Chapter 3: Predicting the Future Carbon Cycle spatially explicit gradients of carbon flux at regional scales. This effort will require combining satellite and ground-based observation of land use patterns and trends. flux measurements,and process-based modeling. Together, information from all these sources will allow the scientific assessment of management scenarios to increase sequestration of carbon on the land. The scientific evaluation of management options requires the direct involvement of social scientists in constructing land use histories. Thus, there is a permeable boundary between this science plan and the relevant social sciences. Careful assessments are needed of the social and economic costs and benefits of proposed carbon sequestration policies. One critical element will be the development of credible land use scenarios for the future. Another important component of carbon management strategies will be the ability to verify commitments. Other sections of this report discuss the estimation of oceanic and terrestrial sinks. An important additional issue is the estimation of fossil fuel emissions. If the global estimate of fossil fuel emissions based on economic accounting methods is off by 10 percent,that would represent an error of 0.65 Gt C/yr-which is more than the reductions envisioned in the Kyoto Protocol. This error is quite large when trying to balance the global carbon budget with oceanic and terrestrial sources and sinks. The financial and economic stakes in correct carbon accounting are of great importance for any country in a world where emis sions permits can be traded. Some measurement strategy is needed, independent from statistical and accounting methods,to determine the magnitude of fossil fuel CO2 emissions on regional and national scales. The only nearly unequivocal (characteristic and stable) tracer for fossil fuel CO2 in the atmosphere is its complete lack of 14C isotopes. These considerations lead to another major near-term goal of the CCSP: Goal 5: Develop a scientific basis to evaluate potential management strategies for enhancing car bon sequestration in the environment and for capture/ disposal strategies. Chapter 4: An Integrated Carbon Cycle The previous two chapters of this report summarized our present understanding of the global carbon cycle and the basic questions that confront present-day efforts to better understand it. This chapter presents an integrated plan to achieve the major goals that have been identified: Near-Term Goals of the U.S. Carbon Cycle The research program outlined here will ultimately be judged by its ability to provide practical answers to both scientific and societal questions. Scientists and policy makers must be able to evaluate 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. Thus,a key moti vation for further research is to develop the predictive capability to define responses of the global carbon cycle to change,as reflected in Goals 4 and 5. Recent assessments of global environmental research have emphasized the need for programs that are both integrated and focused (e.g..National Research Council 1998). This plan puts forth a program that focuses on key problems, yet maintains breadth to reveal new problems and priorities in those areas where the knowledge needed to define focused strategies is currently lacking. In Chapter 6,this report also proposes a management structure for implementation of the research plan and the development of critical partnerships to ensure continuous reassessment and prioritization of goals. Much recent progress in knowledge of the carbon cycle has resulted from studies at the global scale for time periods of years to decades. However, to make significant progress in understanding and quantifying the critical mechanisms that will determine future levels of atmospheric CO2, data must also be obtained for specific geographic regions over a range of time scales. The fingerprints of dominant processes are to be found by studying regional carbon balances and temporal variability. Efforts to address both intermediate spatial scales and longer time scales are thus essential components of the pro posed plan. A U.S. Carbon Cycle Science Plan The program will also study the main processes influ encing 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, evaluate and communicate uncertainties in alter native model simulations,and make these simulations available for public scrutiny and use. Clearly, the systemat ic incorporation of newly understood mechanisms in models must be accompanied by model integration using high quality standard inputs and rigorous consistency tests against an array of benchmark data. Data management and data set construction are sometimes underrepresented in hypothesis-oriented programs. This pitfall must be avoided because, ultimately, it weakens the ability to test hypotheses using comprehensive data and to develop powerful generalizations and new hypotheses. At its most basic level,the global carbon cycle must be viewed as a singular entity. Its various components are so interactive--over so many different scales of time and space-that they cannot conveniently be "isolated" for independent study or modeling. Data are most valuable when combined from a variety of measurements and methods associated with different carbon-cycle components; for example, when oceanic data are applied to help interpret results for the atmosphere and terrestrial bios phere, and vice versa. The present plan, then,proposes three different types of general approach: • Extend observations over the important space and time scales of variability in all active carbon reservoirs • Develop manipulative experiments to probe key mechanisms and their interactions Integrate these data, analysis, and modeling approaches so that they are mutually supportive and can be focused on key problems. The observational strategy described in this chapter is designed to combine atmospheric measurements with observations from space, air, land, and sea to reveal specific processes that affect terrestrial and oceanic carbon exchange at regional as well as global scales. While the goal of the terrestrial component of this strategy is ultimately to understand the Northern Hemisphere terrestrial sink, the continent of North America is an excellent focus for the U.S. research community in developing this research objec tive. North American logistical capabilities are excellent and cost-effective, and there are extensive existing data sets for North American ecosystems, land use, soils,industrial activities, and history. Recent research has pointed to the particular importance of understanding terrestrial carbon exchange in the Northern Hemisphere, and North America's large geopolitical units facilitate the development of an inte grated continental analysis of carbon sources and sinks, Parallel research by European and Asian colleagues will be encouraged to expand the coverage into other parts of the Northern Hemisphere terrestrial biosphere. Similarly, the northern oceans are relatively accessible, and a solid foundation of oceanic data and knowledge is available to support integration of studies of North American atmospheric CO2 exchange with studies of CO2 exchange in the oceanic regions adjacent to it. These foci offer a unique opportunity to combine atmospheric, oceanic,and terrestrial studies in a way that will constrain major components of the global CO2 budget. Goal 1: Understanding the Northern Hemisphere Terrestrial Carbon Sink One principal goal of the CCSP is to establish accurate estimates of the magnitude of the potential Northern Hemisphere terrestrial carbon sink and the biophysical mechanisms that regulate this sink. Several major activities should be conducted in this area: • An expanded program of atmospheric concentration measurements and modeling improvements in support of inverse calculations and global biogeochemical models • A network of integrated terrestrial research sites with eddy-covariance flux measurements and associated process studies, manipulation experiments, and models, which as a whole are sufficient to reduce uncertainty about the current and future carbon cycle to acceptable limits. The two boxes here summarize the specific proposed program elements, which are discussed in detail in the sections that follow. Goal 1a (continued) Chapter 4: An Integrated Carbon Cycle Science Program Atmospheric Measurements and Modeling for Inverse Calculations Atmospheric transport of CO2 integrates the effects of local sources and sinks, with mixing around a latitude cir cle in a few weeks, and between hemispheres in about a year. A primary test of Northern Land Sink hypotheses requires resolving the longitudinal structure of the surface CO2 flux,in addition to the variation in latitude. Clearly, the observations will have to define concentration gradients between the continents and the sea and between the planetary boundary layer and the middle and upper troposphere. These are inherently difficult measurements, because atmospheric mixing is so much more rapid around latitude circles than across them,and measurements near sources or sinks are highly variable. Design of the necessary atmospheric sampling program requires careful attention to the spatial and temporal distribution of sampling, the precision of the atmospheric data,and the details of some boundary-layer atmospheric processes that are not well understood at present. The atmospheric observing system currently consists of roughly 100 sites around the world at which air is collect ed weekly in paired flasks for trace gas analysis at central laboratories,and few sites where observations are made continuously. The sites are intentionally located in remote marine locations to avoid local “contamination" by industrial or terrestrial emissions or uptake,and are operated at low cost using cooperative arrangements with volunteers. There are very few data acquired at altitude and at midcontinental stations. To provide meaningful constraints on net terrestrial CO2 exchange on the regional scale, the observing net work will need to be strengthened considerably to characterize spatial and temporal variations associated with the carbon fluxes that need to be measured. The present network is designed to be insensitive to regional net exchanges. A recent evaluation of 10 global tracer transport models used for CO2 inversions (Denning et al.1999) found that the models converged when compared to the observed values for an inert tracer (SF) at flask stations in the remote marine boundary layer. However, they diverged where the data are sparse (aloft and over the continents). This problem is even worse for CO2 due to the covariance between diurnal and seasonal cycles of CO2 net exchange and rates of atmospheric mixing,often called the "rectifier effect." Expanding the atmospheric observing network to include routine sampling aloft, particularly over the continents,should be one of the highest priorities for the future. Atmospheric sampling over the terrestrial surface must include vertical profiles through a depth sufficient to capture most of the vertical mixing of the surface signal. At a minimum, this vertical sampling must span the depth of the planetary boundary layer (1 to 3 km in warm sunny weather). A primary research goal should be to determine first the optimal sampling density, A U.S. Carbon Cycle Science Plan supporting measurements, and combination of continuous versus flask samples, for long-term airborne sampling. Multiple species, such as tracers of industrial activity, and isotopic ratios must also be measured to obtain the infor mation needed to interpret the observations (e.g., Potosnak et al.1999). The atmospheric boundary layer and the covariance between terrestrial ecosystem processes and near-surface turbulence are not resolved in most of the current generation of global atmospheric models. Most atmospheric tracer transport codes used for CO2 inversion calculations represent subgrid scale vertical transport very crudely, if at all. Likewise, very few of these models include a diur nal cycle of CO2 exchange. However, even if a model could correctly represent the local covariance structure of the fluxes and the turbu lence, the influence of the rectifier effect on the observed concentration field at remote flask stations depends on the persistence of the vertical gradient as the air is transported horizontally for hundreds or thousands of kilometers. This process is very poorly resolved in even the most detailed global models, and is not well understood theoretically. A major effort in understanding the local forcing,spatial scaling, and long distance transport aspects of the rectifier effect is required, through both observations and models. Testing the Northern Land Sink hypothesis also requires filling the gap at the crucial "middle scale" of the flux-transport-concentration problem. This middle scale between local and large-scale observations is completely missing from the current observing system and models. The detailed design of the required atmospheric sam pling program, which must be coordinated with design of the terrestrial flux network and ocean measurements proposed below, is a major scientific endeavor beyond the scope of this document. However some general requirements are clear. A strategy must be developed for atmo spheric sampling over continental regions which takes into account the differences in ecosystem exchanges in stable and convective conditions. This variation must be explored over a range of ecosystems and meteorological regimes by sampling from eddy flux towers,tall towers, balloons,and light aircraft. Continuous long-term meas. urements of the vertical profile of CO2 and other trace gases on tall transmission towers allows "representative" conditions to be defined for the planetary boundary layer (PBL) sampling at the local scale (Bakwin et al. 1995, Bakwin et al.1998,Hurst et al.1997). Similar information can be obtained from continuous long-term measurements of CO, and other trace gases in conjunction with eddycorrelation fluxes defining rates of exchange between the surface layer and the planetary boundary layer (Potosnak et al.1999). Light aircraft can be instrumented with continuous analyzers to determine the boundary layer budgets of trace gases over areas orders of magnitude larger than the footprint of an eddy flux tower (Desjardins et al. 1997, Goulden et al.1998). These types of studies directly address the issues of scalability of tower fluxes, and can be used to design lower cost, routine sampling programs for larger scales. Independent estimates of regional-scale carbon fluxes by inversion of atmospheric data will require a dense network of samples collected by light aircraft. Light aircraft sampling must be dense enough to capture meaningful gra dients in surface fluxes, and must sample both within and above the convective boundary layer. Vertical profiles from light aircraft over a continental region could be coupled with high-altitude transects sampled from appropriately instrumented commercial aircraft (Marenco et al. 1998). Both inversion calculations and forward models will benefit tremendously from additional constraints such as regionally detailed emissions data and multiple tracers. Samples should be analyzed for CO2, as well as CO, CH4, O2/N2, and stable isotopic ratios,all of which provide con straints on the carbon cycle. Ancillary data such as PBL structure (from wind profiling and traditional sounding systems), atmospheric transport (from four dimensional data assimilation systems.4DDA), and the isotopic ratios of other components of the land atmosphere system (plants, soils.precipitation,and groundwater) are needed. Such a system has been proposed using automated sampling equipment and rental aircraft (Tans et al. 1996). Regional observing and modeling programs have been proposed in other countries on a "campaign"basis.and the results of these studies can provide useful constraints on regional flux estimates using inverse modeling. Regional experiments quantifying carbon fluxes or tracer concentrations are currently underway or planned for the near future in Europe,Siberia, Brazil,and Australia. The design and implementation of U.S.observing systems and modeling programs should be optimized to take advantage of these complementary programs. Ideally, inverse calculations of the carbon budget should subsume all available information,including flask samples,in situ data.aircraft sampling,air-sea flux meas urements and eddy covariance data.Carbon budgets calcu lated from inverse methods should not, for example.be inconsistent with measured diurnal cycles of CO2 data collected by regional sampling programs in other parts of the world. The current global observing system is so poorly constrained in the tropics, for example, that tropi cal fluxes are freely estimated in inversion models as a residual, allowing unacceptable freedom of terrestrial fluxes in higher latitudes without violating global mass bal ance.Inclusion of new regional data from experiments in Amazonia in these inverse models would provide stronger constraints on the carbon budget of North America, directly addressing uncertainties in the Northern Hemisphere Land Carbon Sink hypothesis. |