The Goal of the GAIM Task Force is to advance the study of the coupled dynamics of the Earth system using both observational data and numerical models. The challenge to GAIM is to initiate activities that will lead to the rapid development and application of a suite of Global Prognostic Biogeochemical Models. These Global Biogeochemical Models include the terrestrial, oceanic, and atmospheric subsystems and would subsequently be linked to General Circulation Models, thereby contributing to the development of Earth system models. In order to make progress toward its goal, the Task Force must analyze current models and data, interpret the capability of current models and experimental programs against the demands for knowledge, and advance and synthesize our understanding of global biogeochemical cycles and their links to the hydrological cycle and more generally to the physical-climate system as a whole. Much of the progress to date in modeling specific components within the global biogeochemical subsystem sets the context for modeling activities within the various IGBP Core Projects. GAIM is structured as an IGBP Task Force rather than a Core Project because of the specific and cross-cutting nature of its mission. As such, it responds to the needs of Core Project modeling activities and aides in guiding the focus and structure of these activities in the interest of subsequent model coupling.
Much of the progress to date in modelling specific components within the global biogeochemical subsystem sets the context for modelling activities within the various IGBP Core Projects. GAIM recognizes, supports, and will benefit from these efforts. The GAIM activity is by definition cross-cutting; therefore, the activities of GAIM intersect fundamentally with all the Core Projects.
Along with water and oxygen, four elements--carbon, nitrogen, sulfur, and phosphorus--are of particular interest in the study of our planet's environment. Each of these four elements cycle through increasing molecular energy levels as the elements are incorporated into living tissue, and then decreasing energy levels as the organic matter is returned to an inorganic state. The varied dynamic patterns reflected in various states of these cycles are the consequences of a myriad of biological, chemical, and physical processes that operate across a wide spectrum of time and space scales. In the absence of significant disturbances, these processes define a natural cycle for each element with approximate balances in sources and sinks that result in a quasi-steady state for the cycle, at least on time-scales less than a millennium.
Since ancient times humans have modified natural systems. Since the beginning of the Industrial Revolution, human activity has altered significantly biogeochemical cycling at the planetary scale. The magnitude of human disturbance to biogeochemical cycles may be approaching a critical level: the values of important state variables, such as the concentration of atmospheric CO2 and CH4 are moving into a range without historical precedent.
During the nearly 40 years between 1957 and 1996, the global pool of carbon in the atmosphere (in the form of CO2) has increased from 670 to almost 775 Pg (1015 g) C as a result of fossil fuel burning and forest clearing. The annual rate of increase in CO2 is currently about 3 Pg (equivalent to roughly 0.4 percent); during the 1970s methane was increasing by 1 percent per year but this rate has decreased substantially in recent years. Neither the causes of the increase nor the change in rate are well-understood. The picture for methane is complicated by its diverse and dispersed sources, by the complexity of its photochemical sink, and by its poorly understood biological sink in upland soils.
In contrast, we know from the ice core records that the concentration of both carbon dioxide and methane was relatively constant for several thousand years prior to 1700 and that since 1700 the concentrations of CO2 and CH4 have increased by more than 25% and 100%, respectively. The anthropogenic increase in the atmospheric CO2 and CH4 concentrations as well as other greenhouse gases has produced serious concern regarding the energy balance of the global atmosphere.
Human activities have had similar impacts on the nitrogen cycle. For example, the pool size of atmospheric N2O, which is about 1500 Tg (1012 g) N, is increasing by 0.2 to 0.3 percent per year. A large portion of this increase may be due to biomass burning over the past forty years. There have also been large increases in the application of nitrogen fertilizer and in the discharge of sewage. Much of the N in fertilizer and sewage is reaching aquatic systems such as ground water, wetlands, rivers, estuaries, and the coastal ocean. Estimates of the N2O generated as a result of this eutrophication show that total releases have increased by 50 percent over the past 50 years.
There have been substantial changes in the global biogeochemical sulfur cycle as well. Human activity that has increased the present total global flux of sulfur to the atmosphere from a pre industrial level of about 228 Tg S yr-1 to a current flux of approximately 340 Tg S y-1, a 50% increase. Indirect calculations suggest that emissions of gaseous sulfur to the atmosphere from fossil fuel combustion are already on the same order of magnitude as discharges from natural systems. Some of the largest changes have occurred over continental regions where present anthropogenic emissions account for 70% of the total sulfur released to the atmosphere. In industrial areas, sulfur dioxide (SO2) is the major sulfur compound emitted. Sulfur dioxide is rapidly hydrolyzed to sulfuric acid (H2SO4) which is then deposited back to terrestrial and aquatic ecosystems in the form of acid rain. Another possible consequence of changing sulfur emissions is the hypothesized change in the concentration cloud condensation nuclei with possible significant impacts on cloud optical properties and hence a feedback on the physical-climate system.
The biogeochemical system, thus, is not only internally coupled it is also fully intertwined with the planet's physical-climate system. The climate obviously is a strong determinant of vegetation distribution and growth, on litter decomposition and trace gas production, on ocean circulation and marine production, and on atmospheric transport and chemical change. The coupling is, however, not a one-way street. Vegetation strongly controls the exchange of water and energy between the land surface and the atmosphere; there is evidence that marine primary production controls, in part, the depth of the mixed layer in the ocean, and as previously mentioned, many biologically produced gases influence the planet's energy balance through the greenhouse effect. We know from ice-core records that these gases have varied substantially in concentration on glacial and interglacial time scales, and the behavior of the paleo-environmental records provides supporting evidence to indicate that some of them are directly involved in the regulation of natural climate changes.
Understanding the nature of the link between the biogeochemical cycles and the physical-climate system represents a fundamental goal of the International Geosphere-Biosphere Programme: A Study of Global Change (IGBP). This understanding bears directly on key scientific questions concerning the co-evolution of different components of the Earth system including life, as well as on the most pressing environmental questions of our time.
Though coupled, the Earth system can be conceptually partially "decoupled" as a means of studying aspects of the important subsystems. This decoupling is not without strong scientific precedent nor completely devoid of risk. It is without question valuable. Specifically, the Earth system can be viewed as being composed of two interacting subsystems, the Physical-Climate Subsystem and the Biogeochemical Subsystem, linked together by the global hydrological cycle and by subsystem state variables such as greenhouse gas concentrations, surface roughness, and albedo. By exploiting the conceptual decomposition of the Earth system into coupled subsystems, a coordinated attack upon the central problems of Global Change can be formulated. Central to this attack is to realize, test, evaluate, and apply a suite of models and their associated data sets which would be comparable in predictive ability to the current models of the Global Physical-Climate Subsystem, or General Circulation Models (GCMs).
General Circulation Models exist at a variety of institutions around the world, whereas prognostic Global Biogeochemical Models are at a relatively primitive stage. The challenge to GAIM is to initiate activities that will lead to the rapid development and application of a suite of Global Biogeochemical Models. These Global Biogeochemical Models would, in time, be linked, partly through hydrological coupling, to General Circulation Models, thereby, providing models of the Earth system.
In its initial five years, GAIM began to address this challenge. Much of the progress to date in modelling specific components within the global biogeochemical subsystem sets the context for modelling activities within the various IGBP Core Projects. GAIM recognizes, supports, and will benefit from these efforts. The GAIM activity is by definition cross-cutting; therefore, the activities of GAIM intersect fundamentally with all the Core Projects. The birth of the GAIM Task Force was at an opportune time. During the last decade, there has been considerable progress in the development of biogeochemical models for significant components of the Earth System. Significant improvement has been made in process-based models to predict ecosystem metabolism in a variety of terrestrial systems, and the scientific community has begun to extend these models to global scales. Ocean carbon cycle models were developed by incorporating carbon chemistry and crude biological concepts in ocean general circulation models; large drainage basin models were developed with biogeochemical aspects for several areas around the world establishing the capability to extend this work to global scales, and finally, initial steps were taken to begin to link these component models with atmospheric GCMs. It was and is a crucial time; the foundation for substantive progress has been laid by pioneering groups of scientists in several countries, and as a consequence, the GAIM Task Force has had the opportunity to further catalyze model development.
GAIM has encouraged and enhanced the development of key subsystem models and by sponsoring the development of key databases for model calibration and testing. The former activity was coordinated with the relevant IGBP Core Projects and the latter with the IGBP Framework activity on Data and Information Systems (DIS). The three aspects of GAIM (Analysis, Interpretation, Modelling) were developed to varying degrees in the initial stages of GAIM. The Analysis Program consisted initially of a series of short workshops focused upon open scientific issues that limited progress on developing models and deepening our understanding of global biogeochemical cycles and how these cycles and the associated key subsystems may change in response to climate change. The Interpretation Program is a later phase of GAIM, and will be built upon the results of the analysis and modelling activities of GAIM as well as of the IGBP Core Projects. This program will focus upon clarifying specific scientific issues such as those as identified by the IPCC process, the Kyoto Protocol, and additional future links with the Policy sector.
A large focus of initial GAIM activities have centered around model development and intercomparison. The conceptual structure of the initial phase of GAIM modelling activities could be described as a matrix with biogeochemical and time scale axes. The biogeochemical axis consisted of CO2, other Trace Gases, and Climate-Vegetation Interactions. Each of these could be analyzed on various time frames including "paleo" (200 Kyr BP), Historical (200 yr BP), and Contemporary (last 20 years). The Contemporary time frame is one in which there is global remotely sensed data available, providing a comprehensive view of the Earth's atmosphere, oceans and land surface and their changes in real time. The Historical time frame includes the bulk of the period of atmospheric emissions since the industrial revolution. The Paleo time frame is important because many of the anticipated future changes in the Earth system may be of magnitudes unparalleled in recent times, but observable in proxy records of conditions in the geologic past. While the uniformitarianist maxim, "The present is the key to the past" may not apply to the anthropogenically perturbed present day, it may be true that the past may serve as the background matrix to the future, or, "The past is the key to the future."
During the next five years, GAIM will begin its efforts toward biogeochemical subsystem model integration, as mandated by the Scientific Committee of the IGBP in March, 1998. This will entail working with the various IGBP Core Projects to ensure that subsystem models are developed in a way that facilitates model coupling by matching boundary conditions and fluxes, temporal and spatial resolution, and establishing common numerical protocols. In addition, we will expand upon the existing model and data inter-comparison efforts as a basis for model coupling.
The GAIM Task Force began operating in 1993 with the launching of focused modelling projects directed toward understanding various aspects of the global carbon cycle. These were discrete projects which whose purpose was in part to develop the capabilities within GAIM which would be necessary for future integrative activities. As such, the first five years represented a "segmented research" phase of GAIM in preparation for the next five years, which will focus on integration of IGBP research.
The focused modelling and other activities of GAIM during its initial stage were directed toward developing the capability to couple subsystem models as they emerged. In addition, critical knowledge gaps and subsystem linkages were identified and addressed. As the IGBP Core Projects mature, subsystem models are becoming available in robust form, allowing for meaningful coupling and the development of integrated Earth system models. Each of the Core projects will be entering a synthesis phase over the next few years, the results of which should provide conceptual insights necessary for integration across IGBP. While the Core Projects are seeking answers to questions which span their respective subsystems, GAIM will be seeking answers to questions which span the entire Earth system. As such it will rely heavily and work closely with the Core Projects during this phase.
Likewise, now is an opportune time to take stock of the various results generated by GAIM activities in its initial five years. The following sections summarize the results of each of various GAIM activities. Full reports of each activity are being published in the GAIM Report Series (Appendix 1). These activities were conducted for a variety of reasons such as development of model intercomparison techniques, filling cross-disciplinary gaps between Core Project foci, and developing model linkage and coupling techniques. While these are not necessarily related to each other, they all serve to set the stage for synthesis of IGBP science in the coming years.