The ocean also serves as a natural sink for anthropogenic carbon, taking up roughly as much as the terrestrial sink. The Bender group of CMI uses innovative observational techniques to determine current rates of carbon uptake by the ocean, while the Sarmiento group develops cutting-edge models to understand the controls on this uptake and to help predict its response to future climate change.


High-precision analyzer for dissolved inorganic carbon in seawater

Over the course of the CMI grant, the Bender group has developed an instrument to make high-precision measurements of the concentration of dissolved inorganic carbon (DIC) by measuring isotope ratios, which can be done very accurately and rather easily. Through implementation of a new “double-spike” technique, this year the researchers have achieved a measurement precision which is at least 2 times better than the state of the art (0.03% vs. 0.06%). The instrument also has an analysis time of about only 2-8 minutes (up to 10 times shorter than the competition) and requires much less operator attention, making it a very attractive instrument for use in oceanographic cruises.

This instrument has 3 applications. The first is measuring the seasonal cycle of dissolved inorganic carbon in a region to constrain the fertility of the waters. The second involves measuring the rate of increase in the surface water DIC concentration, along a cruise track, over a period of years. The rate of increase is the rate at which the surface ocean is taking up fossil fuel CO2 . The third involves measuring DIC in deep sea waters at selected locations to determine the rate at which fossil fuel CO2 is mixed, very slowly, into this vast reservoir.


Modeling the Southern Ocean CO2 sink at unprecedented resolution

The Sarmiento group, in collaboration with scientists at GFDL, has developed a new high-resolution global earth system model for studying the dynamics of air-sea carbon fluxes and interior ocean carbon transport in the Southern Ocean. At present, the Southern Ocean accounts for up to half of the oceanic uptake of anthropogenic CO2 from the atmosphere. However, it is unknown how today’s uptake rate will respond to changing atmospheric forcing in the region. The uncertainty arises from a scarcity of observations over the Southern Ocean and the difficulty of modeling the small-scale eddy features, which play a pivotal role in the response of the ocean circulation. The old model had a resolution of 1°, whereas the new CM2.6 model has a resolution of 1/10°, which enables it to resolve eddies (~10 km across) and their impact on the response of the carbon uptake in the Southern Ocean to climate change (Figure 4). The model will be used to investigate the dynamical processes controlling the air-sea CO2 flux and to identify if localized carbon transport pathways exist within the ocean.


A simplified ocean biogeochemical model for high resolution simulations

Adding biogeochemistry to ocean models is critical to predicting the future of the ocean carbon sink, but the extra calculations required can make models prohibitively time-consuming to run. The Sarmiento group is testing the sensitivity of carbon uptake modeled to the complexity of the ocean biogeochemical model, evaluating whether the three tracer configuration (the MINIBLING model) used within the 1/10 degree ocean configuration of GFDL’s coupled model CM2.6 is consistent with the more complex published BLING (Biology Light Iron Nutrient and Gas) model. This project will validate the biogeochemistry for the aforementioned project titled “Modeling the Southern Ocean CO2 sink at unprecedented resolution.”

Figure 4. The change in the annual air-sea Southern Ocean CO2 flux between an average preindustrial year and a modern year, as modeled by CM2.6. Red indicates a release of carbon that has been upwelled from the deep ocean into the atmosphere, while blue indicates oceanic uptake of carbon. The Southern Ocean currently moderates atmospheric warming by acting as a large sink of anthropogenic carbon.

Organic carbon in particles sinking from the sunlit surface is a food source for heterotrophic bacteria living in the deep ocean. Using a water column model, the Sarmiento group has found that bacterial colonization rates and activities on particles impact the depth at which organic carbon is transformed to CO2 via bacterial respiration, which can exert significant influence on the residence time and concentration of CO2 in the atmosphere and ocean. This means that as the climate warms, increasing ocean temperatures may alter bacterial activity and growth, with consequent feedbacks on the carbon cycle.

Because bacteria use oxygen and produce CO2 when they respire, changes in ocean oxygen concentrations would also be expected as ocean temperatures warm. In large regions of the ocean, oxygen is depleted to almost zero between 100 and 1000 m depth by respiration as bacteria consume organic carbon. In the future, the Sarmiento group plans to incorporate its water column model into a 3-dimensional ocean model to simulate the effects of bacterial activity on particulate carbon flux and oxygen utilization in the global ocean, and to help assess the impacts of oxygen depletion on marine species (see “Compression of marine habitats” under “Climate change impacts on ecosystems” below).