Patrick Monfray

Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms

Todays surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

Potential impact of climate change on marine export production

Future climate change will affect marine productivity, as well as other many components of Earth system. We have investigated the response of marine productivity to global warming with two different ocean biogeochemical schemes and two different atmosphere-ocean coupled general circulation models (GCM). Both coupled GCMs were used without flux correction to simulate climate response to increased greenhouse gases (+1% CO2/yr for 80 years). At 2×CO2, increased stratification leads to both reduced nutrient supply and increased light efficiency. Both effects drive a reduction in marine export production (−6%), although regionally changes can be both negative and positive (from −15% zonal average in the tropics to +10% in the Southern Ocean). Both coupled models and both biogeochemical schemes simulate a poleward shift of marine production due mainly to a longer growing season at high latitudes. At low latitudes, the effect of reduced upwelling prevails. The resulting reduction in marine productivity, and other marine resources, could become detectable in the near future, if appropriate long-term observing systems are implemented.

Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models

We have compared simulations of anthropogenic CO2 in the four three-dimensional ocean models that participated in the first phase of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP), as a means to identify their major differences. Simulated global uptake agrees to within ±19%, giving a range of 1.85±0.35 Pg C yr−1 for the 1980–1989 average. Regionally, the Southern Ocean dominates the present-day air-sea flux of anthropogenic CO2 in all models, with one third to one half of the global uptake occurring south of 30°S. The highest simulated total uptake in the Southern Ocean was 70% larger than the lowest. Comparison with recent data-based estimates of anthropogenic CO2 suggest that most of the models substantially overestimate storage in the Southern Ocean; elsewhere they generally underestimate storage by less than 20%. Globally, the OCMIP models appear to bracket the real oceans present uptake, based on comparison of regional data-based estimates of anthropogenic CO2 and bomb 14C. Column inventories of bomb 14C have become more similar to those for anthropogenic CO2 with the time that has elapsed between the Geochemical Ocean Sections Study (1970s) and World Ocean Circulation Experiment (1990s) global sampling campaigns. Our ability to evaluate simulated anthropogenic CO2 would improve if systematic errors associated with the data-based estimates could be provided regionally.

Positive feedback between future climate change and the carbon cycle

Future climate change due to increased atmo- sphericCO 2 may affect land and ocean efficiency to absorb atmosphericCO 2. Here, using climate and carbon three- dimensional models forced by a 1% per year increase in at- mosphericCO 2, we show that there is a positive feedback between the climate system and the carbon cycle. Climate change reduces land and ocean uptake of CO2 ,r espec tively by 54% and 35% at 4 × CO2 . This negative impact im- plies that for prescribed anthropogenic CO2 emissions, the atmosphericCO 2 would be higher than the level reached if climate change does not affect the carbon cycle. We esti- mate the gain of this climate-carbon cycle feedback to be 10% at 2 × CO2 and 20% at 4 × CO2 . This translates into a 15% higher mean temperature increase.

Evaluation of ocean carbon cycle models with data-based metrics

New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new data-based metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

Evaluation of ocean model ventilation with CFC-11: comparison of 13 global ocean models

We compared the 13 models participating in the Ocean Carbon Model Intercomparison Project (OCMIP) with regards to their skill in matching observed distributions of CFC-11. This analysis characterizes the abilities of these models to ventilate the ocean on timescales relevant for anthropogenic CO2 uptake. We found a large range in the modeled global inventory (±30%), mainly due to differences in ventilation from the high latitudes. In the Southern Ocean, models differ particularly in the longitudinal distribution of the CFC uptake in the intermediate water, whereas the latitudinal distribution is mainly controlled by the subgrid-scale parameterization. Models with isopycnal diffusion and eddy-induced velocity parameterization produce more realistic intermediate water ventilation. Deep and bottom water ventilation also varies substantially between the models. Models coupled to a sea-ice model systematically provide more realistic AABW formation source region; however these same models also largely overestimate AABW ventilation if no specific parameterization of brine rejection during sea-ice formation is included. In the North Pacific Ocean, all models exhibit a systematic large underestimation of the CFC uptake in the thermocline of the subtropical gyre, while no systematic difference toward the observations is found in the subpolar gyre. In the North Atlantic Ocean, the CFC uptake is globally underestimated in subsurface. In the deep ocean, all but the adjoint model, failed to produce the two recently ventilated branches observed in the North Atlantic Deep Water (NADW). Furthermore, simulated transport in the Deep Western Boundary Current (DWBC) is too sluggish in all but the isopycnal model, where it is too rapid.

Evaluating global ocean carbon models: The importance of realistic physics

A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO2 flux, chlorofluorocarbon and anthropogenic CO2 uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO2. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

Interannual variability of the oceanic sink of CO2 from 1979 through 1997

We have estimated the interannual variability in the oceanic sink of CO2 with a three-dimensional global-scale model which includes ocean circulation and simple biogeochemistry. The model was forced from 1979 to 1997 by a combination of daily to weekly data from the European Centre for Medium-Range Weather Forecast and the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis as well as European Remote Sensing satellite observations. For this period, the ocean sink of CO2 is estimated to vary between 1.4 and 2.2 Pg C yr?1, as a result of annually averaged interannual variability of ±0.4 Pg C yr?1 that fluctuates about a mean of 1.8 Pg C yr?1. Our interannual variability roughly agrees in amplitude with previous ocean-based estimates but is 2 to 4 times less than estimates based on atmospheric observations. About 70% of the global variance in our modeled flux of CO2 originated in the equatorial Pacific. In that region, our modeled variability in the flux of CO2 generally agreed with that observed to ±0.1 Pg C yr?1. The predominance of the equatorial Pacific for interannual variability is caused by three factors: (1) interannual variability associated with El Nino events occurs in phase over the entire basin, whereas elsewhere positive and negative anomalies partly cancel each other out (e.g., for events such as Antarctic Circumpolar Wave and the North Atlantic Oscillation); (2) dynamic processes dominate in the equatorial Pacific, whereas dynamic, thermodynamic, and biological processes partly cancel one another at higher latitudes; and (3) our model underestimates the variability in ocean dynamics and biology at high latitudes.

A three-dimensional synthesis study of δ18O in atmospheric CO2: 1. Surface fluxes

The isotope O-18 in CO2 is of particular interest in studying the global carbon cycle because it is sensitive to the processes by which the global land biosphere absorbs and respires CO2. Carbon dioxide and water exchange isotopically both in leaves and in soils, and the O-18 character of atmospheric CO2 is strongly influenced by the land biota, which should constrain the gross primary productivity and total respiration of land ecosystems, In this study we calculate the global surface fluxes of O-18 for vegetation and soils using the SiB2 biosphere model coupled with the Colorado State University general circulation model. This approach makes it possible to use physiological variables that are consistently weighted by the carbon assimilation rate and integrated through the vegetation canopy, We also calculate the air-sea exchange of O-18 and the isotopic character of fossil emissions and biomass burning. Global mean values of the isotopic exchange with each reservoir are used to close the global budget of O-18 in CO2 results confirm the fact that the land biota exert a dominant control on the delta(18)O of the atmospheric reservoir, At the global scale, exchange with the canopy produces an isotopic enrichment of CO2, whereas exchange with soils has the opposite effect.

The marine source of C2-C6 aliphatic hydrocarbons

C2-C6 Nonmethane hydrocarbon (NMHC) concentrations in the atmospheric boundary layer and in surface seawater were simultaneously measured during an oceanographic cruise in the intertropical Indian Ocean. NMHC were found to be mainly C2-C4 alkenes and C2-C3 alkanes. Their concentrations ranged from 1 to 30×10−9 l/l in the seawater and 0.1 to 15 ppbv in the atmosphere. Seawater appeared to be a source because the C2-C6 NMHC were supersaturated with respect to the atmosphere by 2 or 3 orders of magnitude.After a selection of the pure marine atmospheric samples, performed with the help of stable and radioactive continental tracers, we found an identical composition in NMHC of surface air and seawater. This observation enabled us to establish that the gas transfer between sea and air occurred according to nonsteady state processes, and that the fluxes cannot be deduced only from atmospheric measurements. An order of magnitude value of the oceanic source for the different NMHC is however derived from the comparison of their sea water concentrations to that of propane and an independent evluation of the marine source of this last compound.