Papers by Anders Levermann
Dynamic equilibrium simulation of Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK)
We present a dynamic equilibrium simulation of land ice on Antarctica with the Potsdam Parallel I... more We present a dynamic equilibrium simulation of land ice on Antarctica with the Potsdam Parallel Ice Sheet Model (PISM-PIK). In order to capture slow grounded ice flow, fast ice shelf motion as well as fast flowing ice streams the shallow ice approximation (SIA) is complemented by the shallow shelf approximation (SSA) on a velocity level. The initial PISM code was
ndent Mixing May Increase Sensitivity of Atlantic Overturning to Global Warming
We use the Earth System Model of Intermediate Complexity CLIMBER›3 to investigate the eVect of st... more We use the Earth System Model of Intermediate Complexity CLIMBER›3 to investigate the eVect of stratication›dep endent mixing on the stability of the Atlantic Meridional Overturning Cir› culation under an idealized CO2 increase scenario. The vertical dif› fusivity of the ocean is parameterized as N , where N is the local buoyancy frequency. For all parameter values 0 3, we
Projecting Antarctic ice discharge using response functions from SeaRISE ice-sheet models
Earth System Dynamics, 2014
Future changes in extratropical storm tracks and baroclinicity under climate change
Environmental Research Letters, 2014
Physical Review E, 2000
Diffusion limited aggregation ͑DLA͒ is a model of fractal growth that had attained a paradigmatic... more Diffusion limited aggregation ͑DLA͒ is a model of fractal growth that had attained a paradigmatic status due to its simplicity and its underlying role for a variety of pattern forming processes. We present a convergent calculation of the fractal dimension D of DLA based on a renormalization scheme for the first Laurent coefficient of the conformal map from the unit circle to the expanding boundary of the fractal cluster. The theory is applicable from very small ͑2-3 particles͒ to asymptotically large (n→ϱ) clusters. The computed dimension is Dϭ1.713Ϯ0.003.
Physical Review E, 2002
We study the fractal and multifractal properties (i.e. the generalized dimensions of the harmonic... more We study the fractal and multifractal properties (i.e. the generalized dimensions of the harmonic measure) of a 2-parameter family of growth patterns that result from a growth model that interpolates between Diffusion Limited Aggregation (DLA) and Laplacian Growth Patterns in 2-dimensions. The two parameters are β which determines the size of particles accreted to the interface, and C which measures the degree of coverage of the interface by each layer accreted to the growth pattern at every growth step. DLA and Laplacian Growth are obtained at β = 0, C = 0 and β = 2, C = 1, respectively. The main purpose of this paper is to show that there exists a line in the β − C phase diagram that separates fractal (D < 2) from non-fractal (D=2) growth patterns. Moreover, Laplacian Growth is argued to lie in the non-fractal part of the phase diagram. Some of our arguments are not rigorous, but together with the numerics they indicate this result rather strongly. We first consider the family of models obtained for β = 0, C > 0, and derive for them a scaling relation D = 2D3. We then propose that this family has growth patterns for which D = 2 for some C > Ccr, where Ccr may be zero. Next we consider the whole β − C phase diagram and define a line that separates 2-dimensional growth patterns from fractal patterns with D < 2. We explain that Laplacian Growth lies in the region belonging to 2-dimensional growth patterns, motivating the main conjecture of this paper, i.e. that Laplacian Growth patterns are 2-dimensional. The meaning of this result is that the branches of Laplacian Growth patterns have finite (and growing) area on scales much larger than any ultra-violet cut-off length.
Earth System Dynamics Discussions, 2013
The possibility of an impact of global warming on the Indian monsoon is of critical importance fo... more The possibility of an impact of global warming on the Indian monsoon is of critical importance for the large population of this region. Future projections within the Coupled Model Intercomparison Project Phase 3 (CMIP-3) showed a wide range of trends with varying magnitude and sign across models. Here the Indian summer monsoon rainfall 5 is evaluated in 20 CMIP-5 models for the period 1850 to 2100. In the new generation of climate models a consistent increase in seasonal mean rainfall during the summer monsoon periods arises. All models simulate stronger seasonal mean rainfall in the future compared to the historic period under the strongest warming scenario RCP-8.5. Increase in seasonal mean rainfall is the largest for the RCP-8.5 scenario compared 10 to other RCPs. The interannual variability of the Indian monsoon rainfall also shows a consistent positive trend under unabated global warming. Since both the long-term increase in monsoon rainfall as well as the increase in interannual variability in the future is robust across a wide range of models, some confidence can be attributed to these projected trends.
Biogeosciences Discussions, 2011
Thawing of permafrost and the associated release of carbon constitutes a positive feedback in the... more Thawing of permafrost and the associated release of carbon constitutes a positive feedback in the climate system, elevating the effect of anthropogenic GHG emissions on global-mean temperatures. Multiple factors have hindered the quantification of this feedback, which was not included in the CMIP3 and C 4 MIP generation of AOGCMs 5 and carbon cycle models. There are considerable uncertainties in the rate and extent of permafrost thaw, the hydrological and vegetation response to permafrost thaw, the decomposition timescales of freshly thawed organic material, the proportion of soil carbon that might be emitted as carbon dioxide via aerobic decomposition or as methane via anaerobic decomposition, and in the magnitude of the high latitude amplification of 10 20 thaw and slow but steady soil carbon decomposition means that, by 2300, more than half of the potentially vulnerable permafrost carbon stock in the upper 3m of soil layer (600-1000 PgC) could be released as CO 2 , with an extra 1-3 % being released as methane. Our results also suggest that mitigation action in line with the lower scenario RCP3-PD could contain Arctic temperature increase sufficiently that thawing of the 25 permafrost area is limited to 15-30 % and the permafrost-carbon induced temperature increase does not exceed 0.01-0.07
Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a Coarse Resolution Model
Earth System Dynamics Discussions, 2012
... tion (AMO) (H äkkinen et al., 2011a,b). However, part of the gyre circulation is controlled b... more ... tion (AMO) (H äkkinen et al., 2011a,b). However, part of the gyre circulation is controlled by baroclinic adjustments and thereby the density structure in the region (Greatbatch et al., 1991; Myers et al., 1996; Penduff et al., 2000; Eden and Willebrand ... (2009) and Lohmann et al. ...
Physical Review E, 2002
The method of iterated conformal maps is developed for quasistatic fracture of brittle materials,... more The method of iterated conformal maps is developed for quasistatic fracture of brittle materials, for all modes of fracture. Previous theory, that was relevant for mode III only, is extended here to modes I and II. The latter require the solution of the bi-Laplace rather than the Laplace equation. For all cases we can consider quenched randomness in the brittle material itself, as well as randomness in the succession of fracture events. While mode III calls for the advance ͑in time͒ of one analytic function, modes I and II call for the advance of two analytic functions. This fundamental difference creates different stress distribution around the cracks. As a result the geometric characteristics of the cracks differ, putting mode III in a different class compared to modes I and II.
Interaction of marine ice-sheet instabilities in two drainage basins: simple scaling of geometry and transition time
The Cryosphere Discussions, 2014
Food security: Fertilizing hidden hunger
Nature Climate Change, 2014
Fracture-induced softening for large-scale ice dynamics
The Cryosphere Discussions, 2013
Uncertainty in future solid ice discharge from Antarctica
The Cryosphere Discussions, 2012
ABSTRACT Future solid ice discharge from Antarctica under climate scenarios based on the Extended... more ABSTRACT Future solid ice discharge from Antarctica under climate scenarios based on the Extended Concentration Pathways is investigated with the Potsdam Parallel Ice Sheet Model (PISM-PIK), a shallow hybrid model with a consistent representation of the ice flow in sheet, shelves and the transition zone. Both the uncertainty in the climate forcing as well as the intra-model uncertainty are taken into account: All simulations are performed for a 81-member perturbed- physics ensemble and the likely range of surface and ocean warming under the emission pathways derived from the results of 20 CMIP3-AOGCMS. The effects of surface warming, ocean warming and increased precipitation on solid ice discharge are separately considered. We find that solid ice discharge caused by enhanced basal ice-shelf melting exceeds that caused by surface warming. Increasing precipitation leads to a change from net sea-level rise to sea-level drop. Our results suggest that the history of the ice-sheet plays an important role with respect to projections of solid ice discharge.
Physical Review E, 2002
We study the geometrical characteristic of quasistatic fractures in brittle media, using iterated... more We study the geometrical characteristic of quasistatic fractures in brittle media, using iterated conformal maps to determine the evolution of the fracture pattern. This method allows an efficient and accurate solution of the Lamé equations without resorting to lattice models. Typical fracture patterns exhibit increased ramification due to the increase of the stress at the tips. We find the roughness exponent of the experimentally relevant backbone of the fracture pattern, it crosses over from about 0.5 for small scales to about 0.75 for large scales. We propose that this crossover reflects the increased ramification of the fracture pattern.
Ice plug prevents irreversible discharge from East Antarctica
Nature Climate Change, 2014
Geophysical Research Letters, 2008
Using a coupled model of intermediate complexity the sensitivity of the last glacial maximum (LGM... more Using a coupled model of intermediate complexity the sensitivity of the last glacial maximum (LGM) Atlantic meridional overturning circulation (AMOC) to the strength of surface wind-stress is investigated. A threshold is found below which North Atlantic deep water formation (DWF) takes place south of Greenland and the AMOC is relatively weak. Above this threshold, DWF occurs north of the Greenland-Scotland
Climate Dynamics, 2005
Using the coupled climate model CLIMBER-3a, we investigate changes in sea surface elevation due t... more Using the coupled climate model CLIMBER-3a, we investigate changes in sea surface elevation due to a weakening of the thermohaline circulation (THC). In addition to a global sea level rise due to a warming of the deep sea, this leads to a regional dynamic sea level change which follows quasi-instantaneously any change in the ocean circulation. We show that the
Long legacy of global warming even under negative emissions: 10 times slower cooling than warming
MODELING OF PINE ISLAND GLACIER AND THWAITES GLACIER WITH PISM-PIK
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Papers by Anders Levermann