Strategic Plan for the
Climate Change
Science Program
Review draft, November 2002

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Chapter 9:
Carbon Cycle

Carbon is important as the basis for the food and fiber that sustain and shelter human populations, as the primary energy source that fuels economies, and as a major contributor to the planetary greenhouse effect and potential climate change. Atmospheric concentrations of carbon dioxide (CO₂) and methane (CH₄) have been increasing for about two centuries as a result of human activities. Future atmospheric concentrations of these greenhouse gases will depend on trends and variability in natural and human-caused emissions, and the capacity of terrestrial and marine sinks to absorb and retain carbon.

Elevated atmospheric CO₂ concentrations, additions of nutrients, and changes in land management practices can significantly enhance (and sometimes reduce) ecological carbon sinks. Engineering approaches for carbon sequestration provide additional options to reduce atmospheric greenhouse gas concentrations or reduce their rate of increase. However, uncertainties remain about how much additional carbon storage could be achieved, the efficacy and longevity of carbon sequestration approaches, whether they will lead to unintended environmental consequences, and just how vulnerable or resilient the global carbon cycle is to such manipulations. Successful carbon management strategies will require solid scientific information about the basic processes of the carbon cycle and an understanding of its long-term interactions with other components of the Earth system such as climate and the water and nitrogen cycles. Breakthrough advances in techniques to observe and model the atmospheric, terrestrial, and oceanic components of the carbon cycle have readied the scientific community for a concerted research effort to identify, characterize, quantify, and predict the major regional carbon sources and sinks -- with North America as a near-term priority.

The overall goal for the US Carbon Cycle Science Program research is to provide critical scientific information on the fate of carbon in the environment and how cycling of carbon might change in the future, including the role of and implications for societal actions. In this decade, research on the carbon cycle will focus on two overarching questions:

National and international decisionmakers have called for better information on the global carbon cycle in order to reduce uncertainties concerning the potential for climate change and to evaluate carbon sequestration options for climate change mitigation. A well-coordinated, interagency, and multidisciplinary research strategy, bringing together a broad range of needed infrastructure, resources, and expertise, will be essential in providing this information. Specific research questions that will be addressed in support of the two overarching questions are covered in the following sections.

There is growing evidence of a current Northern Hemisphere terrestrial sink averaging 1.8 billion metric tons of carbon per year. Recent work suggests that this sink may be a result of land use change, including recovery of forest cleared for agriculture in the last century, and land management practices, such as fire suppression. Other studies suggest that elevated CO₂, nitrogen deposition, and changes in regional rainfall patterns also play a role. Atmospheric studies indicate that the terrestrial sink varies significantly from year to year. Current estimates of regional distributions of carbon sources and sinks derived from atmospheric and oceanic data differ from forest inventory and terrestrial ecosystem model estimates. The Carbon Cycle Science Program has created a structure for coordinating observational, experimental, analytical, and data management activities needed to address the discrepancies, to reduce the errors, and produce a consistent result for North America in a North American Carbon Program (NACP). Assuming corresponding international research projects in Europe and Asia, this research will contribute to improving estimates of quantities, locations, and uncertainties of the Northern Hemisphere carbon sink.

Continued and enhanced NACP research will require multidisciplinary investigation of atmospheric concentrations, vertical profiles, and transport of CO₂ and CH₄; micrometeorological estimates of net CO₂ and CH₄ fluxes with accompanying biometric measurements at ecosystem and landscape scales; biomass and soil inventories of carbon in forests, crop and range lands, and unmanaged ecosystems; coastal zone carbon processes; and carbon modeling to integrate and assimilate diverse sources of data. A field program, with intensive campaigns and remote sensing of productivity and land cover, will be conducted initially at a central location in the United States, and subsequently expanded to include the entire continent. Research on ecosystem and ocean margin processes that control carbon exchange, including experimental work, will be needed to explain changes in sources and sinks and to parameterize models. Improved ecosystem, inverse, and data assimilation modeling approaches will be needed to analyze carbon source and sink dynamics.

New data and models will provide enhanced capability for estimating the future capacity of carbon sinks, which will guide full carbon accounting on regional and continental scales. These results are a prerequisite for planning, implementing, and monitoring carbon sequestration practices in North America. Decisionmakers will receive a series of increasingly comprehensive and accurate reports about the status and trends of carbon emissions and sequestration in North America for use in policy formulation and resource management.

The ocean plays a significant role in the global carbon cycle. Globally, the ocean's net uptake of carbon is estimated to be approximately 2 billion metric tons of carbon per year. However, uncertainties remain in this estimate due to regional variations in ocean uptake, seasonal to interannual variation in nutrient supply, and inadequate representation of coastal margins in models. The discovery that iron is a limiting nutrient for major regions of the world's ocean has profound implications for understanding controls on ocean carbon uptake, as well as for evaluating carbon management options. Estimates of regional ocean sinks can now be used in combination with atmospheric data to constrain estimates of terrestrial carbon sinks. Near-term focus will be on the North Atlantic, North Pacific, and Southern Oceans to provide independent constraints on estimates of the Northern Hemisphere carbon sink.

The Carbon Cycle Science Program will need to continue and enhance ocean observations (in situ and remotely-sensed) to track the fate of carbon in the ocean, characterize fluxes of CO₂ from the land and atmosphere to the ocean over large space and time scales, and to achieve process-level understanding of the physical and biological controls on those fluxes now and in the future. The program will generate data required to support the development and implementation of models linking climate, ocean circulation, and ocean carbon biogeochemistry to assess more accurately the relationship of carbon sources and sinks to global and climatic change. Focused process studies in the North Atlantic, North Pacific, and along the margins of those basins, including inputs from rivers, are needed in the next several years to permit quantification of the Northern Hemisphere carbon sink and to develop needed understanding of the mechanisms and magnitudes of carbon exchange between land, sea, and air. In 5-10 years, an intensive Southern Ocean carbon program will be needed to resolve uncertainties in the size, dynamics, and global significance of the Southern Ocean as a carbon sink as well as the processes controlling this sink.

This research will quantify the capacity of the oceans to absorb fossil fuel CO₂ and remove carbon from the Earth's dynamic reservoirs through export to the deep sea. Uncertainties in the size of the global oceanic carbon sink will be reduced. Information will be provided on the effects of deliberate carbon management approaches for the ocean.

A major advance in the past decade has been the ability, enabled by new techniques for atmospheric measurement, to distinguish the roles of the ocean and land in the uptake and storage of atmospheric carbon. Inverse modeling approaches are beginning to allow continental-scale resolution of sources and sinks, but with significant uncertainties. Key processes dominating uptake and release of carbon can vary in different regions of the world, and can change in response to changes in natural and human forcings. New remote sensing observations have engendered a new appreciation for the significant spatial and temporal variability of primary productivity in Earth's ecosystems. There is a growing realization that the carbon cycle must be studied as an integrated Earth system carbon cycle.

Sustained investments will be needed in the collection, reporting, analysis, and integration of relevant global carbon monitoring and inventory data; in our understanding of carbon cycling processes; and in the development of coupled, interactive carbon-climate and, ultimately, Earth system models. New in situ and space-based observational capabilities will be needed. Process studies must focus on characterizing key controls as they vary around the world and on explaining changes in the growth rates of atmospheric CO₂ and CH₄. Improving models will require development of innovative new assimilation and modeling techniques and rigorous testing, evaluation, and periodic intercomparison. The carbon cycle science program will collaborate with all CCSP research elements to assemble, merge, and analyze carbon, biogeochemical, physical, and socioeconomic information for comprehensive reporting on the state of the global carbon cycle. An ongoing dialogue with stakeholders will be essential to ensure that the carbon cycle information provided will be useful. Continued international cooperation will be necessary to achieve results and ensure widespread utility.

Policymakers and resource managers will be provided with consistent, integrated, and quantitative information on global carbon sources and sinks that can be used in national and worldwide carbon accounting and for evaluating carbon management activities. Improved global carbon models and understanding of key process controls on carbon uptake and emissions, including regional variations, will be made available to improve applied climate models and inform scenario development for decision support.

Historic and current land use changes and resource management practices impact the overall carbon cycle. For example, there has been widespread reforestation since 1900 in the eastern United States following the movement of agricultural production toward the Midwest. Forest growth and conversion of forests to long-lived wood products increase the carbon stored in the forest products pool. Better land management practices (e.g., reduced soil tillage in cropping systems), increased agricultural productivity, and conversion from cropland to grassland can increase carbon storage in soil. However, changes in land use and management, such as clearing forests and grasslands and intensive tillage and harvest practices, release CO₂ to the atmosphere. Research in this area will require collaboration with the Land Use and Land Cover Change research element to document global patterns of land use and land cover and to understand changes in them, along with land management practices, as powerful drivers of terrestrial carbon sinks and sources. This information highlights an urgent need for improved understanding of the processes of land use change and the impacts of environmental and resource management decisions.

Maintenance and enhancement of the data collection and synthesis capabilities of national networks of long-term experimental sites in forests, rangelands, wetlands, agricultural lands, and other ecosystems is needed to provide an essential foundation of ecosystem monitoring data. US Carbon Cycle research will collaborate closely with operational resource management and inventory programs to ensure the availability of these needed long-term observations of ecological processes, environmental changes and impacts, and treatment effects. Continued monitoring of carbon storage and fluxes (in soil, litter, vegetation, forest products, and woody debris) and their response to various land use changes and resource management practices will be required to accurately quantify the role of land cover and use change in the global carbon cycle. Continued satellite land cover data products and new remote sensing estimates of aboveground biomass are needed. Process studies linked with observations and long-term manipulative experiments will be required to identify cause-and-effect relationships. Models are needed to link ecosystem, management, policy, and socioeconomic factors to better project future changes in both carbon storage and flux and land use and development.

Quantifying past and current effects of land use change and resource management on the carbon cycle will enable policymakers and resource managers to predict how current activities will affect the carbon cycle at multiple scales and to develop alternative policies and practices to mitigate the continued buildup of atmospheric carbon (e.g., carbon sequestration through agricultural management practices).

Accurate projections of future atmospheric CO₂ and CH₄ levels are critically needed to calculate radiative forcings in models that project changes in climate and their impact on the sustainability of natural resources and human populations. Changes in the size or intensity of terrestrial and marine carbon sinks directly affect the amount of carbon emissions that remain in the atmosphere, and, thus, must be projected as well. There are several different types of carbon models available, but most lack complete integration of all components, interactive coupling, and/or full validation. While no one of these models is ideal, as a group they are becoming quite useful for exploring global change scenarios and bounding potential future CO₂ conditions and responses of ecosystems. Current models are less useful for projecting future CH₄ conditions. Modeling of future carbon conditions will require collaboration with the Human Contributions and Responses and Atmospheric Composition (for CH₄) research elements and rely on scenarios requested by decisionmakers and provided by the Scenario Development element.

Research under this topic area will focus on incorporating improved process understanding into carbon cycle models, developing new generations of terrestrial and ocean carbon exchange models, and developing Earth system models with a dynamic coupling between carbon cycle processes and the climate system. In particular, improved models must address managed as well as natural ecosystems and incorporate the effects of multiple, interacting factors and human influences. Advances in the future will be made through a combination of observations, manipulative experiments, and synthesis via models enabled by increases in computational capabilities. Collaboration with the Ecosystems research element will be essential.

New understanding of the controls on carbon cycle processes will be provided to improve parameterizations and/or mechanistic portrayals in climate models. Projections of future atmospheric concentrations of CO₂ and CH₄ will be made available for use in applied climate models. Both will aid in improving model projections of future climate change and its effects on the Earth system.

Questions about the effectiveness of carbon sequestration, the longevity of storage, the practicality of reducing emissions, technological options, resultant impacts on natural and human systems, and the overall economic viability of carbon management approaches create an imperative for better scientific information to inform decisionmaking to manage carbon. Presently, there is limited scientific information to support carbon management strategies, and little is known about the long-term efficacy of new management practices for enhancing carbon sequestration or reducing emissions or how they will affect components of the Earth system. This element links to the National Climate Change Technology Initiative (NCCTI), which focuses on engineered technologies, carbon offsets, and economic systems.

Research to analyze the effects on terrestrial and marine systems and to scientifically assess the short- and long-term efficacy of carbon management practices is needed. Field studies, manipulative experiments, and model investigations will be needed to evaluate the effectiveness of designed management approaches to manipulate carbon in the ocean, land, and atmosphere, and to assess their impacts on natural and human systems. New monitoring techniques and strategies to measure the efficacy of carbon management activities will also be needed. Experiments and process studies will also be needed to evaluate the likelihood of unintended environmental consequences resulting from enhanced carbon sequestration. Research on the scientific underpinning for carbon management draws upon products from carbon cycle research questions 1-5, and will coordinate with the Ecosystems research element and the NCCTI as well as public and private programs responsible for developing and/or implementing carbon management. Two types of models are required: those that incorporate understanding of basic processes into evaluation of natural and enhanced mechanisms of carbon sequestration, and those that assess the economics of carbon management options in the agricultural and forestry sectors. Research is also needed to support assessments of carbon management and sequestration potentials, decisionmaking processes that involve multiple land management scenarios, and the role of sequestration mechanisms for calculating net carbon emissions intensity.

This research will provide the scientific foundation to inform decisions and strategies for managing carbon stocks and enhancing carbon sinks in terrestrial and oceanic systems. Firm quantitative estimates of key carbon cycle properties (e.g., rate, magnitude, and longevity) will provide fundamental information for projecting carbon sequestration capacity, for calculating net emissions, and for full carbon accounting.

US carbon cycle science will be conducted in cooperation with all the other Climate Change Research Initiative (CCRI) and US Global Change Research Program (USGCRP) research elements as well as other research, operational, infrastructure, and technology development programs. Cooperation with programs that provide national computational infrastructure and data management systems will be essential. Collaboration with the Land Use/Land Cover Change research element (Chapter 9) for Carbon Cycle question 4 will be especially critical. The enhanced observational networks needed to address Carbon Cycle questions 1-3 will need to be planned in close coordination with the Climate Quality Observations, Monitoring, and Data Management element (Chapter 3). Addressing Carbon Cycle question 6 will require scientific studies conducted in close cooperation with the NCCTI and public and private projects that develop and implement management approaches to sequester carbon or reduce emissions. Linkages to Ecosystems (Chapter 10), Water Cycle (Chapter 7), Applied Climate Modeling (Chapter 4), Atmospheric Composition (Chapter 5), Human Contributions and Responses (Chapter 11), Climate Variability and Change (Chapter 6), and Scenario Development (Chapter 4) research elements will also be important.

International cooperation will be necessary to coordinate global observational networks, integrate scientific results from around the world, and ensure widespread utility of the State of the Carbon Cycle Report and model projections. Partnerships are anticipated with Integrated Global Observing Strategy (IGOS) Partners and the global observing systems. Interactions with and contributions to the Global Carbon Project of the International Geosphere-Biosphere Programme, the International Human Dimensions Programme, and the World Climate Research Programme will be important. US carbon cycle research will contribute to bilateral activities being developed by the administration.