Publications of Guy Munhoven
Bookmark and Share    
See detailImpact of future Greenland deglaciation on global weathering fluxes and atmospheric CO2
Munhoven, Guy ULiege; Brovkin, Victor; Ganopolski, A. et al

Conference (2007)

About 1.76×10^6 km2 of Greenland are currently covered by ice. It is expected that this large ice mass will melt away over the next 3000 years if anthropogenic CO2 emissions continue to rise (Alley et al ... [more ▼]

About 1.76×10^6 km2 of Greenland are currently covered by ice. It is expected that this large ice mass will melt away over the next 3000 years if anthropogenic CO2 emissions continue to rise (Alley et al., 2006). As a result, the bedrock currently covered by ice will lie free and become subject to chemical weathering. The resulting weathering fluxes will contribute to increase both the consumption rate of atmospheric CO2 and the production rate of riverine bicarbonate. Increasing these two fluxes will tend to decrease the atmospheric CO2 partial pressure, as a result of the modified ocean-atmosphere carbon cycle. Chemical weathering may thus possibly act as a negative feedback in the Greenhouse World. Other changes (e.g., vegetation cover and additional climate change) concomitant with the melting of the Greenland ice-sheet may either amplify or dampen, if not reverse the weathering effect. Here we use the intermediate complexity Earth System model CLIMBER-2 to quantify and analyse the weathering flux changes that result from the projected melting of the Greenland ice sheet and the implications for atmospheric CO2. The biogeochemical module of CLIMBER-2 has been extended to account for the consumption of atmospheric CO2 and the production of riverine bicarbonate by continental weathering processes, as a function of geographically distributed runoff (interactively provided by the CLIMBER-2 climate module) and lithology (derived from Amiotte Suchet et al., 2003). We find that the increased weathering processes alone would lead to a sustained 0.2 ppm/kyr decrease in atmospheric pCO2. The climate change resulting from the deglaciation of Greenland reduces the magnitude of this trend to 0.1 ppm/kyr. Only in the case where the effect of freshly comminuted bedrock is taken into account (Clark et al., 2006) does the weathering feedback help to reduce atmospheric pCO2 by about 10 ppm in 5000 years. Alley R.B. et al. (2005) Science 310, 456–460. Amiotte Suchet P. et al. (2003) Global Biogeochemical Cycles 17, 1139, doi:10.1029/2002GB001891. Clark P.U. et al. (2006) Quaternary Science Reviews 25, 3150–3184. [less ▲]

Detailed reference viewed: 262 (1 ULiège)
Full Text
See detailModelling the evolution of climate and sea level over the third millennium (MILMO)
Fichefet, Thierry; Driesschaert, Emmanuelle; Goosse, Hugues et al

Report (2007)

A new three-dimensional Earth system model of intermediate complexity was developed. This model, named LOVECLIM, consists of five major components representing the atmosphere (ECBilt), the ocean and sea ... [more ▼]

A new three-dimensional Earth system model of intermediate complexity was developed. This model, named LOVECLIM, consists of five major components representing the atmosphere (ECBilt), the ocean and sea ice (CLIO), the terrestrial biosphere (VECODE), the oceanic carbon cycle (LOCH) and the Greenland and Antarctic ice sheets (AGISM). It also includes a global glacier-melt algorithm which is run in off-line mode. It is worth mentioning that there are very few models of this type worldwide. ECBilt is a quasi-geostrophic atmospheric model with 3 levels and a T21 horizontal resolution. It includes simple parameterisations of the diabatic heating processes and an explicit representation of the hydrological cycle. Cloud cover is prescribed according to present-day climatology. CLIO is a primitive-equation, free-surface ocean general circulation model coupled to a thermodynamic–dynamic sea-ice model. Its horizontal resolution is 3° × 3°, and there are 20 levels in the ocean. VECODE is a reduced-form model of vegetation dynamics and of the terrestrial carbon cycle. It simulates the dynamics of two main terrestrial plant functional types (trees and grassland) at the same resolution as that of ECBilt. LOCH is a comprehensive model of the oceanic carbon cycle that takes into account both the solubility and biological pumps. The version utilised here has the same resolution as the one of CLIO, which greatly facilitates the coupling between both models. Finally, AGISM is composed of a three-dimensional thermomechanical model of the ice sheet flow, a visco-elastic bedrock model and a model of the mass balance at the ice–atmosphere and ice–ocean interfaces. The Antarctic ice-sheet module also contains a model of the ice-shelf dynamics to enable interactions with the ocean and migration of the grounding line. For both ice sheets, calculations are made on a 10 km × 10 km resolution grid with 31 sigma levels. The performance of LOVECLIM was assessed by conducting ensemble simulations over the last few centuries. Starting from different initial conditions, the model was integrated from year 1500 AD up to year 2000 AD with solar irradiance, volcanic activity, tropospheric ozone amount, greenhouse-gas (including CO2) concentrations and sulphate-aerosol load evolving with time according to reconstructions. Over the last 140 years, the model simulates a global surface warming ranging from 0.33°C to 0.43°C, with a mean value of 0.38°C. This value is about 0.15°C lower than the observed one. A detailed analysis of the results has revealed the model behaves reasonably well at mid- and high latitudes. By contrast, at low latitudes, the agreement between the model results and observational estimates is less good, especially in the Southern Hemisphere. In those regions, LOVECLIM significantly underestimates the warming and the climate variability observed during the last few decades. The coarse resolution of the model and the simplified representation of the atmospheric dynamical and physical processes seem to be the two major candidates responsible for this deficiency. Regarding the Greenland ice sheet, we found a slightly increasing ice volume during the period 1700–2000 AD. This trend is largely explained as a residual response to the late Holocene forcing, in particular to the Little Ice Age cooling after year 1500 AD. The effect is not particularly large, however, amounting to only 1.2 cm of global sea-level rise over the entire period. The growing trend stabilizes during the 20th century, with almost no net effect on ice volume. Only during the last decades of the 20th century, the ice volume begins to decrease in response to the imposed warming. We also found the Antarctic ice sheet to be retreating slowly at a rate equivalent to a global sea-level rise of about 1.7 cm during the 20th century. This evolution is mostly due to a long-term background trend of +2.6 cm, mitigated by about 0.9 cm from slightly rising accumulation rates over the same period. The ongoing dominance of past climatic changes on the contemporary ice-sheet evolution is a fine illustration of the inertia encountered when studying the response of large continental ice sheets. In this case, it mainly results from an ongoing grounding-line retreat in West Antarctica following rising sea levels since the Last Glacial Maximum. As far as mountain glaciers and small ice caps are concerned, their area and volume are found to reach a maximum in the late 19th century corresponding to the Little Ice Age, but this maximum and the ensuing 20th century glacier retreat are not very pronounced. Over the last hundred years, the model simulates an ice loss equivalent to only 0.89 cm of sea-level rise. This value is at the lower end compared to other assessments. One reason is the low total ice volume assumed by the global glacier-melt algorithm (about 20 cm of total sea-level rise, a factor 2.5 less than previous estimates). A second reason is the prescribed global ice mass balance for the 1961–1990 reference period, which is also at the lower end of other simulations. For the 20th century, LOVECLIM explains about 7.6 cm of sea-level rise. The bulk of that value, about 4.7 cm, comes from thermal expansion of the World Ocean. The Antarctic and Greenland ice sheets combined lead to a sea-level rise of 2 cm, and glaciers and ice caps are responsible for about 0.9 cm of sea-level rise. These numbers are similar to those that have been derived for the IPCC Third Assessment Report (TAR) for the same components except for the lower glacier contribution as found here. Over the industrial era, the net uptake of carbon by the ocean simulated by LOVECLIM is within the range of current estimates, although at the lower end of this range. It should be noted that a detailed evaluation of the performance of the terrestrial carbon-cycle module was impossible to perform given the very wide range of available data. Experiments with interactive atmospheric CO2 concentration were also carried out with LOVECLIM forced by CO2 emissions from fossil fuel burning and land-use change. Interestingly enough, the atmospheric CO2 level computed by the model in year 2000 AD compares relatively well with the observed one. A series of climate-change projections were then conducted over the 21st century. In these experiments, LOVECLIM was driven by changes in greenhouse-gas (including CO2), tropospheric ozone and sulphate-aerosol concentrations following the IPCC SRES scenarios B1, A1B and A2. In year 2100 AD, the model predicts a globally averaged, annual mean surface warming of 1°C, 1.4°C and 1.8°C for scenarios B1, A1B and A2, respectively, and an associated increase in precipitation of 3.6%, 5.1% and 6.6%, respectively. In agreement with studies performed with climate general circulation models (CGCMs), a weakening of the Atlantic meridional overturning circulation (MOC) is noticed in all runs. At the end of the 21st century, the decrease in the maximum value of the annual mean meridional overturning streamfunction below the surface layer in the Atlantic basin, which is an index of the MOC intensity, reaches 19% for scenario B1, 21% for scenario A1B and 27% for scenario A2. In our model, as in the majority of CGCMs, this decrease is caused more by changes in surface heat flux than by changes in surface freshwater flux. Under the forcing scenario A1B, LOVECLIM simulates a global sea-level rise of 31.3 cm in year 2100 AD. As for the 20th century, the most important contributor is the oceanic thermal expansion (+18.8 cm), followed by the contributions from the Greenland ice sheet (+5.2 cm), glaciers and ice caps (+3.8 cm) and the Antarctic ice sheet (+3.5 cm). The total rise is equivalent to a quadrupling of the sea-level rise simulated for the 20th century. Our sea-level value is somewhat lower than the central estimate for the same four components of about 40 cm in the IPCC TAR predictions. This can be explained by the low climate sensitivity of LOVECLIM, and hence the lower global temperature rise, which mostly affects the largest contribution of thermal expansion of the World Ocean. Another difference with the IPCC TAR predictions is the positive contribution from Antarctica of several cm of sea-level rise. That is in contrast to most other simulations showing a growing ice sheet and a negative contribution to global sea level of typically between -5 and -20 cm. The IPCC TAR also found a generally larger contribution from mountain glaciers and small ice caps. Our glacier-volume loss is smaller because of the lower initial glacier volume assumed by the glacier-melt algorithm. The total projected sea-level rise for the 21st century is only slightly affected by the scenario itself. For the range of SRES scenarios used by LOVECLIM, the total sea-level rise is found to vary between +22 and +35 cm by year 2100 AD. The much larger range of between +9 and +88 cm obtained for the IPCC TAR arose mainly from the inclusion of model uncertainties, and not from the greenhouse-gas-forcing scenarios employed. As expected, climate change impacts the air–sea CO2 exchange in the model by lowering the solubility and hence the net uptake of carbon by the ocean. The effect is however rather modest at the century time-scale given the moderate increase in sea-surface temperature simulated by LOVECLIM. In addition, we do not observe any significant change in the oceanic biology at the global scale during the 21st century. The picture is a bit different regarding the terrestrial biosphere. Both the climate and fertilization effects strongly increase the carbon uptake in VECODE. A number of experiments with interactive atmospheric CO2 concentration were also carried out over the 21st century. Contrary to other modelling studies, LOVECLIM predicts lower atmospheric CO2 levels at the end of the 21st century when the effect of climate change on the carbon cycle is accounted for in the model. The warming enhances the net uptake of carbon by the terrestrial biosphere which more than offsets the reduction in oceanic uptake resulting from the solubility decrease. Finally, we have thoroughly analysed the model response to a range of stabilized anthropogenic forcings over the next millennia. For the variety of forcing scenarios considered, LOVECLIM simulates a globally averaged, annual mean surface warming ranging between 0.55°C and 3.75°C and an associated decrease in Arctic and Antarctic sea-ice extent. However, no simulation predicts an entirely ice-free Arctic Ocean during summertime at the millennium time-scale. In the most pessimistic case, a small ice pack of about 0.5×106 km2 persists. Our results also suggest that it is very likely that the volume of the Greenland ice sheet will largely decrease in the future. After 1000 years of model integration, the ice volume is reduced by more than 20% when the radiative forcing is higher than 6.5 W m-2. Moreover, for a radiative forcing greater than 7.5 W m-2, the ice sheet melts away in less than 3000 years. Note that the ice-sheet disintegration might be even more rapid if processes responsible for the widespread glacier acceleration currently observed in Greenland were taken into consideration in the model. We also found that the freshwater flux from the melting Greenland ice sheet into the neighbouring oceans, which peaks in the most extreme scenario tested at 0.11 Sv (1 Sv = 10^6 m3 s-1) and remains above 0.1 Sv during three centuries, is not large enough to trigger a shutdown of the Atlantic MOC in our model, in contrast to some other models. Those models are however more responsive to freshwater perturbations than ours. Besides, we showed that climate feedbacks play a crucial role in the ice-sheet evolution and that the Greenland deglaciation considerably enhances the greenhouse-gas-induced warming over Greenland and the central Arctic. This stresses the importance of incorporating the two-way interactions between the Greenland ice sheet and climate in climate- and sea-level-change projections at the millennial time-scale. For the Antarctic ice sheet, the response is much less drastic than for the Greenland ice sheet. For instance, after 3000 years of 4×CO2 forcing (∼7.7 W m-2), the Antarctic grounded ice volume and area are reduced in our model by only 8% and 4%, respectively. For a sustained radiative forcing of 8.5 W m-2 (the highest forcing scenario considered in our study), LOVECLIM predicts a global sea-level rise of 7.15 m by year 3000 AD. Most of it is due to melting of the Greenland ice sheet (+4.25 m), followed by melting of the Antarctic ice sheet (+1.42 m), thermal expansion (+1.29 m) and the contribution from mountain glaciers and small ice caps (+0.19 m). Our results show that it will be very difficult to limit the eventual sea-level rise to less than 1 m after 1000 years, unless the atmospheric CO2 concentration can be stabilized to less than twice its pre-industrial level. Such a goal can only be reached by emission reductions far larger than any policy currently pursued. Concerning the carbon cycle, the experiments carried out with LOVECLIM highlight the opposite responses of the terrestrial and oceanic carbon reservoirs to climate change. We also found that, when anthropogenic CO2 emissions cease, the terrestrial biosphere becomes a weak carbon source, while the ocean continues to be a sink. It should be mentioned that no dramatic change in the global marine productivity is observed in our simulations. This arises from the fact that the modifications of the oceanic properties that affect this productivity (stratification, meridional overturning, …) are rather moderate. The effects of climate change are however not negligible. In particular, the decrease in sea-ice extent predicted by the model results in a longer growing season and a larger nutrient uptake (especially silica) in polar regions. As a result, by the end of the 23rd century, silica concentrations in the upper 100 m of the Southern Ocean drop by as much of 30% for the most extreme forcing scenarios. [less ▲]

Detailed reference viewed: 68 (6 ULiège)
Full Text
See detailModeling the influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during the next millennia
Driesschaert, E.; Fichefet, T.; Goosse, H. et al

in Geophysical Research Letters (2007), 34(10),

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 76 (10 ULiège)
See detailAtmospheric Carbon Dioxide and Climate Over Phanerozoic Times
François, Louis ULiege; Lefèbvre, Vincent; Goddéris, Yves et al

Conference (2006, December)

The atmospheric CO2 mixing ratio has fluctuated widely over the Phanerozoic, according to the estimates from available proxy records. Because atmospheric CO2 is a major greenhouse gas, these fluctuations ... [more ▼]

The atmospheric CO2 mixing ratio has fluctuated widely over the Phanerozoic, according to the estimates from available proxy records. Because atmospheric CO2 is a major greenhouse gas, these fluctuations should have led to significant climatic variations. The "classical" view is indeed that atmospheric CO2 has been the main driver of the Earth's climate history. On long-term time scales, the atmospheric CO2 level is the result of the balance between CO2 inputs from volcanoes or oxidation of old organic carbon (kerogen) in exposed rocks and outputs through silicate weathering or organic carbon deposition. Existing model reconstructions of the Phanerozoic history of atmospheric CO2 are based on such budgets. Recent data and model experiments currently challenge these models. First, the carbon cycle may be more complex than represented in the earliest models. In particular, silicate weathering depends on numerous factors, which are not obvious to model or are poorly known over the Phanerozoic. Mountain uplift is one such factor, which has been much debated in the last decade. Lithology is another example: basalts weather much more rapidly than other silicate rocks and the emplacement of large basaltic areas on the continents may trigger glaciations. Continental configuration is also more important than previously thought, as indicated by recent model experiments on super-continent fragmentation coupling geochemical and climate models. Problems of "classical" Phanerozoic CO2 models are also well illustrated by the fact that the most recent estimates of CO2 degassing show very little variation between the Cretaceous and the present, a period when large changes in CO2 have occurred, whereas degassing is the most important forcing of CO2 evolution in long-term carbon cycle models. Second, CO2 is not the only driver of climate evolution. This obvious fact has largely been forgotten in Phanerozoic studies. What the proxies tell us on paleo-atmospheric CO2 is not always in line with what we know about paleoclimatic records. For instance, the proxies suggest relatively high CO2 levels during the Late Ordovician glaciations. Similarly, the Late Jurassic now appears to be colder than earlier thought, while again proxies suggest high atmospheric CO2 at that time. The mid-Miocene climate warming, which occurs simultaneously with a drop in CO2, provides another example. This latter change in CO2 is unanimously reflected in all proxies and, so, this decoupling between CO2 and climate cannot arise from uncertainties on the reconstructed CO2 levels or from dating problems, as might be the case of the former two examples. Other climatic drivers than CO2 clearly need to be considered. In this respect, vegetation- climate feedbacks have been completely disregarded in long-term climatic studies. Cenozoic cooling is, however, accompanied by a progressive transition from closed forests to more widespread grasslands and deserts on the continental areas, a change which must have had major impacts on the surface albedo and the water cycle. [less ▲]

Detailed reference viewed: 51 (4 ULiège)
See detailModeling the interactions between the Greenland ice sheet and climate during the next millennia
Fichefet, T.; Driesschaert, E.; Goosse, H. et al

Conference (2006, October 26)

Detailed reference viewed: 7 (0 ULiège)
Full Text
See detailInteractive comment on “On the application and interpretation of Keeling plots in paleo climate research – deciphering d13C of atmospheric CO2 measured in ice cores” by P. Köhler et al.
Munhoven, Guy ULiege

in Biogeosciences Discussions (2006), 3

The paper "On the application and interpretation of Keeling plots in paleo climate research – deciphering delta13C of atmospheric CO2 measured in ice cores" by P. Köhler et al. is reviewed and commented.

Detailed reference viewed: 28 (2 ULiège)
See detailOn the warming asymmetry between Europe and North America in climate change projections
Driesschaert, E.; Fichefet, T.; Goosse, H. et al

Poster (2006, April 05)

Detailed reference viewed: 26 (1 ULiège)
See detailImpact of a Greenland deglaciation on climate during the next millennia
Driesschaert, Emmanuelle; Brovkin, Victor; Fichefet, Thierry et al

Conference (2006, April 04)

A new Earth system model of intermediate complexity, LOVECLIM, has been developed in order to study long-term future climate changes. It includes an interactive Greenland and Antarctic ice sheet model ... [more ▼]

A new Earth system model of intermediate complexity, LOVECLIM, has been developed in order to study long-term future climate changes. It includes an interactive Greenland and Antarctic ice sheet model (AGISM) as well as an oceanic carbon cycle model (LOCH). Those climatic components can have a great impact on future climate. The few studies in recent literature assessing the impact of polar ice sheets on future climate draw very different conclusions, which shows the need for developing such a model. A set of numerical experiments have been performed in order to study the possible perturbations of climate induced by human activities over the next millennia. A particular attention is given to the Greenland ice sheet. In most of the projections, the Greenland ice sheet undergoes a continuous reduction in volume, leading to an almost total disappearance in the most pessimistic scenarios. The impact of the Greenland deglaciation on climate has therefore been assessed through a sensitivity experiment using the scenario SRES A2. The removal of the Greenland ice sheet is responsible for a regional amplification of the global warming inducing a total melt of Arctic sea ice in summer. The freshwater flux from Greenland generates large salinity anomalies in the North Atlantic Ocean that reduce the rate of North Atlantic Deep Water formation, slowing down slightly the oceanic thermohaline circulation. [less ▲]

Detailed reference viewed: 47 (0 ULiège)
See detailProjections of ice sheet and sea level changes over the next millennia with the LOVECLIM Earth System Model
Janssens, I.; Huybrechts, P.; Raper, S. et al

Poster (2006, April 04)

Detailed reference viewed: 11 (0 ULiège)
See detailGlacial-interglacial rain ratio variations: effect on atmospheric CO2 levels and sedimentary carbonate preservation/dissolution processes
Munhoven, Guy ULiege

Conference (2006, February 10)

A reduction of the carbonate-carbon to organic-carbon export rain ratio during glacial times is commonly advanced to explain an important part of the observed glacial-interglacial atmospheric CO2 ... [more ▼]

A reduction of the carbonate-carbon to organic-carbon export rain ratio during glacial times is commonly advanced to explain an important part of the observed glacial-interglacial atmospheric CO2 variation. This hypothesis was tested and side-effects on the evolution of carbonate preservation/dissolution in the surface sediment explored with a multi-box model (MBM) of the ocean carbon cycle, fully coupled to a new transient advection-diffusion-reaction model (called MEDUSA) representing early diagenesis processes of carbonate minerals in the surface sediment. MEDUSA explicitly considers the role of organic matter remineralisation in the sediment column to enhance calcite (and aragonite) dissolution. It is fully bi-directional and takes chemical erosion into account in times when carbonate dissolution makes the sediment mixed-layer collapse faster than the sediment supply to the surface is able to counterbalance. Coupled model experiments were run for 240,000 years, forced by variable sea-level, temperature and salinity histories, and variable continental weathering inputs. Various scenarios for the evolution of the rain ratio over glacial to interglacial periods were adopted. A peak reduction of the rain ratio by 40% at the Last Glacial Maximum (LGM) was found to produce a net atmospheric pCO2 reduction of about 30ppm, on top of a 60ppm reduction produced by changing continental shelf carbonate accumulation and changing continental weathering inputs. The overall 90ppm oscillation compares well with the observed data. However, the effect on the model sedimentary record is clearly at odds with actual sediment records. The changes related to continental shelf processes and variable weathering flux depress the calcite saturation horizon by about 1km at the LGM; if rain ratio variations are also considered, that depression increases by another km. An assessment of the respective contributions from various model parameters will be presented. [less ▲]

Detailed reference viewed: 25 (0 ULiège)
See detailFuture ocean carbon cycle: a study of feedbacks with the LOVECLIM model
Mouchet, Anne ULiege; Driesschaert, E.; Brovkin, V. et al

Poster (2006, February)

Detailed reference viewed: 17 (0 ULiège)
Full Text
See detailOn the application and interpretation of Keeling plots in paleo climate research - Deciphering δ 13C of atmospheric CO 2 measured in ice cores
Köhler, Peter; Fischer, Hubertus; Schmitt, Jochen et al

in Biogeosciences (2006), 3(4), 539-556

The Keeling plot analysis is an interpretation method widely used in terrestrial carbon cycle research to quantify exchange processes of carbon between terrestrial reservoirs and the atmosphere. Here, we ... [more ▼]

The Keeling plot analysis is an interpretation method widely used in terrestrial carbon cycle research to quantify exchange processes of carbon between terrestrial reservoirs and the atmosphere. Here, we analyse measured data sets and artificial time series of the partial pressure of atmospheric carbon dioxide (pCO(2)) and of delta C-13 of CO2 over industrial and glacial/interglacial time scales and investigate to what extent the Keeling plot methodology can be applied to longer time scales. The artificial time series are simulation results of the global carbon cycle box model BICYCLE. The signals recorded in ice cores caused by abrupt terrestrial carbon uptake or release loose information due to air mixing in the firn before bubble enclosure and limited sampling frequency. Carbon uptake by the ocean cannot longer be neglected for less abrupt changes as occurring during glacial cycles. We introduce an equation for the calculation of long-term changes in the isotopic signature of atmospheric CO2 caused by an injection of terrestrial carbon to the atmosphere, in which the ocean is introduced as third reservoir. This is a paleo extension of the two reservoir mass balance equations of the Keeling plot approach. It gives an explanation for the bias between the isotopic signature of the terrestrial release and the signature deduced with the Keeling plot approach for long-term processes, in which the oceanic reservoir cannot be neglected. These deduced isotopic signatures are similar (-8.6 parts per thousand) for steady state analyses of long-term changes in the terrestrial and marine biosphere which both perturb the atmospheric carbon reservoir. They are more positive than the delta C-13 signals of the sources, e.g. the terrestrial carbon pools themselves (similar to -25 parts per thousand). A distinction of specific processes acting on the global carbon cycle from the Keeling plot approach is not straightforward. In general, processes related to biogenic fixation or release of carbon have lower y-intercepts in the Keeling plot than changes in physical processes, however in many case they are indistinguishable (e.g. ocean circulation from biogenic carbon fixation). [less ▲]

Detailed reference viewed: 118 (10 ULiège)
See detailModelling the Global Riverine U Fluxes to the Oceans
Riotte, Jean; Goddéris, Yves; Chabaux, François et al

Conference (2005, May 21)

Mean U isotopic ratio of the ocean has remained roughly constant since about 600 kyrs (Henderson, 2001). This 1.14 value cannot be explained considering the present day value of the U riverine ratio (1.17 ... [more ▼]

Mean U isotopic ratio of the ocean has remained roughly constant since about 600 kyrs (Henderson, 2001). This 1.14 value cannot be explained considering the present day value of the U riverine ratio (1.17, Chabaux et al., 2001). However, the mean riverine ratio was calculated on half of the total continental runoff. Is this partial mean value really representative of the mean value? If yes, might this value have changed over a glacial-interglacial cycle ? We build up a numerical model calculating the flux of U transfer to the ocean through weathering. The spatial resolution of the model reaches 0.5°lat x0.5°long. Lithology is modified from Amiotte-Suchet et al. (2003). Weathering fluxes are estimated from simple parametric laws, calculating the flux of total dissolved solids from mean annual temperature and runoff. Soil PCO2 is used to estimate carbonate dissolution rates, and is calculated from a simulation of the Caraib model. Uranium fluxes are estimated proportional to the TDS flux, weighted by its abundance in the source rock. CO2 consumption through weathering is simultaneously computed. The 234U/238U ratio of the river is calculated according to a correlation existing between the measured 234U/238U and runoff, showing a decrease of this ratio with increasing runoff. The model is first validated over several large watersheds, including the Amazon, the Ganges-Brahmapoutra, the Mississippi, and the Congo rivers. Global runs are then performed, showing that the modelled mean global value is close to the measured partial mean of 1.17. We explore then possible variations of the modelled ratio at the last glacial maximum. Temperature and runoff fields are taken from LGM simulations of the ECHAM GCM. Extension of ice sheets is assumed to cut off part of the weathering fluxes, producing possible fluctuations in the riverine U isotopic ratio, as well as changes in the regional runoff pattern. [less ▲]

Detailed reference viewed: 21 (3 ULiège)
See detailFuture anthropogenic emissions and climate change impact on the carbon cycle; a study with the LOVECLIM model
Mouchet, Anne ULiege; Driesschaert, E.; Fichefet, T. et al

Conference (2005, May)

Detailed reference viewed: 19 (1 ULiège)
See detailThe combined impact of changing terrestrial organic carbon reservoirs and fractionation effects induced by changing carbonate ion concentrations on the glacial-interglacial marine C-13 record
Munhoven, Guy ULiege; François, Louis ULiege; Ouberdous, Mohamed ULiege

Conference (2005, April 27)

On the basis of the marine and atmospheric glacial-interglacial C-13 isotopic records, it has been calculated that the land biospheric carbon stock must have increased by 270–720 GtC from the Last Glacial ... [more ▼]

On the basis of the marine and atmospheric glacial-interglacial C-13 isotopic records, it has been calculated that the land biospheric carbon stock must have increased by 270–720 GtC from the Last Glacial Maximum (LGM) to the present. Estimates derived from vegetation mapping based on palynological or sedimentological data generally indicate a larger increase of the biospheric stock in the range of 700–1350 GtC and above. Although there is some overlap between the two ranges, they substantially disagree. Further complications arise when carbonate-ion dependent fractionation effects in the marine C-13 record are considered. A detailed budget of the C-13 isotope in the land biosphere at the LGM, as well as in the other reservoirs of the global carbon cycle is therefore required. Here, we analyse the response of the atmosphere–ocean–surface sediment system under the influence of variable release and uptake fluxes of C by the terrestrial biosphere. Glacial-interglacial variations of the carbon stocks and isotopic budgets of the land biosphere were derived from simulation experiments carried out with the global biosphere model CARAIB (CARbon Assimilation In the Biosphere) under boundary conditions typical for the Last Glacial Maximum and for mid-Holocene times. CARAIB uses a mechanistic description of both C3 and C4 photosynthetic pathways. It thus provides information on the C-13 signature of carbon fluxes involved. Using the eleven-box model MBM of the ocean-atmosphere system, we then investigate the effect of these biospheric changes on the oceanic carbon cycle and the CO2 concentration in the atmosphere. MBM has a complete representation of the transfer processes of carbon and alkalinity from the land to the ocean, and between the ocean and the surface sediment, including parameterisations for processes in the shelf area. MBM also considers C-13 signatures of the carbon fluxes and stocks represented. On the basis of empirical relationships for the incorporation of C-13 isotopes in foraminiferal shells as a function of carbonate ion concentration, synthetic carbon isotopic records are generated from the calculated seawater C-13 isotopic evolution, helping to better constrain estimates of the land biosphere carbon stock changes derived from the marine C-13 record. These simulations also test various scenarios for the alkalinity input to the system from weathering, which, through their effect on carbonate ion concentration, may also impinge to a non negligible extent on C-13 variations recorded in deep-sea sediments. [less ▲]

Detailed reference viewed: 18 (0 ULiège)
Full Text
See detailQuantitative interpretation of atmospheric carbon records over the last glacial termination
Köhler, Peter; Fischer, Hubertus; Munhoven, Guy ULiege et al

in Global Biogeochemical Cycles (2005), 19(4), 4020

The glacial/interglacial rise in atmospheric pCO(2) is one of the best known changes in paleoclimate research, yet the cause for it is still unknown. Forcing the coupled ocean-atmosphere-biosphere box ... [more ▼]

The glacial/interglacial rise in atmospheric pCO(2) is one of the best known changes in paleoclimate research, yet the cause for it is still unknown. Forcing the coupled ocean-atmosphere-biosphere box model of the global carbon cycle BICYCLE with proxy data over the last glacial termination, we are able to quantitatively reproduce transient variations in pCO(2) and its isotopic signatures (delta C-13, Delta C-14) observed in natural climate archives. The sensitivity of the Box model of the Isotopic Carbon cYCLE ( BICYCLE) to high or low latitudinal changes is comparable to other multibox models or more complex ocean carbon cycle models, respectively. The processes considered here ranked by their contribution to the glacial/interglacial rise in pCO(2) in decreasing order are: the rise in Southern Ocean vertical mixing rates (> 30 ppmv), decreases in alkalinity and carbon inventories (> 30 ppmv), the reduction of the biological pump (similar to 20 ppmv), the rise in ocean temperatures (15 - 20 ppmv), the resumption of ocean circulation (15 - 20 ppmv), and coral reef growth (< 5 ppmv). The regrowth of the terrestrial biosphere, sea level rise and the increase in gas exchange through reduced sea ice cover operate in the opposite direction, decreasing pCO(2) during Termination I by similar to 30 ppmv. According to our model the sequence of events during Termination I might have been the following: a reduction of aeolian iron fertilization in the Southern Ocean together with a breakdown in Southern Ocean stratification, the latter caused by rapid sea ice retreat, trigger the onset of the pCO(2) increase. After these events the reduced North Atlantic Deep Water (NADW) formation during the Heinrich 1 event and the subsequent resumption of ocean circulation at the beginning of the Bolling-Allerod warm interval are the main processes determining the atmospheric carbon records in the subsequent time period of Termination I. We further deduce that a complete shutdown of the NADW formation during the Younger Dryas was very unlikely. Changes in ocean temperature and the terrestrial carbon storage are the dominant processes explaining atmospheric d13C after the Bolling-Allerod warm interval. [less ▲]

Detailed reference viewed: 32 (4 ULiège)
See detailLOVECLIM, a three-dimensional model of the Earth system for investigating long-term climate changes
Driesschaert, E.; Brovkin, V.; Fichefet, T. et al

Conference (2003, September)

Detailed reference viewed: 24 (0 ULiège)
Full Text
See detailLOVECLIM, a three-dimensional model of the Earth system for investigating long-term climate changes
Driesschaert, E.; Fichefet, T.; Goosse, G. et al

Poster (2003, April 08)

A three-dimensional global model of the Earth system suitable for studying the long-term evolution of climate (LOVECLIM) has been recently developed. This model is made up of a coarse-resolution three ... [more ▼]

A three-dimensional global model of the Earth system suitable for studying the long-term evolution of climate (LOVECLIM) has been recently developed. This model is made up of a coarse-resolution three-dimensional atmosphere-sea-ice-ocean model (ECBILT-CLIO), a dynamical model of the continental biosphere (VECODE), a comprehensive model of the oceanic carbon cycle (LOCH), and a high-resolution thermomechanical model of the Greenland and Antarctic ice sheets (AGISM). The atmospheric component has the big advantage that it has been simplified to a level that makes runs on a multi-century time-scale computationaly feasible, while still being capable of producing results that, on the whole, are comparable to those of atmospheric general circulation models. The performance of the coupled model is evaluated by performing ensemble simulations over the period 1500-2000 and by comparing the model results to available climate reconstructions. In these simulations, the following forcings are taken into consideration : the variations in solar irradiance, the volcanic activity, the anthropogenic emissions of CO2, and the changes in concentration of other greenhouse gases and sulphate aerosols resulting from human activities. In the future, the model will be used to investigate the evolution of climate and sea level over the third millennium. [less ▲]

Detailed reference viewed: 18 (1 ULiège)
See detailThe Glacial Carbon Cycle:Changing continental weathering and glacial-interglacial atmospheric CO2 variations.
Munhoven, Guy ULiege

Conference (2002, December 12)

The role of continental weathering in the global carbon cycle is detailed and a quantitative analysis presented.

Detailed reference viewed: 21 (0 ULiège)
Full Text
See detailGlacial-interglacial changes of continental weathering: estimates of the related CO2 and HCO3- flux variations and their uncertainties
Munhoven, Guy ULiege

in Global and Planetary Change (2002), 33(1-2), 155-176

A range of estimates for the glacial-interglacial variations in CO, consumption and HCO3- production rates by continental weathering processes were calculated with two models of continental weathering ... [more ▼]

A range of estimates for the glacial-interglacial variations in CO, 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 Probst's Global Erosion Model for CO2 Consumption (GEM-CO2) [C. R, Acad. Sci. Paris, Ser. 11317 (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=10(12) 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 HCOT 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-CO, 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-CO, 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 p(CO2) by 5.7+/-1.3 ppmv (GKWM) or 3 12.1+/-1.7 ppmv (GEM-CO2), which might thus represent a non-negligible part of the observed glacial interglacial variation of similar to 75 ppmv. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Detailed reference viewed: 62 (2 ULiège)