The Bender and Sarmiento Groups are improving our understanding of CO2 fluxes into and out of the ocean surface by increasing the accuracy and spatial coverage of carbon measurements and using a combination of data and models to provide estimates of historical fluxes.
Better shipboard measurements of dissolved inorganic carbon
The dissolved inorganic carbon (DIC) concentration of seawater refers to the sum of the concentrations of CO2, HCO3- , and CO32-. This property is interesting for two reasons. First, it registers the invasion of fossil fuel CO2 into the ocean. The DIC concentration of surface water is rising annually by about 0.1 moles m-3 because of fossil CO2 uptake. Measuring DIC in seawater continuously, at high accuracy, over repeated cruise tracks will allow us to better quantify ocean uptake, and will also inform us about where fossil CO2 enters the oceans and mixes after entering. Second, biological activity in the oceans is concentrated in spring and summer, when sunlight is strongest and physical conditions are most favorable. During this time, DIC falls by an amount that depends on the growth of phytoplankton and their extraction of CO2 from the waters to make biomass. Seasonal measurements of DIC thus allow us to quantify biological productivity.
The Bender Group undertook to make a robust, easily operated instrument that would measure DIC concentrations continuously on oceanographic ships from seawater pumped into the lab, with minimal effort to maintain the instrument during daily operation. The team constructed an instrument that works on the following innovative principle. Seawater is mixed with a solution that has a small amount of bicarbonate labeled with the rare stable isotope of carbon, 13C. The mixture is acidified to convert the DIC into CO2, and the isotopic composition of the CO2 is measured using an optical instrument (Picarro Cavity Ringdown Spectrometer). The higher the ratio of natural to spike carbon (12CO2 to 13CO2), the higher the concentration of DIC in the seawater. The instrument works continuously under computer control, does one measurement every 8 minutes, and achieves a precision close to the state-of-the-art for the best measurements of discrete samples, about 0.1%. This coming year they will begin deploying it on research cruises, focusing on the Southern Ocean.
Estimating the time-varying air-sea CO2 flux
Joseph Majkut, a graduate student in the Sarmiento Group, has developed a new method for optimally estimating the historical pCO2 at the sea surface and used it to quantify the air-sea CO2 flux and the time rate of change pCO2 from 1980 through 2010. The method inverts data from surface pCO2 measurement databases using Markov Chain Monte Carlo methods and a simple model for the time evolution pCO2. He used an ocean general circulation model from NOAA GFDL, MOM4p1, forced with atmospheric reanalysis to provide estimates of the seasonal and interannual variability in surface pCO2, which can reduce the bias introduced by the globally sparse sampling. The diagnosed surface fluxes of CO2 are consistent with time-averaged estimates from other methods and also provide estimates of interannual variability, trends and a full treatment of uncertainty. Majkut demonstrated that trends in surface pCO2 generally point to increasing fluxes into the ocean over the last three decades, and also that several trends in CO2 flux previously published are not consistent with this new, globally consistent, estimate.
In the future, this method may be used for detecting and attributing changes in the carbon cycle and the observational schemes necessary to achieve accurate carbon budgets. Every year 30% of the anthropogenic carbon emitted by the burning of fossil fuels dissolves in the ocean and a good estimate of the carbon flux into the ocean is necessary for constraining the global carbon cycle and understanding carbon cycle feedbacks to climate change.