Working Group Members: Ken Brink, Mary-Elena Carr, Francisco Chavez, Scott Doney, Mick Follows, Nicholas Gruber, Paul Robbins, Jorge Sarmiento, John Southon, Taro Takahashi, Rik Wanninkhof
The Solubility Pump Working Group evaluated human-induced and climatic influences on spatial and temporal scales of solubility-driven storage of carbon in the oceans in relation to their impact on the atmospheric CO2. The solubility pump is often defined as the carbon exchange mediated by physical processes such as heat flux, advection, and diffusion. In contrast to the biological pump the fundamental processes controlling the exchange are well understood. The questions thus focus on quantifying and constraining the processes, and assessing possible changes due to climate change. The solubility pump is also intricately linked to the biological pump since the same advective and diffusive processes that transport carbon also control nutrient supply into the euphotic zone and to some extent export of biological products from the mixed layer. The group addressed the perturbations of the solubility pump that arise through changes in seawater temperature and circulation on seasonal, interannual, decadal and centennial time scales. Emphasis was placed on large-scale controls and processes which can provide a framework for the specialized studies in the biological pump section. The group separated the problem into two main directions: (1) processes affecting the air-sea exchange of carbon dioxide; and (2) processes affecting the changes in carbon inventories and transport in the ocean interior.
Primary Question: What is the global oceanic CO2 uptake on the seasonal, interannual, decadal and centennial time scales, and what are the physical controls that will determine future atmospheric CO2 levels?
Primary Hypotheses
Changes in ocean circulation and short-term meteorological forcing are the primary controls on the oceanic variability of CO2 fluxes with the atmosphere on seasonal to interannual time-scales.
Changes in ocean circulation and thermocline ventilation rates due to climate change controls the decadal to centennial scale uptake of CO2.
Changes in ocean circulation primarily controls the strength and efficiency of the biological pump by controlling the supply of nutrients to the euphotic zone on seasonal to interannual time scales.
AIR-SEA EXCHANGE
The ocean plays a critical role in the global CO2 cycle as it is a vast reservoir of CO2, naturally exchanges CO2 with the atmosphere, and takes up a substantial portion of anthropogenically-released CO2 from the atmosphere. In addition, estimates of the ocean sink can be used in conjunction with atmospheric measurements to provide an independent constraint on the terrestrial carbon sink reservoir. Several of the major program elements and activities for the Global Carbon Cycle Plan, therefore, require ocean-based observations. Answering the central questions posed by OCTET requires, in part, systematic observations of surface water CO2 and hydrographic properties, hydrographic sections of CO2, tracer, and associated parameters; and interpolation schemes using remotely sensed parameters such as SST, color, wind and SSS.
The flux of CO2 between surface waters and the atmosphere can be constrained using data of the partial pressure difference between the air and the water, combined with estimates of the gas exchange coefficient (Takahashi et al., 1999). The value of the technique lies primarily in determining the spatial distribution and temporal variability of air-sea CO2 fluxes on short time-scales. It is particularly useful where the signals are large, as in the North Atlantic and Equatorial Pacific, or to study temporal variability such as the contrast between El Niño and non&endash;El Niño years.
One important issue is to gain a better understanding of the spatial patterns of surface ocean pCO2 and its seasonal and interannual variability. Seasonal and interannual variability of pCO2 in the surface ocean is one to two orders of magnitude greater than their annual increase due to uptake of anthropogenic carbon (e.g., Bates et al., 1996, Winn et al., 1994; Feely et al., 1999). Because the signal to be detected is much smaller than this variability, it takes a decade or longer to observe anthropogenic trends in the surface water if we don't have the ability to "subtract" natural variability. In addition, the seasonal and interannual variability in pCO2 gives information on how the carbon cycle functions, and can be used in conjunction with other methods to help understand regional and global patterns of carbon uptake. A very promising development of the last decade is new instrumentation that will make it possible to measure pCO2 (DeGrandpre et al., 1995, Friederich et al., 1995, Goyet et al., 1992, Merlivat and Brault, 1995) and other properties autonomously from moorings, drifters and volunteer observing ships (VOS). OCTET envisions that platforms with these capabilities will allow establishment of many more time-series stations and repeat surface water transects at feasible cost in otherwise remote locations.
Key Questions
Long-term goals for CO2 research in the ocean are, first, to quantify the uptake of anthropogenic CO2 by the ocean, including its interannual variability and spatial distribution; and, second, to understand and model the processes that control the ocean's uptake of CO2. Uptake of anthropogenic CO2 can be determined by measuring either the air-sea flux itself, or the resulting change in carbon inventory for longer term quantitative information on uptake patterns. Both should be carried out, with a strong emphasis on disaggregating the global uptake into contributions from major ocean regions and monitoring temporal variability to facilitate a regional sensitivity analysis to changes in uptake and release rates due to climate change.
In addition to estimating the overall magnitude of the ocean anthropogenic carbon sink (e.g., Takahashi et al., 1999), the spatial pattern of air-sea fluxes can be used to constrain global patterns of carbon sources and sinks on land (Fan et al., 1998). The change in pCO2, and thus the air-sea flux of CO2 is variable in space and time due to changes in circulation, temperature, and salinity, as well as biology. The key to determining this flux and understanding its variations is through in situ monitoring; including both time-series stations and regular measurements along transects using ships of opportunity.
Temporal variations in some areas, such as the Equatorial Pacific, have been identified as major causes of variability in air-sea CO2 fluxes. The limited existing time-series studies are located mostly in the subtropical ocean gyres, while there are major gaps in data on regions of active ocean mixing and high biological variability, especially in subpolar and polar latitudes. Temporal variability is greatest in surface and subsurface layers, locations where biological and physical feedbacks are most likely to alter the ocean's ability to absorb CO2. Characterization and understanding of this "natural" temporal variance is a prerequisite for understanding the processes that limit rates of ocean CO2 uptake.
Remote measurements are an essential tool for extrapolating in situ measurements to the global scale. Several parameters essential for estimating air-sea fluxes can now be observed from space, including wind, sea surface temperature, eddy circulation patterns, and biological productivity. Methods to extrapolate discrete in situ measurements to a larger region will be needed and can be tested against parameters sensed on a coarser resolution. In situ measurements will be critical for validating the accuracy of remote-sensing algorithms.
Strategy
CIRCULATION, CO2 TRANSPORT AND INVENTORIES
The strategy is to put in place a global ocean-observing network for CO2 and tracers to document the continuing large-scale evolution of the CO2 fields. Such a strategy calls for a program of repeat oceanic sampling of carbon system parameters, tracers and hydrography as part of OCTET. The program is critical to our understanding of climate change, both natural and anthropogenic. The objectives are: (1) to quantify changes in the rates and spatial patterns of oceanic carbon uptake, fluxes and storage of anthropogenic CO2; (2) to detect and quantify changes in water mass renewal and mixing rates; and (3) to provide a validation of the time integration of models of natural and anthropogenic climate variability.
Key Questions
Global CO2, tracer and hydrographic surveys are required to monitor the oceanic CO2 inventory and its evolution in space and time. Since the oceans and atmosphere are the primary reservoirs where CO2 is redistributed on earth, it is important, for a better understanding of climate variability, to determine the mechanisms and rates of its redistribution. It is proposed that a set of the hydrographic sections, many of them repeats of WOCE Hydrographic Program sections, be occupied at time intervals of between 5 and 10 years to provide broad-scale global coverage of ocean variability (Fig. 1). The sampling time interval should provide resolution of the local ventilation time-scales within the main thermocline to determine interannual and decadal scale changes in oceanic fluxes. The repeat sections should be integrated with the high-frequency sampling networks (e.g.,VOS ships, drifters, profilers, moored instruments) and process studies in OCTET to quantify seasonal and other high-frequency variability, validate model simulations, and to ground truth the accuracy of the in-situ measurements. If possible, the occupations of these sections should be coordinated with CLIVAR to reduce ambiguities in interpretation of spatial/temporal variations. The measurement suite should include dissolved inorganic carbon and total alkalinity, and should frequently include a third/fourth CO2-system property such as pH and/or pCO2 to assure internal consistency. Other measurements should include 13C/12C ratios and TOM (total organic matter). The hydrographic measurement program should also include transient tracers, which provide temporal information about ocean mixing and water mass history that is essential to interpreting anthropogenic CO2 distributions. In addition to the hydrographic and carbon system parameters including: temperature, salinity, oxygen, nutrients, and CO2 parameters, transient tracers (i.e., 3H/3He, 14C, 13C/12C, CFCs, and HFCs) should be measured on these sections to estimate transport fluxes, provide water mass ages, and document changes in anthropogenic carbon inventories. Some of these tracers reveal mixing over the critical longer (decadal and century) time scales; and some help identify current short-term invasion rates for comparison with older data. Station spacing on the proposed sections should be eddy-resolving to avoid aliasing of eddies and other variability into the climate signal. Meridional sections are important for understanding variations in basin-scale circulation patterns and inventory changes. Repeat occupation of zonal sections allows for the detection of variability in the rates, pathways, and properties of deep and intermediate waters carried towards the equator from the high latitudes. Ideally, they should be located downstream of the deep and intermediate water formation regions.
Because one of the main gaps hindering progress in defining the spatial and temporal variability of carbon uptake in the ocean is lack of data, strategies are needed to increase spatial coverage and frequency at reduced per datum cost. A particular emphasis is placed on the development of new technology, in particular instruments for measurement of CO2 and related quantities on moorings, drifters, VOS and towed vertical samplers, rapid water sampling techniques, and high throughput multi-element analyzers for carbon system measurements, etc. A complementary focus is the identification and utilization of platforms such as ships of opportunity, and enhancing the suite of measurements on other suitable observational platforms to maximize the benefit for both climate and the carbon system.
The lateral transport of carbon by ocean currents plays a key role in the exchange of carbon between the ocean and atmosphere. Accurate measures of the physical transport of carbon are a necessity in order to fully interpret independent estimates of air-sea flux and local accumulation. For example, the southern ocean has been identified as a region of large carbon uptake by the ocean, however, the location of the accumulation of anthropogenic carbon is primarily in the subtropical gyres. Thus, significant meridional advection of carbon is required to link the regions of air-sea flux with those of increasing inventory. High-resolution hydrographic surveys currently offer the only direct method for estimating ocean transport of carbon. Geostrophic currents are determined from the observed density field, often utilizing an inverse box model to estimate unknown reference level velocities. The net carbon transport is the integrated product of the distribution of the currents and carbon concentrations. Previous estimates of carbon transport (Brewer et. al., 1989, Martel and Wunsch 1993, Holfort et al., 1998) reveal that accurate estimates require sufficient spatial sampling of physical parameters to resolve the mesoscale eddy field. While the CTD sampling must be sufficient to resolve the eddy variations (60-100 km), regression and interpolation analysis has demonstrated that the carbon field, especially below the seasonal thermocline, can be accurately reconstructed from a sub-sampled distribution of significantly lower resolution (Goyet et al., 1995).
The WOCE/JGOFS/NOAA CO2 survey provides the first global data set with which to estimate the oceanic transport of dissolved inorganic carbon. Analysis in the South Atlantic (Holfort et al., 1998) reveals a significant southward transport of -0.81 ± 0.08 Pg C/yr. Analysis of the Pacific and Indian basins is underway. The existent data set will be insufficient to answer key questions regarding ocean transport. For example, the WOCE/JGOFS survey can only provide for estimates of the inorganic component of the total carbon flux. Lateral advection of DOC also contributes to the net of carbon (Hansell and Carlson, 1998) and additional measurements are required before accurate calculations of this component are possible.
Temporal variability and evolution of the ocean carbon transport cannot currently be determined from observations. The carbon distribution in the ocean is evolving in response to anthropogenic perturbations. How the ocean transport of carbon will respond to this secular trend is, as yet, undetermined. In addition to the long-term trends in carbon accumulation, temporal variability of transport at shorter time scales (mesoscale, seasonal, inter-annual) are likely to be significant. A single survey of the global dissolved inorganic carbon distribution cannot provide a basis for estimating this variability. Repeat upper ocean temperature measurements have been conducted, however, and offer a illustration of the magnitude of temporal variability. For example, in the North Pacific, over 27 XBT lines have been completed between California and Taiwan. The mean meridional heat transport calculated from this data is 0.77 ± 0.12 PW, with an interannual range of 0.3 PW (Roemmich et al., 2000). In the North Atlantic, analysis of historical hydrography at 36°N indicates the annual mean transport is 1.4 PW but with an interannual range of 0.6 PW (Sato and Rossby, 2000). The temporal variability of the carbon transport is likely to have similar amplitudes.
Biological archives such as banded corals and mollusks represent a potential resource for providing tracer data to help reconstruct spatial and temporal variations in CO2 exchange and intra-annual to centennial-scale ocean circulation changes. For a subset of the parameters which monitor the state of the solubility pump, calcareous recorders of water properties such as known-age annually banded corals and mollusks can provide data at sub-annual resolution to extend the instrumental record in space and time. Records of del18O, trace metals, and the carbon isotopes (del13C and Delta14C) are conventionally obtained from warm-water archives such as corals (Dunbar and Cole, 1993), and existing efforts in this direction should be continued and expanded. In addition, banded mollusks, other calcareous organisms such as bryozoans, rhodoliths, and deep-sea corals, and fish otoliths, are all worthy of investigation as potential carbonate-based recorders of conditions in colder waters, including those of the sub-polar oceans and the main thermocline. These materials represent a unique and underutilized resource for helping to determine the state of the solubility pump in remote and undersampled areas such as the NW Atlantic and Pacific, and the Southern Ocean.
Strategy
Figure 1. Locations of proposed repeat sections for studies of circulation, transport anthropogenic CO2 inventories. The grey lines are proposed CLIVAR cruise tracks and the black lines are committed cruise tracks by similar international programs.
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