MARBL: A novel modular biogeochemistry model

The ocean plays a critical role in the global carbon cycle, with implications for climate. It is therefore important to ensure mechanistic representation of the processes controlling oceanic carbon uptake and redistribution. To this end, scientists at NCAR are developing MARBL (Marine Biogeochemistry Library), a modular model for the simulation of marine ecosystems and biogeochemistry, independent of the ocean general circulation model. The flexible structure of MARBL allows tailored model configurations for addressing specific scientific questions. This includes the addition of novel phytoplankton and zooplankton functional types to address how climate change may be affecting pelagic ocean ecosystems; an example study is described below.

Simulating the effect of anthropogenic ocean acidification on coccolithophores

Increased CO2 emissions from human activity are inundating the upper ocean causing ocean acidification. Coccolithophores, a widespread type of marine algae that play an important role in the ocean carbon cycle through the production of calcium carbonate shells (i.e., calcification), may be particularly affected by ocean acidification (see Figure below).

An illustration of a coccolithophore
Figure: An illustration of a coccolithophore, a type of algae that makes exterior shells made of calcium carbonate (illustration by K.Krumhardt, NCAR).

In a recent study, NCAR scientists and university collaborators added a phytoplankton type representative of coccolithophores into MARBL, run within the Community Earth System Model version 2 (CESM2). They explored how ocean acidification from increasing CO2 affects coccolithophore growth and calcification. By performing CESM2 simulation experiments, they found that, as CO2 rises, coccolithophores increase in abundance in several oceanic regions, including the North Atlantic, Western Pacific, and parts of the Southern Ocean, due to increasing photosynthetic efficiency from additional anthropogenic carbon in surface waters (“carbon fertilization”). However, most areas of the ocean showed decreases in coccolithophore calcification as CO2 increases and ocean acidification becomes more severe. The scientists projected that end-of-the-century CO2 concentrations could result in 11% less oceanic calcification on a global scale relative to preindustrial CO2 levels. Overall, coccolithophores become more abundant in certain regions but are more lightly calcified with increasing CO2, resulting in regions of increasing and decreasing pelagic calcification (see Figure below), which could alter the way the ocean absorbs atmospheric CO2 and exports carbon to the deep sea.

An illustration of a coccolithophore
Figure: Percent change in coccolithophore calcification from preindustrial CO2 levels to end-of-the-century CO2 (900 ppm). Areas in red signify regions where coccolithophore calcification increased from a carbon fertilization effect, while blue areas show the negative effects of ocean acidification on calcification.

Reference

  • Krumhardt, K. M., N. S. Lovenduski, M. C. Long, M. Levy, K. Lindsay, J. K. Moore, and C. Nissen. "Coccolithophore Growth and Calcification in an Acidified Ocean: Insights from Community Earth System Model Simulations." Journal of Advances in Modeling Earth Systems 11, no. 5 (2019): 1418-1437.