Executive Summary

Rationale

Carbon on earth is stored primarily in rocks and sediments. Only a tiny fraction resides in mobile reservoirs (the atmosphere,oceans,and soil and terrestrial biosphere) and is thus available to play a role in biological, physical and chemical processes at the earth's surface. The small fraction of carbon present in the atmosphere as carbon dioxide (CO2) is especially important: it is essential for photosynthesis, and its abundance is a major regulator of the climate of the planet.

The major gases,nitrogen, oxygen,argon, which comprise over 98% of the atmosphere,are transparent to far. infrared (heat) radiation. Trace gases such as carbon dioxide, water vapor, nitrous oxide and methane absorb heat radiation from the surface, warming the atmosphere and radiating heat back to the surface. This process, called the greenhouse effect, is a natural phenomenon without which the Earth's surface would be 30°C cooler than it is at present.

Throughout the climate extremes of the past 400,000 years, during which there were four major glacial cycles, atmospheric CO2 concentrations varied by no more than twenty percent from a mean of about 240 parts per milbion (ppm). Concentrations of CO2 and methane are now higher by more than 30% and 250%, respectively, than the previous maxima. This very rapid increase has occurred since the industrial revolution, raising concerns that the temperature of the atmosphere may rise as a consequence of an increase in the greenhouse effect.

The rapid increase in atmospheric concentrations of CO2 over the past 150 years, reaching current concentrations of about 370 ppm, corresponds with combustion of fossil fuels since the beginning of the industrial age. Conversion of forested land to agricultural use has also redistributed car bon from plants and soils to the atmosphere. There has been growing concern in recent years that these high levels of carbon dioxide not only may lead to changes in the earth's climate system but may also alter ecological balances through physiological effects on vegetation.

Only about half of the CO, released into the atmosphere by human activity ("anthropogenic "CO2 from combustion of fossil and biomass fuels and from land use changes) currently resides in the atmosphere. Over the last 10-20 years,more than half of the CO2 released by burning fossil fuels has been absorbed on land and in the oceans. These uptake and storage processes are called "sinks" for CO2, although the period over which the car. bon will be sequestered is unclear. The efficiency of

global sinks has been observed to change from year to year and decade to decade,due to a variety of mechanisms,only partly understood.

The understanding of carbon sources and sinks has advanced enormously in the last decade. There is now clear evidence that global uptake of anthropogenic CO2 occurs by both land plants and by the ocean. The magni tude of the oceanic sink, previously inferred from models and observations of chemical tracers such as oceanic radiocarbon and tritium distributions, has recently been con firmed by direct observation of the increase in dissolved inorganic carbon. Analysis of new tracers such as chloroflu orocarbons provide further refinements to our understanding of carbon uptake by the oceans. The importance of the sink due to the terrestrial biosphere has emerged from analysis of the global carbon budget,including improved estimates of the ocean carbon uptake, as well as data on 13CO2/12CO2 isotopic ratios and from changes in the abundance of O2 relative to N2. Isotopes can give information on the terrestrial sink, for example, since plants preferentially select certain isotopes during photosynthesis and leave a global signature in the isotopic ratio of carbon dioxide in the atmosphere. Forest inventories and remote sensing of vegetation appear to confirm a significant land sink in the Northern Hemisphere and provide insight into the underly ing mechanisms. However, we cannot yet quantitatively define the global effects of human activities such as agricul ture and forestry or the influence of climate variations such as El Niño. Studies to determine these effects have emerged as critical for understanding long-term changes in atmospheric concentration in the past, and will help to dra matically enhance understanding of how the earth's climate will evolve in the future.

The Carbon Cycle Science Plan (CCSP) presented in this document has several fundamental motivations. First, it is clear that the oceans and land ecosystems have responded in measurable ways to the atmospheric increase in carbon dioxide, but the associated mechanisms are still not well quantified. Second, the land and ocean sinks and sources appear to fluctuate naturally a great deal over time and space,and will likely continue to vary in ways that are still unknown. Third,to predict the behavior of Earth's climate system in the future, we must be able to understand the functioning of the carbon sys tem and predict the evolution of atmospheric CO2. Finally, scientific progress over the past decade has enabled a new level of integrated understanding that is directly relevant to critical societal questions associated with the economic and environmental effects of forestry. agriculture,land use and energy use practices.

The development of the CCSP has been strongly influ enced by the view that carbon cycle science requires an unprecedented coordination among scientists and supporting government agencies. The nature of the problem demands it. Carbon dioxide is exchanged among three major active reservoirs, the ocean,land,and atmosphere, and through a variety of physical, chemical, and biological mechanisms, including both living and inanimate components. Research on atmospheric CO2 therefore encompasses the full Earth system and involves many different research disciplines and approaches.

Consequently, a large number of government agencies and programs are involved in supporting research on the carbon cycle,including data-gathering, fleld research, analysis,and modeling. Thus it is clear that there is extraordinary value to be gained by coordinating research and encouraging disciplinary and organizational crossfertilization through effective program Integration. In addition,a recent report evaluating research programs in global environmental change by the National Research Council highlighted the importance of developing a coordinated, focused,scientific strategy for conducting carbon cycle research (NRC 19981).

The Carbon Cycle Science Plan presents a strategy for a program to deliver credible predictions of future atmos pheric carbon dioxide levels, given realistic emission and climate scenarios, by means of approaches that can incorporate relevant interactions and feedbacks of the carbon cycle-climate system. The program will yield better understanding of past changes in CO2 and will strengthen the scientific foundation for management decisions in numer. ous areas of great public interest.

The intent of the CCSP is to develop a strategic and optimal mix of essential components, which include sustained observations, modeling, and innovative process studies,coordinated to make the whole greater than the sum of its parts. The design of the CCSP calls for coordinated, rigorous, interdisciplinary scientific research that is strategically prioritized to address societal needs. The planned activities must not only enhance understanding of the car. bon cycle, but also improve capabilities to anticipate future conditions and to make informed management decisions.

Basic Strategy

There are two components to the CCSP strategy:

• Developing a small number of potent new research initiatives that are feasible,cost-effective,and compelling, to improve understanding of carbon dynamics in each carbon reservoir and of carbon interactions among the reservoirs.

In the very broadest terms, the present plan addresses two fundamental scientific questions:

• What has happened to the carbon dioxide that has already been emitted by human activities (past anthropogenic CO2)?

• What will be the future atmospheric CO2 concentration trajectory resulting from both past and future emissions?

The first of these questions deals with the past and present behavior of the carbon cycle. Information about this behavior provides the most powerful clues for understanding the disposition of carbon released as a result of human activities, the underlying processes in this disposi tion,and their sensitivity to perturbations. Improved understanding will require targeted historical studies, the development of a sustained and coordinated observational effort of the atmosphere,land and sea,and associated analysis,synthesis,and modeling.

The second question focuses directly on the goal of predicting future concentrations of atmospheric CO2. The study of essential processes in the carbon cycle will be Integrated with a rigorous and comprehensive effort to build and test models of carbon cycle change,to evaluate and communicate uncertainties in alternative model simu lations, and to make these simulations available for public scrutiny and application. The research program outlined in the CCSP will depend on parallel initiatives in human dimensions programs to fully achieve its goals,especially that of predicting future atmospheric CO2 concentrations.

Current estimates of terrestrial sequestration and oceanic uptake of CO2 vary significantly depending on the data and analytical approach used (e.g.,the Forest Inventory Analysis,direct flux measurements in major ecosystems,inverse model analysis of CO2 data from surface stations, changes over time of global CO2, O2, 13CO2/12CO2, measurement based estimates of the air-sea CO2 flux and dissolved inorganic carbon inventory, and ocean and terrestrial vegetation model simulations). The proposed program is designed to reconcile these estimates to a precision adequate for policy decisions by delivering new types of data;applying new, stringent tests for models and assessments;and providing quantitative understanding of the factors that control sequestration of CO2 in the ocean and on land. A sound basis for policy debates and decisions will be provided by the program as fully implemented.