Response of the Greenland and Antarctic Ice Sheets to Multi-Millennial Greenhouse Warming in the Earth System Model of Intermediate Complexity LOVECLIM

Calculations were performed with the Earth system model of intermediate complexity LOVECLIM to study the response of the Greenland and Antarctic ice sheets to sustained multi-millennial greenhouse warming. Use was made of fully dynamic 3D thermomechanical ice-sheet models bidirectionally coupled to an atmosphere and an ocean model. Two 3,000-year experiments were evaluated following forcing scenarios with atmospheric CO₂ concentration increased to two and four times the pre-industrial value, and held constant thereafter. In the high concentration scenario the model shows a sustained mean annual warming of up to 10°C in both polar regions. This leads to an almost complete disintegration of the Greenland ice sheet after 3,000 years, almost entirely caused by increased surface melting. Significant volume loss of the Antarctic ice sheet takes many centuries to initiate due to the thermal inertia of the Southern Ocean but is equivalent to more than 4 m of global sea-level rise by the end of simulation period. By that time, surface conditions along the East Antarctic ice sheet margin take on characteristics of the present-day Greenland ice sheet. West Antarctic ice shelves have thinned considerably from subshelf melting and grounding lines have retreated over distances of several 100 km, especially for the Ross ice shelf. In the low concentration scenario, corresponding to a local warming of 3?C4°C, polar ice-sheet melting proceeds at a much lower rate. For the first 1,200 years, the Antarctic ice sheet is even slightly larger than today on account of increased accumulation rates but contributes positively to sea-level rise after that. The Greenland ice sheet loses mass at a rate equivalent to 35 cm of global sea level rise during the first 1,000 years increasing to 150 cm during the last 1,000 years. For both scenarios, ice loss from the Antarctic ice sheet is still accelerating after 3,000 years despite a constant greenhouse gas forcing after the first 70?C140 years of the simulation.     

利用CryoSat-2波形数据建立南极Lambert冰川流域DEM

Lambert冰川-Amery冰架系统是南极冰盖最大的冰流系统之一,对南极冰盖物质平衡研究有着重要的作用.数字高程模型(DEM)是进行南极冰盖研究的基础.本文基于CryoSat-2 L1b波形数据,研究建立了Lambert冰川流域高分辨率DEM.测高卫星返回波形在冰盖区域存在变形,需进行波形重跟踪处理.利用交叉点分析方法对重心偏移法(OCOG)、阈值法和β参数法等常用的波形重跟踪方法对不同类型的CryoSat-2波形的适用性进行了研究.最后,利用克里金插值方法建立了500 m分辨率的Lambert冰川流域DEM——LAS DEM (Lambert Glacier-Amery Ice Shelf system DEM).利用ICESat卫星测高数据和GPS地面实测数据对LAS DEM进行精度验证,并与另外两种基于CryoSat-2数据的南极DEM进行了对比.结果表明:LAS DEM的整体精度约为0.295±2.7 m,优于另外两种CryoSat-2 DEM;在冰盖内陆地区,LAS DEM的高程误差在2 m之内;在Amery冰架上,LAS DEM的精度优于1 m.     

Climatic changes on orbital and sub-orbital time scale recorded by the Guliya ice core in Tibetan Plateau

Based on ice core records in the Tibetan Plateau and Greenland, the features and possible causes of climatic changes on orbital and sub-orbital time scale were discussed. Orbital time scale climatic change recorded in ice core from the Tibetan Plateau is typically ahead of that from polar regions, which indicates that climatic change in the Tibetan Plateau might be earlier than polar regions. The solar radiation change is a major factor that dominates the climatic change on orbital time scale. However, climatic events on sub-orbital time scale occurred later in the Tibetan Plateau than in the Arctic Region, indicating a different mechanism. For example, the Younger Dryas and Heinrich events took place earlier in Greenland ice core record than in Guliya ice core record. It is reasonable to propose the hypothesis that these climatic events were affected possibly by the Laurentide Ice Sheet. Therefore, ice sheet is critically important to climatic change on sub-orbital time scale in some ice ages.     

PREDICTIONS OF CRUSTAL DEFORMATION CAUSED BY CHANGING POLAR ICE ON A VISCOELASTIC EARTH

Changes in polar ice could cause vertical crustal motion of up to several mm yr-1 along the edge of the Greenland and Antarctic ice caps. Measurements of the uplift could help constrain the changing ice volumes. The problem is complicated by the Earth's visco-elastic response to past loading, including the Late Pleistocene deglaciation. A method is described for removing these visco-elastic effects, by using simultaneous measurements of vertical motion and surface gravity. A linear combination of these two measurement types can be formed which is relatively independent of visco-elastic effects, and which can be interpreted in terms of present-day fluctuations in ice.     

Comments on the influence of a meridionally sloping topography on the development of wave disturbances

Summary The existence of a major zonal mountain belt may be an important climatic agent during glacial periods. The combined effect of a large-scale ice field and a zonal mountain chain south of it, might contribute to the relative warming of the polar regions as the result of the increased poleward heat transport due to enhanced wave activity.     

南极普里兹湾海冰厚度的电磁感应探测方法研究

本文介绍一种由EM31-ICE型电磁感应仪和激光测距仪组合而成的船载电磁感应海冰厚度探测系统. 针对海冰和海水的电学特征,运用电磁感应技术提取探测系统至海冰下底面的距离,运用激光测距仪测量冰面粗糙度和探测系统至海冰上表面的距离,两组数据结合,实现了海冰厚度的探测. 通过南极现场探测数据分析,并与钻孔实测冰厚数据对比研究,定量分析了探测系统距离冰面的高度效应,建立了该系统冰厚测定值随高度变化的修正关系式,并对船载航行数据进行了系统校正. 与SCAR ASPeCt的冰厚数据对比分析,表明该系统能够获得可靠的海冰厚度并具有较高的精度,且能满足对极区大范围海冰厚度观测的需求.     

E) Organisations Nationales

Abstract

Sudden rapid advances or surges of glaciers and sections of smail ice caps are well known. After remaining dormant or in retreat over long periods of time these ice masses suddenly move forward rapidly with speeds about 2 orders of magnitude greater than usual. If such surges were to occur in large sections of the Antarctic ice sheet serious consequences could result. These include a significant rise of sea level, a substantial increase in the high-albedo ice cover around the continent especially in summer, and a cooling of the Antarctic ocean by the additional ice melting.

A numerical model has now been developed which simulates surging in certain glaciers and ice sheets in an apparently realistic manner. This model has been found to give close representations to a number of existing real surging temperate ice masses from small mountain glaciers to large sectors of ice caps. The model reproduces realistically many features of these ice masses such as the period of the surge, the duration, the velocity of advance, the magnitude of the advance, and the changes in ice thickness.

The application of the model to the Antarctic ice sheet is made more difficult by the problem caused by the temperature dependence of the flow properties of ice. This means that for a complete study the interaction with the environment needs to be considered. However, at this stage preliminary calculations indicate a number of features that are relevant to the effect of Antarctic ice surges on the global climate. These include the period between surges, the duration of the surge, the amount of ice advanced and the changes in thickness of the ice sheet.     

利用ICESat数据解算南极冰盖冰雪质量变化

南极冰盖冰雪质量变化反映了全球气候变化,并且直接影响着全球海平面变化.ICESat测高卫星的主要任务之一就是要确定南北两极冰盖的质量变化情况并评估其对全球海平面变化的影响.本文利用2003年10月至2008年12月的ICESat测高数据,针对南极DEM分辨率有限的特殊性,通过求解坡度改正值,解决重复轨道地面脚点不重合的问题,计算了南极大陆(86°S以北区域,后文所述南极冰盖均不包括86°S以南区域)在这5年里的冰雪质量变化情况,得到东南极冰盖的质量变化为-18±20Gt/a,西南极-26±6Gt/a,南极冰盖的冰雪质量变化为-44±21Gt/a,对全球海平面上升的影响约为0.12mm·a~(-1).解算结果表明,南极冰盖质量亏损主要集中在西南极阿蒙森海岸附近冰川以及东南极波因塞特角区域.     

Tephra deposition on glaciers and ice sheets on Mars: Influence on ice survival, debris content and flow behavior

We examine the consequences of pyroclastic deposits being emplaced onto ice layers on Mars, both those in the polar caps and those forming glaciers on the flanks of some of the large shield volcanoes. We show that layers of pyroclasts greater than a few meters in thickness, whether emplaced cold (as fall deposits) or hot (as pyroclastic density current deposits) act almost exclusively to protect ice layers beneath them from sublimation, irrespective of whether they are emplaced at high or low elevations or high or low latitudes. Layers less than about 2 m thick, on the other hand, can cause significant ice loss by raising the surface temperature due to their low albedo and then transmitting that increased temperature to the underlying ice, especially on a diurnal time scale. This can have a significant bearing on the emplacement history of polar water ice and on the survival time of glacial ice on shield volcano flanks. A key factor in the latter case is the timing of the episodic volcanic activity relative to the cycles of climate change driven by Mars' obliquity and eccentricity variations.     

Ground surface temperature history in southern Canada: Temperatures at the base of the Laurentide ice sheet and during the Holocene

We use temperature profiles from 7 deep (≈ 2000 m) boreholes located in southern Canada to infer ground surface temperature histories (GSTH) during the Last Glacial Maximum (LGM) and the Holocene. Visual inspection of the heat flow and of the reduced temperature depth profiles reveals significant regional differences with some sites showing conspicuous signs of post glacial warming, and other indicating only very small changes in ground surface temperature. These differences are confirmed by the inversions of the temperature profiles. The most prominent variations in GST are found at the Sudbury, Ontario, sites where the present ground surface temperature is high. With the exception of Sept-Iles, Quebec, the other sites only show moderate or no variation in GST. For all the sites, except possibly Sept-Iles, temperatures at the base of the ice sheet during the LGM were at or slightly below the melting point of ice. Temperatures might have been lower, a few degrees below 0 °C, at Sept-Iles. These results are consistent with field observations and model predictions suggesting high velocity basal flows in the ice sheet above the studied regions. These new data on basal temperatures will provide better quantitative constraints on glacier flow dynamics. The inversions give a chronology for the retreat of the ice sheet comparable to other proxies. Inversion and direct modeling show that, following the ice retreat, there was a warm period between 2 and 5 ka with temperatures 1–2 K higher than present. The inversion yields a time for this episode 1–2 kyr more recent than that inferred by other proxies for the Holocene climate optimum (HCO).     

基于"嫦娥一号"激光测高数据的月球极区光照条件研究

月球极区光照模型为研究月球车着陆点选择和水冰存在的探测提供了依据.利用"嫦娥一号"探月卫星获取的激光测高数据,得到了全月面高精度的数字高程模型(DEM),特别是在月球两极地区,地形细节清晰可见,为极区光照模型的建立提供了精确的数据基础.本文通过由测高数据建立的DEM和月球轨道参数相结合,采用地形最大高度角法,对月球极区的光照条件及其应用进行了研究和分析.光照率的计算周期为19年,考虑了黄道和白道交点进动的影响(18.6年).计算结果表明:(1)未发现有持续光照区;(2)有长久阴影区存在;(3)在南极或北极的夏季,撞击坑边沿高地处可以享受到连续的光照;(4)Shackleton撞击坑可以作为月球车着陆的首选目标之一.     

Ice cover,landscape setting,and geological framework of Lake Vostok,East Antarctica

Lake Vostok, located beneath more than 4 km of ice in the middle of East Antarctica, is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment. Melting and freezing at the base of the ice sheet, which slowly flows across the lake, controls the flux of water, biota and sediment particles through the lake. The influx of thermal energy, however, is limited to contributions from below. Thus the geological origin of Lake Vostok is a critical boundary condition for the subglacial ecosystem. We present the first comprehensive maps of ice surface, ice thickness and subglacial topography around Lake Vostok. The ice flow across the lake and the landscape setting are closely linked to the geological origin of Lake Vostok. Our data show that Lake Vostok is located along a major geological boundary. Magnetic and gravity data are distinct east and west of the lake, as is the roughness of the subglacial topography. The physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake’s circulation. Lake Vostok is a tectonically controlled subglacial lake. The tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small, but significant amounts of thermal energy into the lake.     

Subglacial Preservation of Valley Morphology at Amundsenisen,Western Dronning Maud Land,Antarctica

On the high altitude polar plateau of Amundsenisen, western Dronning Maud Land, East Antarctica, a subglacial valley, with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta, was studied by radio echo sounding. In addition, a small, probably glacial, valley was mapped within the same subglacial massif. Basal ice temperatures were calculated using field data on precipitation, air temperature and ice sheet thickness. Discoveries of old landforms which have been preserved more or less intact beneath the former Fennoscandian and Laurentide ice sheets have received increasing attention during the last decade. The aim of this study is to investigate whether preservation of landforms occurs under the East Antarctic Ice Sheet, and to discuss under that climatological and glaciological circumstances preservation may take place. The results show that the ice sheet covering the investigated localities is frozen to bed, and therefore has an insignificant erosional capability. The observations suggest that a large-scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold-based ice sheet marginal zone for a long time period. The process of glacial preservation, recognized for bedrock features and tentatively observed for sediment accumulations, should act on similar large-scale landforms under any cold-based ice sheet, present or past. On the basis of existing studies of the age and stability of the East Antarctic Ice Sheet, a Middle Pliocene age is suggested for the preserved landforms. The presence of the presumed sediment-filled valley further indicates that no prolonged periods of basal melting have occurred at the Amundsenisen study area during the ice sheet history, which includes the Quaternary glaciation periods. Finally, calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation. © 1997 by John Wiley & Sons, Ltd.     

Recent advances in understanding ice sheet dynamics

Glaciers and ice sheets play a dynamic role in Earth's climate system, influencing regional- and global-scale climate and responding to climate change on time scales from years to millennia. They are also an integral part of Earth's landscape in alpine and polar regions, where they are an active agent in isostatic, tectonic, and Earth surface processes. This review paper summarizes recent progress in understanding and modelling ice sheet dynamics, from the microphysical processes of ice deformation in glaciers to continental-scale processes that influence ice dynamics. Based on recent insights and research directions, it can be expected that a new generation of ice sheet models will soon replace the current standard. Improvements that can be foreseen in the near future include: (i) the addition of internally-consistent evolutionary equations for ice crystal fabric (anisotropic flow laws), (ii) more generalized flow laws that include different deformation mechanisms under different stress regimes, (iii) explicit incorporation of the effects of chemical impurities and grain size (dynamic recrystallization) on ice deformation, (iv) higher-order stress solutions to the momentum balance (Stokes' equation) that governs ice sheet flow, and (v) the continued merger of ice sheet models with increasingly complex Earth systems models, which include fully-coupled subglacial hydrological and geological processes. Examples from the Greenland Ice Sheet and Vatnajökull Ice Cap, Iceland are used to illustrate several of these new directions and their importance to glacier dynamics.     

Radio‐echo layering in West Antarctica: a spreadsheet dataset

Of the various information recovered from radio‐echo sounding (RES) of polar ice sheets, internal layering is currently under‐utilized by glaciologists, due in part to a lack of available data. Here, RES layering of the West Antarctic Ice Sheet, from the 1970s RES survey of approximately 70 per cent of this ice mass, is made available in a series of spreadsheets. Three types of internal layers are evident in the dataset. The first is continuous layers that have a stratigraphic appearance and can often be traced easily for hundreds of kilometres. The second is buckled layering, which also resembles stratigraphy and can sometimes be traced over tens of kilometres (although layer identification can often be difficult). The roughness of these layers is often greater than the bed at the same wavelength. The third is highly distorted or absent layering, which is not possible to trace laterally. Despite debate concerning the origin of RES layers, they are thought by most glaciologists to represent isochronous surfaces. The pattern of internal layering is potentially of importance to glaciologists for three reasons. (1) The position of undeformed layers below the ice surface is a function of accumulation rate, ice flow and basal melting conditions. Numerical modelling (including new ‘data assimilation’ techniques) could be used to discriminate between these processes, so revealing important information about the ice sheet and its environment. (2) Buckled layers are deformed by ice flow process, and so their occurrence can be related to the flow dynamics of the ice sheet. (3) Very buckled layers are often associated with ice stream flow, which allows their location to mark the positions of past and present fast‐flowing ice. Copyright © 2005 John Wiley & Sons, Ltd.     

Topography and ice sheet growth

This paper uses a numerical ice sheet model to investigate the role of topography in influencing ice sheet growth. The model is applied to the maritime, mid-latitude uplands of Scotland and relies on a series of assumptions about mass balance, topography, and ice flow. It is driven by an imposed pattern of temperature change. The model is able to predict effectively the extent and thickness of the Loch Lomond ice sheet, using a palaeotemperature curve based on Coleoptera assemblages. A series of experiments with a stepped, constant July air temperature depression suggests that in Scotland a change in excess of ?3·0°C is necessary to initiate ice sheet growth; that steady state ice caps build up at changes of ?3 to ? 6·5°C; and that large ice sheets build up at changes of more than ? 6·625°C. The bifurcation revealed by the last two types of behaviour is the result of topography. Both the vertical amplitude and the spatial distribution of bedrock basins and ridges are important in determining the pattern, rate, and extent of ice sheet growth. The implication is that topography plays an important role in determining the dynamics of ice sheet growth.     

Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications

Recent dramatic acceleration, thinning and retreat of tidewater outlet glaciers in Greenland raises concern regarding their contribution to future sea-level rise. These dynamic changes seem to be parallel to oceanic and climatic warming but the linking mechanisms and forcings are poorly understood and, furthermore, large-scale ice sheet models are currently unable to realistically simulate such changes which provides a major limitation in our ability to predict dynamic mass losses. In this paper we apply a specifically designed numerical flowband model to Jakobshavn Isbrae (JIB), a major marine outlet glacier of the Greenland ice sheet, and we explore and discuss the basic concepts and emerging issues in our understanding and modelling ability of the dynamics of tidewater outlet glaciers. The modelling demonstrates that enhanced ocean melt is able to trigger the observed dynamic changes of JIB but it heavily relies on the feedback between calving and terminus retreat and therefore the loss of buttressing. Through the same feedback, other forcings such as reduced winter sea-ice duration can produce similar rapid retreat. This highlights the need for a robust representation of the calving process and for improvements in the understanding and implementation of forcings at the marine boundary in predictive ice sheet models. Furthermore, the modelling uncovers high sensitivity and rapid adjustment of marine outlet glaciers to perturbations at their marine boundary implying that care should be taken in interpreting or extrapolating such rapid dynamic changes as recently observed in Greenland.     

IMF-associated cloudiness above near-pole station Vostok: Impact on wind regime in winter Antarctica

Changes in the cloudiness above the Antarctic station Vostok during the winter season were examined in relation to strong disturbances in the interplanetary magnetic field (IMF). A reliable relationship between cloud formation and IMF has been found: cloudiness increased under the influence of a strong southward IMF and decreased under the northward IMF. The surface temperature at Vostok station, which is derivative of the constant radiation cooling of air situated at the ice sheet and adiabatic warming of the air masses, incoming into the central Antarctica from the middle and upper troposphere, is enhanced or reduced. Quite opposite regularity in the temperature changes is typical of altitudes higher than the suggested cloud layer position (5–8 km). The processes occurring on the Antarctic ridge leads to anomalous winds at the ice dome and decay of the circumpolar vortex at the periphery of the Antarctic continent. As a result, the surface easterlies at the coast stations are replaced by southerlies, and the cold air masses flow from Antarctica out over the Southern Ocean.     

基于远震接收函数的南极大陆冰盖厚度研究

冰盖厚度是研究南极冰盖质量、建立冰盖动力学模型的基本参数,对于冰川均衡调整、冰盖物质平衡及全球气候变化研究具有重要意义.基于地震学的远震接收函数和H-Kappa格网搜索方法可以用于地震台站下方冰盖厚度的可靠探测,不仅能与冰雷达获得的冰盖厚度进行独立对比,还可以与冰雷达方法相互补充,进一步填补南极大陆冰盖厚度探测空白区.本文利用布设于南极大陆冰盖上方的流动地震台阵记录到的远震波形数据,基于接收函数方法对台阵下方的冰盖厚度进行了研究.结果显示:基于远震接收函数方法的冰盖厚度与Bedmap2冰厚格网模型相比,二者差别大多在200 m以内;少数台站差值达到600 m左右,这一差别可能与Bedmap2测线分布空区、冰雷达测深不确定性以及冰盖内部复杂波速结构等因素有关.本文研究结果表明:利用南极大陆冰盖上方的流动地震台阵,基于远震接收函数方法可以获得比较可靠的南极冰盖厚度,为独立验证冰雷达的探测结果并弥补冰雷达探测空白区提供了有效方法.同时,部分台站接收函数波形的复杂性可能暗示了南极大陆数千米厚的冰盖内部结构不是均一的,仍然存在比较复杂的内部结构变化.因此,有必要进一步利用包括接收函数波形拟合、地震面波反演等方法对南极大陆冰盖厚度及其内部精细结构进行更为深入的研究.     

Effect of ice sheet growth and melting on the slip evolution of thrust faults

Field investigations suggest that postglacial unloading and rebound led to the formation or re-activation of reverse faults even in continental shields like Scandinavia. Here we use finite-element models including a thrust fault embedded in a rheologically layered lithosphere to investigate its slip evolution during glacial loading and subsequent postglacial unloading. The model results show that the rate of thrusting decreases during the presence of an ice sheet and strongly increases during deglaciation. The magnitude of the slip acceleration is primarily controlled by the thickness of the ice sheet, the viscosity of the lithospheric layers and the long-term shortening rate. In contrast, the width of the ice sheet, the rate of deglaciation or the fault dip have an only minor influence on the slip evolution. In all experiments, the slip rate variations are caused by changes in the differential stress. The modelled deglaciation-induced slip acceleration agrees well with the occurrence of large earthquakes soon after the melting of the Fennoscandian ice sheet, which led to the formation of spectacular fault scarps in particular in the Lapland Fault Province. Furthermore, our model results support the idea that the low level of seismicity in currently glaciated regions like Greenland and Antarctica is caused by the presence of the ice sheets. Based on our models we expect that the decay of the Greenland and Antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions.