling and calcium carbonate (Sarmiento et al., 2002), silicate (Sarmiento et al., 2004), the efficiency of particle export (Dunne et al., 2005) and its global consequences for organic carbon (Dunne et al., 2007) and calcium carbonate (Dunne et al., 2012) and interior nitrogen and oxygen cycling (Gnanadesikan, 2007; 2012; Bianchi et al., 2012) was undertaken to assure as robust a representation of ocean biogeochemistry as possible. GFDL’s ESM2 generation models have been used in dozens of biogechemical change studies since 2013 with the biogeochemical formulations evolving for more comprehensive Carbon Ocean And Lower Trophics (COBALT) formulations including <a href=”https://www.gfdl.noaa.gov/research_highlight/reconciling-ocean-productivity-and-fisheries-yields/ultra-high resolution ESM2.6 to reconcile ocean productivity and fisheries yields and adaptation to seasonal and multiannual prediction, culminating in GFDL’s 4th generation ESM4.1 participating in the Sixth Coupled Model Intercomparison Project (CMIP6) with data available through ESGF.

Current models include:

COBALTv2: Second generation comprehensive 30 tracer Carbon Ocean And Lower Trophics (COBALT) ecosystem implemented in ESM4.1 (Stock et al., in press)

BLINGv2: Second generation 6 tracer version of TOPAZ-like ecological parameterization of Biogeochemistry with Light, Iron, Nutrients and Gas (BLING) implemented in CM4.0 (Dunne et al., in press)

Previous generation models include:

TOPAZ: the 30 tracer biogeochemically comprehensive Tracers of Ocean Phytoplankton with Allometric Zooplankton Version 2 (TOPAZ2; Dunne et al., 2013; click here for full description) used in GFDL’s ESM2M and ESM2G. This model focuses on multi-elemental biogeochemical coupling while sacrificing ecological comprehensiveness through a high degree of ecological empirical parametrization via Dunne et al. (2005).

BLING: the reduced 6 tracer version of TOPAZ-like ecological parameterization of Biogeochemistry with Light, Iron, Nutrients and Gas (BLING; Galbraith et al., 2009; click here for more information).

COBALT: the 30 tracer Carbon Ocean And Lower Trophics (COBALT; Stock and Dunne 2010; Stock et al., 2013) that focuses on comprehensive representation of pelagic trophic interactions among four phytoplankton and three zooplankton functional groups.

Global land biogeochemistry

On a timeline similar to the ocean biogeochemical model development, land biogeochemical model development has been led by researchers at Princeton University in collaboration with those at GFDL and USGS.

Biogeochemical sensitivity to ocean physical configuration

For the fifth Coupled Model Intercomparison Project (CMIP5) GFDL contributed two new global coupled carbon-climate Earth System Models, ESM2M and ESM2G (Dunne et al., 2012; 2013) in an effort to reduce the uncertainty in ocean biogeochemical response to enhanced CO₂ and climate change coming from the ocean physical representation. These models demonstrate similarly good climate fidelity, though critically distinct circulations of ESM2G have a shallower thermocline, more Antarctic Bottom Water formation, and tropical upwelling, and ESM2M has more Southern Ocean upwelling and downwelling and thermocline ventilation.

ESM2M uses the Modular Ocean Model version 4.1 with vertical pressure layers, whereas ESM2G uses generalized ocean layer dynamics with a bulk mixed layer and interior isopycnal layers. Because these models differ almost exclusively in the physical ocean component and incorporate the same explicit and consistent carbon dynamics, they offer the opportunity to assess the sensitivity of biogeochemistry to physical representation. Because of differences in oceanic ventilation rates, ESM2M has a stronger biological carbon pump but weaker northward implied atmospheric CO₂ transport than ESM2G. The major advantages of ESM2G over ESM2M are improved representation of surface chlorophyll in the Atlantic and Indian Oceans and thermocline nutrients and oxygen in the North Pacific.

While these large mean state differences exist, their climate change response is somewhat uniform. Both oceans respond similarly to atmospheric forcing, with a slow-down in Atlantic Overturning and speed-up of Southern Ocean Overturning results in tropical and subantarctic downwelling anomalies in ocean tracers. As such, these models have constituted a major contribution to reducing the uncertainty in ocean carbon uptake and climate feedbacks as they agree well with historical anthropogenic carbon estimates and project only mild (10% level) differences in future uptake and reduction in carbon uptake due to climate, very similar in scope to the other participating CMIP5 models.

Marine biogeochemical change

GFDL scientists and partners have studied the major stressors on open ocean ecosystems under climate change. These changes include warming, acidification, deoxygenation and changes in primary productivity by marine phytoplankton. Due to rising atmospheric CO₂ in the coming decades, these changes will be amplified. In a recent paper (Bopp et al. 2013) ten Earth System Models contributed to the Coupled Model Intercomparison Project 5 (CMIP5) were used to assess how these stressors may evolve over the course of the 21st century. Two GFDL models were used in this study, GFDL-ESM2G and GFDL-ESM2M (Dunne et al. 2012; 2013). These models project regional ecological change patterns with rich structure in projected surface nutrients, largely similar in scope between models. Broadly, these impacts include increased stratification, decreased nutrient supply leading to a 6% decrease in surface NO₃ globally, 5% decrease in large phytoplankton production, but negligible change in total production with enhanced microbial loop at higher temperatures. Such changes are at the most modest end of the range of CMIP5 models (Bopp et al., 2013).

Regional focus on the North Pacific demonstrated that the poleward wind shift and enhanced stratification lead to a significant expansion of the subtropical oligotrophic biome at the expense of the supolar mesotrophic biome (Polovina et al., 2011). Further regional focus on the mechanisms involved in California Current response demonstrates, for example, that while the large scale gyre response is a decrease in nutrients, the opposite is found in the California Current. While observations show that T and NO3 are negatively correlated seasonally and inter-annually, they are positively correlated under climate change – consistent with multidecadal decreases observed in subsurface oxygen. Investigation of the mechanisms uncovered a dominance of remote forcing on local California Current changes through the interplay of gyre-scale changes in atmospheric winds and heat fluxes, stratification, ventilation, present nutrient limitation versus projected light limitation during subduction, and extended projected watermass pathways modulating biogeochemical response (Rykaczewski and Dunne, 2010).

The complex nature of these changes is further illustrated in terms of tropical oxygen as GFDL ESMs project increased volume of weak hypoxia (<2 ml/l O₂), but decrease in the volume of the most hypoxic waters (<0.2 ml/l O₂) as the slow-down of meridional overturning reduces tropical upwelling, and winter convection off of Chile becomes more robust (Gnanadesikan et al., 2012).

Specific to ocean acidification, this study found strong agreement in the surface acidification of approximately 0.35 pH units over this century under the highest emissions scenario. Additionally, projected acidification was found to have its maximum not at the surface where CO₂ is being taken up from the atmosphere, but rather a few hundred meters below. In a second study (Resplandy et al., 2013), utilized the unparalleled representation of thermocline biogeochemistry in GFDL’s ESM2M to demonstrate how this strongest expression of ocean acidification manifests in the upper tropical thermocline where natural remineralization reduces buffer capacity to acidification and enhanced stratification under climate warming shoals the thermocline and enhances the subsurface remineralization signal. This mechanism of subsurface amplification of ocean acidification was also described in a regional study of the California Current using an early prototype of GFDL’s ESM (Rykaczewski and Dunne, 2010).

Overall, we see a spectrum of perturbations and responses filtering through the various physical and biogeochemical mechanisms as warming increases stratification, resulting in ventilation and nutrient supply decreases globally and a consequent shift to microbial loop with little total productivity change. The poleward expansion and slow-down of subtropical gyres shoals the nutricline in the subtropical gyres leading to enhanced nutrients, hypoxia and intensified acidification in some areas and the beginning of convection off Chile ventilating Pacific low O₂ region, The intensified hydrological cycle reduces North Atlantic overturning, shoals Northern Subpolar Atlantic and deepening tropical physical and biogeochemical properties with this changing balance of processes creating intense regional structure in the net biogeochemical change.

Land biogeochemical change

Efforts have focused on the annual to multi-centennial role of land use with particular emphasis on secondary forest regrowth and harvesting (Shevliakova et al., 2009), and the consequences for the long term dynamic disequilibrium of land fluxes (Sentman et al., 2011) and implications for the last century (Shevliakova et al., 2013) and in future scenarios as part of CMIP5 (Jones et al., 2013). This effort has not been limited to carbon, however, and has included work on the nitrogen modulation of carbon (Gerber et al., 2010; 2013).

Biogeochemical Change

GFDL Research

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