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   1396篇
  免费   75篇
  国内免费   6篇
测绘学   46篇
大气科学   93篇
地球物理   349篇
地质学   487篇
海洋学   138篇
天文学   220篇
综合类   12篇
自然地理   132篇
  2023年   7篇
  2022年   19篇
  2021年   37篇
  2020年   35篇
  2019年   41篇
  2018年   49篇
  2017年   43篇
  2016年   75篇
  2015年   60篇
  2014年   62篇
  2013年   96篇
  2012年   62篇
  2011年   85篇
  2010年   61篇
  2009年   101篇
  2008年   65篇
  2007年   70篇
  2006年   47篇
  2005年   26篇
  2004年   40篇
  2003年   34篇
  2002年   45篇
  2001年   29篇
  2000年   23篇
  1999年   21篇
  1998年   17篇
  1997年   17篇
  1996年   18篇
  1995年   9篇
  1994年   8篇
  1992年   8篇
  1991年   6篇
  1990年   5篇
  1989年   4篇
  1987年   9篇
  1986年   4篇
  1985年   14篇
  1984年   9篇
  1983年   11篇
  1982年   10篇
  1981年   5篇
  1980年   10篇
  1979年   6篇
  1978年   7篇
  1977年   7篇
  1976年   7篇
  1975年   10篇
  1973年   4篇
  1972年   6篇
  1971年   4篇
排序方式: 共有1477条查询结果,搜索用时 15 毫秒
101.
Increasing concentrations of atmospheric CO2 and other greenhouse gases are expected to contribute to a global warming. This paper examines the potential implications of a climatic change corresponding to a doubling of atmospheric concentrations of CO2 on crop production opportunities throughout Ontario, a major food producing region in Canada. The climate is projected to become warmer and drier, but the extent of these shifts are expected to vary from region to region within Ontario. The effect of this altered climate on crop yields and the area of land capable of supporting specific crops varies according to region, soil quality and crop type. Most notable are the enhanced opportunities for grains and oilseeds in the northern regions, and the diminished production prospects for most crops in the most southerly parts of Ontario.  相似文献   
102.
103.
The post-agreement period typically is characterized by negotiations between various stakeholders to reach mutually beneficial and acceptable means to achieve national implementation of, and compliance with, treaty provisions. National ratification of international environmental agreements is often the first subprocess of these ‘postagreement negotiations’. This article places ratification negotiations within the larger conceptual context of postagreement negotiations, with the goal of understanding and explaining problems of treaty Implementation. An empirical analysis reveals that delay in national ratification of environmental agreements is a chronic problem, but is worse for complex, multi-issue treaties. Strong public concern over local environmental issues, low quality of life, low national wealth, and low public research and development expenditures for environmental protection contribute to ratification delay. Ultimately, the authors are interested in identifying ways of improving the international negotiation process that initiated these later problems in implementation. Recommendations are offered in this regard.  相似文献   
104.
Bioconcentration factors (Kbc) for petroleum hydrocarbons, PAHs, LABs and biogenic hydrocarbons in Mytilus edulis were measured in field experiments using time-integrating water samplers. Seven deployments at five sites gave lipid weight Kbcs for total hydrocarbons ranging from 0.99 × 106 to 3.1 × 106 (mean 1.6 × 106)—a narrower range than has been obtained previously. Bioconcentration factors for the PAHs were similar to those for total hydrocarbons where the major hydrocarbon source was oil. However, at other sites the factors for PAHs were an order of magnitude lower than those for petroleum and for hydrocarbons originating from algae. Compositional profiles for the linear alkyl benzenes (LABs) suggested that these compounds were assimilated primarily from the dissolved phase, despite their greater abundance on particles.  相似文献   
105.
In the Thames Estuary at the eastern end of Long Island Sound, most mussels achieve maximal gonadal development during the summer and larval settlement occurs primarily during the fall. The timing of reproductive events is several months later than has been reported for other populations of mussels in Long Island Sound but corresponds to that of mussels inhabiting the south shore of Long Island. The data support the growing body of evidence which indicates that temperature is not a major factor determining the reproductive period of this bivalve.  相似文献   
106.
Benthic fluxes of C, N, P and Si have been measured at two sites in Great Bay Estuary, New Hampshire. Higher fluxes of reactive phosphate, nitrate and reactive silicate were observed at the site where bioturbation is known to occur and the fluxes of NH4 +, PO4 ?3 and reactive silicate at this location were from 3 to 6 times higher than that calculated by simple pore water diffusion models. *** DIRECT SUPPORT *** A01BY019 00010  相似文献   
107.
A study of the oxidation states of metals in selected Pacific ferromanganese nodules by x-ray photoelectron spectroscopy (XPS) indicates the presence of Co(III), Pb(II), Pb(IV), Fe(III) and Ti(IV). These findings are based on measurements of core electron binding energies and an evaluation of the core electron shake-up satellite features.  相似文献   
108.
109.
Reducing model complexity for explanation and prediction   总被引:1,自引:1,他引:1  
A. Brad Murray   《Geomorphology》2007,90(3-4):178
Numerical models can be useful for explaining poorly understood phenomena or for reliable quantitative predictions. When modeling a multi-scale system, a ‘top-down’ approach—basing models on emergent variables and interactions, rather than explicitly on the much faster and smaller scale processes that give rise to them—facilitates both goals. Parameterizations representing emergent interactions range from highly simplified and abstracted to more quantitatively accurate. Empirically based large-scale parameterizations lead more reliably to accurate large-scale behavior than do parameterizations of much smaller scale processes. Conversely, purposefully simplified representations of model interactions can enhance a model's utility for explanation, clarifying the key feedbacks leading to an enigmatic behavior. For such potential insights to be relevant, the interactions in the model need to correspond to those in the ‘real’ system in some straightforward way. Such a correspondence usually holds for models constructed for predictive purposes, although this is not a requirement. The goals motivating a modeling endeavor help determine the most appropriate modeling strategies, as well as the most appropriate criteria for judging model usefulness.  相似文献   
110.
The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide (VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb). A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks.  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