Sensitivity studies of ocean uptake estimates

An important goal of global carbon cycle research is to identify how anthropogenic climate change may impact the exchange of CO2 between the atmospheric and ocean carbon reservoirs. One mechanism for this impact is the response of the ocean circulation to temporal changes in the climate state. In this past year, Joseph Majkut and Jorge Sarmiento have investigated how the uptake and storage of anthropogenic carbon are affected by the forcing applied by the atmosphere to the ocean. Such an understanding is important for quantifying the uncertainties in carbon system projections taken from global circulation models. It is particularly important to model inter-comparisons, such as those included in IPCC-type studies.

Using the GFDL MOM4.1 ocean model coupled to an ocean biogeochemistry model, the researchers generated a suite of simulations to understand the impact of different climatic conditions at the air-sea interface on the carbon balance. A number of data-derived estimates of the atmospheric state (1959 to present) now exist and differ in their representation of the structures and amplitudes of decadal trends in the surface winds and temperatures, each of which has an effect on the CO2 system. Figure 24 shows an initial result from that study. The figure shows a substantial difference in the invasion of anthropogenic carbon into the ocean between simulations forced by the different reanalysis products, with data-based estimates for comparison. Although the distribution of uptake varies in the models, all show total anthropogenic carbon uptake within the limits of uncertainty of data-based estimations.

The two newer reanalysis products in the bottom row show an enhanced uptake in the Southern Ocean and enhanced uptake in the Northern Atlantic. The Sarmiento Group is currently involved in applying new statistical methods to understand how these differences can be understood with respect to observations of the carbon content of the surface ocean and the interior.

Figure 24. A comparison of the anthropogenic carbon inventory from model simulations forced by the reanalysis products: NCEP‐1 (upper right), CORE (lower left) and ERA‐40 (lower right) and data‐derived estimates of the column inventory (upper left).

 

Global climate change is predicted to alter the ocean’s biological productivity with implications for fisheries and climate. While the most comprehensive information available on the global distribution of ocean productivity comes from satellite ocean color data, it has been unclear whether enough satellite ocean color data is currently available to detect trends in ocean productivity.

This year the Sarmiento Group carried out a study of ocean color data and found that the detection of trends in the satellite data will first be possible approximately 30 years from now because of the serial correlation and the large interannual and decadal variability in the ocean productivity time series. This length of time to detection varies according to the SeaWiFs data characteristics and the expected trend magnitude (estimated using three ocean biogeochemical models). To arrive at this time frame, it was assumed that there will be no interruption in satellite data, an unlikely scenario since SeaWiFS stopped emitting data in December 2010 and MODIS and MERIS have already exceeded their expected lifetime.

Sarmiento’s Group member Claudie Beaulieu also assessed how many additional years of observations will be necessary if there are data interruptions due to instrument replacement. She estimated that it could take approximately an additional 13 to 14 years of observations to be able to detect global trends from satellite data (with a probability of 0.9), if there are no continuous measurements available for this time period.

Moving forward, the group plans to study the detection of trends in SeaWiFS data using a Bayesian approach where trends predicted from ocean biogeochemical models are used to fit a prior distribution for the trend magnitude, which will allow a more efficient use of information available from data and models. The team’s work illustrates the importance of launching a new satellite to ensure continuous records suitable for climate change studies.

 

Continuous measurement of dissolved inorganic carbon in surface seawater

The dissolved inorganic carbon concentration of surface seawater is of interest for two reasons. First, fossil fuel CO2 enters the ocean at the surface, and its entry is registered as an increase in the dissolved inorganic carbon concentration (DIC, including dissolved CO2, bicarbonate, and carbonate). Comprehensive data on the concentration of DIC in ocean surface waters are one basic component of an ocean CO2 observing system; local observations concentrated in surface regions where there is intense mixing with the subsurface can give information about the uptake of fossil fuel CO2 and its transport into the ocean interior.

Second, ocean productivity leads to changes in the DIC concentration of surface waters over seasonal timescales. DIC concentrations in the upper ~ 100 m of the water column drop during spring and summer, as organisms assimilate DIC near the surface and remove it by export (sinking). The magnitude of the DIC decrease is a measure of the fertility of the ecosystems, and has been used to constrain ocean productivity over large scales.

Michael Bender’s group is developing an instrument to make continuous measurements of DIC along oceanographic cruise tracks. Their goal is to make an instrument with an accuracy of 0.1% or better that will operate reliably with very little attention underway. The basic approach involves adding the rare heavy stable isotope of carbon (13C) to seawater at a precisely metered rate, and measuring the ratio of natural 12C to added 13C. This ratio rises with the DIC concentration of the seawater.

The researchers have set up a prototype system for the work, and have achieved a precision of ±0.1% or better averaged over a period of hours. The final system has been designed, most of the components have been acquired, and the assembly will begin in the spring of 2011. The DIC instrument should be completed by the summer, and then deployed on cruises where it will be tested against analyses of discrete samples. The plan is to replicate the instrument and integrate it into the global seagoing carbon observing system. The initial focus in this respect will be on Agulhas, the South African icebreaker that operates in the Atlantic sector of the Southern Ocean.

 

Net community production and carbon export in the Southern Ocean

The rate of net organic carbon production by the upper ocean biological community, and the associated “export” (sinking) of organic carbon to deeper, dark waters, is a keystone in our understanding of the ocean carbon cycle. This flux supplies essentially all the food for organisms living in the dark ocean, below about 100 m depth. It is the process that removes carbon and nutrients from the surface ocean and supplies them to the deep; it thereby lowers the DIC and nutrient concentration of the surface ocean, raises the concentrations of DIC and nutrients in the deep ocean, and modulates the CO2 concentration of air.

This past year the Bender Group continued its program to measure net community production in the upper ocean mixed layer by making measurements of dissolved gas concentrations on ships of opportunity operating in the Southern Ocean during spring and summertime. A paper on data collected from the region of the Southern Ocean south of Tasmania illustrates the role of iron and light co-limitation in governing net community production and carbon export over a large oceanic region (Figure 25). Another, dealing with region to the west of the Antarctic Peninsula, shows similar features. These results are leading towards a quantitative understanding of the limitations on ocean fertility and carbon export by light and iron in the Southern Ocean.

Figure 25. Net community production and organic carbon export in the Southern Ocean, south of Tasmania, vs. dissolved iron concentration in the upper ocean mixed layer. The colors represent the depth of the mixed layer. Hot colors (reds) indicate shallow mixed layer; cold colors (blues) indicate deep mixed layers. Productivity is lower when mixed layers are deep, because phytoplankton spend much of their time in the bottom of the mixed layer where they do not receive enough light to grow rapidly. These data show that a combination of mixed layer depth and iron concentration influences productivity in the Southern Ocean.

 

Iron fertilization

Iron fertilization, which is one potential method for increasing CO2 storage in the ocean, has been a primary focus of the Sarmiento Group as part of the Carbon Mitigation Initiative. Earlier work had suggested that the efficiency of iron fertilization at drawing down atmospheric CO2 is questionable, and that, by decreasing the nutrients in waters exported to low-latitudes, iron fertilization would have a deleterious impact on global productivity.

A major new finding is how the location and timing of iron release determines the efficiency and time-scale of CO2 sequestration. Using a biogeochemical model developed at the Geophysical Fluid Dynamics Laboratory, the researchers showed that iron fertilization during the growing season at sites in the Southern Ocean sequestered the most CO2 for the longest periods of time, particularly in the Ross Sea (where winter ice cover prevents evasion of CO2 back into the atmosphere). However, the study showed that the sequestration was not completely permanent, as a back flux of CO2 to the atmosphere resulted in only 50% of sequestered carbon remaining in the ocean a century after fertilization.