A consistent data base for ocean carbon

Rising atmospheric CO2 is affecting ocean chemistry and drives the ocean acidification. All information is now collected in a global data base.

The increase in global temperature is mainly driven by increased concentrations of carbon dioxide (CO2) in the atmosphere; increased CO2 concentrations increases insulation the planet so that less heat is escaping to space. However, all CO2 produced by humans (mainly by burning of fossil fuel, cement manufacturing and land use change) do not stay in the atmosphere, about 30% is taken up by land plants and roughly 30% is taken up by the ocean. The ocean is thus doing an important job in reducing the climate effects of human made CO2 emissions. However it is expected that the ocean sink becomes smaller due to the carbon chemistry of the ocean but since a warmer ocean is expected to be less efficient in transporting surface waters to the interior ocean. It is thus important to quantify the size of the ocean sink, and to monitor to what extent it changes over time. To do this one relies on observations of inorganic carbon and other relevant chemical properties such as oxygen and nutrients. These measurements require taking water samples from a research vessel, and are therefore expensive and require a massive collective effort to sample the world ocean. It is also difficult since the measurements have to be accurate in order to detect small changes to a large number (immense quantities of inorganic carbon are stored in the oceans). This can obviously not be achieved by one country only, but a collective effort of all "ocean carbon" scientists in the world is needed.

Recently scientist has published a data product with interior ocean carbon data called the Global Ocean Data Analysis Project version 2, or short GLODAPv2.  This is a very significant step towards a better understanding of the ocean sink of anthropogenic carbon. Not only did the GLODAPv2 team gather close to 1 million interior ocean carbon relevant measurements from various data bases and scientists, but the team performed a very thorough quality control of the data producing an internally consistent data product. Particularly difficult is the detection, and adjustment, of small biases (off-sets) in the data between different research teams from different countries working on different ships during different decades using different instrumentation. Small biases are very difficult to avoid even with the best methods, and are also very difficult to detect by the individual science team. The solution to the problem is to compare values measured by different teams in the deep ocean at the same location, where changes in carbon content is very slow. Each such comparison results in an offset in the measured values between the two cruises, and the result is a matrix of several hundred thousand offsets. By applying an inverse method to the offset matrix, suggestions are made on which data sets have a significant bias, and should be accordingly adjusted. All of these suggested offsets have been manually inspected by the GLODAPv2 team and adjustments have been collectively agreed on. All the information on the adjustment procedure is available at the GEOMAR Helmholtz Centre for Ocean Research Kiel.

The GLODAPv2 data product has been published and is free and publically downloadable from international data repositories. The product has three components: 1) the data files from the individual cruises, 2) the adjusted data product and 3) the gridded product. GLODAPv2 contains data from 724 scientific cruises with close to 1 million water samples (Figure 1). The data is covering the global ocean from the 1970’s through to recent dates, although the majority of the data comes from the 1990’s onwards with the onset of the World Ocean Circulation Experiment (WOCE) and the follow-up repeat hydrography program GO-SHIP where trans-basin ocean sections are repeated at roughly decadal intervals. 

In addition to the adjusted data product, the GLODAPv2 team produced a mapped product where the measured values have been interpolated to a regular grid (Figure 2). This is particularly valuable for the modelling community that is dependent on such gridded products to initiate biogeochemical ocean models, such as those used for the IPCC report. The availability of the new GLODAPv2 product will be valuable also for that community.

Publications

Lauvset, S. K., Key, R. M., Olsen, A., van Heuven, S., Velo, A., Lin, X., Schirnick, C., Kozyr, A., Tanhua, T., Hoppema, M., Jutterström, S., Steinfeldt, R., Jeansson, E., Ishii, M., Perez, F. F., Suzuki, T., and Watelet, S.: A new global interior ocean mapped climatology: the 1°x1° GLODAP version 2, Earth Syst. Sci. Data, 8, 325-340, 10.5194/essd-8-325-2016, 2016.
Olsen, A., Key, R. M., van Heuven, S., Lauvset, S. K., Velo, A., Lin, X., Schirnick, C., Kozyr, A., Tanhua, T., Hoppema, M., Jutterström, S., Steinfeldt, R., Jeansson, E., Ishii, M., Pérez, F. F., and Suzuki, T.: The Global Ocean Data Analysis Project version 2 (GLODAPv2) – an internally consistent data product for the world ocean, Earth Syst. Sci. Data, 8, 297-323, 10.5194/essd-8-297-2016, 2016.

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