首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   108055篇
  免费   1393篇
  国内免费   1382篇
测绘学   3082篇
大气科学   7474篇
地球物理   20531篇
地质学   41271篇
海洋学   8838篇
天文学   21390篇
综合类   2380篇
自然地理   5864篇
  2022年   633篇
  2021年   1091篇
  2020年   1176篇
  2019年   1254篇
  2018年   6961篇
  2017年   6077篇
  2016年   5132篇
  2015年   1571篇
  2014年   2693篇
  2013年   4511篇
  2012年   3738篇
  2011年   6149篇
  2010年   5234篇
  2009年   6373篇
  2008年   5470篇
  2007年   5992篇
  2006年   3600篇
  2005年   2703篇
  2004年   2829篇
  2003年   2670篇
  2002年   2502篇
  2001年   2013篇
  2000年   2004篇
  1999年   1548篇
  1998年   1617篇
  1997年   1465篇
  1996年   1243篇
  1995年   1243篇
  1994年   1042篇
  1993年   1007篇
  1992年   934篇
  1991年   968篇
  1990年   935篇
  1989年   817篇
  1988年   751篇
  1987年   883篇
  1986年   773篇
  1985年   946篇
  1984年   1071篇
  1983年   1031篇
  1982年   950篇
  1981年   906篇
  1980年   808篇
  1979年   748篇
  1978年   726篇
  1977年   618篇
  1976年   632篇
  1975年   613篇
  1974年   589篇
  1973年   656篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
For agriculture, there are three major options for mitigating greenhouse gas (GHG) emissions: 1) productivity improvements, particularly in the livestock sector; 2) dedicated technical mitigation measures; and 3) human dietary changes. The aim of the paper is to estimate long-term agricultural GHG emissions, under different mitigation scenarios, and to relate them to the emissions space compatible with the 2 °C temperature target. Our estimates include emissions up to 2070 from agricultural soils, manure management, enteric fermentation and paddy rice fields, and are based on IPCC Tier 2 methodology. We find that baseline agricultural CO2-equivalent emissions (using Global Warming Potentials with a 100 year time horizon) will be approximately 13 Gton CO2eq/year in 2070, compared to 7.1 Gton CO2eq/year 2000. However, if faster growth in livestock productivity is combined with dedicated technical mitigation measures, emissions may be kept to 7.7 Gton CO2eq/year in 2070. If structural changes in human diets are included, emissions may be reduced further, to 3–5 Gton CO2eq/year in 2070. The total annual emissions for meeting the 2 °C target with a chance above 50 % is in the order of 13 Gton CO2eq/year or less in 2070, for all sectors combined. We conclude that reduced ruminant meat and dairy consumption will be indispensable for reaching the 2 °C target with a high probability, unless unprecedented advances in technology take place.  相似文献   
92.
Multitemporal remote sensing provides a unique tool to track lake dynamics at the pan-Arctic scale but requires precise registration of thousands of satellite images. This is a challenging task owing to a dearth of stable features to be used as tie points [(TPs), i.e., control points] in the dynamic landscapes. This letter develops an automated method to precisely register images in the lake-rich Arctic. The core premise of the method is that the centers of lakes are generally stable even if their shorelines are not. The proposed procedures first extract lakes in multitemporal satellite images, derive lake centroids and match them between images, and then use the centroids of stable lakes as TPs for image registration. The results show that this approach can achieve subpixel registration accuracy, outcompeting the conventional manual methods in both efficiency and accuracy. The proposed method is fully automated and represents a feasible way to register images for lake change detection at the pan-Arctic scale.   相似文献   
93.
Numerical modeling of the Rideau Valley Watershed   总被引:1,自引:0,他引:1  
Using the Mike11 modeling system by the Danish Hydraulic Institute, a detailed model of the Rideau Valley Watershed was constructed. It includes 532 km of rivers and lakes, 106 basins, 122 bridges and culverts, and 20 water control structures. The model was calibrated using measured streamflow data for a time period of 5 years; additional 5 years of data was used for validation. Various methods, both qualitative and quantitative, were used to evaluate the model performance. It was found that the model can simulate the hydrological response with a reasonable to high degree of accuracy. This model is now being used for various watershed management purposes, including flood forecasting, dam safety assessment, quantification of wetland functions, and derivation of design flows.  相似文献   
94.
The results of geophysical studies conducted with selected electrical and electromagnetic methods in the Kondratowa Valley in the Tatra Mountains (the Carpathian Mountains, Poland) are presented in the article. The surveys were performed with the following methods: electrical resistivity tomography (ERT), georadar (GPR) and conductivity meter (CM). The objective of the noninvasive geophysical measurements was to determine the thickness of the Quaternary postglacial sediments that fill the bottom of the valley and to designate the accumulation of boulders deposited on Quaternary sediments. The results of ERT surveys conducted along the axis of the valley allowed to determine the changeability of the thickness of the postglacial sediments and allowed to designate a few areas of occurrence of boulders. The ERT, GPR and CM surveys conducted across the valley allowed to designate with high accuracy the thickness of the accumulation of boulders sliding down the valley bottom from the couloirs surrounding the valley.  相似文献   
95.
96.
The study of climate impacts on Living Marine Resources (LMRs) has increased rapidly in recent years with the availability of climate model simulations contributed to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Collaboration between climate and LMR scientists and shared understanding of critical challenges for such applications are essential for developing robust projections of climate impacts on LMRs. This paper assesses present approaches for generating projections of climate impacts on LMRs using IPCC-class climate models, recommends practices that should be followed for these applications, and identifies priority developments that could improve current projections. Understanding of the climate system and its representation within climate models has progressed to a point where many climate model outputs can now be used effectively to make LMR projections. However, uncertainty in climate model projections (particularly biases and inter-model spread at regional to local scales), coarse climate model resolution, and the uncertainty and potential complexity of the mechanisms underlying the response of LMRs to climate limit the robustness and precision of LMR projections. A variety of techniques including the analysis of multi-model ensembles, bias corrections, and statistical and dynamical downscaling can ameliorate some limitations, though the assumptions underlying these approaches and the sensitivity of results to their application must be assessed for each application. Developments in LMR science that could improve current projections of climate impacts on LMRs include improved understanding of the multi-scale mechanisms that link climate and LMRs and better representations of these mechanisms within more holistic LMR models. These developments require a strong baseline of field and laboratory observations including long time series and measurements over the broad range of spatial and temporal scales over which LMRs and climate interact. Priority developments for IPCC-class climate models include improved model accuracy (particularly at regional and local scales), inter-annual to decadal-scale predictions, and the continued development of earth system models capable of simulating the evolution of both the physical climate system and biosphere. Efforts to address these issues should occur in parallel and be informed by the continued application of existing climate and LMR models.  相似文献   
97.
Jinshanjiangite (acicular crystals up to 2 mm in length) and bafertisite (lamellar crystals up to 3 × 4 mm in size) have been found in alkali granite pegmatite of the Gremyakha-Vyrmes Complex, Kola Peninsula. Albite, microcline, quartz, arfvedsonite, zircon, and apatite are associated minerals. The dimensions of a monoclinic unit cell of jinshanjiangite and bafertisite are: a = 10.72(2), b=13.80(2), c = 20.94(6) Å, β = 97.0(5)° and a = 10.654(6), b = 13.724(6), c = 10.863(8) Å, β = 94.47(8)°, respectively. The typical compositions (electron microprobe data) of jinshanjiangite and bafertisite are: (Na0.57Ca0.44)Σ1.01(Ba0.57K0.44)Σ1.01 (Fe3.53Mn0.30Mg0.04Zn0.01)Σ3.88(Ti1.97Nb0.06Zr0.01)Σ2.04(Si3.97Al0.03O14)O2.00(OH2.25F0.73O0.02)Σ3.00 and (Ba1.98Na0.04K0.03)Σ2.05(Fe3.43Mn0.37Mg0.03)Σ3.83(Ti2.02Nb0.03)Σ2.05 (Si3.92Al0.08O14)(O1.84OH0.16)Σ2.00(OH2.39F1.61)Σ3.00, respectively. The minerals studied are the Fe-richest members of the bafertisite structural family.  相似文献   
98.
Fossil diatom algae first found in the Paleogene marine succession (Alugivayam Formation) of the Il’pinskii Peninsula, northeastern Kamchatka are studied, and new data on molluscan assemblages from the same sequences are presented. Some of the diatom forms identified suggest the Oligocene age of their host deposits. This is consistent with earlier inference from benthic groups of marine organisms.  相似文献   
99.
100.
Mineralogical alluvial prospecting search for base metals have been carried out from 1968 to 1970 in the Friuli — Venezia Giulia region. The Carnia, a part of this region, was well known from ancient times as an important mining district. Numerous anomalies of the content of some elements and minerals in river sands have been recognized by means of the above mentioned searches. This indicates that important and till now unknown ore concentrations are possible in this region. In fact three new sphalerite and/or galena concentrations and one fluorite concentration in association with As-sulphides have already been discovered during a partial development of these surveys. In the present work the advantages of alluvial prospecting in mining searches are remarked. Moreover method and application of mineralogical alluvial prospecting are described. In the last paragraph the steps which allowed the discovering of the new metal concentrations are reported in detail. The discovered concentrations are shortly analyzed. Strict aim of this note is to indicate the value and scientific importance of the described method.For reasons not within author's control, the results of these researches can be published only to day. But, owing the importance of some recent discoveries, the authors remind that these results have already been discussed at 2nd Int. Symp. on the Mineral Deposits on the Alps Bled, 1971, and at the meeting of Società di Mineralogia e Petrografia Pisa, 1974.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号