北冰洋海平面变化的观测和研究现状

回顾北冰洋海平面观测和研究现状,总结了北冰洋海平面变化特征和变化机制。北冰洋海平面季节变化受海冰生消、蒸发降水和陆地径流季节变化的影响,由比容变化主导;年际到年代际海平面变化受北极涛动影响显著,可用风场异常导致的淡水分布来解释。盐比容变化是深水洋盆海平面变化的主导因素,由之引起的质量变化控制陆架海域和北冰洋平均的海平面变化。近期波弗特环流区域海平面上升极快,与波弗特高压持续增强及淡水积聚有关。气候变暖会导致北冰洋海平面持续上升。海冰快速减退和格陵兰岛冰川融化对北冰洋海平面变化的影响有待深入研究。数据的短缺和观测的不确定性目前仍然制约北冰洋海平面变化的研究工作,高分辨率数值模拟有望成为未来研究的重要工具。     

Reconstructed changes in Arctic sea ice over the past 1,450 years

Arctic sea ice extent is now more than two million square kilometres less than it was in the late twentieth century, with important consequences for the climate, the ocean and traditional lifestyles in the Arctic. Although observations show a more or less continuous decline for the past four or five decades, there are few long-term records with which to assess natural sea ice variability. Until now, the question of whether or not current trends are potentially anomalous has therefore remained unanswerable. Here we use a network of high-resolution terrestrial proxies from the circum-Arctic region to reconstruct past extents of summer sea ice, and show that-although extensive uncertainties remain, especially before the sixteenth century-both the duration and magnitude of the current decline in sea ice seem to be unprecedented for the past 1,450 years. Enhanced advection of warm Atlantic water to the Arctic seems to be the main factor driving the decline of sea ice extent on multidecadal timescales, and may result from nonlinear feedbacks between sea ice and the Atlantic meridional overturning circulation. These results reinforce the assertion that sea ice is an active component of Arctic climate variability and that the recent decrease in summer Arctic sea ice is consistent with anthropogenically forced warming.     

冬季北极海冰面积异常与中国气温变化之间的年际关系

利用1957-2001年冬季的北极海冰资料、中国160站气温资料以及NCEP再分析的大气环流资料分析了冬季北极海冰面积异常与中国气温变化之间的年际关系.过去44年来,北极海冰面积总体上具有减小趋势(鄂霍次克海是例外,那里海冰面积有增加趋势),相应地北极涛动趋于增强,我国大部分地区趋于增暖.叠加这种趋势变化之上的是年际变化.在年际时间尺度上,冬季海冰变化的主要空间型表现为格陵兰海和白令海的海冰异常总是和鄂霍次克海、巴伦支海东部、喀拉海(新地岛附近)以及哈得孙湾的海冰异常符号相反,并且与500 hPa高度场上的EU和WP型遥相关对应.当冬季格陵兰海和白令海的海冰异常偏少,而鄂霍次克海、巴伦支海东部、喀拉海(新地岛附近)以及哈得孙湾的海冰异常偏多时,西伯利亚高压和阿留申低压都偏弱,冬季风减弱,东亚西风增强,我国冬季大部分地区温度升高;反之亦然.     

Effects of autumn-winter Arctic sea ice on winter Siberian High

The intensity of the winter Siberian High has significantly negative correlations with Arctic sea ice concentration anomalies from the previous autumn to winter seasons in the Eastern Arctic Ocean and Siberian marginal seas. Our results indicate that autumn-winter Arctic sea ice concentration and concurrent sea surface temperature anomalies are responsible for the winter Siberian High and surface air temperature anomalies over the mid-high latitudes of Eurasia and East Asia. Numerical experiments also support this conclusion, and consistently show that the low sea ice concentration causes negative surface air temperature anomalies over the mid-high latitudes of Eurasia. A mechanism is proposed to explain the association between autumn-winter sea ice concentration and winter Siberian High. Our results also show that September sea ice concentration provides a potential precursor for winter Siberian High that cannot be predicted using only tropical sea surface temperatures. In the last two decades (1990–2009), a strengthening trend of winter Siberian High along with a decline trend in surface air temperature in the mid-high latitudes of the Asian Continent have favored the recent frequent cold winters over East Asia. The reason for these short-term trends in winter Siberian High and surface air temperature are discussed.     

Changing Arctic Ocean freshwater pathways

Freshening in the Canada basin of the Arctic Ocean began in the 1990s and continued to at least the end of 2008. By then, the Arctic Ocean might have gained four times as much fresh water as comprised the Great Salinity Anomaly of the 1970s, raising the spectre of slowing global ocean circulation. Freshening has been attributed to increased sea ice melting and contributions from runoff, but a leading explanation has been a strengthening of the Beaufort High--a characteristic peak in sea level atmospheric pressure--which tends to accelerate an anticyclonic (clockwise) wind pattern causing convergence of fresh surface water. Limited observations have made this explanation difficult to verify, and observations of increasing freshwater content under a weakened Beaufort High suggest that other factors must be affecting freshwater content. Here we use observations to show that during a time of record reductions in ice extent from 2005 to 2008, the dominant freshwater content changes were an increase in the Canada basin balanced by a decrease in the Eurasian basin. Observations are drawn from satellite data (sea surface height and ocean-bottom pressure) and in situ data. The freshwater changes were due to a cyclonic (anticlockwise) shift in the ocean pathway of Eurasian runoff forced by strengthening of the west-to-east Northern Hemisphere atmospheric circulation characterized by an increased Arctic Oscillation index. Our results confirm that runoff is an important influence on the Arctic Ocean and establish that the spatial and temporal manifestations of the runoff pathways are modulated by the Arctic Oscillation, rather than the strength of the wind-driven Beaufort Gyre circulation.     

Ice-sheet acceleration driven by melt supply variability

Increased ice velocities in Greenland are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice-ocean interactions in water-terminating outlet glaciers and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt but an increase in water input variability that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure. Using a model that captures the dynamic switching between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles and an increase in rain and surface lake drainage events rather than an increase in mean melt supply.     

Low-latitude seasonality of Cretaceous temperatures in warm and cold episodes

The Cretaceous period is generally considered to have been a time of warm climate. Evidence for cooler episodes exists, particularly in the early Cretaceous period, but the timing and significance of these cool episodes are not well constrained. The seasonality of temperatures is important for constraining equator-to-pole temperature gradients and may indicate the presence of polar ice sheets; however, reconstructions of Cretaceous sea surface temperatures are predominantly based on the oxygen isotopic composition of planktonic foraminifera that do not provide information about such intra-annual variations. Here we present intra-shell variations in delta18O values of rudist bivalves (Hippuritoidea) from palaeolatitudes between 8 degrees and 31 degrees N, which record the evolution of the seasonality of Cretaceous sea surface temperatures in detail. We find high maximum temperatures (approximately 35 to 37 degrees C) and relatively low seasonal variability (< 12 degrees C) between 20 degrees and 30 degrees N during the warmer Cretaceous episodes. In contrast, during the cooler episodes our data show seasonal sea surface temperature variability of up to 18 degrees C near 25 degrees N, comparable to the range found today. Such a large seasonal variability is compatible with the existence of polar ice sheets.     

Similar meltwater contributions to glacial sea level changes from Antarctic and northern ice sheets

The period between 75,000 and 20,000 years ago was characterized by high variability in climate and sea level. Southern Ocean records of ice-rafted debris suggest a significant contribution to the sea level changes from melt water of Antarctic origin, in addition to likely contributions from northern ice sheets, but the relative volumes of melt water from northern and southern sources have yet to be established. Here we simulate the first-order impact of a range of relative meltwater releases from the two polar regions on the distribution of marine oxygen isotopes, using an intermediate complexity model. By comparing our simulations with oxygen isotope data from sediment cores, we infer that the contributions from Antarctica and the northern ice sheets to the documented sea level rises between 65,000 and 35,000 years ago were approximately equal, each accounting for a rise of about 15 m. The reductions in Antarctic ice volume implied by our analysis are comparable to that inferred previously for the Antarctic contribution to meltwater pulse 1A (refs 16, 17), which occurred about 14,200 years ago, during the last deglaciation.     

北极航线通航环境的盲数模型评估

为了研究北极航线通航问题,对北极航线的航行技术环境、人文环境和自然环境进行了深入分析,建立了北极航线通航环境评估指标体系,并在此基础上,利用盲数模型对北极航线通航环境进行了安全风险评估,确定了北极航线通航环境安全等级.研究结果表明,盲数理论用于航线通航环境评价,具有一定的理论可行性和推广应用价值     

High temperatures in the Late Cretaceous Arctic Ocean

To understand the climate dynamics of the warm, equable greenhouse world of the Late Cretaceous period, it is important to determine polar palaeotemperatures. The early palaeoceanographic history of the Arctic Ocean has, however, remained largely unknown, because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice. A shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated Upper Cretaceous organic-rich sediment from Alpha ridge, a submarine elevated feature of probable oceanic origin. A lack of carbonate in the sediments from this core has prevented the use of traditional oxygen-isotope palaeothermometry. Here we determine Arctic palaeotemperatures from these Upper Cretaceous deposits using TEX86, a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton, Crenarchaeota. From these analyses we infer an average sea surface temperature of approximately 15 degrees C for the Arctic Ocean about 70 million years ago. This calibration point implies an Equator-to-pole gradient in sea surface temperatures of approximately 15 degrees C during this interval and, by extrapolation, we suggest that polar waters were generally warmer than 20 degrees C during the middle Cretaceous (approximately 90 million years ago).     

Changes in Greenland ice sheet elevation attributed primarily to snow accumulation variability

The response of grounded ice sheets to a changing climate critically influences possible future changes in sea level. Recent satellite surveys over southern Greenland show little overall elevation change at higher elevations, but large spatial variability. Using satellite studies alone, it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability. Here we derive changes in ice-sheet elevation in southern Greenland, for the years 1978-88, using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation. Our patterns of accumulation-driven elevation change agree closely with contemporaneous satellite measurements of ice-sheet elevation change, and we therefore attribute the changes observed in 1978-88 to variability in snow accumulation. Similar analyses of longer ice-core records show that in this decade the Greenland ice sheet exhibited typical variability at high elevations, well within the long-term natural variability. Our results indicate that a better understanding of ice-sheet mass changes will require long-term measurements of both surface elevation and snow accumulation.     

Vertical structure of recent Arctic warming

Near-surface warming in the Arctic has been almost twice as large as the global average over recent decades-a phenomenon that is known as the 'Arctic amplification'. The underlying causes of this temperature amplification remain uncertain. The reduction in snow and ice cover that has occurred over recent decades may have played a role. Climate model experiments indicate that when global temperature rises, Arctic snow and ice cover retreats, causing excessive polar warming. Reduction of the snow and ice cover causes albedo changes, and increased refreezing of sea ice during the cold season and decreases in sea-ice thickness both increase heat flux from the ocean to the atmosphere. Changes in oceanic and atmospheric circulation, as well as cloud cover, have also been proposed to cause Arctic temperature amplification. Here we examine the vertical structure of temperature change in the Arctic during the late twentieth century using reanalysis data. We find evidence for temperature amplification well above the surface. Snow and ice feedbacks cannot be the main cause of the warming aloft during the greater part of the year, because these feedbacks are expected to primarily affect temperatures in the lowermost part of the atmosphere, resulting in a pattern of warming that we only observe in spring. A significant proportion of the observed temperature amplification must therefore be explained by mechanisms that induce warming above the lowermost part of the atmosphere. We regress the Arctic temperature field on the atmospheric energy transport into the Arctic and find that, in the summer half-year, a significant proportion of the vertical structure of warming can be explained by changes in this variable. We conclude that changes in atmospheric heat transport may be an important cause of the recent Arctic temperature amplification.     

Modelled atmospheric temperatures and global sea levels over the past million years

Marine records of sediment oxygen isotope compositions show that the Earth's climate has gone through a succession of glacial and interglacial periods during the past million years. But the interpretation of the oxygen isotope records is complicated because both isotope storage in ice sheets and deep-water temperature affect the recorded isotopic composition. Separating these two effects would require long records of either sea level or deep-ocean temperature, which are currently not available. Here we use a coupled model of the Northern Hemisphere ice sheets and ocean temperatures, forced to match an oxygen isotope record for the past million years compiled from 57 globally distributed sediment cores, to quantify both contributions simultaneously. We find that the ice-sheet contribution to the variability in oxygen isotope composition varied from ten per cent in the beginning of glacial periods to sixty per cent at glacial maxima, suggesting that strong ocean cooling preceded slow ice-sheet build-up. The model yields mutually consistent time series of continental mean surface temperatures between 40 and 80 degrees N, ice volume and global sea level. We find that during extreme glacial stages, air temperatures were 17 +/- 1.8 degrees C lower than present, with a 120 +/- 10 m sea level equivalent of continental ice present.     

Subtropical Arctic Ocean temperatures during the Palaeocene/Eocene thermal maximum

The Palaeocene/Eocene thermal maximum, approximately 55 million years ago, was a brief period of widespread, extreme climatic warming, that was associated with massive atmospheric greenhouse gas input. Although aspects of the resulting environmental changes are well documented at low latitudes, no data were available to quantify simultaneous changes in the Arctic region. Here we identify the Palaeocene/Eocene thermal maximum in a marine sedimentary sequence obtained during the Arctic Coring Expedition. We show that sea surface temperatures near the North Pole increased from 18 degrees C to over 23 degrees C during this event. Such warm values imply the absence of ice and thus exclude the influence of ice-albedo feedbacks on this Arctic warming. At the same time, sea level rose while anoxic and euxinic conditions developed in the ocean's bottom waters and photic zone, respectively. Increasing temperature and sea level match expectations based on palaeoclimate model simulations, but the absolute polar temperatures that we derive before, during and after the event are more than 10 degrees C warmer than those model-predicted. This suggests that higher-than-modern greenhouse gas concentrations must have operated in conjunction with other feedback mechanisms--perhaps polar stratospheric clouds or hurricane-induced ocean mixing--to amplify early Palaeogene polar temperatures.     

Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms

It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean. This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs. Detailed mapping of the ocean floor is the best means to resolve this issue. Although sea-floor imagery has been used to reconstruct the glacial history of the Antarctic shelf, little data have been collected in the Arctic Ocean because of operational constraints. The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 provided the opportunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom profiler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea floor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations.     

冰层厚度自动化观测系统的试验研究

以河冰为例,在实验室中进行了三个冰生消变化过程中直流电微弱传导性质的试验。通过对大量测量数据的抽样分析,验证了冰的直流电传导性;总结了冰在生消变化过程中其直流传导性与冰、传导距离、直流电压的关系,得出了冰具有直流电微弱传导特性。利用这一性质,结合单片机及计算机图形学技术来实现对冰的生消变化及冰层厚度进行自动化观测。该研究成果可用于极地海冰厚度的自动化监测,为极地海冰厚度变化过程监测提供技术基础。     

Low sea level rise projections from mountain glaciers and icecaps under global warming

The mean sea level has been projected to rise in the 21st century as a result of global warming. Such projections of sea level change depend on estimated future greenhouse emissions and on differing models, but model-average results from a mid-range scenario (A1B) suggests a 0.387-m rise by 2100 (refs 1, 2). The largest contributions to sea level rise are estimated to come from thermal expansion (0.288 m) and the melting of mountain glaciers and icecaps (0.106 m), with smaller inputs from Greenland (0.024 m) and Antarctica (- 0.074 m). Here we apply a melt model and a geometric volume model to our lower estimate of ice volume and assess the contribution of glaciers to sea level rise, excluding those in Greenland and Antarctica. We provide the first separate assessment of melt contributions from mountain glaciers and icecaps, as well as an improved treatment of volume shrinkage. We find that icecaps melt more slowly than mountain glaciers, whose area declines rapidly in the 21st century, making glaciers a limiting source for ice melt. Using two climate models, we project sea level rise due to melting of mountain glaciers and icecaps to be 0.046 and 0.051 m by 2100, about half that of previous projections.     

Antarctic ice-sheet loss driven by basal melting of ice shelves

Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.     

Sharply increased mass loss from glaciers and ice caps in the Canadian Arctic Archipelago

Mountain glaciers and ice caps are contributing significantly to present rates of sea level rise and will continue to do so over the next century and beyond. The Canadian Arctic Archipelago, located off the northwestern shore of Greenland, contains one-third of the global volume of land ice outside the ice sheets, but its contribution to sea-level change remains largely unknown. Here we show that the Canadian Arctic Archipelago has recently lost 61?±?7?gigatonnes per year (Gt?yr(-1)) of ice, contributing 0.17?±?0.02 mm?yr(-1) to sea-level rise. Our estimates are of regional mass changes for the ice caps and glaciers of the Canadian Arctic Archipelago referring to the years 2004 to 2009 and are based on three independent approaches: surface mass-budget modelling plus an estimate of ice discharge (SMB+D), repeat satellite laser altimetry (ICESat) and repeat satellite gravimetry (GRACE). All three approaches show consistent and large mass-loss estimates. Between the periods 2004-2006 and 2007-2009, the rate of mass loss sharply increased from 31?±?8?Gt?yr(-1) to 92?±?12?Gt?yr(-1) in direct response to warmer summer temperatures, to which rates of ice loss are highly sensitive (64?±?14?Gt?yr(-1) per 1?K increase). The duration of the study is too short to establish a long-term trend, but for 2007-2009, the increase in the rate of mass loss makes the Canadian Arctic Archipelago the single largest contributor to eustatic sea-level rise outside Greenland and Antarctica.     

Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current

The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line through submarine glacial troughs and causing high rates of melting at the deep ice-shelf bases. The interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice-mass loss. Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice-ocean model forced by a range of atmospheric outputs from the HadCM3 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner-Ronne Ice Shelf represents 80 per cent of the present Antarctic surface mass balance. Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise.