Climate Change in Northern Africa: The Past is Not the Future     

Martin Claussen  Victor Brovkin  Andrey Ganopolski  Claudia Kubatzki  Vladimir Petoukhov 《Climatic change》2003,57(1-2):99-118

By using a climate system model of intermediate complexity, we have simulated long-term natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earth's orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO₂ concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000–6000 years ago), and we show that some expansion of grasslandinto the Sahara is theoretically possible, if the atmospheric CO₂ concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO₂ increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In ourmodel, vegetation expansion into today's Sahara is triggered by an increase in summer precipitation which is amplified by a positive feedback between vegetation and precipitation. This is valid for simulations with orbital forcing and greenhouse-gas forcing. However, we argue that the mid-Holocene climate optimum some 9000 to 6000 years ago with its marked reduction of deserts in northern Africa is not a direct analogue for future greenhouse-gas induced climate change, as previously hypothesized. Not only does the global pattern of climate change differ between the mid-Holocene model experiments and the greenhouse-gas sensitivity experiments, but the relative role of mechanisms which lead to a reduction of the Sahara also changes. Moreover, the amplitude of simulated vegetation cover changes in northern Africa is less than is estimated for mid-Holocene climate.  相似文献   

大兴安岭北部绣峰组下部砾岩的形成时代及其大地构造意义   总被引：21，自引：6，他引：21  

李锦轶和政军  莫申国Andrey A.Sorokin 《地质通报》2004,23(2):120-129

对漠河盆地南缘绣峰组下部砾岩的野外观察，发现其碎屑物除了主要来源于被前人置于新元古代的花岗岩外，还含有火山岩砾石；与上部砂岩之间被断层分隔而不是连续沉积。作为该砾岩主要源岩的花岗岩中的锆石蒸发法年龄为354-164Ma，锆石SHRIMP定年揭示其最后一次岩浆活动可能发生在早侏罗世(180-190Ma)；全岩-长石-黑云母内部Rb-Sr等时线年龄为177～191Ma，黑云母Ar-Ar年龄为171～175Ma，表明该岩基在中侏罗世还深埋地下没有出露地表。其中的火山岩砾石的岩石学和岩石化学特征与区域上晚侏罗世塔木兰沟组火山岩类似。结合区域地质资料，推测该砾岩的形成时代不是中侏罗世，而可能是早白垩世；漠河盆地是与蒙古-鄂霍茨克造山带有关的前陆盆地，而不是侏罗纪陆内断陷盆地。  相似文献   

Transient simulation of the last glacial inception. Part II: sensitivity and feedback analysis     

Reinhard Calov  Andrey Ganopolski  Vladimir Petoukhov  Martin Claussen  Victor Brovkin  Claudia Kubatzki 《Climate Dynamics》2005,24(6):563-576

The sensitivity of the last glacial-inception (around 115 kyr BP, 115,000 years before present) to different feedback mechanisms has been analysed by using the Earth system model of intermediate complexity CLIMBER-2. CLIMBER-2 includes dynamic modules of the atmosphere, ocean, terrestrial biosphere and inland ice, the last of which was added recently by utilising the three-dimensonal polythermal ice-sheet model SICOPOLIS. We performed a set of transient experiments starting at the middle of the Eemiam interglacial and ran the model for 26,000 years with time-dependent orbital forcing and observed changes in atmospheric CO₂ concentration (CO₂ forcing). The role of vegetation and ocean feedback, CO₂ forcing, mineral dust, thermohaline circulation and orbital insolation were closely investigated. In our model, glacial inception, as a bifurcation in the climate system, appears in nearly all sensitivity runs including a run with constant atmospheric CO₂ concentration of 280 ppmv, a typical interglacial value, and simulations with prescribed present-day sea-surface temperatures or vegetation cover—although the rate of the growth of ice-sheets growth is smaller than in the case of the fully interactive model. Only if we run the fully interactive model with constant present-day insolation and apply present-day CO₂ forcing does no glacial inception appear at all. This implies that, within our model, the orbital forcing alone is sufficient to trigger the interglacial–glacial transition, while vegetation, ocean and atmospheric CO₂ concentration only provide additional, although important, positive feedbacks. In addition, we found that possible reorganisations of the thermohaline circulation influence the distribution of inland ice.  相似文献   

Multipath mitigation by conventional antennas with ground planes and passive vertical structures     

Dmitry Tatarnikov  Vladimir Filippov  Igor Soutiaguine  Andrey Astahov  Anton Stepanenko  Pavel Shamatulsky 《GPS Solutions》2005,9(3):194-201

An antenna ground plane (GP) plays a major role in the mitigation of multipath coming from underneath the antenna. A distinct trend in user antenna size and weight reduction has been observed during recent years. With this focus in mind, an overview of conventional flat conductive and impedance GP is provided. Results of modern developments of passive vertically stacked structures are discussed. The possibility of obtaining a reasonable level of multipath mitigation with vertical structures of several centimeters is shown.  相似文献   

Magnetotelluric studies of the East-European Craton and adjacent regions     

Valeriy P. Bubnov  Andrey G. Yakovlev  Elena D. Aleksanova  Denis V. Yakovlev  Mark N. Berdichevsky  Pavel Yu. Pushkarev 《Acta Geophysica》2007,55(2):154-168

Magnetotelluric method is widely applied to study the Russian part of the East-European Craton, as well as the Caucasus and the Urals: several thousand soundings were performed during the last few years. Their periods range is approximately from 0.003 to 3000 seconds, which allows to study the sedimentary cover and the consolidated crust. Resistivity cross-sections along several regional profiles which run across the tectonic structures of the East-European Craton and the adjacent folded systems were obtained, mainly using 1D and 2D interpretational tools. MT investigations provided important information about the structure and reservoir properties of sedimentary complexes, the state of active geodynamic regions, the graphitization and fluid regime of the consolidated crust, and the permeable and fluid-saturated crustal zones.  相似文献   

基于冻融次数-物理时间比拟理论的冻土长期强度预测方法   总被引：1，自引：0，他引：1  

张泽  马巍  ROMAN Lidia  MELNIKOV Andrey  杨希  李宏璧 《岩土力学》2021,(1):86-92

冻融作用可以改变土的结构,引起其力学特征的变化,从而影响到冻土工程的稳定性。由于冻融周期的设定不尽相同,因此大量的试验结果不能有效地进行对比分析。另外,对于冻融作用下土体力学特征的预测也成为研究的难点。在冻土遗传蠕变理论的基础上,利用球型模板压入仪作为测试方法,提出了一种冻融次数-物理时间比拟的方法。把冻融周期(次数)转化成物理时间(min),将不同冻融周期土的长期强度曲线簇映射到同一坐标中,利用该归一化曲线进行长期抗剪强度的预测,并选取了两种土样进行了测试,分别得到了相关的长期抗剪强度的预测方程。该方法对冻融作用下土体力学行为的成果对比研究以及长期强度的预测有重要的理论意义,对寒区工程稳定性预测分析也具有工程实践价值。  相似文献   

Two New Species of Eretmophyllum (Ginkgoales) from the Jurassic of Eastern Siberia     

Andrey FROLOV  Irina MASHCHUK 《《地质学报》英文版》2023,97(4):1014-1025

Two new species of Eretmophyllum Thomas are described from the lower (Toarcian) and upper (Aalenian) Prisayan Formation of the Irkutsk Basin, Eastern Siberia, Russia based on distinct morphological and cuticular features of the leaves that distinguish them from other species of the genus. Eretmophyllum polypapillosum sp. nov. is characterized by the presence of 2–3 papillae on the lower and upper epidermis of ordinary cells. Taphonomically, the leaf burials of E. polypapillosum sp. nov. indicate that it preferred mesophilic forests developed on river terraces and/or low watersheds. Eretmophyllum yershowskiensis sp. nov. is represented exclusively by leaf cuticles extracted by coal maceration, and it probably inhabited the low banks of peat bogs. The presence of the genus Eretmophyllum in the Early Jurassic of France and Eastern Siberia shows that it was already widespread in the territory of Eurasia, distributed from Western Europe to Eastern Siberia. This indicates the possible existence of Eretmophyllum species elsewhere in the Early Jurassic such as Central Asia and Northwest China.  相似文献   

Magnetite-apatite-dolomitic rocks of Ust-Chulman (Aldan shield,Russia): Seligdar-type carbonatites?     

Prokopyev  Ilya R.  Doroshkevich  Anna G.  Redina  Anna A.  Obukhov  Andrey V. 《Mineralogy and Petrology》2018,112(2):257-266

Mineralogy and Petrology - The Ust-Chulman apatite ore body is situated within the Nimnyrskaya apatite zone at the Aldan shield in Russia. The latest data confirm the carbonatitic origin of the...  相似文献   

Transient simulation of the last glacial inception. Part I: glacial inception as a bifurcation in the climate system   总被引：2，自引：2，他引：0  

Reinhard Calov  Andrey Ganopolski  Martin Claussen  Vladimir Petoukhov  Ralf Greve 《Climate Dynamics》2005,24(6):545-561

We study the mechanisms of glacial inception by using the Earth system model of intermediate complexity, CLIMBER-2, which encompasses dynamic modules of the atmosphere, ocean, biosphere and ice sheets. Ice-sheet dynamics are described by the three-dimensional polythermal ice-sheet model SICOPOLIS. We have performed transient experiments starting at the Eemiam interglacial, at 126 ky BP (126,000 years before present). The model runs for 26 kyr with time-dependent orbital and CO₂ forcings. The model simulates a rapid expansion of the area covered by inland ice in the Northern Hemisphere, predominantly over Northern America, starting at about 117 kyr BP. During the next 7 kyr, the ice volume grows gradually in the model at a rate which corresponds to a change in sea level of 10 m per millennium. We have shown that the simulated glacial inception represents a bifurcation transition in the climate system from an interglacial to a glacial state caused by the strong snow-albedo feedback. This transition occurs when summer insolation at high latitudes of the Northern Hemisphere drops below a threshold value, which is only slightly lower than modern summer insolation. By performing long-term equilibrium runs, we find that for the present-day orbital parameters at least two different equilibrium states of the climate system exist—the glacial and the interglacial; however, for the low summer insolation corresponding to 115 kyr BP, we find only one, glacial, equilibrium state, while for the high summer insolation corresponding to 126 kyr BP only an interglacial state exists in the model.

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Reinhard CalovEmail:

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Synergy of satellite remote sensing and numerical modeling for monitoring of suspended particulate matter     

Andrey?Pleskachevsky  Gerhard?GayerEmail author  Jochen?Horstmann  Wolfgang?Rosenthal 《Ocean Dynamics》2005,55(1):2-9

Monitoring and modeling of the distribution of suspended particulate matter (SPM) is an important task, especially in coastal environments. Several SPM models have been developed for the North Sea. However, due to waves in shallow water and strong tidal currents in the southern part of the North Sea, this is still a challenging task. In general there is a lack of measurements to determine initial distributions of SPM in the bottom sediment and essential model parameters, e.g., appropriate exchange coefficients. In many satellite-borne ocean color images of the North Sea a plume is visible, which is caused by the scattering of light at SPM in the upper ocean layer. The intensity and length of the plume depends on the wave and current climate. It is well known that the SPM plume is especially obvious shortly after strong storm events. In this paper a quasi-3-D and a 3-D SPM transport model are presented. Utilizing the synergy of satellite-borne ocean color data with numerical models, the vertical exchange coefficients due to currents and waves are derived. This results in models that for the first time are able to reproduce the temporal and spatial evolution of the plume intensity. The SPM models consist of several modules to compute ocean dynamics, the vertical and horizontal exchange of SPM in the water column, and exchange processes with the seabed such as erosion, sedimentation, and resuspension. In the bottom layer, bioturbation via benthos and diffusion processes is taken into account.Responsible Editor: Jörg-Olaf Wolff  相似文献