OCTET Prospectus

The Carbon Cycle Science Plan (CCSP) prepared for the U.S. Global Change Research Program has outlined a strategic mix of terrestrial, oceanic and atmospheric research dedicated to answering two fundamental questions:

• What has happened to the carbon dioxide that has been emitted by human activities?

• How will the atmospheric carbon dioxide concentration evolve in the future?

One of the critical components needed to answer these questions is an improved understanding of the past, present and future variability of the ocean carbon cycle especially as it relates to the air-sea exchange of carbon. Credible projections of the ocean carbon cycle response to climate perturbation will not be possible without a much more detailed, mechanistic understanding of the processes controlling the global sequestration of carbon, both natural and anthropogenic.

The CCSP has identified several key areas to pursue in advancing our understanding of the ocean carbon sink: constraining interannual variability of air-sea carbon fluxes, deducing their spatial distribution, determining the sensitivity of carbon fluxes and storage to changes in climate. All this requires an understanding and quantitative description of the mechanisms controlling fluxes and transformations of ocean carbon. Thus, the oceanographic community sees the urgent need to maintain scientific focus and observational capabilities for continued investigation of the marine carbon cycle. As one mechanism for meeting this need, we present here OCTET, a new research initiative to understand carbon dynamics in the ocean within the framework of the CCSP.

The ocean carbon cycle influences atmospheric CO₂, and thus the physical climate system, via changes in the net air-sea CO₂ flux that are driven by differences in pCO₂ between the surface ocean and lower atmosphere. The inventory of dissolved CO₂ in the oceans is 50-60 times greater than that in the atmosphere, so a small perturbation of the ocean carbon cycle can result in a substantial change in the concentration of CO₂ in the atmosphere. The near-term focus of OCTET is to develop the improved conceptual framework and tools required for estimating basin-scale patterns, seasonal cycles, and interannual variability of the sea surface pCO₂ distribution. These distributions will lead to better quantitative measures for the net anthropogenic CO₂ uptake into the different ocean basins and provide important constraints on the atmospheric and terrestrial components of the global carbon system. An initial OCTET emphasis on the North Atlantic and North Pacific Oceans will contribute to broader CCSP efforts to quantify regional magnitudes and variability of Northern Hemisphere carbon sinks. A longer term focus of OCTET is to determine the role of the ocean carbon cycle in amplifying or ameliorating natural and anthropogenic variation in atmospheric CO₂, and thus climate change. The Southern Ocean is thought to be key in this regard. An important outcome of OCTET will be an assessment, based on more detailed process-level understanding, of proposed mechanisms by which the ocean carbon sink evolves in the future.

The regional and vertical partitioning of carbon in the ocean is dominated by two interdependent carbon pumps that deplete the ocean surface of total CO₂ relative to deep water. Because the solubility of CO₂ in seawater increases with decreasing temperature, the SOLUBILITY PUMP transfers CO₂ to the deep sea as the formation of cold deep waters at high latitudes acts as a temperature-dependent sink for atmospheric CO₂. The BIOLOGICAL PUMP removes carbon from surface waters by gravitational settling, diffusion, and active biotransport of organic and inorganic carbon derived from biological production. OCTET seeks to understand the influence of climate on the processes controlling these carbon pumps, as well as potential feedbacks on the climate system via air-sea carbon exchange.

Perturbations of the solubility pump arise through changes in seawater temperature and circulation, so it is inevitable that global warming will alter the solubility-driven storage of carbon in the ocean. The magnitude of changes in atmospheric CO₂ following perturbations of the solubility pump can be large. For example, by restricting the magnitude of equatorial upwelling, El Niño events reduce the net flux of CO₂ from the ocean to the atmosphere in the equatorial Pacific. Large, concurrent changes in biological production illustrate the coupling between the solubility pump and the biological pump. Natural variability of ocean circulation in other regions may likewise carry significant consequences for ocean-atmosphere CO₂ partitioning on interannual scales. As an example, deep water formation in the North Atlantic Ocean has changed in recent years in response to changes in the North Atlantic Oscillation (NAO). The North Atlantic is a strong and persistent sink of CO₂, but how the magnitude of CO₂ uptake has changed in response to the shift in NAO is unknown. On the time scale of anthropogenic climate change, model projections suggest significant alterations in ocean circulation including increased surface stratification, slower vertical exchange, particularly in the Southern Ocean, and reduced deep water formation in the North Atlantic. We must determine the effect these changes in circulation have on ocean carbon storage.

The biological pump transports to deeper water the organic carbon and CaCO₃ produced by organisms in the surface ocean. In deeper waters, this fixed carbon is largely dissolved or remineralized, adding to the total CO₂ reservoir that is isolated from the atmosphere. There are only a few basic mechanisms by which changes in the ocean's biological pump can alter the partitioning of CO₂ between the ocean and the atmosphere: (1) a change in the inventory, supply or uptake efficiency of limiting nutrients; (2) changes in stoichiometric ratios of organic matter produced and retained in the surface layer, or exported from it; (3) changes in the form of organic carbon produced and exported biologically (with consequent changes in the depth scales of remineralization for C, N and P), and (4) changes in the organic carbon/CaCO₃ ratio of biogenic debris sinking through the water column. Alterations in the biological pump in the Southern Ocean, where there is a large inventory of available nutrients in the euphotic zone, might have a large impact on carbon sequestration. Elsewhere, euphotic zone nutrient inventories are small, and the assumption has generally been made that perturbations in pathways would not have an important effect on atmospheric CO₂. We need to determine quantitatively the impact of such perturbations on evolution of atmospheric CO₂.

OCTET must examine processes that can significantly alter partitioning of CO₂ between the ocean and atmosphere on time scales of societal importance; we must do so in a way that allows quantitative projections of the impact and uncertainty on atmospheric CO₂ levels of plausible perturbations of the physical, chemical, and biological processes regulating the ocean carbon cycle. At the same time, we must consider the reverse, the effect of climate change on ocean processes. Thus, OCTET will examine basic processes in ocean physics and biogeochemistry that are important for understanding the carbon cycle and that may be altered by global change. Such processes include:

• mesoscale eddy pumping of nutrients to the euphotic zone;

• large-scale exchange of nutrients between coastal waters and the open ocean;

• subsurface particle and dissolved organic matter transport and remineralization;

• variations in Fe (and other limiting trace element) fluxes to the ocean;

• rates of nitrogen fixation and denitrification;

• increased surface stratification, hence slower vertical exchange and reduced deep water formation.

In considering future research priorities, we drew from previous NSF-coordinated planning efforts within the oceanographic community, FOCUS and OEUVRE, and several other community planning efforts.

As will the entire CCSP with its emphasis on ocean, land and atmosphere, studies on the ocean carbon cycle will require coordination among scientists from many research disciplines and with many approaches. Modeling will be conducted during the development and implementation of OCTET, to promote effective design and to refine the models through direct comparison with observations. As have past research programs on ocean biogeochemistry, we expect OCTET to involve:

• Development and use of modern observing systems: developing new in-situ analytical systems (e.g., autonomous samplers; rapid water samplers and sensors) while exploiting the remote sensing and modeling tools necessary for implementing a global ocean carbon monitoring program. Co-ordination with GOOS is anticipated on this front.

• Historical and paleoclimate variability studies: synthesis of existing data sets and design of new, targeted field studies to evaluate the ocean carbon cycle response and feedbacks to climate variability.

• Process studies: traditional and "manipulative" studies directed specifically at current (and evolving) hypotheses for how the ocean carbon system can significantly impact climate.

• Ocean carbon cycle climate projections: synthesis and modeling studies to tie ocean processes into the global carbon cycle and physical climate with a specific emphasis on modeling paleoclimate and anthropogenic climate change.

Three categories of ocean biogeochemical processes need explicit attention:

• Processes that are at steady state (or nearly so) on time scales of a year or more. These may have a large impact on regional net air-sea fluxes of CO₂, but they will have little impact on ocean uptake of anthropogenic CO₂. For example, seasonal cycles in oligotrophic gyres (e.g., Bermuda) involve a deepening of the mixed layer in winter that injects nutrients into surface waters, leading to a phytoplankton bloom. Superimposed on the biological processes, which alter surface pCO₂, is the seasonal cooling and heating of the surface layer. While the air-sea CO₂ flux may be substantial at times, sources should equal sinks when averaged over an entire year, and the net flux is zero.

• Processes that exhibit natural interannual variability, unrelated to human activities. It is important to assess natural interannual variability for several reasons; e.g., (a) so that natural interannual variability is not confused with a response to global warming or other human perturbations; (b) so that the ocean-terrestrial mass balance approach used to evaluate terrestrial uptake of CO₂ in North America isn't biased severely by ocean budgets constructed in an anomalous year or years; and (c) because natural variability can be used to study the response of biogeochemical processes to variable physical forcing.

• Processes that have not been important in the past, but may become important as the result of global warming or other human perturbation, where one or more of the factors determining the efficiency of the biological pump are perturbed, with a consequent feedback on the ocean's uptake of anthropogenic CO₂. These might include the response to physical changes in the ocean environment, such as increased stratification, and to chemical changes such as increased input of fixed nitrogen (both via runoff and via the atmosphere) or increased input of iron (dust) due to desertification.

Specific research strategies for OCTET will naturally arise from more encompassing community planning efforts. Other research programs (e.g., SOLAS and EDOCC) are also beginning which should tie in well with OCTET. Observations key to the carbon cycle may find a place in the Global Ocean Observing System (GOOS). No doubt there will also be similarities between past research efforts on biogeochemical cycling and those proposed by OCTET. Both USJGOFS and IRONEX, for example, are successful programs that dealt with aspects of the carbon cycle. In addition, USJGOFS conducted a global CO₂ survey and maintains two long-time series observation stations.

JGOFS grew out of a NAS workshop initiated following developments in ocean observing capability which made a global scale program possible. Moreover, the oceanography community recognized that an international and truly interdisciplinary approach was needed to address biogeochemical processes at global and decadal scales. The development of a multidisciplinary community of ocean scientists, both observationalists and modelers, that values collaborative research is a principal achievement of JGOFS which makes OCTET feasible. OCTET will build on the scientific findings, political lessons and the human heritage of JGOFS and other programs of the past decade (WOCE, GLOBEC, IRONEX, CLIVAR, etc.).

Among the lessons learned is the realization that large programs build up momentum and develop inertia that can inhibit creativity and stifle individual spontaneity which are necessary parts of scientific inquiry. It is hard to shift the direction of big programs. OCTET needs mechanisms to encourage and embrace smaller, cutting-edge projects and programs. IRONEX is an example of a new program which was not easily accommodated by JGOFS, and was only implemented through other means. Another lesson from JGOFS and many other efforts is that large programs absolutely require rigorous analytical intercomparison/calibration exercises, but these remain expensive and difficult to organize and complete. JGOFS initiated successful development of reference materials for organic and inorganic carbon species and an intercomparison of dissolved organic carbon analyses which remain as signal achievements of the program, but significant disagreement remains for other key state variables (e.g., iron, particulate organic carbon), not to mention rate processes (e.g., particle fluxes). OCTET will need to make better allowance for these smaller activities. However a main positive lesson of JGOFS is that big questions require big programs. A new, broadly collaborative and interdisciplinary, coordinated research entity is needed to attack the ocean carbon cycle.

OCTET should be an integrated and directed research program that, while it has a variety of elements, is focused on addressing key uncertainties in ocean carbon cycling that are relevant to understanding anthropogenic global change. Such direction is seen as necessary to advance our understanding sufficiently to help with future policies. The next step should be the scheduling of a workshop to identify what we perceive as the highest priority issues and to design science and implementation plans for addressing them.

The CCSP planning effort highlighted two critical activities that could serve as an initial starting point for the OCTET community discussion. A near-term goal is constraining the spatial and temporal patterns of surface CO₂ concentrations and fluxes in the North Atlantic and North Pacific Oceans on seasonal and interannual timescales. The variability of the air-sea CO₂ fluxes in the northern hemisphere is an important element in reconstructing the patterns and magnitude of terrestrial carbon uptake using the atmospheric CO₂ monitoring network and transport inversion models. In an appropriate mechanistic context, such an integrated field, remote sensing and modeling effort could form a pilot regional theme within OCTET addressing a number of key scientific and technical issues: biological vs. physical carbon pumps; interannual variability; trace metal limitation; deep-water ventilation; autonomous sampling capability; connections to global carbon cycle. A long-term view towards the Southern Ocean was also defined in CCSP based on its size, the large inventory of available but unused nutrients in its euphotic zone, the more limited historical data base, and modeling studies. These show that the Southern Ocean is presently the dominant region taking up anthropogenic CO₂ and is, at least in some models, disproportionately sensitive to climatic perturbations. Such views are broadly consistent with paleoceanographic studies. An immediate task when developing the OCTET science and implementation plans will be to reexamine these two foci and address whether other equally compelling arguments can be made for other regions.