About a quarter of annual anthropogenic CO₂ emissions are taken up by the ocean, thereby significantly reducing CO₂-induced greenhouse warming.  Emission reduction strategies for limiting atmospheric CO₂ rest upon quantitative knowledge of the magnitude of ocean uptake as well as an understanding of how it may change as atmospheric CO₂ levels and climate conditions change.  The same information is also needed to predict how acidification of the ocean due to CO₂ uptake will change in the future.

Key factors controlling air-sea CO₂ fluxes are the difference between atmospheric and oceanic concentrations of CO₂ (typically expressed as partial pressures of CO₂, pCO₂), sea surface temperature (SST), and local winds, all of which vary regionally and temporally across the ocean.  While SST and winds can be determined from remote, space-based observations, pCO₂ must be measured directly.  To that end, NOAA makes repeated measurements of pCO₂ from research vessels and commercial ships of opportunity along transects spanning the world’s oceans.  Additional measurements are made continuously from moored buoys.  The findings are combined with remote sensing to monitor fluxes of CO₂ into and out of the ocean, and to determine how those fluxes vary across regions and seasons, and in response to patterns of climate variability such as El Niño.

The NOAA Underway pCO₂ program is managed in close cooperation with about a dozen other countries that contribute under the framework of the International Ocean Carbon Coordination Panel, which is co-sponsored by the Intergovernmental Oceanographic Commission and the Scientific Committee on Oceanographic Research. NOAA field studies of ocean carbon uptake are managed by the NOAA Atlantic Oceanographic Meteorology Laboratory together with partners at the NOAA Pacific Marine Environmental Research Laboratory, the Lamont-Doherty Earth Observatory of Columbia University, the Bermuda Institute of Ocean Studies, and the Rosenstiel School of Marine and Atmospheric Science (RSMAS) at the University of Miami.

Surface pCO₂ data are managed and processed on behalf of this consortium and provided on a web site maintained by the the Lamont-Doherty Earth Observatory group. Delayed mode data are made available subsequently through the Carbon Dioxide Information and Analysis Center, in Oak Ridge, TN.

To understand the role of the oceans as a sink for anthropogenic CO₂, it is necessary to determine the distribution of carbon species in the ocean interior and the processes affecting the transport and storage of CO₂ taken up from the atmosphere. Carbon dioxide emissions from human activities are parcelled out among three global reservoirs for carbon: the atmosphere, the terrestrial biosphere and the ocean. The vast majority of carbon is stored in the ocean, where CO₂ reacts with seawater to form carbonic acid and its dissociation products, bicarbonate and carbonate ions, collectively known as Dissolved Inorganic Carbon (DIC). To better understand how the ocean carbon sink may change in a future, warmer, higher CO₂ world, it is necessary to monitor how the inventory of carbon stored in the ocean is changing, and to make adequate measurements of related chemical, physical and biological factors in the ocean interior to discern why it is changing.

To this end, in collaboration with the National Science Foundation Physical Oceanography and Chemical Oceanography Divisions, NOAA deploys oceanographic cruises aboard research vessels to make measurements from the surface to the bottom of the ocean at regularly repeated intervals, with the objective of monitoring how critical, climatically-sensitive variables are changing over time in the interior of the ocean. The measurement suite includes pCO₂, DIC, total alkalinity and pH for the study of ocean carbon, as well as a broad range of correlative observations including inert tracers, which are critical to understanding how ocean currents and mixing affect the observations and how they change in their own right; dissolved oxygen and ocean nutrients, which are critical to understanding related biological activity that strongly affects carbon storage and uptake; and physical oceanographic measurements such as temperature and salinity profiles, which provide information on changes in the deep ocean (beneath the depths reached by Argo profiling floats or XBTs). Because of the vastness of the ocean it takes about a decade to complete each set of cruises, which are then repeated to monitor change.

U.S. and international repeat hydrography programs continue the reoccupation of ocean transects begun under earlier programs (in particular, the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS) during the 1990s) that provided full depth data sets against which current observations are now being compared.

The NOAA/NSF Repeat Hydrography program is managed in close cooperation with about a dozen other countries that collaborate through the GO-SHIP program of the International Ocean Carbon Coordination Panel, which is co-sponsored by the Intergovernmental Oceanographic Commission and the Scientific Committee on Oceanographic Research. NOAA repeat hydrography observations are implemented jointly by the NOAA Pacific Marine Environmental Research Laboratory and the NOAA Atlantic Oceanographic Meteorology Laboratory, under the guidance of the U.S. CO₂/Repeat Hydrography Program.

U.S. repeat hydrography data are archived at the CLIVAR/CO₂ Repeat Hydrography data centers, including the CLIVAR and Carbon Hydrographic Data Office housed at the University of California San Diego, and the Carbon Dioxide Information and Analysis Center, in Oak Ridge, TN.