Objective:

To understand the causes of inferred changes in atmospheric trace gas and aerosol content and composition for the past 200,000 years.

Rationale:

One of the most outstanding challenges facing the science of global change is to understand the forcing, regulation and climate feedbacks of the natural variations in the chemical composition of the atmosphere. Variations of CO₂, N2O, CH4 and other reactive trace gases, marine biogenic S compounds, and mineral dust, need to be understood on decadal to multi-millennial time scales before the consequences of recent and future anthropogenic perturbations of atmospheric chemistry can be fully understood. Data from ice cores are continuing to yield new discoveries, particularly with regard to rapid changes in atmospheric composition. Some examples are 1) the remarkably rapid response of atmospheric CH4 to climate changes on the Dansgaard-Oeschger time scale; 2) the apparently considerably slower response of atmospheric CO₂ and its continuing rise during the Younger Dryas cold period; 3) enormous temporal variability in mineral aerosol fluxes to high latitudes during glacial periods; and 4) linked Holocene variations (± 20 ppmv) in atmospheric CO₂ and 13CO₂ that suggest mechanisms involving changes in the terrestrial biosphere. It could be argued that without a predictive understanding of the dramatic changes in atmospheric composition that have taken place during the past 200,000 years, we have little hope of foreseeing the long-term consequences of human activities that are now so greatly altering the atmosphere.

Approach:

This project will mount a concerted attack on the paleo trace gas and aerosol problem, using a combination of ice-core atmospheric composition data (the primary "target" for explanation), state-of-the-art models, and the best available global paleodata sets to constrain the models. It is a joint initiative of GAIM, PAGES and IGAC and will build on the more disciplinary achievements of the various IGBP core projects, and on the focused activities carried out during the first phase of GAIM, to attain the major integrative goals of IGBP. Our approach will involve both data and models:

• Modelling- Efforts are underway to build ever more comprehensive models of the Earth system, including biogeochemical as well as physical components and including a range of approaches, from full-resolution, three-dimensional coupled model frameworks (such as CSM and ECHAM) to reduced-form models (such as CLIMBER) that can execute multi-millennial simulations of the coupled atmosphere-ocean-biosphere system and its response to continuous forcing. The full three-dimensional approach is essential for "snapshot" analysis of key time periods, where it alone can speak to such issues as the regional consequences of changes in atmospheric and ocean circulation patterns, or the effects of changing spatial locations of sources on reactive gas chemistry and aerosol distribution. The intermediate-complexity approach is also now yielding a rich harvest of new insights: for example the recent simulation of the abrupt collapse of the "green Sahara" and the associated enhanced African monsoon during the 6000-5000 yr. BP interval, which was shown to depend on synergistic mechanisms involving the atmosphere, ocean and land.

• Data- A rapidly growing body of paleodata offers an extremely rich source of constraints for Earth system models. The relevant paleodata include not only the records from ice-cores themselves, but also the spatially distributed records of terrestrial and marine conditions that are being assembled in globally consistent form by projects such as the BIOME 6000, PMIP, and Epilog. These projects have focused so far on selected time slices (for comparison with the snapshot simulation approach), but data bases are now being developed and technologies being put in place to allow multitemporal sampling of terrestrial and marine proxy records under the aegis of IGBP activities such as PMAP and IMAGES. Top

Objectives:

To quantify the contribution of simulated tracer transport, inversion methodology, and choice of data to the overall uncertainty associated with regional source/sink estimates produced by inversion calculations

Identify the mechanisms responsible for the inversion differences

Recommend/prioritize improvements to models and the existing carbon observing system to refine constraints on simulated inversions

Rationale:

A key component in the projection of future global change is the ability to predict future concentrations of atmospheric greenhouse gases such as carbon dioxide (CO₂). Unfortunately, the current state of the science cannot completely account for the growth rate and interannual variations of atmospheric CO₂ with confidence, so accurate prediction of future concentrations is difficult.

Approach:

TransCom 3 will focus on a model comparison of CO₂ sources and sinks derived from inversion of the modeled CO₂ concentrations at observed locations. This will allow transport modellers to directly quantify the contribution of simulated tracer transport to the overall uncertainty associated with regional carbon source/sink estimates produced by inversion calculations. There will be three levels to the TransCom 3 experiment. The first level will be an annual mean inversion, the second will include seasonality (using 12 one month pulses), and the third will incorporate different inversion methods into a comparison. Details can be found at: http://transcom.colostate.edu. A series of experiments in which leading chemical tracer transport models from around the world will be used to calculate the global carbon budget of the atmosphere.

There will be 22 basis regions (11 terrestrial/11 oceanic) for the inversions with the addition of seasonal sea ice maps to provide spatial structure to the polar oceanic fluxes. Pre-subtractions will include fossil fuel emissions, a neutral biosphere (data to be provided by monthly NEP from the satellite-driven CASA model), and ocean carbon fluxes. In addition, a terrestrial NPP map from the CASA model will provide spatial structure of terrestrial carbon exchanges. There will be no seasonality to the spatial pattern. There will be one forward run of SF6 for each of the 11 terrestrial basis regions. An emissions map with time variation will be supplied.

Input and output datsets will be formatted with NetCDF. Model output will be submitted on standard pressure levels. Modelers will report 3D fields for u, v, w (plus the 3D concentration fields). In addition, the submitted concentration fields for those grid cells with monitoring stations will be subsampled such that smaller files can be generated for alternative analysis by others. Modelers will report high-frequency concentration and wind components at a chosen set of locations when conditional subsampling to match the sampling decisions at the monitoring locations can then be performed.

TransCom inversion results should be ready for comparison and discussion by January 1, 2000. Top

To evaluate comprehensive, geographically explicit model of the global carbon cycle in an integrative way by exploring the model predictions with large- and global-scale observational constraints.

Rationale:

In many respects the global carbon cycle constitutes a paradigm for GAIM:

It is an interdisciplinary field cross-cutting several IGBP core project activities.

It is strongly coupled to the physical climate system.

The main processes controlling the storage and cycling of carbon are known to a degree that allows the development of global prognostic models.

A comprehensive model of the carbon cycle requires components of the carbon systems in the atmosphere, the ocean and on land. A fundamental question concerns the accurate quantification of the fate of the anthropogenic CO₂ in the global carbon cycle - where is it being stored and for how long? The global carbon cycle is also strongly coupled to the physical climate system, but the various feedback mechanisms between the two systems are still only poorly understood. In order to investigate, quantify and ultimately predict the response of the global carbon cycle to natural and anthropogenic perturbations, global models are now being developed.

Approach - Phase 1 (1993-1998)
During phase 1, the initiative had an emphasis on the evaluation of terrestrial biogeochemical carbon cycle models ("TBMs"). Seven different terrestrial models, 1 atmospheric transport model and 1 ocean carbon cycle model were used in the simulation experiments in various combinations. The simulation experiments included:

1. Simulation of the seasonal cycle of atmospheric CO₂,
2. Response of the TBMs to the observed historical rise in atmospheric CO₂ (1765-1990)
3. Response of the TBMs to observed historical climate fluctuations (1900-1995).
4. Response of the TBMs to historical changes in land-use in combination of (2) and (3).
5. Uptake and exchange of radiocarbon in the terrestrial carbon cycle.
6. Simulations of the 13C/12C ratio in atmospheric CO₂ (discrimination, Suess effect, seasonal cycle).
7. Response experiments to artificial impulse perturbations (e.g. step changes in temperature).

Approach - Phase 2 (1999-2001):

A second phase of CCMLP is currently being initiated. Key foci of phase 2 will include:

A comprehensive assessment of the simulated present-day carbon cycle. This will include evaluation experiments similar to those conducted during phase 1 but with more advanced carbon cycle simulation models. In addition, detailed model evaluations against new observational data (e.g. from the flux measurement networks, and from regional and global atmospheric inverse modelling studies).

Simulations and evaluations of coupled carbon cycle - climate models. These experiments will allow an assessment of the climatic feedbacks on the terrestrial carbon cycle in a more rigorous way as compared to presently conducted off-line experiments with prescribed climate.

A comprehensive assessment of the human impacts on the terrestrial carbon cycle. These include changes in land use, food production, fire management and nitrogen fertilization.

Similar to CCMLP phase 1, the new project will include only a limited number of participating groups and models, and will be conducted as a pilot project. This mode of operation allows for a much more comprehensive model evaluation and intercomparison than would be possible in a general model intercomparison framework. The present plan includes 5 participating groups, which all are running terrestrial biogeochemical models that are able to significantly contribute to the main goals of the project. The products to be developed within the project, however, e.g. the experiment protocols, the input forcing data sets and the observational data for the model evaluation, will be made available to the scientific community at large.

A more detailed workplan is currently being designed. A schedule of activities will be available in spring 1999, while a kickoff meeting and initial simulation experiments are being envisaged in summer/fall of 1999. Top

Objective:

To develop Earth System models of intermediate complexity which will enable modelers to explore the couplings and interactions between subsystems (e.g. atmosphere, ocean, land) using modest computers and rapid run times.

Rationale:

Investigating the dynamic behavior of the total Earth System remains a "grand challenge" for the scientific community. It is motivated by our limited knowledge about the consequences of large?scale perturbations of the Earth System by human activities, such as fossil?fuel combustion or the fragmentation of terrestrial vegetation cover. Will the system be resilient with respect to such disturbances, or will it be driven towards qualitatively new modes of planetary operation?

Earth System analysis primarily relies on a hierarchy of simulation models. Depending on the nature of questions asked and the pertinent time scales, there are, at the one extreme, zero?dimensional tutorial or conceptual models like those in the "Daisyworld" family. At the other extreme, there are three?dimensional comprehensive models (such as in the "Great Leap" experiment, this issue), which couple atmospheric and oceanic circulation with explicit geography and high spatiotemporal resolution. The former, simple heuristic models can be perceived as hypotheses?generating machines that allow one to explore major geosphere?biosphere feedbacks such as insolation?albedo?vegetation dynamics. The latter, comprehensive models are capable of zooming into brief time slices of Earth history, and will ultimately be used for biogeochemical prognosis. Hence, there is a need for "Earth?System Models of Intermediate Complexity" (EMIC's) which address two fundamental requirements: first, being simple enough to permit numerical integration over many millennia and, second, being complex enough to yield a somewhat realistic picture of the Earth System by the inclusion of more interactions than are achieved in the comprehensive models.

Approach:

This activity will be initiated with a kickoff workshop (Potsdam, Germany) which aims at bringing together scientists in order to review the present range of EMICs. In this project, we do not intend to conduct a Ômodel competition', but rather to exchange ideas about the various approaches and their basic differences. The key theme of the initial stages of the project will be: "How far can we reduce a comprehensive Earth?System model without losing the crucial feedbacks that govern the overall systems dynamics? " The answer will certainly depend on the spatiotemporal scales under consideration. In order to be specific, we will focus on two? and three?dimensional global models spanning time windows of several thousand years and longer. Top

To project future climate change in response to a specified scenario of fossil fuel emission and land use as well as the biogeochemical feedbacks between climate, land cover, and human activity. The effort will entail development of the a fully coupled biogeochemical Earth system model.

Rationale:

This project has been dubbed a conceptual "Great Leap" toward GAIM's ultimate goal of promoting the development of a suite of prognostic biogeochemical Earth System models to emphasize the uncertainties and excitement of the endeavor. It was determined at the November 1998 GAIM meeting that it would be instructive to begin the development of fully coupled models, even though the subsystem models themselves may not yet be completely robust. Interactions and feedbacks revealed by the coupling process may help guide refinements and improvements of the models so that subsequent efforts at fully coupled models will be more meaningful. The project will be parallel to the WCRP/WGCM Coupled Model Intercomparison Project (CMIP) wherein coupled atmosphere-ocean circulation models are forced by a specified rate (1%/yr) of CO₂ increase in the atmosphere. To carry out the " Great Leap" experiment, terrestrial and oceanic carbon modules need to be integrated into the coupled GCMs. Carbon uptake would estimated on-line, and would vary with the instantaneous climate. The radiation code in the atmospheric GCM would "see" the atmospheric CO₂ levels that increase with the airborne fraction of fossil fuel CO₂, computed as the "residual" after the land and oceans have absorbed a portion of the fossil fuel CO₂.

Approach:

To start, CO₂ release from fossil fuel combustion would be specified as a global value (PgC/yr.) as a function of time based on a scenario that would have given a 1%/yr. increase in the absence of climate feedbacks on the carbon uptake. The terrestrial and oceanic modules would be geographically referenced to take account the differential ecosystem/circulation effects on the carbon exchange. The terrestrial and oceanic uptake would be summed over area to yield annual values (PgC/yr.) such that

dCatm/dt = FF(t)-S[land uptake(x,y,t] - S[ocean uptake(x,y,t)]

where FF(t) is the rate of fossil fuel emissions. Carbon uptake by the biosphere and oceans would respond to the instantaneously simulated climate. In this way, carbon-climate interactions are included to determine the rate of CO₂ increase and consequently the rate of climate warming.

The "Great Leap" experiment is timely. Several GCM groups have already incorporated the oceanic and terrestrial carbon cycles. Others have tested the carbon cycle models using output from the GCM simulations. There are clearly many intermediate steps that must be made. On the terrestrial side, these are focused on the consistency and compatibility between the treatment of water and CO₂ exchange through stomates, and between the treatment soil water in the GCM and terrestrial carbon models. The experience of climate drift in coupled atmosphere-ocean GCMs warns us of potential similar excursions of the carbon-climate system which may result because the terrestrial and oceanic modules may not be capable of simulating the contemporary carbon budget.

It has been suggested that a first intercomparison of the carbon-climate experiment should focus on the interannual variations in the last 20 years (the AMIP period). The interannual CO₂ variations of the last 20 years are not large enough to have an impact on interannual climate variations. Interannual variations in CO₂ can be investigated using observed and/or off-line climate information. Coupling to the GCM is not necessary. Consequently, we encourage the participating groups to carry out the interannual experiment to test the implementation of the carbon models in the GCMs. The "Great Leap" would focus on the future, when carbon-climate interactions are large in both directions.

Future climate change will involve land-cover changes in response to human action and climate feedbacks. Increases in methane, dust and other trace constituents in the atmosphere will also influence the rate of climate change. These are not included in the experiment design at this stage. To keep the "Great Leap" experiment simple and interpretable, we have chosen to specify only the evolution of fossil fuel emission. In this way, we deliberately highlight the sensitivity of CO₂ processes to climate feedback.

One should treat the "Great Leap" as a grand challenge to our understanding of the carbon cycle as well as of carbon-climate interactions. It should be the stimulus to take biogeochemical and climate models to another level of sophistication and compatibility. Top

Objective:

To develop a complete picture of the global carbon cycle through synthesis of our understanding of carbon budgets and reservoirs in each part of the Earth's biogeochemical systems. This "grand synthesis" project involves all parts of IGBP and is being driven by the SC-IGBP. While GAIM will play a key role in the synthesis, all Core Projects will be fully involved.

Rationale:

A number of factors, most notably the Kyoto Protocol, are focusing the attention of both the policy and scientific communities on the global carbon cycle. For example, the IPCC has been asked by SBSTA (Subsidiary Body for Scientific and Technical Advice to the Framework Convention on Climate Change) to produce, in addition to the Third Assessment Report, a special report on terrestrial carbon sinks, which will include an overview of the carbon cycle. In addition, a number of national initiatives to develop an overall, integrated understanding of the carbon cycle are being proposed.

Approach:

The project will built around an initial set of three dedicated workshops and three other activities which are directly relevant. These are outlined the table below. The project will begin by producing a "fast-track" overview paper as a contribution to the IPCC process. It will also lay the foundation for a broader, more in-depth IGBP synthesis of the carbon cycle, to be undertaken over a 12-18 month time frame. As the project's activities proceed, emphasis will shift from a strong focus on the synthesis towards putting in place, based on the new understanding coming out of the synthesis, a more coordinated international framework for research on the global carbon cycle.

The IGBP Synthesis Project over the next 2-3 years provides an ideal opportunity for IGBP to undertake an initial synthesis of its work on the carbon cycle. The framework for the synthesis could be based on (i) a Ôbox and arrow' diagram of the global carbon cycle showing key compartments and fluxes, and (ii) an overarching suite of questions and a set of more focused questions. In addition to a review of past and present IGBP and related research on the global carbon cycle, the synthesis effort may include some additional short-term activities specifically aimed at contributing to the synthesis: -Sensitivity analyses based on current models to determine the critical parameters in the system and any critical thresholds or nonlinearities. -Incorporation of experimentally based new understanding of components of the carbon cycle (arising from the Core Projects) into a global carbon modelling framework. Test the implications of new assumptions and new findings through model sensitivity runs. -Possible development of an IGBP Ôcommunity' global carbon cycle model, based on work within IGBP for wide use in global change studies (e.g., in developing and testing «what if' questions and scenarios). This could either be de novo or based on the modification of an existing global carbon cycle model.

These tasks could be tackled by a two-tiered structure within the IGBP Carbon Synthesis Project:

A GAIM-led group which develops the community model and which undertakes the sensitivity studies and the Ômodel experiments'.

Focused project teams which tackle specific issues (e.g., saturation of the terrestrial carbon sink) and which facilitate incorporation of new process understanding into global carbon models.

In parallel with the synthesis effort, the project will develop an international framework for collaborative research on the carbon cycle. Much of the work will be built around existing components of the international programs, but where critical gaps exist, we will work with agencies to initiate new work. Top

By Kathy Hibbard

Objective:

To compare model estimates of terrestrial carbon fluxes Net Primary Production (NPP and net ecosystem production NEP, where available) with estimates from ground-based measurements, and improve our understanding of environmental controls of carbon allocation.

Rationale:

Progress in modeling the global carbon cycle has previously been inhibited by the lack of adequate observational data for model parameterization and validation, such as NPP from field measurements.

In October 1998, a planning meeting was held to discuss opportunities to dovetail NPP data sets from the Global Primary Productivity Data Initiative (GPPDI) (see GAIM Newsletter summer 1998) with a global model/data intercomparison. Models that were included in the 1995 PIK workshop will be invited to participate in the EMDI exercise. A special volume of Global Change Biology devoted to these models will be published this Spring.

The experimental design will consists of a multitiered approach to make maximum use of the available NPP and NEE measurements. The NPP data sets emerging from GPPDI are derived from both point and spatially explicit sampling designs, thus enabling a valid comparison between point and area-based models and data. Analyses and visualizations will be carried out within each tier to investigate the model controls on NPP and their underlying formulations as follows:

Site model-data comparisons - The first tier of the comparison will be to compare the globally distributed intensive point data to models (e.g. biogeochemical) that predict carbon fluxes at a single point, or 1-D models.

Grid-cell model-data comparisons - The second tier of the comparison will be to compare the globally distributed spatial data to models. Areal NPP data ware derived from county and polygonal for boreal and temperate forests, grasslands, and crops. Approximately ten global sites have been identified as candidates for spatial modeling comparison. The inclusion of spatially-extant data will require models to simulate carbon fluxes and make valid comparisons with the measured NPP data.

Global model-data comparisons - The third level of comparison would take advantage of the large collections of extensive NPP estimates (approximately 2500-3000 sites) to provide a broader comparison between patterns of measured NPP and model outputs. The extensive collections represent a mix of years, methods, and available documentation - often not known or readily available. Carbon allocation rules developed in the GPPDI may be applied to these data to provide a reasonably consistent set of NPP estimates when only partial information is available.

FLUX Data - An important source of ecosystem production measurements (with monthly time resolution) to include in model-data intercomparison is being compiled by the community of scientists associated with the global network of CO₂ flux towers. It is anticipated that 10-12 AmeriFlux sites and 10-15 EUROFLUX sites will be able to provide monthly, multiple-year estimates of NEE for the EMDI exercise.

Modeling groups interested in participating in the Ecosystem Model/Data Intercomparison please contact Dr. Kathy Hibbard at: kathyh@eos.sr.unh.edu. Top

To identify the principal differences between global-scale, three-dimensional, ocean carbon-cycle models, to accelerate their development, and to improve their predictive capacity.

Rationale:

Ocean carbon-cycle models offer a means to help synthesize our understanding about the redistribution of carbon in the ocean and the resulting effect on the global carbon cycle. The need to improve these models and to ultimately incorporate them into a composite description of the earth's complex biogeochemical and physical climate system motivated GAIM to launch OCMIP. Modelers saw the need for OCMIP as being fundamental for two reasons: (1) the basic need to compare results between models, and (2) the awareness that models improve more rapidly when resources are pooled and understanding is shared between modelling groups. For consistency, standard boundary conditions and protocols were developed for OCMIP and analysis was centralized (at IPSL/LSCE).

OCMIP's main interest is the carbon cycle. Hence the primary concern has been to focus on the abilities of models to predict ocean carbon distributions and air-sea fluxes of CO₂. The first phase of OCMIP was recently concluded (see GAIM Report #7, 1998; http://gaim.unh.edu/Projects/OCMIP), and OCMIP-2 (http://www.ipsl.jussieu.fr/OCMIP/) is now underway. The OCMIP-1 strategy was to study (1) natural CO₂, with simulations which were allowed to reach equilibrium with pre-industrial atmospheric CO₂ (at 278 ppm), and (2) anthropogenic CO₂, with simulations forced by observed atmospheric CO₂ from pre-industrial time to present. In addition, to evaluate model behavior, OCMIP-1 compared simulated vs. observed 14C. A global network of 14C samples was taken during GEOSECS in the 1970's and more recent sections from WOCE are now beginning to become available. Natural 14C offers a powerful test of an ocean model's deep ocean circulation; bomb 14C helps constrain the modeled circulation of surface and intermediate waters. Bomb 14C also appears to exhibit similar behavior to anthropogenic CO₂, under certain conditions. Exploiting the 14C- CO₂ relationship, when appropriate, would offer one way to circumvent the difficulty of directly measuring the small anthropogenic change in dissolved inorganic carbon (DIC) in the ocean, relative to the large DIC pool which is naturally present.

Approach:

In OCMIP-1, we demonstrated that predictions from ocean carbon-cycle models differ regionally by a substantial amount, particularly in the Southern Ocean, where modeled air-sea fluxes of anthropogenic CO₂ are also largest. The recently launched OCMIP-2 involves 13 models and additional simulations. The focus remains on CO₂, but OCMIP-2 also includes emphasis on new circulation tracers, such as CFC-11 and CFC-12, and new biogeochemical tracers such as O₂. OCMIP-2 will also include simulations with a common biogeochemical model so that participants can better study effects due to differences in modeled ocean circulation. Furthermore, OCMIP-2 includes data specialists who are leading the JGOFS and WOCE synthesis for CO₂, 14C, and CFC's, which will strengthen model validation efforts.

Studies during the first two phases of OCMIP have relied on ocean models run under present climatological conditions, where circulation patterns do not evolve with time. Beyond OCMIP-2, future work will probably focus on the impact of changing climate on marine biogeochemistry as well as the feedback of changes in marine biogeochemistry on climate. To validate such simulations, it will be crucial to focus on how well models are able to reproduce observed interannual variability. Top

To understand the causes of variations in atmospheric methane concentrations on various timescales in response to changes in sources, sinks, and atmospheric reactions.

Rationale:

Atmospheric methane contributes directly to the Earth's greenhouse effect. It is a precursor for water vapor and ozone and it affects atmospheric oxidation rates. Methane levels have changed markedly in the past with both anthropogenic and natural causes. This project attempts to explain such observed changes, on decadal, century, and glacial to interglacial time scales, through a synthesis of all relevant biogeochemical processes. A key outcome will be improved predictions of future methane levels, taking into account climatic and atmospheric chemistry feedbacks.

Changes in atmospheric methane concentrations during glacial - interglacial transitions appear closely correlated with climate, probably due to the effect of changes in temperature and hydrological regimes on wetlands. Detailed records from ice-cores indicate a marked onset of anthropogenic sources over the last 200 years and growth rates at times exceeding 1% /yr. Observations over the last two decades have revealed a downward trend in growth rates consistent with near constant total sources, but significant inter-annual variations in emissions and or removal rates are also evident.

The Atmospheric Methane Synthesis will bring together recent advances in understanding methane emission processes, new analyses of atmospheric oxidation chemistry, and improved atmospheric chemical tracer models. Analysis of the contemporary methane budget will use geographic and climatic data in process models to set "bottom up" constraints on emissions, and atmospheric data to set "top-down" constraints. The bottom-up approach alone runs into difficulties with scaling and data sparsity. The top-down approach is limited by dispersion in the atmosphere and the difficulty of identifying specific emission processes. The combined approach used here has the potential to exploit the strengths of each.

Understanding the past history of atmospheric methane will build on the same process based approach as used for the contemporary budget. Recent work on past changes in biogeochemical cycles, land use, and atmospheric chemistry will be used to estimate changes in sources and sinks/reactions, and relate these to long term changes in methane concentrations. Extending our analysis to cover the Earth's past will verify our understanding over a much wider range of climatic conditions, and throughout the transition from a natural to an anthropogenically dominated regime. The results will complement and contribute to the "Paleo Trace Gas and Aerosol Challenge" (this issue).

Approach:

A process-based understanding of the atmospheric methane budget over a wide range of conditions will provide a robust basis for projecting future methane concentrations. However, as anthropogenic emissions become increasingly important with respect to agriculture and land use, prediction of future methane concentrations will also require assessment of likely changes in human needs and behavior.

The GAIM Methane Synthesis project will be closely linked to other IGBP Projects including, among others:

• the IGAC theme of atmospheric oxidation and the GEIA activity;
• wetlands studies involving IGAC, GCTE, BAHC, and the IGBP Water Project;
• PAGES work on past biogeochemical cycles and their interaction with climate;
• the GAIM TransCom project;
• IHDP work on human changes related to agricultural and land use emissions. Top