Long-term biogeochemical impacts of liming the ocean

Conference (2011, December 08)

Fossil fuel CO2 emissions result in large-scale long-term perturbations in seawater chemistry. Oceans take up atmospheric CO2, and several geo-engineering approaches have been suggested to mitigate ... [more ▼]

Fossil fuel CO2 emissions result in large-scale long-term perturbations in seawater chemistry. Oceans take up atmospheric CO2, and several geo-engineering approaches have been suggested to mitigate impacts of CO2 emissions and resulting ocean acidification that are based on this property. One of them is to enhance weathering processes to remove atmospheric CO2. This method involves dissolving rocks (i.e. limestone) or adding strong bases (i.e. calcium hydroxide) in the upper ocean and is termed as liming the oceans. The net effect of this approach is to increase ocean alkalinity, thereby increasing the oceanic capacity to store anthropogenic CO2. Another effect of adding alkalinity would be to drive seawater to higher pH values and thus counteract the ongoing ocean acidification. However, whereas adding bases only alter alkalinity of seawater, dissolution of carbonates perturb both, alkalinity and dissolved inorganic carbon budgets. Thus, on longer time scales, these two methods will likely have different biogeochemical effects in the ocean. Here we test enduring implications of the two approaches for marine carbon cycle using the global ocean biogeochemical model HAMOCC. In our model scenarios we add alkalinity in the amounts proportional to fossil fuel emissions. We compare the longterm effectiveness of the two geo-engineering approaches to decrease atmospheric CO2. [less ▲]

Climate-carbon cycle feedback during glacial cycles

Conference (2011, August 15)

Paleoclimate records reveal a close link between global ice volume and atmospheric CO2 concentration, at least, through the last 800,000 years. Despite many efforts over the last two decades, mechanisms ... [more ▼]

Paleoclimate records reveal a close link between global ice volume and atmospheric CO2 concentration, at least, through the last 800,000 years. Despite many efforts over the last two decades, mechanisms of glacial-interglacial CO2 variability and its role for the glacial cycles remain elusive. Here using the Earth system model of intermediate complexity CLIMBER-2 which includes all major components of the Earth system – atmosphere, ocean, land surface, ice sheets, terrestrial biota, eolian dust and marine biogeochemistry – we performed simulations of the last glacial cycles employing variations in the Earth’s orbital parameters as the only prescribed climatic forcing. In the experiments with constant CO2 concentration, temporal dynamics of the simulated glacial cycles strongly depend on the CO2 level. For CO2 concentrations about and above preindustrial one, the model simulates only short glacial cycles with precessional and obliquity frequencies. However, for lower CO2 concentrations the model simulates long glacial cycles with dominant 100 kyr periodicity. Simulated glacial cycles agreed favorably with paleoclimate reconstructions, but their amplitude is underestimated compared to those of the simulations with time-dependent CO2 concentration. These results confirm that the positive climate-carbon cycle feedback plays an important role in amplification of long glacial cycles. Experiments with fully interactive CO2 shed some light on the mechanism of climate-carbon cycle feedback during glacial cycles. Forced by orbital variations only, the model is able to reproduce the main features of CO2 changes: the 40 ppmv CO2 drop during glacial inception, the minimum concentration at the last glacial maximum being 80 ppmv lower than the Holocene value, and the relatively abrupt CO2 rise during the deglaciation. The main drivers of atmospheric CO2 evolve with time: changes in sea surface temperature and volume of bottom water of southern origin exert CO2 control during glacial inception and deglaciation, while changes in carbonate chemistry and marine biology are dominant during the first and second parts of the glacial cycles, respectively. [less ▲]

The interglacial carbon cycle

Poster (2011, April 07)

Explaining the difference in carbon cycle dynamics (and hence atmospheric CO2) between various interglacials is an elusive issue. Several biogeochemical mechanisms of different origin are involved in ... [more ▼]

Explaining the difference in carbon cycle dynamics (and hence atmospheric CO2) between various interglacials is an elusive issue. Several biogeochemical mechanisms of different origin are involved in interglacial CO2 dynamics, leading to a CO2 release from the ocean (carbonate compensation, coral growth) compensated by a land carbon uptake (biomass and soil carbon buildup, peat accumulation). The balance between these fluxes of CO2 is delicate and time-dependent, and it is not possible to provide firm constraints on these fluxes from proxy data. The best framework for quantification of all these mechanisms is an Earth System model that includes all necessary physical and biogeochemical components of the atmosphere, ocean, and land. To perform multi-millennial model integrations through the Holocene, Eemian, and MIS11, we use an earth system model of intermediate complexity, CLIMBER-2, coupled to the dynamic global vegetation model LPJ with a recently implemented module for boreal peatland dynamics. During glacial-interglacial cycles, the carbon cycle never is in complete equilibrium due to a number of small but persistent fluxes such as terrestrial weathering. This complicates setting up interglacial experiments as the usual approach to start model integrations from an equilibrium state is not valid any more. In order to circumvent the problem of non-equilibrium initial conditions, the model is initialised with the oceanic biogeochemistry state taken from a transient simulation through the last glacial cycle with CLIMBER-2 only. In this simulation, the CLIMBER-2 model was run through the last glacial cycle with carbon cycle in “offline mode” as interactive components of the physical climate system (atmosphere, ocean, ice sheets) were driven by concentration of greenhouse gases reconstructed from ice cores. Using these initial conditions, we performed coupled climate carbon cycle experiments for the Holocene, the Eemian and MIS11, driven by orbital forcing. Contrary to the results we published previously (Kleinen et al. 2010), peat accumulation was not prescribed, but rather determined dynamically, making this model setup applicable to previous interglacials as well. For the Holocene, our results resemble the carbon cycle dynamics as reconstructed from ice cores quite closely, both for atmospheric CO2 and delta13CO2. These experiments will be presented, analysing the role of different forcing mechanisms. The land surface appears to be an overall sink for CO2, due to carbon accumulation in the soil, as well as peat accumulation, and oceanic contributions due to temperature and circulation changes are quite small. Finally, results for MIS11 and the Eemian will be shown. [less ▲]

Modelling Climate and Vegetation Interactions at the Middle Miocene with the Planet Simulator and CARAIB

Conference (2011, January 18)

Carbon cycle dynamics during interglacials

Conference (2010, December 16)

Explaining a difference in atmospheric CO2 dynamics among interglacials is an elusive issue. Several biogeochemical mechanisms of different origin are involved in interglacial CO2 dynamics leading to a ... [more ▼]

Explaining a difference in atmospheric CO2 dynamics among interglacials is an elusive issue. Several biogeochemical mechanisms of different origin are involved in interglacial CO2 dynamics leading to a CO2 release from the ocean (carbonate compensation, coral growth) compensated by a land carbon uptake (biomass and soil carbon buildup, peat accumulation). The balance between these fluxes of CO2 is delicate and time-dependent, and it is not possible to provide firm constraints on these fluxes from proxy data. The best framework for quantification of all these mechanisms is an Earth System model that includes all necessary physical and biogeochemical components of the atmosphere, ocean, and land. To perform multi-millennial model integrations through the Holocene and Eemian, we use an intermediate complexity climate model, CLIMBER-2, coupled to the LPJ DGVM model with recently implemented boreal peatland module. The global carbon cycle is never in complete equilibrium during the glacial cycles due to changes in small but persistent fluxes such as terrestrial weathering. This complicates setting up the interglacial runs as the usual approach to start model integration from equilibrium state is not valid anymore. To by-pass this problem of non-equilibrium initial conditions, the model is initialized with the oceanic biogeochemistry state taken from a transient CLIMBER-2 simulation through the last glacial cycle. In this simulation, the CLIMBER-2 model was run through the last glacial cycle with carbon cycle in “offline mode” as interactive components of the physical climate system (atmosphere, ocean, ice sheets) were driven by concentration of greenhouse gases reconstructed from ice cores. In response to simulated climate change, the carbon cycle model was able to reproduce the main features of glacial CO2 dynamics reconstructed from ice cores. Results of the CLIMBER-LPJ model integrations through the Holocene and Eemian interglacials in terms of climate changes and atmospheric CO2 and d13CO2 dynamics will be presented. [less ▲]

Interactive comment on “A permafrost glacial hypothesis to explain atmospheric CO2 and the ice ages during the Pleistocene” by R. Zech et al.

in Climate of the Past Discussions (2010), 6

The paper "A permafrost glacial hypothesis to explain atmospheric CO2 and the ice ages during the Pleistocene" by R. Zech et al. is reviewed.

Simulation of glacial-interglacial atmospheric CO2 variations using a comprehensive Earth system model of intermediate complexity

Conference (2010, May 04)

The mechanisms of strong glacial-interglacial variations in the atmospheric CO2 concentration and the role of CO2 in driving glacial cycles still remain debatable. Here using the model of intermediate ... [more ▼]

The mechanisms of strong glacial-interglacial variations in the atmospheric CO2 concentration and the role of CO2 in driving glacial cycles still remain debatable. Here using the model of intermediate complexity CLIMBER-2 which includes all major components of the Earth system – atmosphere, ocean, land surface, ice sheets, terrestrial biota and weathering, aeolian dust and marine biogeochemistry – we performed simulation of the last glacial cycle using variations in the Earth’s orbital parameters as the only prescribed climatic forcing. The model simulates rather realistically temporal and spatial dynamics of the Northern Hemisphere glaciation and temporal dynamics of the atmospheric CO2 concentration. During the glacial inception, the model is able to simulate a decrease in the atmospheric CO2, despite of release of terrestrial biosphere carbon. The drop in CO2 concentration during the first part of the glacial cycle is between 20 and 40 ppmv. It is related primarily to the physical mechanisms – increase of the ocean solubility and relative volume and the age of the Antarctic bottom water masses. The latter is related to increased sea ice formation in the Southern Ocean and lowering of the surface salinity in the northern North Atlantic. During the second part of the glacial cycle, the atmospheric CO2 concentration decreases towards the level of 200 ppmv. A part of this drop is due an increase of biological productivity in the Southern Ocean which is directly related in the CLIMBER-2 model to increase of aeolian dust supply into the Southern Hemisphere via the iron fertilization mechanism. Significant part of the decreasing CO2 trend is also explained by increased weathering on land, especially on the exposed tropical shelves. A decrease in shallow water carbonate sedimentation and shift of CaCO3 sedimentation towards the deep ocean also plays important role in CO2 decrease. With the onset of the glacial termination, initial rise in the atmospheric CO2 concentration is explained by a weakening of the Atlantic thermohaline circulation due to increased freshwater input into the northern North Atlantic. The model is able to simulate the return of CO2 concentration to its interglacial value after termination of the glacial cycle but simulated CO2 concentration still lags considerably behind the ice core reconstructions. [less ▲]

Holocene carbon cycle dynamics

in Geophysical Research Letters (2010), 37

We are investigating the late Holocene rise in CO2 by performing four experiments with the climate-carbon-cycle model CLIMBER2-LPJ. Apart from the deep sea sediments, important carbon cycle processes ... [more ▼]

We are investigating the late Holocene rise in CO2 by performing four experiments with the climate-carbon-cycle model CLIMBER2-LPJ. Apart from the deep sea sediments, important carbon cycle processes considered are carbon uptake or release by the vegetation, carbon uptake by peatlands, and CO2 release due to shallow water sedimentation of CaCO3. Ice core data of atmospheric CO2 between 8 ka BP and preindustrial climate can only be reproduced if CO2 outgassing due to shallow water sedimentation of CaCO3 is considered. In this case the model displays an increase of nearly 20 ppmv CO2 between 8 ka BP and present day. Model configurations that do not contain this forcing show a slight decrease in atmospheric CO2. We can therefore explain the late Holocene rise in CO2 by invoking natural forcing factors only, and anthropogenic forcing is not required to understand preindustrial CO2 dynamics. [less ▲]

Effects of CO2, continental distribution, topography and vegetation changes on the climate at the Middle Miocene: a model study

in Climate of the Past (2010), 6

The Middle Miocene was one of the last warm periods of the Neogene, culminating with the Middle Miocene Climatic Optimum (MMCO, approximatively 17–15 Ma). Several proxy-based reconstructions support ... [more ▼]

The Middle Miocene was one of the last warm periods of the Neogene, culminating with the Middle Miocene Climatic Optimum (MMCO, approximatively 17–15 Ma). Several proxy-based reconstructions support warmer and more humid climate during the MMCO. The mechanisms responsible for the warmer climate at the MMCO and particularly the role of the atmospheric carbon dioxide are still highly debated. Here we carried out a series of sensitivity experiments with the model of intermediate complexity Planet Simulator, investigating the contributions of the absence of ice on the continents, the opening of the Central American and Eastern Tethys Seaways, the lowering of the topography on land, the effect of various atmospheric CO2 concentrations and the vegetation feedback. Our results show that a higher than present-day CO2 concentration is necessary to generate a warmer climate at all latitudes at the Middle Miocene, in agreement with the terrestrial proxy reconstructions which suggest high atmospheric CO2 concentrations at the MMCO. Nevertheless, the changes in sea-surface conditions, the lowering of the topography on land and the vegetation feedback also produce significant local warming that may, locally, even be stronger than the CO2 induced temperature increases. The lowering of the topography leads to a more zonal atmospheric circulation and allows the westerly flow to continue over the lowered Plateaus at mid-latitudes. The reduced height of the Tibetan Plateau notably prevents the development of a monsoon-like circulation, whereas the reduction of elevations of the North American and European reliefs strongly increases precipitation from northwestern to eastern Europe. The changes in vegetation cover contribute to maintain and even to intensify the warm and humid conditions produced by the other factors, suggesting that the vegetation-climate interactions could help to improve the model-data comparison. [less ▲]

Middle Miocene climate and vegetation modelling with PLASIM and CARAIB

Conference (2009, April 21)

In a long-term climatic cooling trend, the Middle Miocene represents one of the last warm periods of the Neogene, culminating with the Miocene Climatic Optimum, MCO (17-15 My). Palynological studies ... [more ▼]

In a long-term climatic cooling trend, the Middle Miocene represents one of the last warm periods of the Neogene, culminating with the Miocene Climatic Optimum, MCO (17-15 My). Palynological studies suggest that the warmer climatic conditions prevailing during the MCO allowed warm forests to expand poleward of the subtropical zone, with evergreen forests proliferating in North America and Europe (Jimenez-Moreno and Suc, 2007, Palaeogeogr. Palaeoclimatol. Palaeoecol. 253: 208-225). In this work, we used the Planet Simulator (Fraedrich et al., 2005, Meteorol. Z. 14: 299-304 and 305-314), an Earth system model of intermediate complexity, to carry out several simulation experiments, where we have assessed the effects of the absence of ice on the continents, the opening of the Central American and Eastern Tethys seaways, the lowering of the topography on land and the effect of various atmospheric CO2 concentrations, in agreement with the values reported in the litterature. We then produced several vegetation distributions, using the dynamic vegetation model CARAIB (Galy et al., 2008, Quat. Sci. Rev. 27: 1396-1409), to analyse if the climatic forcings considered are sufficient to explain the expansion of warmer forest types to higher latitudes. Our results indicate that an increase of atmospheric CO2 concentration, higher than the present-day one, is necessary to allow subtropical forest types to expand poleward. This result agrees with recent paleo-atmospheric CO2 reconstruction from stomatal frequency analysis, which suggests 500 ppmv of CO2 during the MCO. However, the required warming may be due to processes not considered in our setup (e.g. full oceanic circulation or other greenhouse gases). [less ▲]