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   1296篇
  免费   44篇
  国内免费   33篇
测绘学   33篇
大气科学   342篇
地球物理   49篇
地质学   196篇
海洋学   132篇
天文学   3篇
综合类   70篇
自然地理   548篇
  2024年   3篇
  2023年   6篇
  2022年   35篇
  2021年   30篇
  2020年   38篇
  2019年   61篇
  2018年   63篇
  2017年   65篇
  2016年   67篇
  2015年   58篇
  2014年   48篇
  2013年   236篇
  2012年   41篇
  2011年   52篇
  2010年   47篇
  2009年   47篇
  2008年   54篇
  2007年   64篇
  2006年   46篇
  2005年   42篇
  2004年   34篇
  2003年   33篇
  2002年   28篇
  2001年   24篇
  2000年   27篇
  1999年   27篇
  1998年   21篇
  1997年   13篇
  1996年   5篇
  1995年   8篇
  1994年   4篇
  1993年   5篇
  1992年   2篇
  1991年   7篇
  1989年   1篇
  1988年   7篇
  1987年   5篇
  1986年   3篇
  1985年   5篇
  1984年   4篇
  1983年   4篇
  1982年   1篇
  1980年   1篇
  1978年   1篇
排序方式: 共有1373条查询结果,搜索用时 0 毫秒
341.
陈世栋  袁奇峰 《地理科学》2015,35(7):852-859
在城市区域化下,基本农田保护区(农保)成为都市区生态保育的重要空间载体及划定城市增长边界依据。以广州市白云区为例,以镇和行政村为基本空间单元,研究了1997年以来基本农田保护区空间演变过程,并构建指标体系,建立不同产权主体农保转用的数理模型,揭示农保政策对不同主体管制的差异及其影响因素。研究发现:① 广州都市边缘区农保被大量转用,并以转为集体建设用地为主;② 农保政策对不同主体的空间管制效果差异较大,可分为管制有效型、集体转用失效型、国有转用失效型三种类型;③ 国有转用主要位于镇区周边,是城镇化所需代价,集体转用主要分布于靠近城市中心而远离镇区的村庄;④ 区位及村庄对非农经济收入的追求是农地集体转用的主要原因,而社会变迁因素影响并不显著。总之,基本农田制度对政府的管制有效,对集体则失效,说明了对基本农田的保护不仅需要划定刚性的物理边界,还需协调集体产权所有者的利益。  相似文献   
342.
Abstract

Scholarship on collaboration in natural resource management is restricted by a lack of large-N assessments and mixed methods approach to examine relationships between collaborative structures and processes, and management actions and outcomes. This paper examines the relationship between perceived levels of collaboration, collaborative process indicators, and benefits of USDA Forest Service stewardship contracts at a broad spatial and temporal scale using mixed methodology. This study found higher levels of collaboration were strongly associated with jointly initiated processes that included a breadth of interests and utilized a broad range of outreach mechanisms and opportunities for engagement. Highly collaborative processes were closely linked with attaining project objectives and social and economic benefits. Findings contribute to defining collaboration by uncovering the relationship between perceived levels of collaboration, the role of process characteristics, and perceived benefits of collaborative stewardship contracting processes through a large-N dataset (n?=?1,064) and case study (n?=?61) findings.  相似文献   
343.
Most countries implementing an emissions trading system (ETS), such as EU member states, California in the US, or South Korea, are generally targeting large sized companies, which consume energy above a specific threshold. However, previous studies using computable general equilibrium (CGE) models have analyzed climate policies without considering company size. This may have led to inaccurate results because the impacts of climate policy would differ depending on the coverage of regulated companies. Accordingly, this study examines the environmental and economic impacts of greenhouse gas emission reduction policies, assuming policy results vary by firm size, as covered by the Korean emission trading system. To this end, a CGE model with a separate social accounting matrix based on company size is used to compare three scenarios that reflect different types of carbon pricing methods. The results show that greenhouse gases will be reduced to a lower extent and utility will decrease more if mitigation policies are only imposed to large companies.

Key policy insights

  • Carbon pricing policies should consider the different impacts on companies of different sizes and industry sectors.

  • Without considering the different sizes of companies covered by an ETS, the expected carbon price and its economic impact will be underestimated.

  • Small and medium-sized companies will face more negative impacts than large companies in some industry sectors under an ETS, even if the mitigation burden is only faced by large companies.

  相似文献   
344.
Increases in sea surface temperature have led to distributional changes in many commercially exploited fish species. These changes have already led to conflict over mackerel fisheries, arising from demand for fair resource apportionment and desire to manage the fishery sustainably. In order to develop adaptable management strategies for complex ocean fishery systems, policymakers and researchers must move beyond a reactive producer–consumer relationship to develop proactive, supportive collaborations. The history of U.S. national fisheries management is presented as an example of this transition. Building cooperative capacity over the last two centuries has lead to a more systematic understanding of the oceans, and has led to success in reducing the number of overfished stocks. Similar development of cooperation between policymakers and researchers on an international scale may be the surest way to develop management strategies adaptable enough to withstand challenges posed by future climate change.  相似文献   
345.
The objectives are to (1) quantify, map, and analyze vegetation cover distributions and changes across Accra, Ghana, for 2002 and 2010; and (2) examine the statistical relationship between vegetation cover and a housing quality index (HQI) for 2000 at the neighborhood level. Pixel-level vegetation cover maps derived using threshold classification of 2002 and 2010 QuickBird normalized difference vegetation index images have very high overall accuracies and yield an estimate of 5.9 percent vegetation cover reduction over the study area between 2002 and 2010. A high degree of variance in vegetation cover for individual dates is explained by HQI at the neighborhood level, although minimal covariability between absolute or relative vegetation cover change and HQI for 2000 was observed.  相似文献   
346.
China announced plans on April 1, 2017 to create Xiong’an New Area, a new economic zone about 110 km southwest of Beijing. The new area of national significance covers the three counties of Xiongxian, Rongcheng and Anxin in Hebei Province, and is home to Baiyangdian, a major wetland in northern China. It will cover around 100 km2 initially (called “Starting Zone”) and will be expanded to 200 km2 in the mid-term and about 2000 km2 in the long term. In this context, it is important to assess the land use pattern for the new area’s planning and development. Based on remote sensing interpretation of Landsat OLI images, we examine the current land use features, the potential for built-up land saving and intensive use, and the spatial variation of arable land quality. The results indicate that the arable lands, built-up lands and wetlands account for around 60%, 20% and 10%, respectively. The potential for rural built-up land saving and intensive use is quite large. The arable land quality and use intensity in Rongcheng and Anxin counties are much better than that of Xiongxian. The pros and cons for three potential options for the Starting Zone are listed and compared. The eastern area of Rongcheng county, to the north of Baiyangdian, is believed to be the most suitable Starting Zone. Policy recommendations for built-up land expansion, farmland saving and flood risk mitigation for Xiong’an New Area are also proposed in this paper.  相似文献   
347.
Newton  John 《Natural Hazards》1997,16(2-3):219-241
Natural Hazards - Exposure to natural hazards tends to be poorly understood and yet implicitly accepted by a large proportion of populations at risk. Some people through years of experience and...  相似文献   
348.
Flared slopes are smooth concavities caused by subsurface moisture-generated weathering in the scarp-foot zone of hillslopes or boulders. They are well represented in granitic terrains but also developed in other massive materials such as limestone, sandstone, dacite, rhyolite, and basalt, as well as other plutonic rocks. Notches, cliff-foot caves, and swamp slots are congeners of flared slopes. Though a few bedrock flares are conceivably caused by nivation or by a combination of coastal processes, most are two-stage or etch forms. Appreciation of the origin of these forms has permitted their use in the identification and measurement of recent soil erosion and an explanation of natural bridges. Their mode of development is also germane to the origin of the host inselberg or bornhardt and, indeed, to general theories of landscape evolution. But certain discrepancies have been noted concerning the distribution and detailed morphology of flared slopes. Such anomalies are a result of structural factors (sensu lato), of variations in size of catchment and in degree of exposure, and of several protective factors. Notwithstanding, the original explanation of flared slopes stands, as do their wider implications. [Key words: flared slope, inselberg, soil erosion, weathering, fractures.]  相似文献   
349.
贫困山区的开发建设,对进一步密切党群关系,加强民族团结,实现四化建设宏伟目标,都有着重要的战略意义。全国贫困山区有18片。国家对贫困山区采取了一系列扶持措施,如放宽政策、扩大农民经营主动权和减轻农民负担等。目前这方面的工作已取得一些成绩。为加快贫困山区开发建设步伐,要采取五种政策性措施。  相似文献   
350.
This article analyses the interactions between agricultural policy measures in the EU and the factors affecting GHG emissions from agriculture on the one hand, and the adaptation of agriculture to climate change on the other. To this end, the article uses Slovenia as a case study, assessing the extent to which Slovenian agricultural policy is responding to the challenges of climate change. All agricultural policy measures related to the 2007–2013 programming period were analysed according to a new methodological approach that is based on a qualitative (expert evaluation) and a quantitative (budgetary transfers validation) assessment. A panel of experts reached consensus on the key factors through which individual measures affect climate change, in which direction and how significantly. Data on budgetary funds for each measure were used as weights to assess their relative importance. The results show that there are not many measures in (Slovenian) agricultural policy that are directly aimed at reducing GHG emissions from agriculture or at adaptation to climate change. Nevertheless, most affect climate change, and their impact is far from negligible. Current measures have both positive and negative impacts, but overall the positive impacts prevail. Measures that involve many beneficiaries and more budgetary funds had the strongest impact on aggregate assessments. In light of climate change, agricultural policy should pay more attention to measures that are aimed at raising the efficiency of animal production, as it is the principal source of GHG emissions from agriculture.

Policy relevance

Agricultural policy must respond to climate challenges and climate change impact assessment must be included in the process of forming European agricultural policy. Agricultural policy measures that contribute to the reduction of emissions and adaptation, whilst acting in synergy with other environmental, economic and social goals, should be promoted. The approach used in this study combines qualitative and quantitative data, yielding an objective assessment of the climate impact of agricultural policy measures and providing policy makers with a tool for either ex ante or ex post evaluations of climate-relevant policy measures.  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