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Leistungen
To mitigate rising levels of atmospheric CO2, it has been proposed to purposefully sequester CO2 generated by power plants in the deep ocean. Five of the GOSAC groups made simulations, as specified in the Injection HOWTO, to evaluate the efficiency of the ocean in retaining this sequestered CO2; motivated by the GOSAC effort, three non-European OCMIP modelling groups also contributed. Injection was carried out simultaneously at seven separate sites; these 7-injection site simulations were made at three different depths. At 3000 m, all models reveal at least a 85% global efficiency in year 2200, i.e., 100 years after the end of the 100-year injection period); at the same time, 1500-m injection is 60-80% efficient and 800-m injection is only 42-61% efficient. A sensitivity test in the SOC model reveals that continuing injection after year 2100 increases global efficiency from 66% to 77% in 2200 and from 38% to 58% in 2500. Most of themodels simulate that for the 1500-m injection, the Pacific sites are more efficient than those in the Atlantic, and that injection in the Indian Ocean yields intermediate efficiencies. Such would be expected based upon our understanding of the age of deep waters in each of these three basins. The models generally predicted that Injection just offshore of New York was amongst the least efficient sites and that injection just offshore of San Francisco was most efficient. Most of the CO2 injected at 3000 m was lost from the Southern Ocean (the principal region by which the deep ocean is ventilated); for the shallower injections, relatively more was lost sooner, from the northern hemisphere and the tropics. It appears that the range of simulated efficiencies for the 3000-m injection brackets what would be real ocean behaviour (under an identical injection scenario), based on correlations of the global injection efficiency at that depth with the global mean CFC-11 inventory as well as with the global mean, deep-ocean natural C-14; such would also be expected based on the large diversity of models that participated in this exercise. For shallower injections, the correlation is less clear (1500-m) or nonexistent (800-m).
Simulated results from 11 models were compared to database estimates of the air-sea CO2 flux. The latter was obtained by multiplying the observed estimates of the difference between surface ocean and atmospheric pCO2 (i.e., Delta-pCO2), times the OCMIP-2 fields of gas exchange. To compare the full space-time distribution, we used the recently developed Taylor diagram to assess model performance in regards to the zonal annual mean, zonal anomalies, and seasonalanomalies. The models all performed well in regard to their ability to simulate the zonal annualmean. However, none of the models showed skill in simulating the zonal or seasonal anomalies. The seasonality problem may be linked to the common Biotic model used in OCMIP-2, which has no explicit plankton dynamics. Sensitivity tests with one model will be necessary to further investigate the causes of this problem.
All 13 OCMIP-2 groups made the Abiotic simulation for the natural component of CO2, and all but one of the groups made the optional Biotic simulation (proposed by U.S. OCMIP after the start of GOSAC). For the latter, all models used a common biogeochemical model that carried five tracers: alkalinity (Alk), oxygen (O2), dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and the nutrient phosphate (PO4). New production in each model was determined by restoring simulated surface PO4 to the observed surface seasonal distribution. GOSAC analyzed the CO2 fluxes from these runs, whereas U.S. OCMIP took on related analysis for PO4, new production, and O2. The OCMIP-2 models show consistent patterns for thermal driven CO2 fluxes, with uptake due to cooling in the high latitudes and outgassing due to warming in the tropics. In sharp contrast, air-sea fluxes due to the biological component have outgassing in the Southern Ocean and uptake in the low latitudes.
The Southern Ocean is the site for the largest uptake for the thermal-driven component and the largest outgassing for the biological component. When combined, the magnitude of the total pre-industrial air-sea flux in the Southern Ocean is much smaller. Models disagree about whether that region was a net sink or a net source during pre-industrial time. The OCMIP-2 pre-industrial air-sea fluxes were also used to evaluate global meridional transport in the pre-industrial ocean. This initial condition of the ocean must be properly quantified to predict the magnitude of the terrestrial carbon sink in the northern hemisphere. The global southward transport in the OCMIP-2 models ranged from 0.1 to 0.7 Pg C/yr, the high end of which is much larger than the prediction from OCMIP-1 (<0.1 Pg C/yr). For the first time, the meridional transport simulated by ocean carbon cycle models (without including riverine carbon) is as large as that estimated from measurements in the Atlantic Ocean (0.3 to 0.5 Pg C/yr). Additionally, pre-industrial fluxes from OCMIP-2 were used as boundary conditions to two atmospheric tracer transport models (TM2 and TM3) in order to estimate the marine pre-industrial component of CO2 in the lower troposphere. Pre-industrial air-sea fluxes from the ocean models alone explain up to about half to the 0.8 ppm north-south pre-industrial difference, based on time extrapolation of measurements at two stations Mauna Loa (MLO, Hawaii) and South Pole (SPO) to zero emissions.
Accounting for river carbon loop may explain the remaining difference, but currently only one model has simulated this effect. The models show a high correlation between pre-industrial meridional transport and the pre-industrial MLO-SPO difference in marine atmospheric CO2.
Most modelling groups made simulations for two future scenarios from the IPCC (IS92a and S650) Models agree for the modern era within +/-22%, but diverge in the future in both scenarios. In year 2100, agreement about the mean was +/-30% for the less severe S650 scenario (where atmospheric CO2 was stabilized at 650 ppm) and +/-33% for scenario C-IS92a (where atmospheric CO2 reached 800 ppm in 2100). For the S650 run, which was continued until year 2300, model agreement remained near +/-30% , reaching at most +/-33% in year 2200. Neglecting the high and low models (based on CFC-11 and natural C-14 tracer constraints) the agreement between models improves considerably. It remains at +/-18% through year 2100, under both future scenarios. In the longer S650 run, agreement after year 2100 worsens slightly to +/-22%.
The GOSAC-OCMIP protocols for the CFC, Abiotic, Biotic, Injection, and Helium simulations were developed and made available through the OCMIP Web page as detailed HOWTO documents; these documents also provide the boundary conditions and code to make these simulations and store output in standard OCMIP format. They will remain available, as an important legacy of GOSAC and OCMIP.
For the GOSAC project, the coordinating group IPSL developed a Global Analysis Package (GAP) to analyze model output generated by OCMIP and to compare it to data. During the 3-year GOSAC project, the first stable version was released for other OCMIP groups to allow their participation in the OCMIP analysis. At the end of the project, a second stable version of GAP was released at the end of the project. It is freely available to all who are interested by simply contacting the project coordinator. GAP offers unique features that facilitate model-model and model-data comparison. Unlike other software, it can perform such comparison even though the model grids are not rectangular; the GOSAC group included models with irregularly spaced, contorted, and unstructured grids. GAP provides for model-data comparison along sections, multi-variable analysis, and zone mean analysis, all of which were crucial for the GOSAC/OCMIP-2 comparison.
All groups made simulations for the anthropogenic CO2, forced by observed atmospheric CO2 during the historical period. The anthropogenic component is the change from the pre industrial state to present. For the 1980's average, the 13 OCMIP-2 models and 3 additional sensitivity runs simulated a range of 1.99 +/- 0.43 Pg C/yr (half the range over the mean). For the 1990's, the OCMIP-2 models predict a 24% increase in ocean uptake (2.38 +/- 0.53 Pg Cyr), in contrast with the 1980s-to-1990s decrease predicted by the most recent IPCC TAR report [Houghton et al., 2001, Chapter 3]. The IPCC results were based on a budget determined largely from atmospheric O2 measurements. However, it has been shown by two separate studies that errors in thatbudget compromise the IPCC evaluation of the 1980's to 1990's increase. The GOSAC/OCMIP-2 range for the modern uptake of anthropogenic CO2 probably brackets real ocean uptake because
(a) the OCMIP-2 model range for the CFC-11 inventory brackets the real ocean inventory (see above),
(b) the OCMIP-2 models bracket the observed deep-ocean C-14 (see above), and
(c) the modern global uptake of anthropogenic CO2 is correlated with both of these global criteria.
Model output from each of the thirteen OCMIP groups was provided in a standard predefined format based on netCDF and GDT. That standard output was consolidated into one standard hierarchy of OCMIP-2 model output files, separated by group and simulation, and archived at IPSL. That archive occupies about 50 Gb, and has been made available to other OCMIP participants. Portions of the OCMIP-2 model output database are available electronically via the WWW (DODS protocol); the remainder will also become publicly available as soon as final quality control is completed. OCMIP members will continue to exploit the rich OCMIP-2 model output database that was generated during GOSAC; the coordinating group IPSL will also promote andfacilitate its use by others. The OCMIP-2 model output database provides as an essential benchmark for community, and is thus another important legacy of OCMIP.
All 13 OCMIP-2 model groups submitted standardized output for standard simulations for two tracers of ocean circulation, CFC-11 and CFC-12. Simulated distributions were compared to the database of these inert ocean circulation tracers, measured precisely throughout the world ocean. Model output was quality controlled, archived, and made available electronically. The OCMIP-2 simulated range of global CFC-11 uptake varied by 30% around the mean, pointing out the need for an estimate of the observed global inventory. Differences in the modelled global inventory were found to be largely due to differences in uptake in the Southern Ocean. Much uptake occurs in the Southern Ocean due to ventilation of intermediate waters. Ventilation via a major component of Southern intermediate waters, Sub-Antarctic Mode Water (SAMW), was realistic only in models with lateral diffusion oriented along surfaces of constant density (isopycnals). Furthermore, ventilation of the deep Southern Ocean by Antarctic Bottom Water (AABW) was inadequate except in circulation models that were coupled to a sea-ice component model. In the north, models systematically under-predicted CFC-11 inventories in the subtropics;these coarse-resolution models also fared poorly in resolving the CFC-11 distribution in the Deep Western Boundary Current in the Western Atlantic. Model-data comparison along sections reveals that some models generally under-predict the CFC-11 observations whereas others generally overpredicted. The OCMIP-2 models probably also bracket the real ocean's global CFC-11 inventory. The CFC-11 analysis and publication were led by a GOSAC project scientist[Dutay et al., 2002].
A tracer that bears some qualitative resemblance to injected CO2 is Helium-3, which is measured as the He-3/He-4 ratio. Oceanographic He-3 measurements offer an independent constraint to help characterize ocean circulation. As opposed to the relatively homogeneous sea-surface input of C-14, He-3 is naturally injected at point sources in the deep ocean and is eventually lost to the atmosphere through air-sea gas exchange. Due to this qualitative similarity to injected CO2, GOSAC also made He-3 simulations and compared results to observations. For the GOSACsimulations, natural He-3 was injected along axes of mid-ocean ridges, with fluxes that were linearly proportional to observed spreading rates. Unlike in the only previous study, two of the GOSAC models simulated He-3/He-4 ratios that were similar in magnitude and structure with observed values in the Atlantic Basin. Thus GOSAC showed for the first time that it is indeed possible to simulate reasonable levels of He-3 in the Atlantic basin when using a simple boundary condition that is proportional to the ridge-spreading rate. Nonetheless, based on the GOSAC model-data comparison for natural C-14, there remains a problem with the He-3 boundary condition. That is, both these models that have "reasonable" levels of He-3/He-4 in the North Atlantic also have corresponding deep waters that are much too rich in natural C-14. Thus there is excessive ventilation of the deep ocean by surface waters, which are rich in natural C-14. In other words, waters in the deep North Atlantic in these two models are too young. Furthermore, there appears to be a wider problem with the boundary condition: the magnitude of simulated. He-3/He-4 ratio in these two models is generally too high throughout the deep ocean. If the He-3 source function that was used, were correct, a general excess of He-3 would suggests that a model's deep-ocean circulation or ventilation was systematically too sluggish. Yet, natural C-14, with its better-known boundary condition, indicates that none of the models involved in the He-3 comparison have deep waters that are much older than those in the real ocean. The overall correlation between He-3 and natural C-14 is being investigated. This work should help improve our understanding of the source function used to drive such He-3 simulations. Lessening uncertainties in the He-3 boundary condition will improve its utility as anevaluation tool for modelled deep-ocean circulation.
All 13 OCMIP-2 groups made standard simulations for natural and anthropogenic C-14. Responsibilities for analyzing C-14 were split according to the nature of each component: GOSAC adopted the responsibility of analyzing simulated vs. observed natural C-14 (a tracerof deep-ocean circulation); efforts within U.S. OCMIP focused on the anthropogenic bomb component (a tracer of near surface circulation). Here we focus only on results from analysis of the natural component. The range of results from the OCMIP-2 models for deep C-14(deeper than 1000 m) were found to bracket the observed global mean for deep C-14 (-150 permil); we further used this constraint to help setting limit on modern ocean uptake of anthropogenic CO2. Distribution patterns of natural C-14 were compared, after first removing the mean natural C-14 bias from each model. All models simulated the major meridional features of the C-14 distribution pattern in the deep Pacific Ocean; conversely, few models properly simulated the more complicated, meridional C-14 distribution in the deep Atlantic. In the Pacific Ocean, the new WOCE C-14 data set also allowed us to evaluate longitudinal variability as well as latitudinal variability of natural C-14. The near bottom longitudinal (zonal) structure in the South Pacific appears partly controlled by east-west differences in bottom bathymetry: young dense waters from the south fill bathymetric lows in the west; older waters are returned, eventually, in the east (shallower bathymetry). OCMIP exploited a new observationalconstraint for C-14 along the sigma-2 = 36.925 density surface. On that sole surface is found the entire range of C-14 in the deep ocean: in the real ocean, one end-member lies at the core of the youngest deep water (North Atlantic Deep Water NADW) and another end memberlies at the core of the oldest deep water (North Pacific Deep Water NPDW). A fair number of models have difficulty simulating the proper depth of this surface, which itself is a useful diagnostic of model performance. This model evaluation reveals the interest in extending these new observed diagnostics for observed C-14 (i.e., the bottom map and sigma-2 =36.925 surface map) beyond the Pacific into the Atlantic.
Verwertbare Ergebnisse
Summary : By combining measurements at sea and from satellites with coupled ocean models of the circulation and biogeochemical cycles, we have improved our knowledge of the air-sea exchange of carbon and of its fate in the ocean.
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