other elements of the CCSP. To succeed, responsible agencies will need to develop a managerial framework with a unified vision of the program and with greatly enhanced mutual collaboration and strategic planning.

Goal 5: The Entire CCSP (all elements, integrated and coordinated)

Goal 5-developing the scientific basis for evaluating management decisions relating to CO2 in many critical ways represents the culmination of the entire Carbon Cycle Science Plan. The ultimate measure of a successful carbon cycle research program will be found in its ability to provide practical answers to both scientific and societal questions.

Implementation Principles

Past experience with large-scale, multidisciplinary global change research programs (e.g. the TOGA Program) has demonstrated that a coherent,integrated approach to program implementation is essential for optimal execution and delivery of products designed to serve societal needs. The principles for successful implementation of the CCSP program, discussed in detail in Chapter 6 of the report,are: • A scientific vision shared by the broad community and by the participating agencies to develop consistency and focus.

• Shared programmatic responsibility to insure coherence.coordination,and strategic pursuit of program goals.

• Program integration to bring together interdisciplinary aspects of the strategy.

• Scientific guidance and review to provide feedback and support to program managers, to help foster innovation and creativity, and to create an environment where program elements and goals evolve as new knowledge is obtained.

• Links to international programs to maximize the benefits from efforts in all countries.

• Access to data and communication of research results to insure timely communication of knowledge to the general public and to enhance the scientific utility of new knowledge.

With these principles in mind,the Working Group has recommended in Chapter 6 specific steps to establish a collaborative management structure for the Carbon Cycle Science Program, with strong interagency commitment to joint implementation of the program,including common development of requests for proposals and coordination review and funding activities. The program is built around a tripartite,collaborative management structure for

integrated carbon cycle research consisting of a scientific steering committee (SSC),interagency working group (IWG),and Carbon Cycle Science Program office. The intent is to strengthen relevant federal agency activities by improved coordination,integration,coherence,prioritization, focus, and adherence to conceptual goals. The pro posed structure will provide the basis for issuing interagency research announcements to stimulate a broad range of important new research, with agreed-upon goals,using a coordinated interagency merit review process of propos als and overall agency programs. The importance of fostering partnerships among Federal laboratories, the extramural research community, and the private sector is stressed, as is the need to communicate effectively the evolving understanding of carbon sources and sinks to the public and policy makers.

Initial Funding Priorities

Most of the program elements outlined above serve more than one of the major long-term and five-year goals. Again, the program requires a coordinated, integrated approach by the responsible agencies: the value of an integrated program will greatly exceed the return from uncoordinated program elements.

The individual program elements described in this plan, however, are not at equal stages of maturity and readiness. Some program elements represent intellectually-ready work that has been constrained by limited resources in the past and could begin immediately with a near-term infusion of funding. New technology enterprises, for example, have suffered disproportionately in the recent past due to such resource constraints. Since many of the program ele ments in the CCSP call for focused technology development prior to large-scale implementation, we recommend that a high priority be placed on funding those technology development efforts that have been deferred due to prior insufficient funding. More specifically, the following program elements should be considered as high priorities for initial funding.

• Both facility and technology development for the enhanced flux network,airborne sampling,and automated and streamlined ocean sampling for long time-series and underway measurements

• Airborne CO2 monitoring programs, both dispersed weekly measurements and in support of regional studies

• An expanded and enhanced surface monitoring network for atmospheric CO2

• Improved forest inventories (with carbon measure ments a key focus and an explicit goal) and development of new techniques for remote sensing of aboveground biomass

Rationale

Future concentrations of atmospheric carbon dioxide (CO) must be known to characterize and predict the behavior of the Earth's climate system on decadal to centennial time scales. Predicting these future CO2 concen trations in turn requires understanding how the global carbon cycle has functioned in the past and how it functions today. For these reasons, a variety of federal agencies have funded scientific research into the oceanic, atmospheric,and terrestrial components of the global car. bon cycle. This research has been an important element of the U.S.Global Change Research Program (USGCRP) since its inception. The carbon cycle consists of an inte grated set of processes affecting closely coupled carbon reservoirs in the atmosphere and ocean and on land. Successful predictions require considering all the impor tant processes that affect these reservoirs. Programs must therefore support a well-integrated approach,with links fostered among atmospheric,oceanic,terrestrial,and human sciences.

Recently, interest in the global carbon cycle has intensified. Progress in scientific understanding and technology has given rise to new scientific opportunities to address critical components of the atmosphere-ocean-land system. Policy makers have also been seeking the appropriate responses to the United Nations Framework Convention on Climate Change and to the underlying societal and scientific concerns. There is particular impetus to understand the sources and sinks of carbon on continental and regional scales and the development of these sources and sinks over time. Also critical is elucidating how ocean carbon uptake and marine and terrestrial ecosystems might respond to changing climate and ocean circulation, or to the enhanced availability of CO2 and fixed nitrogen compounds associated with human activities such as the burning of fossil fuels. Changes in ecosystems may affect climate and may have important resource and societal impli cations. Another topic of increasing interest is the purposeful sequestration,or storage, of carbon by burial below ground or in the ocean, or by land management practices,to keep some amount of carbon from entering the atmosphere in the form of CO2.

The ultimate measure of the success of the carbon cycle research program will be its ability to provide pragmatic answers to both scientific and societal questions. Scientists and policy makers must be able to evalu ate alternative scenarios for future emissions from fossil fuels.effects of human land use,sequestration by carbon sinks, and responses of carbon cycling to potential climate change.

This document outlines a U.S.Carbon Cycle Science Plan (CCSP) with the view that credible predictions of future atmospheric carbon dioxide levels can be made given realistic emission and climate scenarios that incorporate relevant interactions and feedbacks. The problems of understanding the carbon cycle and improving related predictive capabilities are complex. These problems require coordinated interdisciplinary research that is scientifically rigorous and that at the same time advances knowledge and serves societal needs. Achieving all this together is clearly a challenge. The present plan aims at specifying an optimal mix of sustained observations,modeling, and innovative process studies and manipulative experiments such that the whole is greater than the sum of parts. This strategy for an inte grated CCSP has two basic thrusts:

• Developing a small number of new research initiatives that are feasible,cost-effective, and compelling, to address difficult,linked scientific problems

• Strengthening the broad research agendas of the agen cies through better coordination, focus,conceptual and strategic framework, and articulation of goals.

In sum, the rationale for a CCSP is severalfold. Future concentrations of atmospheric CO2 must be projected to characterize and predict the behavior of the Earth's climate system on decadal to centennial time scales. Moreover, the scientific community is in a good position to make important progress in this area. But making such progress will require an unprecedented level of coordina tion among the scientists and government agencies that support this research.

Outline of the Research Program

The CCSP is designed to address two basic questions: 1. What has happened to the CO2 that has already been emitted through human activities (anthropogenic car. bon dioxide)?

2. What will be the future atmospheric carbon dioxide concentrations resulting from both past and future emissions?

The first of these questions concerns the history and the present behavior of the carbon cycle. Information about the past and present provides us with the most powerful clues for understanding the behavior of anthro pogenic CO2 and the processes that control it,as well as its sensitivity to perturbations. Achieving a better

understanding of these phenomena will require a sustained observational effort.

The second question directly concerns the goal of predictability. The CCSP will study the essential processes that influence how carbon cycling may change in the future. These studies will be integrated in a rigorous and comprehensive effort to build and test models of carbon cycle change,to evaluate and communicate uncertainties in alternative model simulations, and to make these simulations available for public scrutiny and application.

The long-term goals and implementation objectives that flow from the two questions above are shown in Table 1.1 and reviewed in the sections that follow below on sus tained observations,manipulative experiments,and model development.

In addition to the physical, biogeochemical, and ecolog ical processes that are traditionally studied in carbon cycle research,the integrated CCSP must address human influences as well, with special emphasis on understanding the consequences of land use and land cover changes;the

Rifle Question

What has happened to the carbon dioxide already emitted by human activities (anthropogenic CO2) including that emitted through combustion of fossil fuels, deforestation, and agriculture?

What will be the future atmospheric carbon dioxide concentrations resulting from both past and future emissions?

effects of various management strategies such as no-til agriculture, long- vs short-rotation forestry, and deep ocean CO2 injection; and the effectiveness of response/mitigation options such as controlling carbon emissions or enhancing carbon sinks. The CCSP addresses the interactions between human systems and the carbon cycle in much the same way that it does interactions between the climate tem and the carbon cycle. While the program does not encompass either broad socioeconomic research or di mate research, it does consider problems of the interac tions of human systems and climate with the carbon cycle.

Sustained Observations

A program of sustained observations of sufficient spatial and temporal resolution is essential for determining interannual variability and long-term trends in terrestrial and oceanic carbon sources and sinks,both globally and regionally. Such a program is also required to monitor the effectiveness and stability of any purposeful carbon sequestration activities,as well as to detect major shifts in

the functioning of the carbon system that might lead to major changes in atmospheric CO2.

Data on global and regional variability and trends in CO2 concentrations provide us with the most valuable information on the response of the global carbon cycle to climate changes and to processes such as forest regrowth or fertilization owing to increased CO2 and nitrogen oxides (NO) from fossil fuels. Monitoring programs should take advantage of the recent insights from inverse models to combine limited direct observations with model estimates to determine the spatial and temporal distribution of carbon sources and sinks. The program of observations must also include a strong focus on under standing critical processes that determine the long-term sequestration of anthropogenic CO2, as well as its interannual variability.

The Mauna Loa and South Pole CO2 time series of C. D. Keeling provided the first unambiguous evidence of increasing atmospheric CO2. Time series data for CO2, combined with tracers, constituents of the atmosphere that have a specific origin or change in an understood way, such as 14CO2. 13CO2, and O2, provide strong quantitative constraints which help confirm the factors regulating the global balance of carbon. Expanded atmospheric observa tion networks,including airborne measurements and a growing number of flux towers,which measure the amount of CO2 going into or out of a certain area of land over a period of time, have enabled scientists to begin defining the regional distribution of carbon sources and sinks. This work is still rudimentary, but already indicates great potential for assessing source and sink distributions at smaller (e.g., regional) scales. A major hypothesis issuing from the past decade of research is that there exists a large terrestrial sink for anthropogenic CO2 in the Northern Hemisphere. Much of the near-term observational work proposed in this plan aims to test this hypothesis.

A critical task is to improve the modeling and statistical tools needed to infer sources and sinks and to otherwise interpret these observations. Additional oceanic and terrestrial observations must also be defined to complement global monitoring data so that better temporal and spatial resolution is achieved. Such observations should improve understanding of seasonal and year-to-year variability, and they should monitor regions identified as significant sources and/or sinks.

Three areas in particular-areas that the scientific community is positioned to address-require near-term emphasis:

• Establishing accurate estimates of the magnitude and partitioning of the current Northern Hemisphere terrestrial carbon sink.

Manipulative Experiments

Manipulative experiments play a unique role in global change research. They allow direct study of many key ecosystem processes that have strong control over the carbon cycle. These experiments can contribute to carbon cycle research in at least three ways. First,global changes in the coming decades will create a range of novel conditions,some of which will very likely be far enough outside the envelope of current and past conditions that observational data alone cannot provide a sufficient basis for credible modeling. Experiments will be especially important in assessing responses to multiple,interacting changes, and in helping to assess slowly developing responses,such as changes in biodiversity. Second,model development and testing solely against observations of current patterns cannot provide the rigor and level of credibility that comes from validation against carefully designed experiments. And third, explaining and illustrating the future trajectory of the carbon cycle can be substantially enhanced through experimental manipulations. Even if the scientific community believes the models, the illustrative value of a solid experiment to the broader public is invaluable.

For manipulative experiments to yield large payoffs, they will need to be tightly integrated with observations and models. Targeted work must address key uncertainties. Interpreting results from manipulative expertments has presented major challenges in the past,especially those relating to the experiments'small spatial scale,short temporal scale, and incomplete coverage of relevant ecosystem processes. Interpretations will certainly continue to be difficult, but these difficulties can be managed through careful selection of study systems,precise definition of key questions,and strong emphasis on integrating the experiments with one another and with other compo nents of carbon cycle research.

Experiments can play an essential role in reducing uncertainties about the location of and the mechanisms underlying current terrestrial and oceanic sinks. Critical targets for experimental work include evaluating the