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Report of the CSM Biogeochemistry Working Group Meeting

Fourth Annual Climate System Model Workshop

By Scott Doney and Inez Fung, Co-chairs

Breckenridge, CO

June 24, 1999

 

The overall goal of the CSM Biogeochemistry (BGC) Working Group is to improve our understanding of the interactions and feedbacks between the physical and biogeochemical climate systems under past, present, and future climates. A key, short-range objective for the group is to develop a suite of global, fully coupled, prognostic, biogeochemical models incorporated within the CSM to study both natural interannual variability and forced decadal-to-centennial response to perturbations (e.g., climate warming, land-use change). Extensive data analysis and diagnostic modeling studies for the period of the last two decades, when good atmospheric composition and satellite remote sensing data exist, will also be used to evaluate model skill and determine underlying processes. An early focus on a set of fully interactive carbon cycle simulations is based on the primary role of anthropogenic CO2 emissions in potential climate change, the availability of a global network for atmospheric CO2 and related compounds, and on the readiness of the various component models for the ocean, atmosphere, and land. The BGC Working Group has every intention to expand into other radiatively and chemically important species, such as CH4, O3, organic halides, and sulfur species as improved models and resources become available.

Growing concern over the issue of climate change has focussed efforts on understanding the temporal evolution, climate impact, and potential feedbacks for biogeochemical forcing factors, such as radioactively active trace gas species (e.g., CO2, CH4, N20), natural and anthropogenic aerosols, and land-use change (Schimel et al., 1996). With recent improvements in numerical climate models and a general convergence in estimated climate sensitivities, the future levels of atmospheric greenhouse species, such as CO2, have become one of the major uncertainties associated with climate predictions through the next few centuries (Hansen et al., 1998). Fossil fuel emissions and anthropogenic land-use change have resulted in an increase in atmospheric CO2 concentrations over the last century and a half from a pre-industrial level of about 280 parts per million (ppm) to 365 ppm at present, and business as usual scenarios project values as high as 700 to 800 ppm by the end of the 21st century.

For the recent decade of the 1980's, fossil fuel emissions and tropical deforestation released roughly 5.5 giga tons carbon per year (GtC/yr) and 1.5 GtC/yr, respectively, of which about 40% or 3 GtC/yr remained in the atmosphere. The difference must be removed either by oceanic and terrestrial sinks, each estimated to contribute about -2 GtC/yr over this period. The physical mechanism for the dissolution of excess CO2 into the ocean is reasonably well understood if not fully quantified, while the terrestrial sink is thought to result from a poorly determined mix of forest regrowth, CO2 and N fertilization, and climate effects.

Future projections of atmospheric CO2 levels are relatively sensitive to assumptions about the behavior of the land and ocean carbon sinks, which are expected to change due to saturation effects and responses to the modified physical climate. On interannual timescales, the absolute magnitude and portioning between land and ocean regions for the CO2 sinks show considerable interannual variability as expressed in the atmospheric CO2 growth rate and other measures, such as the atmospheric CO2 isotopic or O2 compositions. Paleoclimate records of atmospheric CO2 also suggest that the global carbon cycle has not and is not likely to remain static.

Until recently, research in climate sensitivity and climate change has revolved around the response of the physical climate system to specified increases in atmospheric CO2. Climate model experiments have investigated the equilibrium and transient responses to prescribed changes in atmospheric CO2 concentration and have not taken into account the effects of the terrestrial and oceanic systems on the CO2 growth rate. The interactions with the terrestrial and oceanic carbon systems may accelerate or decelerate the atmospheric CO2 growth rate and, hence, the climate change. In turn, climate change has serious implications for the carbon dynamics of the terrestrial and oceanic systems and may alter the carbon sequestration potential of these systems.

The proposed CSM BGC Working Group scientific effort over the next 5 years will attack several scientific questions that address national needs and that may provide the scientific basis for international agreements. The foremost issues are: What are the controls on the observed interannual variability of land and ocean CO2 sinks over the last few decades? How will the terrestrial and oceanic carbon cycles change with the changing climate? How will these feedbacks alter the growth rate of atmospheric CO2 and the rate of climate evolution? How will the changes in the climate (and in climate extremes) and in the biosphere influence human welfare? Can and should humans manage the terrestrial and oceanic carbon cycles to obtain the optimal economic, social, and climatic future?

The BGC Working Group research activities can be split roughly into four areas: completion of the initial carbon cycle land, ocean, and atmosphere component models; application of prognostic and diagnostic models for evaluation and process studies; ongoing component model development; and exploration of the carbon-climate interaction using uncoupled and fully coupled carbon system experiments leading up to and including the so-called "Flying Leap" carbon experiment (an idealized, centennial climate change sensitivity experiment with a prognostic carbon model and full interactive physical climate). Over the last year, the BGC Working Group has focussed efforts on building the individual land, ocean, atmosphere carbon component models necessary for a complete prognostic global carbon cycle simulation. Essentially all of the required models are in place or will be by Fall 1999, and we expect to begin the first coupled experiments in the Winter, 2000.

The main elements controlling the evolution of the atmospheric CO2 distribution can be expressed as:

dCO /dt + Transport[CO2] = Fossil Fuel + Deforestation - Photosynthesis + Respiration + Air-Sea Flux,

where the righthand side terms are the surfaces fluxes coming from the land and ocean, respectively. From left to right, the current status with respect to a prognostic model capability is as follows. The atmospheric CO2 transport comes directly from the general circulation model, in this case the Community Climate Model version 3 (CCM3). Existing CO2 transport calculations with CCM2 (Erickson et al., 1996) and CCM3 given prescribed surface CO2 flux forcing replicate the main features of the observed seasonal CO2 cycles and meridional gradients at representative NOAA/CMDL network stations (see also TransCom discussion below). The time-space distributions of the fossil fuel emissions are reasonably well constrained from historical reconstructions, and the same is true for the deforestation term but with significantly larger uncertainties. A number of models are available within the CSM framework for predicting the terrestrial photosynthesis (LSM, SiB, BATS, IBIS) and respiration (Century, CASA, IBIS) fluxes. The plan is to use initially a new land biogeochemical model under development by Kergoat, Schimel, and Bonan that combines elements of LSM (photosynthesis, biophysics, LAI), Century (soil microbial respiration and nitrogen dynamics), and a new plant model (growth, plant respiration, allocation, nitrogen limitation). On the ocean side, Doney, Lindsay, DeConto, and Najjar have created and implemented the Ocean Carbon Model Intercomparison Project (OCMIP) simple, full-depth carbon biogeochemical model with diagnostic surface production in the NCAR CSM Ocean Model (NCOM). A fully prognostic version is also under development.

Starting with these existing carbon cycle models, the BGC Working Group proposes a focussed study of carbon-climate interactions and their effect on future aspects of global change. The group will carry out a series of global, three-dimensional, numerical experiments on carbon-climate interactions. We note at the outset that we do not view the model experiments as simulations. The goal of the experiments is to articulate what we actually know and do not know and to formulate and prioritize the agenda for future research. The strategy is to build the understanding of carbon-climate interactions systematically, progressing with experiments/hypotheses that can be evaluated by ancillary information (e.g., on interannual timescales), to experiments that project the future interactions in carbon-climate space not experienced in the past 400,000 years. Key to the analysis within each theme will be the sensitivity experiments that capture our uncertainties in the processes. A set of three experimental themes are envisioned:

1. Interannual Experiments. Causes of the atmospheric CO2 variations since the 1980ís will be investigated. Terrestrial carbon models will be forced by the observed climate statistics of the period, while oceanic carbon modules will respond to the circulation variations forced by the variations in surface exchanges of momentum, energy, and freshwater.

2. Permissible Emissions Experiment. Atmospheric CO2 concentration (with e.g., 1%/year growth rate) will be specified, and the terrestrial and oceanic carbon modules will be separately forced to estimate the uptake. Two sets of climates will be used: one corresponding to the current climate (from the control run of the CSM) and one corresponding to the climate evolving with a fixed 1%/year CO2 growth rate. The residual between the specified growth rate and calculated terrestrial and oceanic sinks would be the anthropogenic emissions permissible to maintain the specified growth rate. This experiment will also provide a first estimate of the effect of changing climate on the uptake.

3. Flying Leap Experiment. A fossil fuel emission scenario is prescribed and the atmospheric radiation will be forcing the residual of the fossil fuel CO2, after terrestrial and oceanic uptake has been accounted for. The terrestrial and oceanic carbon uptake will be calculated using prognostic carbon modules that are responsible to changes in climate and circulation.

The research area of carbon-climate interaction is new: the proposed work has not been carried out anywhere in the world. The "Flying Leap Experiment" discussed above was approved in early 1999 as a joint project between the World Climate Research Programme (WCRP) and the International Geosphere-Biosphere Programme (IGBP). It will define the scientific framework for understanding how the terrestrial and oceanic biospheres may be affected by, and in turn, alter the course of atmospheric and oceanic circulation and climate change.

A complementary research path is being developed by the BGC Working Group in diagnostic or inverse modeling. The magnitude, spatial patterns, and underlying mechanisms for the land and ocean CO2 surface fluxes (sinks) are not well understood in detail. However, given an atmospheric transport field and observational constraints, such as the CO2 distribution, 13C composition of CO2 and O2/N2 ratio (13C, and O2/N2 constrain the partitioning the CO2 sink between the ocean and land), a relatively well developed set of numerical techniques can be applied to solve or invert for the unknown surface fluxes. Sufficient atmospheric data exist to study the mean surface fluxes and interannual variability over the last two decades, determine the relationship of the CO2 sinks with climate parameters and indices (e.g., temperature, precipitation, SOI), and evaluate the performance and response of the forward, prognostic models to variability.

Ongoing research for the ocean, atmosphere, and land biogeochemical component models is also required with respect to model evaluation, process and sensitivity studies, and model development. Several cross-domain issues have been identified for near-term effort. The incorporation of carbon isotopes (13C and 14C) and atmospheric O2/N2 is crucial for partitioning between ocean and land carbon sinks and for tracking the anthropogenic carbon signal (fossil fuel carbon is isotopically light in 13C and has effectively no radiocarbon or 14C). The terrestrial production, atmospheric transport, and subsequent oceanic deposition of iron containing dust has been identified as an important factor modulating marine productivity and carbon storage through micronutrient and nitrogen fixation limitations. The 18O isotopic composition of atmospheric water provides a direct coupling of the hydrological and biogeochemical cycles and is a good measure of model behavior given the well known global 18O precipitation distribution. Finally, the atmospheric distributions of the radiatively important species CH4 and O3 are a complex balance of surface biogeochemical fluxes, atmospheric transport, and chemical dynamics (especially the tropospheric oxidative and stratospheric reactive chlorine and nitrogen states). A reduced form version of the chemical dynamics model MOZART is being developed by NCAR's Atmospheric Chemistry Division for incorporation into the CCM.

Specific research tasks are also outlined for the individual domain component models. For the ocean, there is a clear need for a more sophisticated treatment of ecological processes (upper ocean production and export and subsurface remineralization) beyond that in the current NCOM biogeochemical model. A subgroup plans to develop and evaluate a "core" marine ecosystem model linked with full-depth biogeochemistry, along the lines of Doney et al. (1996), augmented with multiple phytoplankton and zooplankton size classes, multiple nutrient limitations (N, P, Si, Fe), and dissolved organic matter. Both the NCOM biogeochemical model and the "core" ecosystem model will be migrated to the new Parallel Ocean Program (POP) CSM ocean model when that is complete. A series of ocean tracer model-data comparison studies (CFCs, Tritium-He3, 14C, abiotic and biotic carbon, anthropogenic carbon) are underway within NCOM as part of the IGBP/GAIM OCMIP. These simulations will help to evaluate the circulation characteristics of the ocean model and will be used in an ongoing fashion as benchmarks for the evolving physical model framework. Although the present CSM ocean model does not adequately resolve from a biogeochemical perspective the role of mesoscale eddies or the coastal oceans, preliminary process studies in higher resolution basin scale models are in progress, the results of which will feed into parameterizations for the full global model.

The current emphasis within the atmospheric biogeochemical component is on model evaluation and application rather than development. A main objective is to complete a suite of model tracer transport validation exercises as part of the IGBP/GAIM TransCom activity. The TransCom 1 model intercomparison, which included the NCAR CCM3, clearly demonstrated the qualitatively different character of atmospheric general circulation model (GCM) solutions in models with and without a planetary boundary layer scheme for so-called "rectified" tracers, such as vegetative CO2, whose surface fluxes are strongly correlated on diurnal and seasonal timescales with boundary mixing. The second phase of TransCom (Denning et al., 1999) deals with the transport of a passive, anthropogenic tracer species SF6, and Scott Denning, Colorado State University, has received limited funding from NSF to complete this phase with the NCAR CCM3. The third TransCom effort focuses on simulating the atmospheric CO2 distributions from a large set of spatial and temporal surface flux basis functions. Inversions based on the observed atmospheric CO2 fields will then be completed for each model to determine the spatial patterns and magnitudes of the implied ocean and land CO2 sinks. The BGC Working Group is actively working to ensure that TransCom-3 is completed with the NCAR CCM3.

On the terrestrial side, the BGC Working Group has identified a number of needs with respect to model development. One clear area for work is to incorporate a mechanistic phenology into LSM or CLM that is a prognostic rather than a specified seasonal cycle of vegetative leave area index. The inclusion of a prognostic vegetation scheme similar to that in the IBIS model would also be desirable, as it would allow for variations in land surface cover and plant type in response to climate forcing. Finally, the active transport of biogeochemical species by rivers would allow the group to study a number of issues ranging from the recent hypothesis by Stallard (1998) regarding carbon sequestration in reservoirs to anthropogenic eutrophication of the coastal oceans to riverine iron input.

The success of the CSM Biogeochemistry Working Group depends on maintaining strong linkages with other CSM working groups. The biogeochemical models by necessity are set in the physical climate system as represented by the modeling of the Land Model, Ocean Model, and Atmosphere Model Working Groups. In particular, good cross working group communication is required to ensure that the appropriate hooks for the biogeochemical modules are retained (particularly in the development of the new CLM) and that physical/biogeochemical model development progresses in tandem for a number of key areas (e.g., mechanistic terrestrial phenology, river routing, atmospheric boundary layer, coastal ocean). The other obvious interaction for the BGC Working Group is with the Chemistry and Climate Working Group, who has laid the groundwork to explore the sensitivity of the CSM over the 20th and 21st centuries using prescribed forcing functions.

List of Participants:

Johanna Balle-Beganton, Center for Atmospheric Sciences
Maurice Blackmon, NCAR
Lori Bruhwiler, NOAA
Kenneth Caldeira, Lawrence Livermore National Laboratory
Yi Chao, Jet Propulsion Lab
Shaoping Chu, Los Alamos National Laboratory
Robert DeConto, University of Massachusetts
Christine Delire, University of Wisconsin
Robert Dickinson, University of Arizona
Scott Doney, NCAR
Scott Elliott, Los Alamos National Laboratory
Jon Foley, University of Wisconsin
Inez Fung, University of California, Berkeley
Peter Gent, NCAR
Nicolas Gruber, Princeton University
Matthew Hecht, NCAR
Jasmin John, University of California, Berkeley
Al Kellie, NCAR
Laurent Kergoat, NCAR and CNRS
Joan Kleypas, NCAR
Keith Lindsay, NCAR
Ferial Louanchi, Pennsylvania State University
Natalie Mahowald, University of California, Santa Barbara
Robert Malone, Los Alamos National Laboratory
Laurie McNair, Los Alamos Natinal Laboratory
Norikazu Nakashiki, NCAR and CRIEPI
Chris Poulsen, Pennsylvania State University
James Randerson, University of California, Berkeley
Philip Rasch, NCAR
Steven Running, University of Montana
Charles Zender, NCAR