Observing Systems for Constraining Ocean Carbon Uptake and Acidification

The ocean removes roughly a quarter of total human carbon dioxide (CO₂) emissions from fossil fuel combustion and deforestation, slowing global climate change. However, ocean uptake may become less effective in the future due to warming, increased vertical stratification, and altered ocean circulation. Carbon uptake also leads to wholesale changes in the seawater chemistry, a process termed ocean acidification that may negatively impact a wide-range of marine organisms. No single observational approach can address all aspects of ocean carbon uptake, requiring an integrated sampling strategy constraining both ocean carbon inventories and air-sea fluxes.

Monitoring the slow rate of change in ocean carbon inventory requires high-quality, full-depth measurements for at least 2 of 4 carbon parameters (dissolved inorganic carbon, alkalinity, pH, and CO₂ partial pressure, pCO₂) as well as temperature and salinity. The anthropogenic carbon component cannot be measured directly but can be constrained with additional information on hydrography, nutrients, oxygen, and other chemical tracers. The global WOCE/JGOFS ship survey (late-1980s and 1990s) provided a baseline of the cumulative anthropogenic CO₂ uptake from pre-industrial conditions and for tracking future change. The Repeat Hydrography/CO₂ Program is reoccupying select hydrographic sections, and early results include estimates of the decadal growth of anthropogenic carbon.

Constraining the global integral of air-sea CO₂ exchange is challenging because it involves estimating the small net imbalance between large positive and negative fluxes. Air-sea flux can be derived from measured ocean and atmosphere pCO₂ difference and empirical gas-transfer velocities derived from wind speed. Spatial mapping of surface water pCO₂ is greatly facilitated by underway sampling on VOS research, commercial, and Antarctic resupply vessels. Global ocean carbon uptake estimates are broadly consistent across methods including air-sea fluxes, ocean inventories, forward and inverse ocean models, and atmospheric inverse models.

New ocean observing systems are emerging to constrain ocean carbon uptake and acidification based on ship-based measurements, moorings, time-series stations, and arrays of autonomous platforms such as floats and gliders. Rapid progress is being made on in-situ biogeochemical sensors for Argo profiling floats, opening up the possibility of companion global biogeochemical array. Currently available sensors include dissolved oxygen, nitrate and bio-optics; inorganic carbon system sensors are under development.