Ernst Maier-Reimer

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.

Transport and storage of CO2 in the ocean ——an inorganic ocean-circulation carbon cycle model

Inorganic carbon in the ocean is modelled as a passive tracer advected by a three-dimensional current field computed from a dynamical global ocean circulation model. The carbon exchange between the ocean and atmosphere is determined directly from the (temperature-dependent) chemical interaction rates in the mixed layer, using a standard CO2 flux relation at the air-sea interface. The carbon cycle is closed by coupling the ocean to a one-layer, horizontally diffusive atmosphere. Biological sources and sinks are not included. In this form the ocean carbon model contains essentially no free tuning parameters. The model may be regarded as a reference for interpreting numerical experiments with extended versions of the model including biological processes in the ocean (Bacastow R and Maier-Reimer E in prep.) and on land (Esser G et al in prep.). Qualitatively, the model reproduces the principal features of the observed CO2 distribution bution in the surface ocean. However, the amplitudes of surface pCO2 are underestimated in upwelling regions by a factor of the order of 1.5 due to the missing biological pump. The model without biota may, nevertheless, be applied to compute the storage capacity of the ocean to first order for anthropogenic CO2 emissions. In the linear regime, the response of the model may be represented by an impulse response function which can be approximated by a superposition of exponentials with different amplitudes and time constants. This provides a simple reference for comparison with box models. The largest-amplitude (∼0.35) exponential has a time constant of 300 years. The effective storage capacity of the oceans is strongly dependent on the time history of the anthropogenic input, as found also in earlier box model studies.

Time-dependent greenhouse warming computations with a coupled ocean-atmosphere model

Climate changes during the next 100 years caused by anthropogenic emissions of greenhouse gases have been simulated for the Intergovernmental Panel on Climate Change Scenarios A (“business as usual”) and D (“accelerated policies”) using a coupled ocean-atmosphere general circulation model. In the global average, the near-surface temperature rises by 2.6 K in Scenario A and by 0.6 K in Scenario D. The global patterns of climate change for both IPCC scenarios and for a third step-function 2 x CO2 experiment were found to be very similar. The warming delay over the oceans is larger than found in simulations with atmospheric general circulation models coupled to mixed-layer models, leading to a more pronounced land-sea contrast and a weaker warming (and in some regions even an initial cooling) in the Southern Ocean. During the first forty years, the global warming and sea level rise due to the thermal expansion of the ocean are significantly slower than estimated previously from box-diffusion-upwelling models, but the major part of this delay can be attributed to the previous warming history prior to the start of present coupled ocean-atmosphere model integration (cold start).

Geochemical cycles in an ocean general circulation model. Preindustrial tracer distributions

A state-of-the-art report is given of the Hamburg model of the oceanic carbon cycle. The model advects geochemical tracers important to the carbon cycle by the currents of a general circulation model. The geochemical cycling is driven by a Michaelis-Menten type production kinetics. The model is an extension of the Bacastow and Maier-Reimer (1990) model. It is based on a more realistic current field and includes a mechanism of lysocline-sediment interaction. Principal variables are ∑CO2, alkalinity, phosphate, oxygen, and silicate. The carbon variables are defined for 12C, 13C, and 14C separately. In addition to these carbon isotopes, 39A and δ18O of dissolved oxygen are predicted. The model predicts realistic global patterns of tracer distribution. In the equatorial eastern Pacific, however, the structures are exaggerated due to a strong upwelling which is a common feature of coarse resolution models of the general circulation of the ocean.

Mean Circulation of the Hamburg LSG OGCM and Its Sensitivity to the Thermohaline Surface Forcing

Abstract The sensitivity of the global ocean circulation to changes in surface heat flux forcing is studied using the Hamburg Large Scale Geostrophic (LSG) ocean circulation model. The simulated mean ocean circulation for appropriately chosen surface forcing fields reproduces the principal water mass properties, residence times, and large-scale transport properties of the observed ocean circulation quite realistically within the constraints of the model resolution. However, rather minor changes in the formulation of the high-latitude air–sea heat flux can produce dramatic changes in the structure of the ocean circulation. These strongly affect the deep-ocean overturning rates and residence times, the oceanic heat transport, and the rate of oceanic uptake of CO2. The sensitivity is largely controlled by the mechanism of deep-water formation in high latitudes. The experiments support similar findings by other authors on the sensitivity of the ocean circulation to changes in the fresh-water flux and are cons...

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.

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.

The influence of air and sea exchange on the carbon isotope distribution in the sea

We explore here the influence of the temperature dependence of isotope fractionation between atmospheric CO2and ocean ΣCO2 on the distribution of carbon isotopes in the ocean. This is accomplished by an analysis of departures from the expected Redfield tie between PO4 and δ13C. We find that for the surface ocean, the temperature influence largely compensates for the biologic influence. In the deep ocean, the temperature influence imprinted at the sites of deepwater formation reduces somewhat the biologically induced difference between the carbon isotope ratios for deep waters produced in the northern Atlantic and in the Antarctic. These same features are reproduced in the Hamburg ocean model. In order to assess the impact of changes in the ratio of ocean mixing rate to wind speed, we have made a model run in which CO2 exchange rates between air and sea were everywhere doubled. As expected, the influence of the thermodynamic effect on the oceanic carbon isotope distribution is magnified.

Effects of plankton dynamics on seasonal carbon fluxes in an ocean general circulation model

We discuss the effect of embedding a simple plankton model in the Hamburg model of the oceanic carbon cycle (HAMOCC3) [Maier-Reimer, 1993]. The plankton model consists of five components: phytoplankton, zooplankton, detritus, dissolved organic carbon, and nutrients. Interactions between compartments are described by one global set of parameters. Despite its simplicity the plankton model reproduces regional differences in seasonal oceanic pCO2 and improves the biogeochemical tracer distributions at the depth of the oxygen minimum in the Pacific Ocean. The predicted seasonal turnover of organic material is consistent with recent atmospheric O2 measurements in the remote areas of the Southern Ocean.

Multiple timescales for neutralization of fossil fuel CO2

The long term abiological sinks for anthropogenic CO2 will be dissolution in the oceans and chemical neutralization by reaction with carbonates and basic igneous rocks. We use a detailed ocean/sediment carbon cycle model to simulate the response of the carbonate cycle in the ocean to a range of anthropogenic CO2 release scenarios. CaCO3 will play only a secondary role in buffering the CO2 concentration of the atmosphere because CaCO3 reaction uptake capacity and kinetics are limited by the dynamics of the ocean carbon cycle. Dissolution into ocean water sequesters 70–80% of the CO2 release on a time scale of several hundred years. Chemical neutralization of CO2 by reaction with CaCO3 on the sea floor accounts for another 9–15% decrease in the atmospheric concentration on a time scale of 5.5–6.8 kyr. Reaction with CaCO3 on land accounts for another 3–8%, with a time scale of 8.2 kyr. The final equilibrium with CaCO3 leaves 7.5–8% of the CO2 release remaining in the atmosphere. The carbonate chemistry of the oceans in contact with CaCO3 will act to buffer atmospheric CO2 at this higher concentration until the entire fossil fuel CO2 release is consumed by weathering of basic igneous rocks on a time scale of 200 kyr.