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1.
A set of simple scaling formulas related to ice sheet evolution is derived from the dynamic and thermodynamic equations for ice and is used to consider two common situations: (a) when we wish to estimate potential ice sheet characteristics given the prescribed net snow accumulation over an area; and (b) when we wish to reconstruct net snow accumulation and vertical temperature difference within the ice sheet given empirical data only concerning ice sheet area and volume. The scaling formulas are applied to the present day Antarctic and Greenland ice sheets, as well as to some ancient ice sheets, and are used to estimate the potential global sea level change due to greenhouse warming.  相似文献   

2.
Greenland ice cores offer seasonal to annual records of δ18O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ18O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ18O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO? regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ18O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ18O variance explained of similar magnitude for the whole set of ice core records. Greenland δ18O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region.  相似文献   

3.
Here we present a set of regional climate scenarios of sea level rise for the northeast Atlantic Ocean. In this study, the latest observations and results obtained with state-of-the-art climate models are combined. In addition, regional effects due to ocean dynamics and changes in the Earth’s gravity field induced by melting of land-based ice masses have been taken into account. The climate scenarios are constructed for the target years 2050 and 2100, for both a moderate and a large rise in global mean atmospheric temperature (2 °C and 4 °C in 2100 respectively). The climate scenarios contain contributions from changes in ocean density (global thermal expansion and local steric changes related to changing ocean dynamics) and changes in ocean mass (melting of mountain glaciers and ice caps, changes in the Greenland and Antarctic ice sheets, and (minor) terrestrial water-storage contributions). All major components depend on the global temperature rise achieved in the target periods considered. The resulting set of climate scenarios represents our best estimate of twenty-first century sea level rise in the northeast Atlantic Ocean, given the current understanding of the various contributions. For 2100, they yield a local rise of 30 to 55 cm and 40 to 80 cm for the moderate and large rise in global mean atmospheric temperature, respectively.  相似文献   

4.
The model of Paillard and Parrenin (Earth Planet Sci Lett 227:263–271, 2004) was modified to obtain a closer fit to δ18O and CO2 time series for the last 800 kyr. The model performance can be improved if its CO2 sensitivity to I65 insolation is eliminated and if different response times are assumed for ablation/accumulation of ice. Correlations between simulated and experimental time series for CO2 and ice volume V increase from 0.59 and 0.63 to 0.79 and 0.88, respectively. According to these models, terminations are produced by I65 amplification through CO2-T and T-CO2 feedbacks, in synergy with an extra CO2 contribution from the deep ocean. This contribution is strongly dependent on ice-sheet extent and ice volume (or alternatively, CO2 concentration, which is a good proxy of Antarctic temperature) but is insensitive to Southern Ocean (SO) insolation on 21 February (I60). Change of deep SO state may be the “order parameter” for nonlinear deglacial changes. According to these models, 100 kyr periodicity of glacial cycles arises from the characteristic time of Antarctic ice sheet advance to the continental slope.  相似文献   

5.
Thresholds for irreversible decline of the Greenland ice sheet   总被引：1，自引：0，他引：1  
The Greenland ice sheet will decline in volume in a warmer climate. If a sufficiently warm climate is maintained for a few thousand years, the ice sheet will be completely melted. This raises the question of whether the decline would be reversible: would the ice sheet regrow if the climate cooled down? To address this question, we conduct a number of experiments using a climate model and a high-resolution ice-sheet model. The experiments are initialised with ice sheet states obtained from various points during its decline as simulated in a high-CO2 scenario, and they are then forced with a climate simulated for pre-industrial greenhouse gas concentrations, to determine the possible trajectories of subsequent ice sheet evolution. These trajectories are not the reverse of the trajectory during decline. They converge on three different steady states. The original ice-sheet volume can be regained only if the volume has not fallen below a threshold of irreversibility, which lies between 80 and 90% of the original value. Depending on the degree of warming and the sensitivity of the climate and the ice-sheet, this point of no return could be reached within a few hundred years, sooner than CO2 and global climate could revert to a pre-industrial state, and in that case global sea level rise of at least 1.3 m would be irreversible. An even larger irreversible change to sea level rise of 5 m may occur if ice sheet volume drops below half of its current size. The set of steady states depends on the CO2 concentration. Since we expect the results to be quantitatively affected by resolution and other aspects of model formulation, we would encourage similar investigations with other models.  相似文献   

6.
The future rate of Greenland Ice Sheet (GrIS) deglaciation and the future contribution of GrIS deglaciation to sea level rise will depend critically on the magnitude of northern hemispheric polar amplification and global equilibrium climate sensitivity. Here, these relationships are analyzed using an ensemble of multi-century coupled ice-sheet/climate model simulations seeded with observationally-constrained initial conditions and then integrated forward under tripled preindustrial CO2. Polar amplifications and climate sensitivities were varied between ensemble members in order to bracket current uncertainty in polar amplification and climate sensitivity. A large inter-ensemble spread in mean GrIS air temperature, albedo and surface mass balance trends stemming from this uncertainty resulted in GrIS ice volume loss ranging from 5 to 40 % of the original ice volume after 500 years. The large dependence of GrIS deglaciation on polar amplification and climate sensitivity that we find indicates that the representation of these processes in climate models will exert a strong control on any simulated predictions of multi-century GrIS evolution. Efforts to reduce polar amplification and equilibrium climate sensitivity uncertainty will therefore play a critical role in constraining projections of GrIS deglaciation and sea level rise in a future high-CO2 world.  相似文献   

7.
Summary Meteorological and glaciological analyses are integrated to examine the precipitation trends during the last three decades over the ice sheets covering Antarctica and Greenland. For Antarctica, the best data source is provided by glaciologically-measured trends of snow accumulation, and for limited sectors of East Antarctica consistency with precipitation amounts calculated from the atmospheric water balance equation is obtained. For Greenland, precipitation rates parameterized from atmospheric analyses yield the only comprehensive depiction. The precipitation rate over Antarctica appears to have increased by about 5% over a time period spanning the accumulation means for the 1955–65 to 1965–75 periods, while over Greenland it has decreased by about 15% since 1983 with a secondary increase over the southern part of the ice sheet starting in 1977. At the end of the 10-year overlapping period, the global sea-level impact of the precipitation changes over Antarctica dominates that for Greenland and yields a net ice-sheet precipitation contribution of roughly 0.02 mm yr–1. These changes are likely due to marked variations in the cyclonic forcing affecting the ice sheets, but are only weakly reflected in the temperature regime, consistent with the episodic nature of cyclonic precipitation. These conclusions are not founded on high quality data bases. The importance of such changes for understanding global sea-level variations argues for a modest research effort to collect simultaneous meteorological and glaciological observations in order to describe and understand the current precipitation variations over both ice sheets. Some suggestions are offered for steps that could be taken.With 8 Figures  相似文献   

8.
 The climate during the Last Glacial Maximum (LGM) has been simulated using the UK Universities Global Atmospheric Modelling Programme (UGAMP) general circulation model (GCM) with both prescribed sea surface temperatures (SSTs) based on the CLIMAP reconstruction and computed SSTs with a simple thermodynamic slab ocean. Consistent with the Paleoclimate Modelling Intercomparison Project (PMIP), the other boundary conditions include the large changes in ice-sheet topography and geography, a lower sea level, a lower concentration of CO2 in the atmosphere, and a slightly different insolation pattern at the top of the atmosphere. The results are analysed in terms of changes in atmospheric circulation. Emphasis is given to the changes in surface temperatures, planetary waves, storm tracks and the associated changes in distribution of precipitation. The model responds in a similar manner to the changes in boundary conditions to previous studies in global mean statistics, but differs in its treatment of regional climates. Results also suggest that both the land ice sheets and sea ice introduce significant changes in planetary waves and transient eddy activity, which in turn affect regional climates. The computed SST simulations predict less sea ice and cooler tropical temperatures than those based on CLIMAP SSTs. It is unclear as to whether this is a model and/or a data problem, but the resulting changes in land temperatures and precipitation can be large. Snow mass budget analysis suggests that there is net ice loss along the southern edges of the Laurentide and Fennoscandian ice sheets and net ice gain over other parts of the two ice sheets. The net accumulation is mainly due to the decrease in ablation in the cold climate rather than to the changes in snowfall. The characteristics of the Greenland ice-sheet mass balance in the LGM simulations is also quite different from those in the present-day (PD) simulations. The ablation in the LGM simulations is negligible while it is a very important process in the ice mass budget in the PD simulations. Received: 10 January 1997 / Accepted: 11 December 1997  相似文献   

9.
Several multi-century and multi-millennia simulations have been performed with a complex Earth System Model (ESM) for different anthropogenic climate change scenarios in order to study the long-term evolution of sea level and the impact of ice sheet changes on the climate system. The core of the ESM is a coupled coarse-resolution Atmosphere–Ocean General Circulation Model (AOGCM). Ocean biogeochemistry, land vegetation and ice sheets are included as components of the ESM. The Greenland Ice Sheet (GrIS) decays in all simulations, while the Antarctic ice sheet contributes negatively to sea level rise, due to enhanced storage of water caused by larger snowfall rates. Freshwater flux increases from Greenland are one order of magnitude smaller than total freshwater flux increases into the North Atlantic basin (the sum of the contribution from changes in precipitation, evaporation, run-off and Greenland meltwater) and do not play an important role in changes in the strength of the North Atlantic Meridional Overturning Circulation (NAMOC). The regional climate change associated with weakening/collapse of the NAMOC drastically reduces the decay rate of the GrIS. The dynamical changes due to GrIS topography modification driven by mass balance changes act first as a negative feedback for the decay of the ice sheet, but accelerate the decay at a later stage. The increase of surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long term. Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced to 3/4 of the original size. From then, the reduction in the albedo of Greenland strongly accelerates the decay of the ice sheet.  相似文献   

10.
An attempt is made to simulate the Pleistocene glacial cycles with a numerical model of the Northern Hemisphere ice sheets. This model treats the vertically-integrated ice flow along a meridian, including computation of bedrock adjustment and temperature distribution in the ice. Basal melt water is traced and controls ice-mass discharge. The model produces asymmetric glacial cycles, even when it is not forced. Model parameters can be chosen such that cycles with a duration of about 100 000 yr occur. Due to the production of basal melt water and bedrock sinking, deglaciations are very rapid. The occurrence of glacial cycles in the model is a stable feature, but thephase of the cycles is very sensitive to the model parameters. The main conclusion is that ice-sheet dynamics may provide an explanation for the Pleistocene glacial cycles. However, the ‘predictability’ of the ice-volume record appears to be small.  相似文献   

11.
We use the Earth system model of intermediate complexity LOVECLIM to show the effect of coupling interactive ice sheets on the climate sensitivity of the model on a millennial time scale. We compare the response to a 2×CO2 warming scenario between fully coupled model versions including interactive Greenland and Antarctic ice sheet models and model versions with fixed ice sheets. For this purpose an ensemble of different parameter sets have been defined for LOVECLIM, covering a wide range of the model??s sensitivity to greenhouse warming, while still simulating the present-day climate and the climate evolution over the last millennium within observational uncertainties. Additional freshwater fluxes from the melting ice sheets have a mitigating effect on the model??s temperature response, leading to generally lower climate sensitivities of the fully coupled model versions. The mitigation is effectuated by changes in heat exchange within the ocean and at the sea?Cair interface, driven by freshening of the surface ocean and amplified by sea?Cice-related feedbacks. The strength of the effect depends on the response of the ice sheets to the warming and on the model??s climate sensitivity itself. The effect is relatively strong in model versions with higher climate sensitivity due to the relatively large polar amplification of LOVECLIM. With the ensemble approach in this study we cover a wide range of possible model responses.  相似文献   

12.
This paper investigates the possible implications for the earth-system of a melting of the Greenland ice-sheet. Such a melting is a possible result of increased high latitude temperatures due to increasing anthropogenic greenhouse gas emissions. Using an atmosphere-ocean general circulation model (AOGCM), we investigate the effects of the removal of the ice sheet on atmospheric temperatures, circulation, and precipitation. We find that locally over Greenland, there is a warming associated directly with the altitude change in winter, and the altitude and albedo change in summer. Outside of Greenland, the largest signal is a cooling over the Barents sea in winter. We attribute this cooling to a decrease in poleward heat transport in the region due to changes to the time mean circulation and eddies, and interaction with sea-ice. The simulated climate is used to force a vegetation model and an ice-sheet model. We find that the Greenland climate in the absence of an ice sheet supports the growth of trees in southern Greenland, and grass in central Greenland. We find that the ice sheet is likely to regrow following a melting of the Greenland ice sheet, the subsequent rebound of its bedrock, and a return to present day atmospheric CO2 concentrations. This regrowth is due to the high altitude bedrock in eastern Greenland which allows the growth of glaciers which develop into an ice sheet.  相似文献   

13.
On the longer climatic time scales, changes in the elevation and extent of the Antarctic ice sheet have an important role in modulating global atmospheric and oceanographic processes, and contribute significantly to world-wide sea levels. In this paper, a 3-D time-dependent thermomechanical model for the entire ice sheet is presented, that is subsequently used to examine the effects of glacial-interglacial shifts in environmental boundary conditions on its geometry. The model takes into account a coupled ice shelf, grounding-line dynamics, basal sliding and isostatic bed adjustment and considers the fully coupled velocity and temperature fields. Ice flow is calculated on a fine mesh (40 km horizontal grid size and 10 layers in the vertical) for grounded and floating ice and a stress transition zone in between at the grounding line, where all stress components contribute in the effective stress in the flow law. There is free interaction between ice sheet and ice shelf, so that the entire geometry is internally generated. A simulation of the present ice sheet reveals that the model is able to yield realistic results. A series of sensitivity experiments are then performed, in which lower temperatures, reduced accumulation rates and lower global sea level stands are imposed, either singly or in combination. By comparing results of pairs of experiments, the effects of each of these environmental changes can be determined. In agreement with glacial-geological evidence, we found that the most pronounced changes show up in the West Antarctic ice sheet configuration. They appear to be essentially controlled by variations in eustatic sea level, whereas typical glacial-interglacial changes in temperature and ice deposition rates tend to balance one another. These findings support the hypothesis that the Antarctic ice sheet basically follows glacial episodes in the northern hemisphere by means of sea-level teleconnections. Grounding occurs more readily in the Weddell sea than in the Ross sea and long time scales appear to be involved: it may take up to 30–40000 years for these continental shelf areas to become completely grounded after an initial stepwise perturbation in boundary conditions. According to these reconstructions, a steady state Antarctic ice sheet may contribute some 16 m to global sea level lowering at maximum glaciation.  相似文献   

14.
The two-step shape and timing of the last deglaciation in Antarctica   总被引：5，自引：0，他引：5  
The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record.  相似文献   

15.
Response of the Antarctic ice sheet to future greenhouse warming   总被引：2，自引：0，他引：2  
Possible future changes in land ice volume are mentioned frequently as an important aspect of the greenhouse problem. This paper deals with the response of the Antarctic ice sheet and presents a tentative projection of changes in global sea level for the next few hundred years, due to changes in its surface mass balance. We imposed a temperature scenario, in which surface air temperature rises to 4.2° C in the year 2100 AD and is kept constant afterwards. As GCM studies seem to indicate a higher temperature increase in polar latitudes, the response to a more extreme scenario (warming doubled) has also been investigated. The mass balance model, driven by these temperature perturbations, consists of two parts: the accumulation rate is derived from present observed values and is consequently perturbed in proportion to the saturated vapour pressure at the temperature above the inversion layer. The ablation model is based on the degree-day method. It accounts for the daily temperature cycle, uses a different degree-day factor for snow and ice melting and treats refreezing of melt water in a simple way. According to this mass balance model, the amount of accumulation over the entire ice sheet is presently 24.06 × 1011 m3 of ice, and no runoff takes place. A 1°C uniform warming is then calculated to increase the overall mass balance by an amount of 1.43 × 1011 m3 of ice, corresponding to a lowering of global sea level with 0.36 mm/yr. A temperature increase of 5.3°C is needed for the increase in ablation to become more important than the increase in accumulation and the temperature would have to rise by as much as 11.4°C to produce a zero surface mass balance. Imposing the Bellagio-scenario and accumulating changes in mass balance forward in time (static response) would then lower global sea level by 9 cm by 2100 AD. In a subsequent run with a high-resolution 3-D thermomechanic model of the ice sheet, it turns out that the dynamic response of the ice sheet (as compared to the direct effect of the changes in surface mass balance) becomes significant after 100 years or so. Ice-discharge across the grounding-line increases, and eventually leads to grounding-line retreat. This is particularly evident in the extreme case scenario and is important along the Antarctic Peninsula and the overdeepened outlet glaciers along the East Antarctic coast. Grounding-line retreat in the Ross and Ronne-Filchner ice shelves, on the other hand, is small or absent.  相似文献   

16.
 The atmospheric CO2 concentrations have been reconstructed over the past 600 ka based on regression between the Vostok CO2 data and the SPECMAP oxygen isotope values. A lag of 4.5 ka (CO2 preceding δ18O) gives the best results. A polynomial of order 5 explains 66% of the Vostok CO2 variance over the last 220 ka. The Northern Hemisphere ice-sheet volume was simulated over the past 575 ka using the LLN 2-D model, forced by insolation and these statistically reconstructed atmospheric CO2 concentrations. The simulated ice volume fluctuations resemble the deep-sea oxygen isotope variations. CO2 of interglacial level is necessary for explaining both the interglacial at oxygen isotopic stage 11 and our present-day interglacial.  相似文献   

17.
The total mass budget of the Antarctic ice sheet is studied with a simple axi-symmetrical model. The ice-sheet has a parabolic profile resting on a bed that slopes linearly downwards from the centre of the ice sheet into the ocean. The mean ice velocity at the grounding line is assumed to be proportional to the water depth. The accumulation rate is a linear function of the distance to the centre. Setting the total mass budget to zero yields a quadratic equation for the steady-state ice-sheet radius R. Analysis of the equilibrium states sheds light on the sensitivity of the ice-sheet radius to changes in sea level (S) and precipitation with respect to the present state (Prel). For model parameters obtained by matching the analytical model to the present state of the Antarctic ice sheet, the sensitivity values are dR/dS = -2400 and dR/dPrel = 4000 m/%. The model can also be used to study transient behaviour of the ice sheet. The characteristic relaxation time (e-folding time scale) is about 3500 years. Forcing the model with a sea-level and accumulation history over the past few hundred thousands of years yields Antarctic ice-volume curves that are similar to those obtained by comprehensive numerical modelling. The current imbalance predicted by the model corresponds to a sea-level rise of 0.25 mm yr-1.  相似文献   

18.
 The great continental ice sheets of the Pleistocene represented a significant topographic obstacle to the westerly winds in northern midlatitudes. This work explores how consequent changes in the atmospheric stationary wave pattern might have affected the shape and growth of the ice sheets themselves. A one dimensional (1-D) model is developed which permits an examination of the types and magnitudes of the feedbacks that might be expected. When plausible temperature perturbations are introduced at the ice-sheet margin which are proportional to the stationary wave amplitude, the equilibrium shape of the ice sheet is significantly altered, and depends on the sign of the perturbation. The proposed feedback also affects the response of the ice sheet to time-varying climate forcing. The results suggest that the evolution of a continental-scale ice sheet with a land-based margin may be significantly determined by the changes it induces in the atmospheric circulation. Received: 1 October 1999 / Accepted: 17 July 2000  相似文献   

19.
The large uncertainty in future global glacier volume projections partly results from a substantial range in future climate conditions projected by global climate models. This study addresses the effect of global and regional differences in climate input data on the projected twenty-first century glacier contribution to sea-level rise. Glacier volume changes are calculated with a surface mass balance model combined with volume-area scaling, applied to 89 glaciers in different climatic regions. The mass balance model is based on a simplified energy balance approach, with separated contributions by net solar radiation and the combined other fluxes. Future mass balance is calculated from anomalies in air temperature, precipitation and atmospheric transmissivity, taken from eight global climate models forced with the A1B emission scenario. Regional and global sea-level contributions are obtained by scaling the volume changes at the modelled glaciers to all glaciers larger than 0.1 km2 outside the Greenland and Antarctic ice sheets. This results in a global value of 0.102 ± 0.028 m (multi-model mean and standard deviation) relative sea-level equivalent for the period 2012–2099, corresponding to 18 ± 5 % of the estimated total volume of glaciers. Glaciers in the Antarctic, Alaska, Central Asia and Greenland together account for 65 ± 4 % of the total multi-model mean projected sea-level rise. The projected sea-level contribution is 35 ± 17 % larger when only anomalies in air temperature are taken into account, demonstrating an important compensating effect by increased precipitation and possibly reduced atmospheric transmissivity. The variability in projected precipitation and atmospheric transmissivity changes is especially large in the Arctic regions, making the sea-level contribution for these regions particularly sensitive to the climate model used. Including additional uncertainties in the modelling procedure and the input data, the total uncertainty estimate for the future projections becomes ±0.063 m.  相似文献   

20.
采有英国Hadley中心的GISST海冰面积资料，NCFP／NCAR再分析资料以及中国160站气温和降水资料，分析了春季格陵兰海冰面积与夏季中国区域气温和降水的关系。初步研究表明，春季格陵兰海冰面积变化和随后夏季我国黄河长江中下游之间地区气温以及8月份华北和西南地区降水呈明显正相关，而和6月黄河中上游地区降水则具有明显的负相关。同时，春季格陵兰海冰异常时期对应着北半球大气环流的明显主为化，表明海冰与我国气温及降水之间的联系具有一定的环流背景。  相似文献   

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