Guy Munhoven

Modeling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia

A three-dimensional Earth system model of intermediate complexity including a dynamic ice sheet component has been used to investigate the long-term evolution of the Greenland ice sheet and its effects on the Atlantic meridional overturning circulation (AMOC) in response to a range of stabilized anthropogenic forcings. Our results suggest that the Greenland ice sheet volume should experience a significant decrease in the future. For a radiative forcing exceeding 7.5 W m(-2), the modeled ice sheet melts away within 3000 years. A number of feedbacks operate during this deglaciation, implying a strong nonlinear relationship between the radiative forcing and the melting rate. Only in the most extreme scenarios considered, the freshwater flux from Greenland into the surrounding oceans ( of ca. 0.1 Sv during a few centuries) induces a noticeable weakening of the AMOC in the model.

Atmospheric CO2 consumption by continental erosion: present-day controls and implications for the last glacial maximum

The export of carbon from land to sea by rivers represents a major link in the global carbon cycle. For all principal carbon forms, the main factors that control the present-day fluxes at the global scale have been determined in order to establish global budgets and to predict regional fluxes. Dissolved organic carbon fluxes are mainly related to drainage intensity, basin slope, and the amount of carbon stored in soils. Particulate organic carbon fluxes are calculated as a function of sediment yields and of drainage intensity. The consumption of atmospheric/soil CO2 by chemical rock weathering depends mainly on the rock type and on the drainage intensity. Our empirical models yield a total of 0.721 Gt of carbon (Gt C) that is exported from the continents to the oceans each year. From this figure, 0.096 Gt C come from carbonate mineral dissolution and the remaining 0.625 Gt C stem from the atmosphere (FCO2). Of this atmospheric carbon, 33% is discharged as dissolved organic carbon, 30% as particulate organic carbon, and 37% as bicarbonate ions. Predicted inorganic carbon fluxes were further compared with observed fluxes for a set of 35 major world rivers, and possible additional climatic effects on the consumption of atmospheric CO2 by rock weathering were investigated in these river basins. Finally, we discuss the implications of our results for the river carbon fluxes and the role of continental erosion in the global carbon cycle during the last glacial maximum.

Glacial–interglacial changes of continental weathering: estimates of the related CO2 and HCO3− flux variations and their uncertainties

A range of estimates for the glacial–interglacial variations in CO2 consumption and HCO3− production rates by continental weathering processes were calculated with two models of continental weathering: the Gibbs and Kump Weathering Model (GKWM) [Paleoceanography 9(4) (1994) 529] and an adapted version of Amiotte Suchet and Probsts Global Erosion Model for CO2 Consumption (GEM-CO2) [C. R. Acad. Sci. Paris, Ser. II 317 (1993) 615; Tellus 47B (1995) 273]. Both models link CO2 consumption and HCO3− production rates to the global distributions of lithology and runoff. A spectrum of 16 estimates for the runoff distribution at the Last Glacial Maximum (LGM) was constructed on the basis of two different data sets for present-day runoff and climate results from eight GCM climate simulation experiments carried out in the framework of the Paleo Modelling Intercomparison Project (PMIP). With these forcings, GKWM produced 3.55–9.0 Tmol/year higher and GEM-CO2 4.7–13.25 Tmol/year higher global HCO3− (1 Tmol=1012 mol) production rates at the LGM. Mean variations (plus/minus one standard error of the mean with 7 df) were 6.2±0.6 and 9.4±1.0 Tmol/year, respectively. The global CO2 consumption rates obtained with GKWM were 1.05–4.5 Tmol/year (mean: 2.8±0.4 Tmol/year) higher at the LGM than at present. With GEM-CO2, this increase was 1.95–7.15 Tmol/year (mean: 4.8±0.6 Tmol/year). The large variability in the changes obtained with each weathering model was primarily due to the variability in the GCM results. The increase in the CO2 consumption rate due to continental shelf exposure at the LGM was always more than 60% larger than its reduction due to ice cover. For HCO3− production rates, the increase related to shelf exposure was always more than twice as large as the decrease due to ice cover. Flux variations in the areas exposed both now and at the LGM were, in absolute value, always more than 3.5 times lower than those in the shelf environment. The calculated CO2 consumption rates by carbonate weathering were consistently higher at the LGM, by 2.45–4.5 Tmol/year (mean: 3.4±0.2 Tmol/year) according to GKWM and by 2.75–6.25 Tmol/year (mean: 4.6±0.4 Tmol/year) according to GEM-CO2. For silicate weathering, GKWM produced variations ranging between a 1.9 Tmol/year decrease and a 0.4 Tmol/year increase for the LGM (mean variation: −0.7±0.2 Tmol/year); GEM-CO2 produced variations ranging between a 0.8 Tmol/year decrease and a 1.05 Tmol/year increase (mean variation: +0.2±0.2 Tmol/year). In the mean, the calculated variations of CO2 and HCO3− fluxes would contribute to reduce atmospheric pCO2 by 5.7±1.3 ppmv (GKWM) or 12.1±1.7 ppmv (GEM-CO2), which might thus represent a non-negligible part of the observed glacial–interglacial variation of ∼75 ppmv.

Modelling the glacial-interglacial changes in the continental biosphere

Abstract A new estimate of the glacial–interglacial variations of the terrestrial carbon storage was obtained with the CARAIB biosphere model. The climatic data for the Last Glacial Maximum (LGM) necessary to drive the biosphere model are derived from results of the ECHAM2 General Circulation Model (GCM). Six model simulations (four under typical interglacial and two under typical glacial climatic conditions) were performed to analyse the roles of different environmental changes influencing the biospheric net primary productivity (NPP) and carbon stocks. The main differences between these simulations come from the adopted CO 2 levels in the atmosphere, the presence or absence of crops and from changing continental boundaries. The variation of the terrestrial carbon stocks since the LGM are estimated by comparing the pre-agricultural (280 ppm of CO 2 , no crops, modern climate) and the full glacial simulations (200 ppm of CO 2 , LGM climate reconstruction). Our model predicts a global NPP increase from 38 Gt C year −1 to 53 Gt C year −1 during the deglaciation, a substantial part of that change being due to CO 2 fertilization. At the same time, the terrestrial biosphere would have fixed between 134 (neglecting CO 2 fertilization effects) and 606 Gt C. The treatment of both the C 3 and C 4 photosynthetic pathways in the CARAIB model enabled us further to reconstruct the partitioning between C 4 and C 3 plants. Following our experiments, 29.7% of the total biospheric carbon stock at the LGM was C 4 material, compared to an interglacial fraction of only 19.8%. The average biospheric fractionation factor was ∼1.5‰ less negative at LGM than it is today. Considering an atmospheric δ 13 C 0.5±0.2‰ lower at LGM than at pre-industrial times, the 606 Gt C transfer would lead to a global ocean δ 13 C shift of roughly −0.41‰, fully consistent with currently available data. For the smaller change of 134 Gt C obtained without the CO 2 fertilization effect, this shift would only be on the order of −0.10‰.

High-resolution spectra of Jupiter's northern auroral ultraviolet emission with the Hubble Space Telescope

The first spectroscopic observations of planetary aurora with the Hubble Space Telescope (HST) are reported. These include spectral regions centered on the H2 Lyman and Werner bands of a region of Jupiters northern aurora. The observations were made with the Goddard High Resolution Spectrograph (GHRS) using the Large Science Aperture as part of a campaign to study Jupiter at the time of the Ulysses flyby. The individual rotational-vibrational bands are resolved and the observed emissions are essentially all from H2. A rotational-vibrational temperature for H2 of 530 +/- 100 K is derived, a value significantly less than the 850-1100 K reported for Jovian H3(+) in the near-infrared but consistent with the temperature reported for fundamental-band quadrupole H2 emission. Comparison with the Faint Object Camera (FOC) images shows that the observed region was not one of the hot spots of the aurora. The results are interpreted in trms of electron impact excitation of H2 from secondary particles generated by primaries precipitating into Jupiters atmsophere from the magnetosphere. In the region of the aurora observed, the homopause level is found to be significantly hotter but not necessarily higher than observed at nonauroral latitudes. The equatorial H2 dayglow spectrum was also detected; its intensity was 3.2 kR or 13% of the strength of the observed auroral emission.

Glacial-interglacial variability of atmospheric CO2 due to changing continental silicate rock weathering: A model study

An 11-box model of the oceanic carbon cycle including sedimentary processes is used to explore the role chemical weathering of continental silicate rocks might play in driving atmospheric CO2 levels on glacial-interglacial timescales. Histories for the consumption of CO2 by silicate rock weathering processes are derived from the marine Ge/Si record. Taking the major uncertainties in the knowledge of the Ge and Si cycles into account, several histories for the evolution of the riverine dissolved silica fluxes are calculated from this record. The investigation of the systematics between riverine dissolved silica and bicarbonate fluxes under different weathering regimes leads us to the tentative conclusion that although there is no correlation between dissolved silica and total bicarbonate concentrations in the major rivers, there may exist a negative correlation between weathering intensity and the ratio of dissolved silica to bicarbonate derived from silicate weathering alone. With this correlation as a working hypothesis, it is possible to interpret the dissolved silica fluxes in terms of equivalent CO2 consumption rates. The calculated histories indicate that glacial rates of CO2 consumption by chemical silicate rock weathering could have been twice, and possibly up to 3.5 times, as high as they are today. When used to force the carbon cycle model, they are responsible for glacial-interglacial pCO2 variations in the atmosphere of typically 50–60 ppm and up to 95–110 ppm. These variations are superimposed to a basic oscillation of 60 ppm generated by the model, mainly in response to coral reef buildup and erosion processes. The total pCO2 signal has an amplitude of about 80–90 ppm and up to 125–135 ppm. Although these large amplitudes indicate that silicate weathering processes should be taken into account when studying glacial-interglacial changes of CO2 in the atmosphere, it also raises new problems, such as too high CO2 levels during the period from 110–70 kyr B.P., requiring further study.

Direct effect of ice sheets on terrestrial bicarbonate, sulphate and base cation fluxes during the last glacial cycle: minimal impact on atmospheric CO2 concentrations

Chemical erosion in glacial environments is normally a consequence of chemical weathering reactions dominated by sulphide oxidation linked to carbonate dissolution and the carbonation of carbonates and silicates. Solute fluxes from small valley glaciers are usually a linear function of discharge. Representative glacial solute concentrations can be derived from the linear association of solute flux with discharge. These representative glacial concentrations of the major ions are f25% of those in global river water. A 3-D thermomechanically coupled model of the growth and decay of the Northern Hemisphere ice sheets was used to simulate glacial runoff at 100-year time steps during the last glacial cycle (130 ka to the present). The glacially derived fluxes of major cations, anions and Si over the glaciation were estimated from the product of the glacial runoff and the representative glacial concentration. A second estimate was obtained from the product of the glacial runoff and a realistic upper limit for glacial solute concentrations derived from theoretical considerations. The fluxes over the last glacial cycle are usually less than a few percent of current riverine solute fluxes to the oceans. The glacial fluxes were used to provide input to an oceanic carbon cycling model that also calculates changes in atmospheric CO2. The potential change in atmospheric CO2 concentrations over the last glacial cycle that arise from perturbations in glacial solute fluxes are insignificant, being < 1 ppm. D 2002 Elsevier Science B.V. All rights reserved.

Modelled glacial and non-glacial HCO3−, Si and Ge fluxes since the LGM: little potential for impact on atmospheric CO2 concentrations and a potential proxy of continental chemical erosion, the marine Ge/Si ratio

The runoff and riverine fluxes of HCO3−, Si and Ge that arise from chemical erosion in non-glaciated terrain, are modelled at six time steps from the Last Glacial Maximum (LGM) to the present day. The fluxes that arise from the Great Ice Sheets are also modelled. Terrestrial HCO3− fluxes decrease during deglaciation, largely because of the reduction in the area of the continental shelves as sea level rises. The HCO3− fluxes, and the inferred consumption of atmospheric CO2 are used as inputs to a carbon cycle model that estimates their impact on atmospheric CO2 concentrations (atmsCO2). A maximum perturbation of atmsCO2 by ∼5.5 ppm is calculated. The impact of solutes from glaciated terrain is small in comparison to those from non-glaciated terrain. Little variation in terrestrial Si and Ge fluxes is calculated (<10%). However, the global average riverine Ge/Si ratio may be significantly perturbed if the glacial Ge/Si ratio is high. At present, variations in terrestrial chemical erosion appear to have only a reduced impact on atmsCO2, and only little influence on the global Si and Ge cycle and marine Ge/Si ratios during deglaciation.

HST spectra of the Jovian ultraviolet aurora: Search for heavy ion precipitation

Ultraviolet spectra using Hubble Space Telescope sampled between 1250 and 1680 A at spectral resolution ≤0.57 A are reported for characteristically bright regions of Jupiters morning and afternoon northern aurora. Several observed spectra exhibit sharply enhanced resolution. We interpret this as bright auroral emission foreshortened on the morning limb with a maximum intensity at least as high as 2000 kR. We have searched for evidence that the primary precipitating particles exciting the aurora include the heavy ions known to exist in Jupiters plasma torus and magnetosphere. We have also searched for such ambient heavy ions and neutrals at rest in the auroral ionosphere, the end products of previous precipitation, excited by the auroral cascade. We argue that primary emission would be characterized by a dramatically Doppler-broadened (~10-15 A) and redshifted line profile resulting from the cascade process and the angle between the line of sight and the magnetic field lines in the atmosphere. In contrast, ambient emission would be distinguished by narrow emission lines. We have modeled the theoretical sulfur and oxygen line shapes for ion precipitation and conclude that electron precipitation is responsible for most of the H2 emissions. O ions contributed <13% of the precipitating energy flux, and S ions contributed <50%. This dominance suggests that field-aligned magnetospheric currents are more important than energetization of energetic ions and subsequent scattering by plasma waves as a mechanism for generating the Jovian aurora. We set an upper limit over our spectra of 35-43 R to the emission from ambient oxygen and sulfur ions and their neutrals, except that for the S II 1256 triplet, the upper limit for the nominally brightest line, at 1260 A, is 74 R. Hence, we find no evidence for the accumulation of sulfur in the auroral ionosphere. A single narrow emission line from an unidentified ambient specie near 1254 A may be detected at the 4 σ level, introducing the possibility of complex auroral aeronomy. Differences were observed in the auroral spectral hydrocarbon absorption at different locations, which cannot be interpreted without ambiguity between auroral and atmospheric structural causes. We have found that the brighter emission in an auroral sector consistently shows more spectral hydrocarbon absorption than the dimmer emission. We suggest two alternative physical explanations for this phenomenon.

Assessing the potential of calcium‐based artificial ocean alkalinization to mitigate rising atmospheric CO2 and ocean acidification

Enhancement of ocean alkalinity using calcium compounds, e.g., lime has been proposed to mitigate further increase of atmospheric CO2 and ocean acidification due to anthropogenic CO2 emissions. Using a global model, we show that such alkalinization has the potential to preserve pH and the saturation state of carbonate minerals at close to today’s values. Effects of alkalinization persist after termination: Atmospheric CO2 and pH do not return to unmitigated levels. Only scenarios in which large amounts of alkalinity (i.e., in a ratio of 2:1 with respect to emitted CO2) are added over large ocean areas can boost oceanic CO2 uptake sufficiently to avoid further ocean acidification on the global scale, thereby elevating some key biogeochemical parameters, e.g., pH significantly above preindustrial levels. Smaller-scale alkalinization could counteract ocean acidification on a subregional or even local scale, e.g., in upwelling systems. The decrease of atmospheric CO2 would then be a small side effect.