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  • 1
    Call number: ZS-190(81) ; ZSP-625-81
    In: PIK report
    Type of Medium: Series available for loan
    Pages: 136 S.
    Series Statement: PIK report 81
    Classification: D.4.
    Location: Lower compact magazine
    Location: Lower compact magazine
    Branch Library: GFZ Library
    Branch Library: GFZ Library
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  • 2
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    Copernicus Publications (EGU)
    In:  Climate of the Past, 13 (7). pp. 819-832.
    Publication Date: 2019-02-01
    Description: Glacial cycles of the late Quaternary are controlled by the asymmetrically varying mass balance of continental ice sheets in the Northern Hemisphere. Surface mass balance is governed by processes of ablation and accumulation. Here two ablation schemes, the positive-degree-day (PDD) method and the surface energy balance (SEB) approach, are compared in transient simulations of the last glacial cycle with the Earth system model of intermediate complexity CLIMBER-2. The standard version of the CLIMBER-2 model incorporates the SEB approach and simulates ice volume variations in reasonable agreement with paleoclimate reconstructions during the entire last glacial cycle. Using results from the standard CLIMBER-2 model version, we simulated ablation with the PDD method in offline mode by applying different combinations of three empirical parameters of the PDD scheme. We found that none of the parameter combinations allow us to simulate a surface mass balance of the American and European ice sheets that is similar to that obtained with the standard SEB method. The use of constant values for the empirical PDD parameters led either to too much ablation during the first phase of the last glacial cycle or too little ablation during the final phase. We then substituted the standard SEB scheme in CLIMBER-2 with the PDD scheme and performed a suite of fully interactive (online) simulations of the last glacial cycle with different combinations of PDD parameters. The results of these simulations confirmed the results of the offline simulations: no combination of PDD parameters realistically simulates the evolution of the ice sheets during the entire glacial cycle. The use of constant parameter values in the online simulations leads either to a buildup of too much ice volume at the end of glacial cycle or too little ice volume at the beginning. Even when the model correctly simulates global ice volume at the last glacial maximum (21 ka), it is unable to simulate complete deglaciation during the Holocene. According to our simulations, the SEB approach proves superior for simulations of glacial cycles.
    Type: Article , PeerReviewed
    Format: text
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  • 3
    Publication Date: 2018-12-06
    Description: Multiple proxy data reveal that the early to middle Holocene (ca. 8–6 kyr B.P.) was warmer than the preindustrial period in most regions of the Northern Hemisphere. This warming is presumably explained by the higher summer insolation in the Northern Hemisphere, owing to changes in the orbital parameters. Subsequent cooling in the late Holocene was accompanied by significant changes in vegetation cover and an increase in atmospheric CO2 concentration. The essential question is whether it is possible to explain these changes in a consistent way, accounting for the orbital parameters as the main external forcing for the climate system. We investigate this problem using the computationally efficient model of climate system, CLIMBER‐2, which includes models for oceanic and terrestrial biogeochemistry. We found that changes in climate and vegetation cover in the northern subtropical and circumpolar regions can be attributed to the changes in the orbital forcing. Explanation of the atmospheric CO2 record requires an additional assumption of excessive CaCO3 sedimentation in the ocean. The modeled decrease in the carbonate ion concentration in the deep ocean is similar to that inferred from CaCO3 sediment data [Broecker et al., 1999]. For 8 kyr B.P., the model estimates the terrestrial carbon pool ca. 90 Pg higher than its preindustrial value. Simulated atmospheric δ13C declines during the course of the Holocene, similar to δ13C data from the Taylor Dome ice core [Indermühle et al., 1999]. Amplitude of simulated changes in δ13C is smaller than in the data, while a difference between the model and the data is comparable with the range of data uncertainty.
    Type: Article , PeerReviewed
    Format: text
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  • 4
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract A 2.5-dimensional climate system model of intermediate complexity CLIMBER-2 and its performance for present climate conditions are presented. The model consists of modules describing atmosphere, ocean, sea ice, land surface processes, terrestrial vegetation cover, and global carbon cycle. The modules interact through the fluxes of momentum, energy, water and carbon. The model has a coarse spatial resolution, nevertheless capturing the major features of the Earth's geography. The model describes temporal variability of the system on seasonal and longer time scales. Due to the fact that the model does not employ flux adjustments and has a fast turnaround time, it can be used to study climates significantly different from the present one and to perform long-term (multimillennia) simulations. The comparison of the model results with present climate data show that the model successfully describes the seasonal variability of a large set of characteristics of the climate system, including radiative balance, temperature, precipitation, ocean circulation and cryosphere.
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 2017-05-08
    Description: Over the past two decades net mass loss from the Greenland ice sheet quadrupled, caused by enhanced surface melting and speedup of the marine-terminating outlet glaciers. This speedup has been related, among other factors, to enhanced submarine melting, which in turn is caused by warming of the surrounding ocean and by increased subglacial discharge. For the future and recent mass balance changes of the Greenland Ice Sheet, ice-ocean processes potentially play an important role, yet they are not properly represented in contemporary Greenland Ice Sheet models. In this work we performed numerical experiments with a one-dimensional plume model coupled to a one-dimensional model of outlet glacier. We investigate the response of a coupled ice-flow plume model to possible outcomes of climate change. In particularly, we examine the transient and equilibrium response of the outlet glaciers to changes in ocean temperature and subglacial discharge which affects both: glacier geometry and submarine melt rates.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2019-02-04
    Description: We introduce the coupled model of the Green- land glacial system IGLOO 1.0, including the polythermal ice sheet model SICOPOLIS (version 3.3) with hybrid dy- namics, the model of basal hydrology HYDRO and a param- eterization of submarine melt for marine-terminated outlet glaciers. The aim of this glacial system model is to gain a better understanding of the processes important for the future contribution of the Greenland ice sheet to sea level rise under future climate change scenarios. The ice sheet is initialized via a relaxation towards observed surface elevation, impos- ing the palaeo-surface temperature over the last glacial cycle. As a present-day reference, we use the 1961–1990 standard climatology derived from simulations of the regional atmo- sphere model MAR with ERA reanalysis boundary condi- tions. For the palaeo-part of the spin-up, we add the temper- ature anomaly derived from the GRIP ice core to the years 1961–1990 average surface temperature field. For our pro- jections, we apply surface temperature and surface mass bal- ance anomalies derived from RCP 4.5 and RCP 8.5 scenar- ios created by MAR with boundary conditions from simula- tions with three CMIP5 models. The hybrid ice sheet model is fully coupled with the model of basal hydrology. With this model and the MAR scenarios, we perform simulations to estimate the contribution of the Greenland ice sheet to future sea level rise until the end of the 21st and 23rd centuries. Fur- ther on, the impact of elevation–surface mass balance feed- back, introduced via the MAR data, on future sea level rise is inspected. In our projections, we found the Greenland ice sheet to contribute between 1.9 and 13.0 cm to global sea level rise until the year 2100 and between 3.5 and 76.4 cm until the year 2300, including our simulated additional sea level rise due to elevation–surface mass balance feedback. Translated into additional sea level rise, the strength of this feedback in the year 2100 varies from 0.4 to 1.7 cm, and in the year 2300 it ranges from 1.7 to 21.8 cm. Additionally, taking the Helheim and Store glaciers as examples, we inves- tigate the role of ocean warming and surface runoff change for the melting of outlet glaciers. It shows that ocean temper- ature and subglacial discharge are about equally important for the melting of the examined outlet glaciers.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2017-01-02
    Description: Here, we define a test bed for fast flow regions and its vicinity embedded in an ice sheet. This test bed is designed for outlet glaciers and ice streams of the Greenland ice sheet. It consists of a fine resolution part with a manufactured basal trough over which the professional software COMSOL (Multiphysics Modeling Software) operates as a full- Stokes model. Results by COMSOL are compared with coarse resolution simulations with the ice-sheet model SICOPOLIS operating in shallow-ice-approximation mode and using parameterizations of the fast flow effects. For simplification, in this preliminary approach, both models run in isothermal mode. Definition of surface mass balance follows the EISMINT intercomparison project with parameters adapted to the Greenland ice sheet. In particular, we inspect with this test bed upstream and lateral flow effects of ice streams and outlet glaciers. We present first simulations with this approach, although presentation of the test bed itself is the main emphasis of this presentation.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2018-04-16
    Description: Climate change projections for the future are uncertain, also due to inter-model differences. The application of these models to paleo times, which can be constrained by reconstructions, is therefore essential, not only to gain a better understanding of past climate changes, but also for model validation purposes. In this respect both data- and model-based approaches have been used to generate time series of global temperature changes, ∆Tg. The ratio of ∆Tg over radiative forcing, ∆R, defines the specific equilibrium climate sensitivity S, and has been suggested to be state-dependent, potentially increasing towards warming climates, and therefore suggesting climate sensitivity for the future to be at the upper end of the range of published results (Köhler et al., 2015, 2017). Here we reanalyse existing time series of ∆Tg and ∆R for the last 800,000 years and show that this proposed state-dependency of S is only found if ∆Tg is based on data (reconstructions), and not if ∆Tg is based on models (simulations). We furthermore identify that in data-based reconstructions ∆Tg is decoupled from atmospheric CO2 predominantely during times of decreasing obliquity (identical to periods of land-ice sheet growth and sea level fall), while in model simulations ∆Tg and CO2 vary in phase throughout. This multi-millennial decoupling of CO2 and temperature has been suggested to be partially caused by a sea level-induced surge in magma and CO2 fluxes from oceanic hotspot volcanoes and mid ocean ridges (Hasenclever et al., 2017). The neglection of these feedbacks between the solid Earth and the climate system in recent Earth system models is partly responsible for the data/model misfit, and illustrates our current limitation in the model-based interpretation of the paleo records. Paleo-based estimates of S might be restricted to data without this ∆Tg-CO2-decoupling leading to a 20% smaller quantification of S for interglacial conditions of the late Pleistocene.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called Dansgaard–Oeschger events, with a quasi-periodicity of ∼1,470 years for the late glacial period. Various hypotheses have been suggested to explain these rapid ...
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] In the context of gradual Cenozoic cooling, the timing of the onset of significant Northern Hemisphere glaciation 2.7 million years ago is consistent with Milankovitch's orbital theory, which posited that ice sheets grow when polar summertime insolation and temperature are low. However, the ...
    Type of Medium: Electronic Resource
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