Ocean-Atmosphere Carbon Dioxide Flux
Quantifying ocean carbon uptake is essential to understanding how much of the carbon dioxide (CO2) emitted as a consequence of human activities is sequestered in the ocean, and how much remains in the atmosphere to cause greenhouse warming. Determining the rate at which CO2 dissolves in the ocean is also critical to monitoring ocean acidification. To these ends, it is important to resolve how ocean carbon uptake varies from location-to-location, and from season-to-season.
Algorithms, derived from ship and co-located satellite data, are applied to regional fields of remotely sensed sea surface temperature and ocean color to produce ∆pCO2 fields, which, combined with wind speeds, determine regional fluxes.
Air-sea exchange of carbon dioxide is governed by the difference between atmosphere and ocean in the concentration of CO2 (measured as the difference in partial pressure, pCO2), which provides a thermodynamic driving force for equalization of the concentrations. Where there is more CO2 in the air, it tends to diffuse into the ocean; where there is more CO2 in the upper layer of the ocean, it tends to be emitted to the atmosphere. While the amount of CO2 in the air varies gradually from location to location, the amount of CO2 in the surface water varies strongly with sea surface temperature, strength of upward or downward mixing, salinity, and extent of biological activity, all of which exhibit significant spatial and temporal variability. In addition, the rate at which CO2 crosses the boundary between atmosphere and ocean depends upon the strength of the wind at the ocean surface, which in turn is highly variable.
Sea surface temperatures and winds can be retrieved directly from remote observations, but pCO2 cannot be measured directly from space. Therefore, NOAA sponsors in situ measurements of pCO2 from research vessels and commercial ships of opportunity along transects spanning the world’s oceans. Additional pCO2 measurements are made continuously from moored buoys. However, even this extensive network of in situ pCO2 measurements covers only a small fraction of the ocean's surface, far too little to resolve the high variability of ocean carbon fluxes. By developing empirical relationships, however, between in situ pCO2measurements and space-based parameters such as SST and winds, scientists at the NOAA Atlantic Oceanographic Meteorological Laboratory have extended the spatial domain of the ocean carbon observing system adequately to produce seasonal estimates of ocean-atmosphere CO2 fluxes over the surface of the world's ocean. Using these techniques it is now possible to visualize how ocean carbon uptake has varied in space and time over recent decades.