全文获取类型
收费全文 | 9449篇 |
免费 | 1783篇 |
国内免费 | 2300篇 |
专业分类
测绘学 | 266篇 |
大气科学 | 3859篇 |
地球物理 | 1933篇 |
地质学 | 3183篇 |
海洋学 | 1072篇 |
天文学 | 240篇 |
综合类 | 424篇 |
自然地理 | 2555篇 |
出版年
2024年 | 52篇 |
2023年 | 112篇 |
2022年 | 297篇 |
2021年 | 456篇 |
2020年 | 469篇 |
2019年 | 491篇 |
2018年 | 417篇 |
2017年 | 474篇 |
2016年 | 455篇 |
2015年 | 492篇 |
2014年 | 649篇 |
2013年 | 1027篇 |
2012年 | 619篇 |
2011年 | 588篇 |
2010年 | 535篇 |
2009年 | 683篇 |
2008年 | 692篇 |
2007年 | 630篇 |
2006年 | 602篇 |
2005年 | 528篇 |
2004年 | 480篇 |
2003年 | 434篇 |
2002年 | 368篇 |
2001年 | 299篇 |
2000年 | 296篇 |
1999年 | 233篇 |
1998年 | 226篇 |
1997年 | 196篇 |
1996年 | 167篇 |
1995年 | 125篇 |
1994年 | 112篇 |
1993年 | 72篇 |
1992年 | 57篇 |
1991年 | 48篇 |
1990年 | 37篇 |
1989年 | 20篇 |
1988年 | 26篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 15篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
971.
利用线性分析方法,分析了濮阳近50 a平均风速、大风日数的变化特征,并利用滑动的t检验法分析了风速突变,结果显示:年平均风速以0.498(m/s)/10 a的倾向率减少,大风和扬沙日数分别以2.7 d/10 a和5.6 d/10 a的倾向率减少;平均风速,大风、扬沙日数在20世纪80年代初期发生突变。 相似文献
972.
The Stern Review on the Economics of Climate Change concluded that there can be “no doubt” the economic risks of business-as-usual (BAU) climate change are “very severe” [Stern, 2006. The Economics of Climate Change. HM Treasury, London, p. 188]. The total cost of climate change was estimated to be equivalent to a one-off, permanent 5–20% loss in global mean per-capita consumption today. And the marginal damage cost of a tonne of carbon emitted today was estimated to be around $312 [p. 344]. Both of these estimates are higher than most reported in the previous literature. Subsequently, a number of critiques have appeared, arguing that discounting is the principal explanation for this discrepancy. Discounting is important, but in this paper we emphasise that how one approaches the economics of risk and uncertainty, and how one attempts to model the very closely related issue of low-probability/high-damage scenarios (which we connect to the recent discussion of ‘dangerous’ climate change), can matter just as much. We demonstrate these arguments empirically, using the same models applied in the Stern Review. Together, the issues of risk and uncertainty on the one hand, and ‘dangerous’ climate change on the other, raise very strongly questions about the limits of a welfare-economic approach, where the loss of natural capital might be irreversible and impossible to compensate. Thus we also critically reflect on the state-of-the-art in integrated assessment modelling. There will always be an imperative to carry out integrated assessment modelling, bringing together scientific ‘fact’ and value judgement systematically. But we agree with those cautioning against a literal interpretation of current estimates. Ironically, the Stern Review is one of those voices. A fixation with cost-benefit analysis misses the point that arguments for stabilisation should, and are, built on broader foundations. 相似文献
973.
974.
975.
976.
中国西南天山西域砾岩的磁性地层年代与地质意义* 总被引:11,自引:5,他引:11
西域组是我国西部一重要并广泛引用的晚新生代地层,关于其年代和成因至今尚存争议。在西南天山喀什远源盆地喀什-阿图什褶皱带不同构造部位选择有代表性的5~6个晚新生代地层剖面开展了详细的沉积学、磁性地层年代学对比研究,据此限定了不同构造的起始变形时间以及西域砾岩的沉积年代。西域砾岩并非一年代地层单位,作为一岩石地层单位,其底界具有穿时特征,从山体(北)向喀什前陆盆地(南)逐渐变新。其底界年龄在盆地北部近源区约为15.5Ma[1],在盆地中部中源区约为8.6Ma[1],在盆地南部远源区的阿图什背斜为1.9Ma,喀什背斜为1.6~0.7Ma。这一穿时的砾岩沉积楔体的起始堆积起因于盆地北部边界逆冲断层(KBT)的活动。构造变形是由北南脉冲式迁移扩展的,其速率是非均匀的,在约15.5Ma至4.0Ma期间为1.4~3.4mm/a,在约4.0Ma以来剧增至>10mm/a。西域砾岩沉积前缘向南进积速率与构造变形前缘迁移速率有很好的一致性,但在时间上较构造变形可能滞后2.0Ma。这表明构造变形前缘向南的脉冲式扩展是西域砾岩进积并发生侧向和垂向上岩相突变的主因。 相似文献
977.
Variations in dust event frequency over the past century reflected by ice-core and lacustrine records in north China 总被引:1,自引:0,他引:1
WANG NingLian YAO TanDong YANG XiangDong SHEN Ji & WANG Yong Key Laboratory of Cryosphere Environment Cold Arid Regions Environmental Engineering Research Institute Chi- nese Academy of Sciences Lanzhou China Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing China Nanjing Institute of Geography Limnology Chinese Academy of Sciences Nanjing China 《中国科学D辑(英文版)》2007,50(5):736-744
In this paper, we analyzed the variations of dust proxies in the Dunde, Malan and Chongce ice cores from the northern Tibetan Plateau and the Hongjiannao lacustrine sediment core from north Shaanxi Province, and found that they all showed a general decrease trend over the past century. Owing to the fact that all these ice cores and lacustrine core were retrieved from the margins and/or the leeward sides of the major areas of dust events in north China, their records could suggest that the dust event frequency in north China declined over the study period. This decrease trend might be attributed to increasing precipitation and weakening westerly. However, human activities have made the areal extent of desertification expand acceleratingly in north China. This status could make it possible that dust events would occur on a large spatial scale under the future climate change, which would be a big environmental issue we shall face. 相似文献
978.
Liu WenHui Chen MengJin Guan Ping Zheng JianJing Jin Qiang Li Jian Wang WanChun Hu GuoYi Xia YanQing Zhang DianWei 《中国科学D辑(英文版)》2007,50(10):1494-1503
The establishment of geochemical-tracing system of gas generation and accumulation is helpful to re-elucidating the gas migration and accumulation in time and space. To deduce the complex process of gas accumulation, a ternary geochemical-tracing system is set up, according to stable isotope inheritance of source rocks, kinetic fractionation of stable isotopes, time-accumulating effect of noble gas isotopes, mantle-derived volatile inheritance, and organic molecule inheritance of light hydrocarbons and thermally kinetic fractionation in their generation, in combination with the previous achievements of gas geochemistry and geochemical parameters of gas-source correlation. There are tight interactions for the geochemical parameters with much information about parent inheritance and special biomarkers, in which they are confirmed each other, reciprocally associated and preferentially used for the requirement so that we can use these geochemical parameters to effectively demonstrate the sources of natural gas, sedimentary environments and thermal evolution of source rocks, migration and accumulation of natural gas, and rearrangement of natural gas reservoirs. It is necessary for the ternary geochemical-tracing system to predict the formation of high efficient gas reservoir and their distribution in time and space. 相似文献
979.
Pleistocene Lake Bonneville of western Utah contains a variety of spits associated with shorelines and other features that formed between 21,000 and 12,000 14C yr BP. Field studies in the low-lying mountain ranges of the central portion of Lake Bonneville identified 17 spits of various types. The spits are connected to small mountain ranges and islands, vary in size from 0.02 to 0.5 km2, and are composed of coarse-grained, well-rounded, poorly-sorted sedimentary material. Sixteen of the 17 spits have a northeasterly to southwesterly orientation implying that winds were from the northwest to northeast, approximately 180° out of phase with modern winds in the eastern Great Basin. Lake Bonneville spit orientation is best explained as the result of persistent northerly winds caused by the high atmospheric pressure cell of the continental ice sheet and passage of low pressure extratropical storms south of the lake. Similar, strong persistent winds are a common feature of modern continental ice sheets and passing low pressure systems. If so, the North American jet stream tracked south of Lake Bonneville as recently as 12,000 14C yr BP, well past the height of the last glacial maximum. 相似文献
980.
A. V. Lozhkin P. M. Anderson T. V. Matrosova P. S. Minyuk J. Brigham-Grette M. Melles 《Russian Journal of Pacific Geology》2007,1(6):550-555
The comprehensive study of the upper 1283 cm of sediment from Lake El’gygytgyn, which formed nearly 4 Ma ago following a meteorite impact in northern Chukotka, yielded the first continuous record of the extreme changes in the Beringian climate and vegetation from the middle Middle Pleistocene to recent time (equivalent of marine isotope stages of 1–7 and the upper part of isotope stage 8). During this period, the climate was warmer than at present between 8600 and 10 7000 14C years and during the Late Pleistocene (isotope substage 5e, 116–128 ka ago). In 2003, the German-Russian-USA expedition continued studying sediments of Lake El’gygytgyn to obtain new evidence of the change in the vegetation cover in the Middle Pleistocene and the first information on the Middle Pleistocene interglacial (isotope stage 9; 297–347 ka ago). Pollen spectra characterizing the Middle Pleistocene interglacial are similar to spectra of the early stage of the Early Pleistocene interglacial and the climatic optimum in the Pleistocene to Holocene transitional period. The climatic history of Lake El’gygytgyn is basic for stratigraphic interpretations and correlations in the eastern sector of the Arctic. These data also expand our understanding of climatic changes that are studied within the framework of the “Pole-Equator-Pole Paleoclimate,” “Past Global Changes,” and other international projects. 相似文献