AN ACCURATE KNOWLEDGE OF THE GLOBAL CARBON CYCLE HAS BECOME POLICY IMPERATIVE FOR THIS AND THE FORTHCOMING DECADES, BOTH GLOBALLY AND FOR INDIVIDUAL COUNTRIES. INCREASING ATMOSPHERIC CO2 CONCENTRATION DUE TO HUMAN ACTIVITY IS AN IMPORTANT CAUSATIVE FACTOR FOR POTENTIAL CLIMATE VARIABILITY AND CHANGE. This recognition has placed the global carbon cycle at the forefront of policy debates and scientific studies. The priority being given to an improved understanding of the carbon cycle is very likely to continue into the foreseeable future. For example, the Kyoto Protocol recognizes the role of terrestrial systems as carbon sinks and sources, and it provides a basis for developing future emission trading arrangements that involve forests and potentially other ecosystems. Understanding of the pathways through which the anthropogenic CO2 leaves the atmosphere and enters into ecosystems, thus offsetting a portion of the human-caused emissions is incomplete, at best. For example, about 15-30% of the anthropogenic carbon emissions cannot presently be accounted for (decadal average; annual uncertainties are higher), and are presumed to be absorbed by terrestrial ecosystems. A more accurate knowledge of the sequestration of the carbon emissions is therefore critical to implementing effective carbon-related policies. Of equal importance is the ability to predict the evolution of the atmospheric CO2 concentration in order to optimise mitigation strategies.
In addition to the environmental policy dimension, the distribution and quantification of terrestrial carbon (i.e. carbon in the vegetation or soils) has important economic and resource management implications. For example, the yield of agricultural, forest, and rangeland resources is directly related to the amount of carbon tied up in the aboveground biomass. Thus, improved information on the distribution and changes in carbon content and better understanding of the terrestrial carbon cycle will contribute to more effective use and management of agricultural, forest, and rangeland resources.
The global carbon cycle connects the three major components of the earth system: the atmosphere, oceans, and land. In each domain, large pools of readily exchangeable carbon are stored in various compartments (pools or stocks). Large amounts of carbon (fluxes) are transferred between the pools over various time periods (daily, annual, decadal, etc.). Although some of the fluxes are very large, the net change over a given time period need not be. For many centuries prior to the industrial revolution the carbon pools were more or less in equilibrium, and the net transfer was close to zero over sufficiently large areas or, in case of smaller areas, over sufficiently long periods of time. The major change occurred following industrialization, with the accelerated transfer from the geological pool (fossil fuels) to the atmosphere. Because of the connections among pools, the increased atmospheric carbon concentration affects the other connected pools in oceans and on land. The processes governing the fluxes between the pools take place at various speeds, from daily to centennial and longer. These factors and dependencies make the quantification and study of the carbon cycle very challenging, and they need to be taken into consideration when designing a strategy for a carbon observing system.
The environmental, economic and societal importance of the perturbation of the carbon cycle led to numerous activities at national and international levels. It has become widely appreciated that appropriate responses to this issue require better understanding of the current and future evolution of the atmospheric, land and ocean pools, supported by accurate observations of the magnitudes and trends of the fluxes. Recognizing these factors, IGOS-P responded to a proposal to consider the establishment of systematic terrestrial carbon observation which was submitted to the November 1999 meeting by GTOS. IGOS-P decisions were:
Action 4/5 GTOS with FAO support to lead the Terrestrial Carbon Cycle theme and to present a report to the Partners along the lines of the Oceans theme report.
Action 4/6 GCOS, FAO, IGBP, ICSU, UNESCO, and CEOS to nominate representatives for the Terrestrial Carbon Cycle team by the end of November 1999.
Action 4/9 GOOS, GCOS, GTOS, IGBP, NASA to prepare proposals for the overarching Global Carbon Theme and to decide amongst themselves who should lead this activity.
In response to Action 4/5 and 4/6, a Terrestrial Carbon Theme Team representing various IGOS Partners has been established and produced this report. The report describes the terrestrial and the associated atmospheric components of the global carbon cycle. It incorporates inputs from several science workshops focused on this topic: GTOS-IGBP Terrestrial Carbon Observation Synthesis Workshop in Ottawa, Canada (8-11 February; Cihlar, Denning and Gosz 2000); the EU-IGBP-GTOS Terrestrial Carbon Meeting in Costa da Caparica, Portugal (22-26 May 2000) and the EU-IGBP-IHDP-WCRP international workshop on carbon cycle research in Durham, New Hampshire (16-20 October 2000), in addition to the Team members contributions. Other GTOS and GCOS documents were also used in preparing this report. In parallel, a report on ocean carbon observation is being prepared for IGOS-P.
Action 4/9 led to the proposal for an Integrated Global Carbon Observation (IGCO) theme (Steffen, 2000). IGCO is designed as an overarching theme which will integrate terrestrial, atmospheric and ocean components of the global C cycle as well as ensure linkages to socioeconomic and other relevant issues. The integration of the terrestrial and oceanic reports will be subject of future efforts as part of IGOS-P planning.
Consistently with IGOS-P aims, this report focuses on systematic, long-term global observation requirements and thus does not present all the observations needed about the carbon cycle; the above source reports present a more complete picture in this respect. In addition, the report focuses on CO2 as the most important gas from the perspective of climate change, with the major exception of methane (p.20). In the future, it may be possible to address other gases relevant to the carbon cycle in a more systematic manner.
The main sections of the report are: motivation for systematic terrestrial carbon observation (Chapter 2); existing observing activities and capabilities (Chapter 3); major, urgent continuity and consistency issues requiring immediate attention (Chapter 4, p.12); important long-term issues (p.20); data/products considerations (Chapter 5); and an outline of the initial observing system (Chapter 6). Appendix 1 highlights important users of information on terrestrial carbon. Appendix 2 contains information on the relevant current observation programmes and activities.