Estimating the Carbonate System and its Anthropogenic Spatio-Temporal Variability

Abstract

The increase of anthropogenic carbon has become an integral component for understanding the ocean and how it is changing over time. The uptake of anthropogenic CO₂ at the ocean surface has been well quantified due to the availability of surface observations. However, in the interior ocean, observations of dissolved inorganic carbon (DIC) are spatially sparse and with an irregular sampling rate. We used statistical time series methods to estimate the state changes through time, with uncertainties, for the carbonate system and DIC’s natural and anthropogenic components. Three studies have been completed, with a theme of what is the story hidden within the data. The first study estimates the carbonate system in the Labrador Sea, producing monthly time series for multiple depths. A state space model combined GLODAP, SOCAT, and CO2SYS calculated values using the Kalman smoother algorithm. Through the water column DIC increases at rates of 0.34 to 1.13 μmol/kg/year and fCO₂ at 0.79 to 2.52 μatm/year. Acidification of pH declines at rates of -0.0017 to -0.0005/year and alkalinity decreases with the decline of salinity. Our time-dependent method has lowered the uncertainty for pH by 70% and for fCO₂ by 64%, compared to time-independent methods. The second study focused on DIC in the northwest Atlantic, separating DIC into natural and excess components using linear regression. The Kalman smoother algorithm then estimates the monthly values of the excess component. This method is a time series generalization of the extended multiple linear regression (eMLR) method. The rate of excess carbon increase in the northwest Atlantic was roughly 0.57 μmol/kg/year for all depths. Monthly averaging the data was also key to mitigating the strong summer sampling bias in the data, which led to underestimating the trend. The third study expanded to a near-global analysis, estimating the DIC natural and excess components for 25 Longhurst Provinces. The anthropogenic component of DIC is increasing globally, notably in the mid-latitudes (20°S-60°S), North Atlantic and North Pacific. The natural component was found to be declining in much of the global ocean, some regions have components that change linearly, and some have inter-annual variations.