Research Article
Riukojietna’s Sensitivity To Climatic Changes
- Gunhild Rosqvist
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 357
-
- Article
-
- You have access Access
- Export citation
-
Riukojietna (lat. 68°N., long. 18°E.), which is classified as an ice cap, is located 35 km north-west of Kebnekaise, northern Sweden. The glacier is situated between 1140 and 1456 m a.s.l. and covers an area of 4.6 km2. The surrounding mountains reach the 1600 m level. Two maps, based on air photographs taken 1960 and 1978, have been produced. A study of sediments from two lakes receiving meltwater from Riukojietna has yielded information on Riukojietna’s ability to produce rock flour during the Holocene. Several factors affect the production and removal of rock flour of which the most important are ice thickness, basal ice temperature and water discharge. It is assumed that maximum in silt production for a warm-based glacier will closely follow or coincide with maximum in ice volume. Thus the variation of the relative amount of silt in proglacial lacustrine sediments provides a continuous record of fluctuations in glacier activity. Riukojietna either was small and inactive or may have disappeared totally during a long period between 9500 and 2500 years B.P. Between 2500 and 2000 years B P. the climatic conditions were such that a reactivation of Riukojietna could occur. The Scandinavian glaciers reached a distinct maximum at the beginning of the 20th century According to topographical maps, Riukojietna was then more than 10 km2 larger in extent than at present. In order to understand the pattern of glacier variation during the Holocene, the relation between climatic fluctuations and behaviour of Riukojietna is under observation. The purpose is to define those factors that make Riukojietna more sensitive to climatic changes than neighbouring glaciers.
The mass balance of Riukojietna has been investigated during the balance years 1985–86, 1986–87 and 1987–88. In spring 1986 the subglacial topography was monitored with a low frequency radio-echo sounder. Based on these results, holes were drilled in August 1988 for temperature recordings. Winter balances have been rather equal over the years. Differences in net balance values are primarily caused by fluctuating summer balances. A high degree of correlation between summer balance and summer temperature can be expected and has been calculated for Storglaciären. Since the net and summer balances of Riukojietna fluctuate in phase with those from Storglaciären, a similar dependence of the mass balance on summer temperature may exist. Because of the gently-sloping surface and even distribution of the accumulation, a rather uniform and negative summer balance occurs over the whole glacier surface.
During years with some net accumulation on the glacier, the accumulation area is located on the easterly, lee side of the ice cap, in the height interval 1360–1400 m a.s.l. The lowering of the surface profile between 1960 and 1978 was negligible between 1360 and 1400 m as compared to the lowering of the rest of the glacier surface. The maximum ice thickness, 105 m, also occurs in this interval, whereas the mean ice thickness of Riukojietna is only 36 m.
Mass-balance studies have continuously been carried out on Storglaciären since 1945. Between 1959 and 1980 the mean value of the net balance for Storglaciären was −0.33 m w.eq. By using maps from 1960 and 1978 a corresponding value for Riukojietna can be calculated. The result, −0.6 m w.eq., shows that Riukojietna is far from being in balance with the existing climate, while Storglaciären is close to a steady state. According to the “summit method” the glaciation limit is located at 1550 m a.s.l. in the vicinity of Riukojietna. Since the ice-covered bedrock only reaches 1400 m a.s.l., Riukojietna will not reform after a disappearance unless a climatic deterioration generates an approximately 150 m lower glaciation limit. Since the glacier does not experience any net accumulation at present, it will finally disappear if present trends continue; its present condition is probably similar to that experienced during the early Holocene. A distinct climatic deterioration, like the one that occurred between 2500 and 2000 years B.P., would allow a reactivation and expansion of the ice cap.
Riukojietna, which covers a mountain plateau, comprises a relatively small vertical extent. Since it is relatively low-lying as compared to cirque glaciers, which often have a larger vertical extent, it is much more sensitive to changes in the climate. Once the ELA rises over 1400 m a.s.l. or is depressed below 1300 m a.s.l. a major part of the ice cap becomes either ablation or accumulation area. After a presumed disappearance, Riukojietna has to reform at a much lower altitude as compared to a cirque glacier. While a minor lowering of the glaciation limit is enough to reactivate cirque glaciers, a more distinct lowering is necessary before a reformation and a reactivation of Riukojietna can occur. If the climatic deterioration is severe enough, Riukojietna will quickly expand over the plateau. The areal extent of the ice cap then becomes much larger as compared to cirque glaciers that are forced to expand to lower altitudes where melting increases.
A 3500-Year Ice Chemistry Record From The Dominion Range, Antarctica: Linkages Between Climatic Variations and Precipitation Chemistry
- Mary Jo Spencer, Paul A. Mayewski, W. Berry Lyons, Mark S. Twickler, Pieter Grootes
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 358
-
- Article
-
- You have access Access
- Export citation
-
In 1984 a 200-m ice core was collected from a local accumulation basin in the Dominion Range, Transantarctic Mountains, Antarctica. A complete oxygen isotope record has been obtained and a considerable portion of the core has been analyzed in detail for chloride, nitrate, sulfate, and sodium. About half of the chloride is due to sea salt with the remainder originating as gaseous HCl. Nitrate levels have increased markedly over the last 1000 years whereas the levels of the other constituents have remained fairly constant.
The oxygen isotope results suggest that this region of Antarctica is responding to long-term global climate forcing as well as to shorter-term climatic variations. This data will be compared with the anion and sodium records in order to determine the effects of climatic forcing on these other records. In particular, nitrate appears to vary in concert with fluctuations in long-term climate. Additionally, variations in each constituent over the 3500 year period will be examined in detail to determine the influence of other processes which affect their concentrations.
Instability Of The Global Greenhouse Gas System As A Cause Of The Ice Ages: A Low-Order Dynamical Model
- B. Saltzman, K.A. Maasch
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 358
-
- Article
-
- You have access Access
- Export citation
-
Due to the extremely low rate at which the major glacial variations occurred during the Pleistocene it is not possible to explain these variations by direct calculation of the fundamental fluxes of heat, momentum, and water mass that must be involved. Instead, we approach the problem in a more inductive manner by trying to formulate a physically plausible dynamical system, representing the net effects of these fluxes, that can account for the observed variance with a minimum number of adjustable parameters, given the known forcing due to Earth-orbital (Milankovitch) variations. Our model involves three “slow-response” variables: the global ice mass, the concentration of atmospheric greenhouse gases (notably CO2, methane, and water vapor), and a measure of the thermal-biological-chemical state of the ocean (perhaps measured by the stratification) that may control the Earth’s carbon cycle over the Pleistocene time period. These variables are connected by a nonlinear dynamical system comprised of three ordinary differential equations that can exhibit instability and free oscillatory behavior of a period close to 100 000 years (the period at which the maximum ice-age variability is found to exist in the late Pleistocene) despite relatively small external Earth-orbital forcing. We calculate the rate constants needed for the model to account for the main features of the ice variations, including the mid-Pleistocene transition from a period of low global ice mass devoid of the 100 000 year oscillation. The implied variations of the greenhouse gases over the past 150 000 years are in good agreement with the recent Vostok ice-core analyses.
Changes Of Atmospheric Methane Concentration Parallel To Climatic Changes
- B. Stauffer, H. Oeschger, J. Schwander
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 359
-
- Article
-
- You have access Access
- Export citation
-
Measurements on ice-core samples showed that atmospheric methane concentration changed with the large climatic cycles during the last two glaciations (Stauffer and others, 1988; Raynaud and others, 1988). The methane concentration is lower in cold periods and higher in warm periods. In this paper we discuss the results of CH4 measurements of samples from periods of minor climatic change, like the climatic optimum 8000 years B.P. and the Younger Dryas period about 10 000 to 11 000 years B.P.. The data are interpreted in terms of the present understanding of methane sources and sinks.
Holocene Paleoenvironmental Reconstruction From Deep Ground Temperatures, Canadian Arctic Archipelago: A Comparison With Climatic Inferences From The δ18O Record Of Ice Cores
- Alan E. Taylor
-
- Published online by Cambridge University Press:
- 20 January 2017, pp. 359-360
-
- Article
-
- You have access Access
- Export citation
-
The δ18O record from ice cores serves as a proxy paleoclimatic temperature record, through the association of isotopic ratio to air temperatures at time of precipitation. Climatic change may be preserved also as a signal in ground temperatures, not as a proxy indicator of past climate but as a direct consequence of the effect of past air temperature variations and associated physical processes at the ground surface. In the Canadian Arctic Archipelago, δ18O records are available from the Devon and Agassiz ice caps, and precision ground temperatures to depths of up to 1000 m are available from 40 petroleum exploration wells, about one third of which are suitable for paleoenvironmental reconstruction. There is an opportunity to compare these two methods of looking at the paleoenvironment, and to show how complementary they are to each other.
Geothermal analysis is predicated on the fundamental hypothesis that the terrestrial heat flow, which arises largely from the decay of radioactive elements within the crust, does not vary measurably in the upper few km. But at many wells, the heat flow, calculated as the product of the measured temperature gradient and rock thermal conductivity, does vary systematically with depth in the well. While more random variations may be attributed to measurement errors, and corrections may be made for such known effects as local topography, the residual coherent “long wavelength” variation may be ascribed to effects arising from climate change.
Can we, then, determine whether a particular temperature history is consistent with the geothermal record, or ideally, invert the geothermal data to reveal a record of past surface temperatures? Attempts with varying success at paleoclimatic reconstruction from ground temperatures have been reported in the literature (e.g. Lane, 1923; Hotchkiss and Ingersoll, 1934; Birch, 1948; Cermak, 1971; Vasseur and others, 1983; Lachenbruch and others, 1986) and from temperature profiles in ice sheets (e.g. Paterson, 1968; Weertman, 1968; Budd and Young, 1982).
In this study, standard techniques in geothermics (e.g. Jaeger, 1965) have been used (1) to show the effect of any hypothesized surface paleotemperature model upon subsurface temperatures, or (2) on the hypothesis that the variation in heat flow is attributed to paleoclimatic effects, to derive a surface temperature model at each well that minimizes the variation in a statistical sense. The resolution of the method and limitations in our measured temperature and rock thermal conductivity data restrict the application of the second method to the past few hundred to one thousand years. The paper considers the first approach for the period 1 ka-10 kaB.p. at about a dozen wells and gives an example of the second approach at a well west of the Agassiz Ice Cap.
Aproach (1). In studying the Devon Island ice core, Fisher and Koerner (1979) present a detailed record of the mean annual air temperature at the site throughout the Holocene, based on the δ18O record. A simplified time-temperature model of this record is applied to the ground temperature data set for the period 1 ka-10 ka B.P. Although the effect on the ground temperatures is only subtly perceptible, the model has the effect of reducing the apparent climatically-related curvature in the data, as reflected in an improvement in the standard deviation in the calculated heat flow profile by 5% to 30%. Hence, the geothermal record provides quantitative support for Holocene climatic information derived from the ice core record.
Approach (2). This inversion technique is analogous to Paterson’s (1968) reconstruction of the surface temperature during the past century from a temperature profile taken in the small Meighen Ice Cap, Arctic Canada. A unique model is not obtained; rather, a small set of possible surface temperature variations consistent with the deeper subsurface temperatures is produced. Such modelling suggests that subsurface temperatures at a well 180 km west of the Agassiz Ice Cap are consistent with ground surface temperatures some 4–6 Κ lower at the well during the Little Ice Age; this is considerably more severe than the mean annual air temperatures projected from the δ18O record at Agassiz. It is possible that the large increase in ground surface temperature at the wellsite since the Little Ice Age may be attributed to some climatically-related phenomena such as increased incidence of snow cover coherent with the changing climate. A well on Devon Island is not deep enough for a comparison to that ice cap.
The oxygen isotope data provide a valuable estimate of Holocene climate with which to correct ground temperature data for terrestrial heat flow, or other studies. However, examination of the signal of more recent events suggests that ground temperatures may be considerably modified by associated transient phenomena such as snow cover, vegetation, etc. Hence, one would expect that such a Holo¬cene correction might either understate or overstate the actual experience of the ground surface at a site.
Increased Accumulation On The Antarctic Ice Sheet Due To Climatic Warming
- Stephen Warren, Susan Frankenstein
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 361
-
- Article
-
- You have access Access
- Export citation
-
Climatic warming due to increased greenhouse gases is expected to cause increased precipitation in the next century because of the increased water content of the air, assuming constant relative humidity. Since temperatures over most of Antarctica are far below freezing even in the warmest month of the year, the increase in melting is probably negligible compared to the increase in precipitation.
Oerlemans (1982) showed that this increase of precipitation would cause a growth of the ice sheet, tending to lower sea level. This would partially counteract the rise of sea level due to increased melting on mountain glaciers and Greenland, and to a possible (and more difficult to predict) surge of ice from West Antarctica.
Oerlemans may have underestimated the increase in accumulation. He used results of General Circulation Models (GCMs) which indicated an increase of precipitation by only 12% for a temperature change ΔΤ = 3 Κ and 30% for ΔΤ = 8 K. In contrast, the change in accumulation rate at Dome C (Lorius and others, 1979) accompanying the warming from the recent ice age to the present was in accord with the simple assumption that accumulation is proportional to saturation vapor pressure at the temperature of the inversion layer, i.e. a 30% increase for ΔΤ = 3 K.
The experimental results are to be preferred to the climate model results because GCMs do not represent ice-sheet accumulation processes well. Most of the accumulation is not snow falling from clouds but instead results from clear-sky ice-crystal formation in near-surface air, or hoarfrost deposition on the surface. GCMs lack sufficient vertical resolution to represent the strong temperature inversion on which these accumulation mechanisms depend.
The figure shows that the increase of vapor pressure due to ΔΤ = 5 Κ varies from a factor of 1.9 at Τ = −60°C to a factor of 1.6 at Τ = −20°C. A climatic warming of 5 K. over Antarctica, which is possible during the next century, could thus increase the Antarctic accumulation from its present 17g cm−2 yr−1 to 30 g cm−2 yr−1, leading to a 50 cm drop in sea level in 100 years. This assumes that the simple proportionality of precipitation rate to saturation vapor pressure applies as well to the coastal regions, which is doubtful because the accumulation processes are not the same as on the plateau.
The potential importance of Antarctic accumulation changes in contributing to changes of sea level argues for further study of the mechanisms of Antarctic precipitation and for their improved representation in climate models.
Satellite and Oceanographic Observations Of Large Ice-Edge Eddies In The Kuril Basin Region Of The Okhotsk Sea
- Masaaki Wakatsuchi, Seelye Martin, Esther Munoz
-
- Published online by Cambridge University Press:
- 20 January 2017, pp. 360-361
-
- Article
-
- You have access Access
- Export citation
-
We examined the behavior of the sea ice in the Okhotsk Sea which formed over the deep Kuril Basin during the period 1978–82. When ice extended over the basin, we observed the formation of large eddies with diameters of order 200 km. We determined the size and duration of these eddies through use of the 37 GHz channel on the Nimbus 7 Scanning Multichannel Microwave Radiometer, and with the visible channel on the geostationary Himawari satellite. Within the ice cover, the satellite data show that these eddies produced open-water regions which persisted for 4–6 weeks, and that the eddies recurred year after year, even though their relative position changed. Comparison of eddy positions determined from satellite data with oceanographic positions shows that the oceanography drives the eddies. An estimate of heat loss from these eddies shows that the role of the ocean eddies is to keep the region ice-free until heat loss approaches zero, so that fluxes over the eddies primarily cool the water column without adding salt. Then as the atmosphere begins to warm in spring, the eddies tend to become ice-covered, so that melt water is introduced to their surface. Examination of the oceanography shows that the early summer water-column structure depends on the heat loss from the region during the preceding ice season, the amount of ice over the basin, and the total amount of ice formation in the Okhotsk Sea. During the heavy ice year of 1979, the upper 200–300 m were cooler, less saline, and highly oxygenated. This modification appears to be a local process, driven by eddy-induced mixing, local cooling, and ice melting. At 300–1200 m depths, water modification is caused by advection of water from outside the Kuril Basin. During heavy ice years with strong cooling, this water is more saline, colder, and richer in oxygen than during lighter ice years. The water modified in the basin can be traced into the North Pacific, where it cools and dilutes the surface water, and oxygenates the upper 200–400 m.
Impurity Distributions In Ice Under Different Environmental Conditions
- Eric Wolff, Robert Mulvaney
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 362
-
- Article
-
- You have access Access
- Export citation
-
We have shown previously (Mulvaney and others, 1988; Wolff and others, 1988) that some of the impurities in ice are localised. For samples from Dolleman Island in the Antarctic Peninsula, sulphuric acid was found at very high concentrations at the triple junctions (where three grains meet). No such localisation was found for sea salt elements, which are the other major soluble impurity. We believe that the acid is sufficiently concentrated at ice-sheet temperatures to remain liquid, forming a network of sub-micron veins through the ice.
We used a scanning electron microscope (SEM) fitted with an X-ray microanalysis system and a cold stage that holds samples below −160°C. Located at the University of Lancaster, the instrument allows frozen samples to be investigated with elemental analysis carried out at a resolution of the order of 1 micron.
Further experiments have yielded similar results for other samples from the same ice core. However, we have not yet found a method of cutting and cooling the samples that gives quantitatively reproducible data, so that it is too early to say what proportion of the acid in the sample is at the triple junctions.
Nonetheless, we have now also seen S at several triple junctions in ice from Site G in central Greenland. The sample includes part of the material from the 1783 Laki volcanic eruption. We have still to look at samples from other sites, but are reassured that the positive result is not confined to one ice core.
This work, still at a formative stage, has posed some important questions:
(1) For us there is the technical question of how we obtain reproducible quantitative results.
(2) How widespread is the phenomenon, and how much of the acid is at triple junctions? This is the next phase of studies at Lancaster, and is likely to include a study of older ice, and of temperate ice.
(3) Why is the acid at triple junctions, and why is sea salt not found there? This must be due to processes in the atmosphere or snowpack, and is likely to be related to the eutectic temperatures of impurity/water mixtures. Thus the distribution may influenced by changes in climate or chemistry. For instance, Wisconsin-age ice in Greenland is neutral, any acid having reacted with alkali dusts. How did this affect the impurity distribution?
(4) If the distribution does change as a result of a changed environment, does this affect the physical properties of the ice itself? In particular, is the presence or absence of liquid at the junctions a contributory factor to the changes in rheology between Wisconsin and Holocene ice? We are far into the realms of speculation here, but this does have the potential to be an interesting long-timescale feedback to climatic and environmental changes.
Detecting Global Change In The Arctic
- Gunter Weller
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 362
-
- Article
-
- You have access Access
- Export citation
-
Numerical models have predicted global temperature increases due to rising atmospheric CO2 levels, which should be detectable by now, but have not yet been identified in an unambiguous manner. This detection is complicated by inadequate data and by the fact that climate can be changed by factors other than CO2 increases. A systematic monitoring strategy is therefore needed to assess global change. In the Arctic, cryospheric parameters, including sea ice, snow cover, glaciers and permafrost are sensitive indicators of climate change and their monitoring by satellites and surface observations is of particular interest. Sea ice and snow cover are perhaps the most important of these parameters. They respond quickly to climate change, and in turn directly affect the climate through feedback processes; major changes in ice and snow extent and thickness can be expected as a consequence of climate change. Glaciers also respond to climatic variability by changes in their mass balance which can be monitored. Melting glaciers raise the level of the world ocean, and the glaciers of the sub-Arctic, particularly in the Alaskan coastal mountains, have been major contributors to the observed sea-level rise of about 20–30 cm over the last century. Past temperature changes are recorded in glacier ice and permafrost and techniques are now available to reconstruct past climates from these sources.
The numerical models of the CO2 greenhouse effect show the polar regions to be affected most strongly by greenhouse warming, and sea ice, snow, glaciers and permafrost should be good indicators of such a global change. The known responses and sensitivities of cryospheric parameters to climate change are reviewed, and a monitoring strategy is suggested. The Alaska SAR Facility, utilizing synthetic aperture radar from several spacecraft scheduled for launch in the 1990s, will be a key facility for collecting and analyzing climate-related satellite data. Its monitoring capabilities are briefly reviewed.
Soluble Impurities In Ice Core D-1 Of Dunde Ice Cap, China, Over The Last 500 Years
- Wu Xiaoling, Liu Jingsona, Yang Qinzhou
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 363
-
- Article
-
- You have access Access
- Export citation
-
This paper gives the preliminary results of 26 trace element measurements of ice cores from Dunde Ice Cap, China. The chemical composition of soluble impurities along ice core D-1 covering the last 500 years B P., is reported and interpreted in terms of atmospheric contributions. The dust content in ice cores of Dunde Ice Cap is 36 times higher than in Byrd Station, Antarctica. Variations of soluble elements such as Ca, Mg, Κ and Na, in Dunde Ice cores are very sensitive to climatic and environmental changes. The 25 trace elements in ice core D-1 (K, Na, Ca, Mg, Cd, Cr, Co, Cu, Fe, Mn, Mo, Ni, Pb, Al, Sr, Ti, V, Zn, As, Ba, Β, Li, Ρ, S, Sn) were measured. Cationicions arranged in order of content are as follows: Ca > Na > Mg > Κ > Αl > Fe > Ζn > Cu > Μn > Pb > Cr > Ni > Co > Cd etc. The content of soluble impurities has typical terrestrial features. Rock-forming elements such as Ca, Mg, Κ, Na, Si, Al, and Fe make up 99% in the core samples.
Particular attention is given to the possible impact of the so-called “pre-Industrial Revolution period” and man’s influence on the atmospheric chemistry. The spectral analysis of time series for the variation of each of the 26 contributions show a 92 year cycle that is present in the variation of all 26 ions with depth. Short-term variations, such as 23, 31, 48, 81 year cycles, are also discussed. The ice-core research program has been supported by the Chinese National Foundation of Natural Science under Grant DO125-4860011.
Secular Trends Of Accumulation Rate On Ice Cores From Dunde Ice Cap, China Over The Last 1000 Years
- Wu Xiaoling, Li Zhongqin, Xie Zichu
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 363
-
- Article
-
- You have access Access
- Export citation
-
Cooperative research programs were conducted on the Dunde Ice Cap (38°06′N, 96°26′E), China, in 1984, 1986, 1987, by the Lanzhou Institute of Glaciology and Geocryology (LIGG), China and the Byrd Polar Research Center (BPRC), U.S.A. This paper gives the preliminary results of the analysis on accumulation rate of the ice cap over the last 1000 years. Three ice cores were recovered to bedrock from the ice-cap summit (5324 m a.s.l.). Core D-1 (139.8 m long) was divided in the field along the entire length and was shared equally between LIGG and BPRC. Core D-2 (136.6 m long) was returned frozen complete to the LIGG for ice-core measurements. In Core D-3 (138.4 m long) the upper sectors were melted and bottled in the field and the lower sectors were returned frozen to the BPRC, U.S.A. Core D-1 was analyzed in China along the entire length for oxygen isotope, liquid conductivity and pH. A year-by-year dating of the ice cores has been made with Dansgaard-Johnsen’s flow pattern by using the data of surface strain-rate (August 1986 to August 1987) and tritium measurements. The resulting time-scales of the ice cores in Dunde Ice Cap yield an age of 4600 yr B.P. The annual layer thicknesses of core D-1 were measured mainly by δ18O analysis and liquid conductivity. The lower δ18O is generally associated with higher electrical conductivity. Annual layer thickness was converted to accumulation rates and compared with meteorological records from Delingxa Meteorological Station. The mean accumulation rate is 518 mm in ice-equivalent. Particular attention is given to the possible impact of the Little Ice Age. Based on spectral analysis of time series for the accumulation variation with depth, short-term (30, 33 year at 0.01 level) and intermediate-term variation (120 year) were discussed. The ice-core research program has been supported by the Chinese National Foundation of Natural Science under Grant DO125-4860011.
Effect Of Eurasian Snow Cover On Summer Climate Of The Northern Hemisphere: A GCM Study
- Tetsuzo Yasunari, Akio Kitoh, Tatsushi Tokioka
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 364
-
- Article
-
- You have access Access
- Export citation
-
Observational studies have shown that Eurasian snow-cover anomalies during winter-through-spring seasons have a great effect on anomalies in atmospheric circulation and climate in the following summer season through snow albedo feedback (Hahn and Shukla, 1976; Dey and Bhanu Kumar, 1987). Morinaga and Yasunari (1987) have revealed that large-scale snow-cover extent over central Asia in late winter, which particularly has a great effect on the circulation over Eurasia in the following season, is closely related to the Eurasian pattern circulation (Wallace and Gutzler, 1981) in the beginning of winter.
Some atmospheric general circulation models (GCM) have suggested that not only the albedo effect of the snow cover but also the snow-hydrological process are important in producing the atmospheric anomalies in the following seasons (Yeh and others, 1984; Barnett and others, 1988).
However, more quantitative evaluations of these effects have not yet been examined. For example, it is not clear to what extent atmospheric anomalies are explained solely by snow-cover anomalies. Spatial and seasonal dependencies of these effects are supposed to be very large. Relative importance of snow cover over Tibetan Plateau should also be examined, particularly relevant to Asian summer monsoon anomalies. Moreover, these effects seem to be very sensitive to parameterizations of these physical processes (Yamazaki, 1988).
This study focuses on these problems by using some versions of GCMs of the Meteorological Research Institute. The results include the evaluation of total snow-cover feedbacks as part of internal dynamics of climatic change from 12-year GCM integration, and of the effect of anomalous snow cover over Eurasia in late winter on land surface conditions and atmospheric circulations in the succeeding seasons.
Evidence Of Environmental Change Since The Last Glacial Maximum Inferred From Chemical Analysis Of An Ice Core From Law Dome, Antarctica
- N.W. Young, M. De Angelis, D. Davies
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 365
-
- Article
-
- You have access Access
- Export citation
-
An ice core, drilled near the margin of the Law Dome ice cap at Cape Folger, has been analysed for trace chemical content. The concentration of the major anions and cations has been measured on samples selected from the ice core to give information on the major environmental changes which have occurred in the period 6–26 ka B.P. The chemical species can be divided into two fractions representing the two major sources of trace chemicals; marine and continental sources. Four species are chosen to illustrate the main features in the record; aluminium as an indicator of the continental fraction, sodium and magnesium as indicators of the marine fraction and methane sulphonic acid (MSA). Sodium and magnesium concentrations in the Law Dome core are predominantly derived from marine sources, although they usually include also small contributions from the continental sources. MSA has a marine biogenic source and exhibits a pattern which is generally unrelated to the variations in the two main fractions. Measured oxygen isotope ratios provide an additional data source. Concentrations of the same species in the Dome C core (De Angelis and others, 1982; Saigne and Legrand, 1987) are used as indicators of the global background atmospheric chemical content, and by inter-comparison of the records from the two cores are used to derive a proxy chronology for the Law Dome core.
The interval in each core corresponding to the final stages of the Last Glacial Maximum (LGM) can be identified from the oxygen isotope records (Budd and Morgan, 1977; Lorius and others, 1984). Both cores have high aluminium concentrations in this interval reducing to very low concentrations towards the end of the transition to the Holocene. A similar sharp change from high to very low concentration is also observed for MSA. Very low concentrations of other species are also observed in this interval in the transition period. By assuming that these changes in the two cores are contemporaneous, the age scale from the Dome C core (Lorius and others, 1984) can be applied to the Law Dome core. An age of 13 ka B.p. is assigned to the very clean interval near the end of the transition. Other, less obvious, events in the chemical and isotope records distinguish intervals corresponding to ages of approximately 7.5, 15.5, and 26 ka B.P. Ages for intermediate intervals are derived by interpolation and reference to a modelled age-depth relation.
The records from each of the cores for MSA and the continental fraction, represented by aluminium, show similar features at the Law Dome site as at Dome C. But the records for the marine fraction show distinct differences. On Law Dome there is a clear trend of decreasing concentration with depth, consistent with the ice at greater depth having an origin at higher elevation further inland on the ice cap. Very low concentrations occur in the lower part of the core, which includes the interval corresponding to the LGM. By way of contrast, at Dome C the concentration of sodium in the interval corresponding to the Holocene is low, but relatively higher in the LGM interval. The concentrations during the LGM, of both the marine and continental fractions, are lower in Law Dome by a factor generally between 1 and 2 than those at Dome C as a result of dilution caused by the higher precipitation and snow accumulation rates near the coast.
For interpretation of the records, the concentrations in the Dome C core are assumed to indicate changes in the global background atmospheric loading and atmospheric circulation. On Law Dome, the general trend of decreasing concentra- tion with depth for the marine fraction is modulated by variations in the background atmospheric loading, and the effect of variations in past ice sheet and sea ice extent and thus distance to the source. At about 11 ka B.P., sodium and magnesium concentrations increase sharply to about three times the background level, and are maintained till about 9.5 ka B.P. This event is not apparent in the Dome C record. During the period 6–8 ka B P., sodium and magnesium concentrations are higher by a factor between 1.5 and 2 in conjunction with colder (more negative) values of the oxygen isotope ratio. There is some evidence of similar variations in the Dome C record.
This suggests two separate scenarios. For the period 9.5–11 ka B P., one or more of the following events probably occurred: a change in the seasonal pattern of variation in sea ice extent and distribution; lesser sea ice extent; more open water closer to the coast; increased storminess in the coastal region, each of which could lead to an increased supply of material with marine source (sodium and magnesium) by either more vigorous atmospheric circulation or less distance to the source. Coincidentally, increased storminess is consistent with an increased fraction of open water in the sea ice zone. But there is apparently no change in the concnetration of MSA above background levels during this period. This could provide a constraint on the possible mechanisms causing the observed event. For the more recent period, 6–8 ka B.P., the changes found in both cores probably reflect climatic variation on a broader hemispheric or global scale, involving lower temperatures in at least the high latitudes, probably increased zonal atmospheric circulation, and perhaps changes in the seasonal sea ice distribution and total extent.
Ice-Sheet Elevation Change
- H. Jay Zwally
-
- Published online by Cambridge University Press:
- 20 January 2017, p. 366
-
- Article
-
- You have access Access
- Export citation
-
Over century time scales, the primary effect of ice-sheet/climate-change interactions is vertical growth or shrinkage of the ice in response to changes in precipitation and surface heat flux. Because the dynamic response of large ice masses is generally slower than climate variations experienced during the last few centuries, the ice sheets are unlikely to be in equilibrium with today’s climate. Uncertainty in the current mass balance has been large, at least ±30% or ±2 mm yr−1 in sea-level equivalent. Estimates of annual snow accumulation, iceberg discharge, and peripheral melting of the Antarctic ice sheet (personal communication from S. Jacobs) would suggest a negative mass-balance equivalent to +2 mm yr−1 of sea-level rise, in contrast to other estimates of a small positive balance for both Antarctica (−0.6 ±0.6 mm yr−1 sea level) and Greenland (−0.1 ±0.4 mm yr−1 sea level) (Meier and others, 1985). For some years, measurement of changes in ice-sheet surface elevation by satellite altimetry has been noted as a potential means of determining the overall ice-sheet mass balance and investigating regional variations. Difficulties in deriving elevation change from a set of sequential measurements from several satellites have been primarily a result of the limited precision (about 1–2 m) of satellite radar altimetry, residual orbit errors, and relative uncertainties in the gravity fields and geoid reference levels used for different satellites. However, recent radar altimeter measurements by the U.S. Navy Geosat to 72°N and 72°S provide a sufficient density of repeated measurements of ice elevation for analysis of elevation change during the life of the satellite. The average elevation change at 224 267 orbital crossovers over southern Greenland is +28.3 ±0.4 cm yr−1. The largest values are observed near the summit and over the southern dome, with smaller values in the saddle region and toward the margins. Local gridded values of thickening and thinning rates agree with estimates from surface studies in the vicinity of the EGIG traverse (Steckel, 1977), Dye 3 (Reeh, 1985), and the OSU survey (Kostecka and Whillans, 1988) within the error limits of the respective measurements. Comparisons with elevations measured by the Seasat altimeter in 1978 and continuing Geosat measurements provide information on the temporal continuity of the thickening. The spatial distribution of the elevation changes is used to estimate an average thickening rate and the current rate of oceanic depletion.