全文获取类型
收费全文 | 1396篇 |
免费 | 75篇 |
国内免费 | 6篇 |
专业分类
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 |
出版年
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.
Barry Smit Michael Brklacich Robert B. Stewart Ray McBride Murray Brown Deborah Bond 《Climatic change》1989,14(2):153-174
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
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