Ulrich Siegenthaler

A perturbation simulation of CO2 uptake in an ocean general circulation model

The uptake of anthropogenic CO2 by the ocean is simulated using a perturbation approach in a three-dimensional global general circulation model. Atmospheric pCO2 is prescribed for the period 1750–1990 using the combined Siple ice core and Mauna Loa records. For the period 1980 to 1989, the average flux of CO2 into the ocean is 1.9 GtC/yr. However the bomb radiocarbon simulation of Toggweiler et al. (1989b) shows that the surface to deep ocean exchange in this model is too sluggish. Hence the CO2 uptake calculated by the model is probably below the actual value. The observed atmospheric increase in 1980 to 1989 is 3.2 GtC/yr, for a combined atmosphere-ocean total of 5.1 GtC/yr. This is comparable to the estimated fossil CO2 production of 5.4 GtC/yr, implying that other sources and sinks (such as from deforestation, enhanced growth of land biota, and changes in the ocean carbon cycle) must be approximately in balance. The sensitivity of the uptake to the gas exchange rate is small: a 100% increase in gas exchange rate gives only a 9.2% increase in cumulative oceanic uptake. Details of the penetration into different oceanic regions are discussed.

Production and stable isotopic composition of CO2 in a soil near Bern, Switzerland

During a period of one year, we measured the concentration and the stable isotopic composition of CO2 in weekly soil air samples from two depths in a grass-covered soil. The CO2 production rate, as estimated from the diffusion equation and the observed concentration gradient, was 15 mmol/m-2/h-1, as an annual mean. During the cold season, the production rate varied with soil temperature, but no correlation existed with temperature above about 10 °C. Concentrations and fluxes of N2O in the soil were also determined. Observed δ13C values were in the expected range, and δ18O of Soil CO2 was found to be in isotopic equilibrium with the soil water. δ18O of the CO2 released to the atmosphere is expected to be lower by roughly 8‰ due to diffusive isotope fractionation; the precise value depends on the competition between CO2 production, equilibrium with soil water and diffusion.

super 14) C dating of plant macrofossils in lake sediment.

Macrofossils of terrestrial plants have been picked from a sediment core taken in Lake Lobsigen, a small lake on the Western Swiss Plateau. The sediments were previously analyzed for pollen composition, plant and animal macrofossils, and stable isotopes. Plant macrofossils were selected near pollen zone boundaries in Late Glacial and early Postglacial sediment for C-14 dating by AMS. In the same lake carbonate and gyttja (aquatic plant) samples were dated by decay counting. The dates on terrestrial material are generally younger than those on carbonate and gyttja, ie, material reflecting the C-14/C ratio of dissolved bicarbonate in lake water. This is probably due to a contribution of dissolved limestone carbonate and thus a somewhat reduced C-14/C, ratio in the lakes water (hard water effect).

Production and stable isotopic composition of CO 2 in a soil near Bern, Switzerland

During a period of one year, we measured the concentration and the stable isotopic composition of CO 2 in weekly soil air samples from two depths in a grass-covered soil. The CO 2 production rate, as estimated from the diffusion equation and the observed concentration gradient, was 15 mmol/m -2 /h -1 , as an annual mean. During the cold season, the production rate varied with soil temperature, but no correlation existed with temperature above about 10 °C. Concentrations and fluxes of N2O in the soil were also determined. Observed δ 13 C values were in the expected range, and δ 18 O of Soil CO 2 was found to be in isotopic equilibrium with the soil water. δ 18 O of the CO 2 released to the atmosphere is expected to be lower by roughly 8‰ due to diffusive isotope fractionation; the precise value depends on the competition between CO 2 production, equilibrium with soil water and diffusion. DOI: 10.1034/j.1600-0889.1991.00013.x

Pollen and oxygen isotope analyses on late- and post-glacial sediments of the Tourbière de Chirens (Dauphiné, France)

The sequence of vegetation phases in the late Glacial was studied in a sediment section from the bog Tourbiere de Chirens by means of pollen analysis. 18O16O ratios of samples of lacustrine marl, obtained from the same profile, reflect variations of 18O16O in precipitation and thus provide an additional, independent paleoclimatic record. The observed 18O16O variations agree well with the climatic history as deduced from pollen analysis. The climatic transition from the Oldest Dryas to the Bolling period sensu lato, as well as the beginning and end of the Younger Dryas cold phase, is marked by abrupt changes in the 18O16O ratio which were observed also in other regions of the Alps. These drastic climatic changes probably took place simultaneously over large areas of Central Europe and occurred within short time spans.

New Production and the Global Carbon Cycle

The export of newly produced organic carbon from the surface ocean and its regeneration at depth account for an estimated three-quarters of the vertical ΣCO2 gradient shown in Fig. 1 (Volk and Hoffert, 1985). If these processes, often referred to as the “biological pump,” had ceased operating during the pre-industrial era, the increase in surface ΣCO2 resulting from upward mixing of high ΣCO2 deep waters would have raised atmospheric pCO2 from 280 ppm to the order of 450 ppm (Sarmiento and Toggweiler, 1984) over a period of centuries. Vertical exchange, which gives an estimated upward flux of 100 GtC/yr (Fig. 2), works continuously to bring about just such a scenario. The biological pump prevents it by stripping out about 10 GtC/yr, so that the water arriving at the surface has a concentration equal to that which is already there.

Possible effects of iron fertilization in the Southern Ocean on atmospheric CO2 concentration

Recently, it was proposed (Baum, 1990 and Martin et al, 1990a, 1990b) that the southern ocean should be fertilized with iron to stimulate biological productivity, thus enhancing the flux of organic carbon from surface to depth, thereby lowering the concentration of inorganic carbon in surface water and in turn the atmospheric CO2 concentration. We explore the possible impact of a hypothetical iron fertilization on atmospheric CO2 levels during the next century using a high-latitude exchange/interior diffusion advection model. Assuming as an upper-limit scenario that it is possible to stimulate the uptake of the abundant nutrients in the southern ocean, the maximum atmospheric CO2 depletion is 58 ppm after 50 years and 107 ppm after 100 years. This scenario requires completely effective Fe fertilization to be carried out over 16% of the world ocean area. Sensitivity studies and comparison with other models suggest that the errors in these limits due to uncertainties in the transport parameters, which are determined by calibrating the model with radiocarbon and validated with CFC-11 measurements, range from −29% to +17%. If iron-stimulated biological productivity is halted during the six winter months, the additional oceanic CO2 uptake is reduced by 18%. Possible changes in surface water alkalinity alter the result of iron fertilization by less than +9% to −28%. Burial of the iron-induced particle flux as opposed to remineralization in the deep ocean has virtually no influence on the atmospheric response for the considered time scale of 100 years. If iron fertilization were terminated, CO2 would escape from the ocean and soon cancel the effect of the fertilization. The factors which determine the atmospheric CO2 reduction most strongly are the area of fertilization, the extent to which biology utilizes the abundant nutrients, and the magnitude of future CO2 emissions. The possible effect of fertilizing the ocean with iron is small compared to the expected atmospheric CO2 increase over the next century, unless the increase is kept small by means of stringent measures to control CO2 emissions.

The camp century 10Be record: Implications for long-term variations of the geomagnetic dipole moment

Abstract 10 Be concentrations measured in ice samples from Camp Century, Greenland, show short term variations which in general correspond to the 100–200 year “wiggles” in the 14 C tree ring record. There is, however, no evidence for a long term variation over the last 5000 years. This constancy is in contrast to the approximately sinusoidal variation of the atmospheric 14 C concentration which has generally been attributed to a changing geomagnetic dipole moment. This discrepancy implies that the 14 C trend might stem from other causes such as changes of oceanic circulation processes or from higher production rates during the Wisconsin rather than from variation in the geomagnetic field.

Carbon Dioxide: Its Natural Cycle and Anthropogenic Perturbation

Carbon dioxide is, besides water, the main nutrient for plants and therefore for life on earth. In consequence of fossil fuel burning and human impact on the land biota, the atmospheric concentration of carbon dioxide is steadily increasing, which may lead to long-lasting changes of the global climate. These two facts explain the strong interest of scientists from many disciplines in this gas and its natural cycle.

Ocean carbon transport in a box-diffusion versus a general circulation model

We have compared vertical transport of temperature, anthropogenic CO2, natural radiocarbon (14C), and bomb 14C in a global box-diffusion model (B-D) and a three-dimensional (3-D) ocean general circulation model from the Geophysical Fluid Dynamics Laboratory. Our main objectives were (1) to test the eddy diffusion parameterization of large-scale vertical transport in ocean box models and (2) to assess the utility of bomb-produced and natural 14C observations to validate ocean models used to estimate anthropogenic CO2 uptake. Prom the 3-D models distributions and fluxes of natural 14C, bomb 14C, and anthropogenic CO2, we have calculated apparent diffusivities (Kap) vertically over the global ocean that range mostly between 4000 and 8000m2 yr−1. These Kap agree quantitatively with diffusivities found by fitting B-D models to observed distributions of natural and bomb 14C We then used these sets of Kap in different runs of a global B-D model. Results from all B-D models runs matched to within 13% those from the 3-D model for global uptake of anthropogenic CO2 and bomb-14C penetration depth. Although Kap from 3-D simulations for bomb 14C vary with time, those from 3-D runs for anthropogenic CO2 are essentially constant. Still, we found nearly the same results with the B-D model when Kap from 3-D bomb 14C simulations are approximated as time invariant. The best agreement (within 3%) between 3-D CO2 simulations and B-D model runs was found when applying Kap derived from bomb 14C in the surface and from natural 14C in the deep. Agreement was worse when using Kap from 3-D simulations for anthropogenic CO2itself, mostly because in this case deeper Kap could only be extrapolated from higher surface values. We have found it appropriate to study global oceanic uptake of anthropogenic CO2 with B-D model and to validate anthropogenic carbon uptake models using natural and bomb 14C observations. For bomb 14C in the 3-D model, convective transport was most important during 1955–1964 while atmospheric levels were rising; afterward, atmospheric levels drop, and advective overturning dominates as for natural 14C. Thus 14C seems less than ideal to validate the convective scheme of general circulation models.