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   744篇
  免费   66篇
  国内免费   7篇
测绘学   24篇
大气科学   53篇
地球物理   148篇
地质学   306篇
海洋学   69篇
天文学   91篇
综合类   1篇
自然地理   125篇
  2024年   2篇
  2023年   4篇
  2022年   6篇
  2021年   22篇
  2020年   29篇
  2019年   25篇
  2018年   34篇
  2017年   47篇
  2016年   43篇
  2015年   32篇
  2014年   32篇
  2013年   45篇
  2012年   45篇
  2011年   54篇
  2010年   50篇
  2009年   81篇
  2008年   50篇
  2007年   45篇
  2006年   27篇
  2005年   14篇
  2004年   23篇
  2003年   26篇
  2002年   14篇
  2001年   11篇
  2000年   10篇
  1999年   2篇
  1998年   9篇
  1996年   6篇
  1995年   3篇
  1994年   3篇
  1993年   2篇
  1992年   1篇
  1991年   3篇
  1990年   1篇
  1989年   3篇
  1988年   1篇
  1986年   1篇
  1985年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1979年   1篇
  1977年   1篇
  1974年   1篇
  1971年   1篇
  1969年   1篇
  1968年   1篇
排序方式: 共有817条查询结果,搜索用时 15 毫秒
711.
Sarah Elwood 《GeoJournal》2002,58(2-3):121-130
With the increasing involvement of local citizens and community organizations in carrying out urban planning and service delivery functions formerly handled by state institutions, questions have emerged about their implications for the urban political role and influence of community level actors. Some scholars identify these purportedly collaborative neighborhood revitalization initiatives as part of a neoliberal policy program of downsizing the state, while others argue that the new roles assumed by civic institutions offer new opportunities for citizen involvement in urban policy making and priority setting for revitalization. Drawing evidence from the case of a collaborative revitalization program in Minneapolis, Minnesota, this paper suggests that collaborative revitalization efforts may simultaneously foster both tendencies. By way of the new roles they are assuming in urban governance, grassroots organizations may become engaged in reproducing neoliberal priorities and policies at a highly localized level. At the same time, this involvement does not necessarily eliminate possibilities for community organizations to challenge and revise a neoliberal revitalization agenda.  相似文献   
712.
713.
A driving force of urban development is “accumulation by dispossession” which occurs through expropriation, privatization, and commodification of land. While the macro theory is now well refined, there is still much we do not know about how the underlying processes occur and how they shape economic development and urbanization. In this study, we analyze 2009 survey data of land expropriation cases across 12 Chinese cities, and find that expropriation takes different forms leading to specific outcomes. In half the cases, local governments followed central government policies requiring them to pay standard rates of compensation, but in the other half, local governments negotiated with farmers over the terms of expropriation. In the latter scenario, farmers were more likely to receive compensation higher than the standard rate, particularly if they were embedded in local power structures, which we argue could be counteracted if all households in a rural collective negotiate one agreement rather than individual agreements. These findings move us closer to understanding why accumulation through dispossession sometimes contributes to economic growth and social development, and at other times undermines urban development leading to social instability.  相似文献   
714.
As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km2) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes.  相似文献   
715.
Investment in the release of open data has become increasingly central to the implementation of smart city programs by governments around the world. Though originally arising out of a push towards “open government” and the pursuit of more transparent decision-making by public authorities at multiple scales, open data programs have more recently been adopted by municipal governments to support entrepreneurial goals of enhanced competitive positioning and attracting investment. As urban scholars now subject the smart city project to critical scrutiny for its role in advancing urban entrepreneurialism, this article considers the relevance of the open data agenda as it shapes wider understandings of the smart city. In particular, I address the collection of policy practices, aspirations, stakeholders and entrepreneurs active in framing the opportunities and values of open data for urban governments. Both the momentum of support for open data, along with a recent shift in the rhetorical aspirations of the open data movement away from the values of openness and transparency and towards a more confined focus on value generation, raise important critical questions for urban geographers concerned with the nature of urban governance in an age of big data.  相似文献   
716.
Agriculture on the fringes of cities across the Global North is increasingly perceived as making an important contribution to urban sustainability. As Australian cities continue to expand and encroach on their peri-urban peripheries, there is rising concern about loss of farmland to housing. Such concerns are especially urgent in the Sydney Basin, due to population growth, and topographical and land-use constraints. Accounting for the Basin's farmlands, however, remains opaque, not unrelated to difficulties in acquiring reliable data on the area and value of Sydney's agricultural industries. The problem is not simply that there are no data available but rather that the nature of existing data is (often hotly) contested. Critical questions for urban planners therefore remain unanswered, including: is peri-urban agriculture as important as advocates suggest? Are metropolitan food supplies under threat? If peri-urban farmland is important, what should be done to preserve it? In collating and analysing existing Australian Bureau of Statistics (ABS) and non-ABS data on Sydney agriculture between 1992 and 2011, we outline the need for more reliable and consistent longitudinal data to enable better planning for Sydney's farmland into the future. Notwithstanding limitations of available data sources, our findings reveal trends in Sydney Basin agriculture that invite debate on many assumptions about the nature of peri-urban agriculture. These findings emphasise the importance of geographically specific, evidence-based analysis as a basis for planning for peri-urban agriculture.  相似文献   
717.
Social capital has the potential to influence the success of biodiverse carbon plantings in the face of uncertainty amongst rural landholders about the need or efficacy of efforts to address climate change through tree planting. We conducted 17 face-to-face semi-structured interviews with landholders in Victoria, Australia who voluntarily participate in biodiverse carbon plantings on their land, focusing in particular on the role of social capital for understanding how ‘early adopters' can advocate for programs locally. The interviews revealed the importance of social networks and the profound impact of trusted peers on the diffusion of carbon planting schemes. These social capital dimensions are especially important for shaping ongoing participation and the ways in which participants become active agents in trusted relationships that influence the participation of others. Our results suggest that the positive impact of social networks can counteract doubts about the validity of climate adaptation responses such as carbon planting, and enable landholders to connect the program with their existing stewardship motivations. The ability for early adopters of the program to demonstrate the physical materialisation of their plantings to others was vital to this process. We propose that targeting champions and trusted peers in local communities could accelerate the proliferation of biodiverse carbon planting schemes.  相似文献   
718.
Linking surficial sediment patterns in reef environments to the processes that underlie their depositional dynamics enables predictions to be made of how environmental changes will influence reef‐associated sedimentary landforms, such as islands and beaches. Geomorphic linkages between sediment deposition patterns and the biophysical processes that drive them are often poorly resolved, particularly at broad landscape scales where tangible statements can be made about structural changes to landforms. The present study applies geospatial techniques to link patterns in reef sediment dynamics at Lady Musgrave Island to the underlying processes driving them. In situ calcification is characterized by developing a high resolution map of the surficial calcium carbonate producing communities inhabiting the reef platform, and associated sediments across the reef flat are analysed for grain size, kurtosis, sorting and threshold bed shear stress to explore transport pathways across the reef flat and lagoon. Wave energy is modelled across the entire reef platform as a potential driver of sediment dynamics, and morphometric linkages are empirically defined between wave energy and grain size. Findings indicate that carbonate sediments are primarily sourced from calcifying communities colonizing the outer periphery of the reef platform and that sediment grain size can be reliably linked to wave energy by virtue of a linear model.  相似文献   
719.
Natural Hazards - The rip current hazard on beaches is a global public health issue. While physical controls on rip current formation and flow behavior are relatively well understood, there has...  相似文献   
720.
Aquaculture currently provides half of all fish for human consumption, and this proportion is expected to increase to meet the growing global demand for protein. As aquaculture, including oyster farming, expands, it is increasingly important to understand effects on coastal ecosystems. The broad-scale ecological effects of oyster aquaculture are well documented; however, less is known regarding the influence of oyster aquaculture on sediment bacterial communities. To better understand this relationship, we compared three different oyster farming practices that varied in oyster biomass and proximity of oysters to the sediment. We used high-throughput sequencing and quantitative polymerase chain reaction to examine the effect of oyster farming on sediment bacterial communities. We examined the entire bacterial community and looked specifically at bacteria that support essential estuarine ecosystem services (denitrifiers), as well as bacteria that can be detrimental to human health (members of the Vibrio genus). We found that oyster biomass increased Vibrio richness and sediment carbon content, which influenced bacterial community composition. When compared to reference sites, the overall abundance of bacteria was increased by the bottom planting method, but the associated increases in denitrifiers and Vibrio were not significant. We were unable to detect V. parahaemolyticus, V. vulnificus, or V. cholera, the three most common Vibrio pathogens, in any sample, suggesting that oyster farming did not enhance these potential human pathogens in sediments at the time of sampling. These results highlight how differences in oyster farming practice can affect sediment bacterial communities, and the ecosystem services they provide.  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