全文获取类型
收费全文 | 1528篇 |
免费 | 171篇 |
国内免费 | 161篇 |
出版年
2024年 | 11篇 |
2023年 | 43篇 |
2022年 | 66篇 |
2021年 | 78篇 |
2020年 | 88篇 |
2019年 | 103篇 |
2018年 | 117篇 |
2017年 | 102篇 |
2016年 | 93篇 |
2015年 | 90篇 |
2014年 | 93篇 |
2013年 | 230篇 |
2012年 | 110篇 |
2011年 | 59篇 |
2010年 | 48篇 |
2009年 | 67篇 |
2008年 | 60篇 |
2007年 | 71篇 |
2006年 | 64篇 |
2005年 | 41篇 |
2004年 | 28篇 |
2003年 | 30篇 |
2002年 | 20篇 |
2001年 | 19篇 |
2000年 | 14篇 |
1999年 | 18篇 |
1998年 | 18篇 |
1997年 | 13篇 |
1996年 | 12篇 |
1995年 | 6篇 |
1994年 | 9篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1988年 | 1篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有1860条查询结果,搜索用时 15 毫秒
111.
The reform in environmental regulations being considered at both federal and state levels is intended to enhance the value and effectiveness of a rule by incorporating risk assessment and cost benefit analysis in the rule making process or regulatory implementation. Although a risk based approach may not provide a panacea to all environmental problem solving, it offers some obvious advantages over the status quo. In particular, it establishes a scientifically defensible basis for evaluating the trade off between risks, costs and benefits in making prudent environmental decisions and developing effective regulatory policies. This paper presents a conceptual framework for risk reduction, summarizes the current status in risk-based legislation at the federal level, provides examples of how various states are using risk based approaches in their regulatory programs, addresses aspects of technical and policy challenges in rule making and other policy and enforcement decisions and provides suggestions for meeting these challenges. 相似文献
112.
Clustering Timber Harvests and the Effect of Dynamic Forest Management Policy on Forest Fragmentation 总被引:1,自引:1,他引:0
Eric J. Gustafson 《Ecosystems》1998,1(5):484-492
To integrate multiple uses (mature forest and commodity production) better on forested lands, timber management strategies
that cluster harvests have been proposed. One such approach clusters harvest activity in space and time, and rotates timber
production zones across the landscape with a long temporal period (dynamic zoning). Dynamic zoning has been shown to increase timber production and reduce forest fragmentation by segregating uses in time without reducing the spatial extent of timber production. It is reasonable
to wonder what the effect of periodic interruptions in the implementation of such as strategy might be, as would be expected
in a dynamic political environment. To answer these questions, I used a timber harvest simulation model (HARVEST) to simulate
a dynamic zoning harvest strategy that was periodically interrupted by changes in the spatial dispersion of harvests, by changes
in timber production levels, or both. The temporal scale (period) of these interruptions had impacts related to the rate at
which the forest achieved canopy closure after harvest. Spatial dynamics in harvest policies had a greater effect on the amount
of forest interior and edge than did dynamics in harvest intensity. The periodically clustered scenarios always produced greater amounts of forest interior and less forest edge than did their never clustered counterparts. The results suggest that clustering of harvests produces less forest fragmentation than dispersed cutting alternatives,
even in the face of a dynamic policy future. Although periodic episodes of dispersed cutting increased fragmentation, average
and maximum fragmentation measures were less than if clustered harvest strategies were never implemented. Clustering may also
be useful to mitigate the fragmentation effects of socially mandated increases in timber harvest levels. Implementation of
spatial clustering during periods of high timber harvest rates reduced the variation in forest interior and edge through time,
providing a more stable supply of forest interior habitat across the landscape.
Received 19 September 1997; accepted 6 August 1998. 相似文献
113.
Reinout Heijungs Arjan de Koning Sangwon Suh Gjalt Huppes 《Journal of Industrial Ecology》2006,10(3):147-158
Integrated product policy, according to the European Union, requires reliable data on the impact of consumer products along their life cycles. We argue that this necessarily requires the development of an information tool for hybrid analysis, combining aspects of life-cycle assessment and input-output analysis. A number of requirements in the development of such a hybrid information tool are identified, mainly concerning data and computational structure. For the former, some important points of attention are discussed, whereas for the latter, operational formulas are developed. 相似文献
114.
在医学教育模式多元化背景下,通过将"集中强化培训"和"点线式网络化实战训练"等教学方法相结合,本文着重探索基层卫生机构中影像学医务人员继续教育新模式。通过为基层影像学医务人员提供快速扎实继续教育通道和平台,并在其实际工作中建立实用型会诊学习体系,以形成"精良多元、持续有序"的教育模式,解决基层人员"培训难,提高难,会诊难"的问题。新培训模式能够不断提高基层卫生机构影像学医务人员业务水平,为根本提升基层医疗诊断水平奠定基础。 相似文献
115.
Global solid biomass trade for energy by 2020: an assessment of potential import streams and supply costs to North-West Europe under different sustainability constraints
下载免费PDF全文
![点击此处可从《Global Change Biology Bioenergy》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Patrick Lamers Ric Hoefnagels Martin Junginger Carlo Hamelinck André Faaij 《Global Change Biology Bioenergy》2015,7(4):618-634
The expected use of solid biomass for large-scale heat and power production across North–West Europe (NW EU) has led to discussions about its sustainability, especially due to the increasing import dependence of the sector. While individual Member States and companies have put forward sustainability criteria, it remains unclear how different requirements will influence the availability and cost of solid biomass and thus how specific regions will satisfy their demand in a competitive global market. We combined a geospatially explicit least-cost biomass supply model with a linear optimization solver to assess global solid biomass trade streams by 2020 with a particular focus on NW EU. We apply different demand and supply scenarios representing varying policy developments and sustainability requirements. We find that the projected EU solid biomass demand by 2020 can be met across all scenarios, almost exclusively via domestic biomass. The exploitation of domestic agricultural residue and energy crop potentials, however, will need to increase sharply. Given sustainability requirements for solid biomass as for liquid biofuels, extra-EU imports may reach 236 PJ by 2020, i.e., 400% of their 2010 levels. Intra-EU trade is expected to grow with stricter sustainability requirements up to 548 PJ, i.e., 280% of its 2010 levels by 2020. Increasing sustainability requirements can have different effects on trade portfolios across NW EU. Excluding pulpwood pellets may drive the supply costs of import dependent countries, foremost the Netherlands and the UK, whereas excluding additional forest biomass may entail higher costs for Germany and Denmark which rely on regional biomass. Excluding solid biomass fractions may create short-term price hikes. Our modeling results are strongly influenced by parameterization choices, foremost assumed EU biomass supply volumes and costs and assumed relations between criteria and supply. The model framework is suited for the inclusion of dynamic supply–demand interactions and other world regions. 相似文献
116.
117.
118.
Donna L. Peppin Peter Z. Ful Janet C. Lynn Anne L. Mottek‐Lucas Carolyn Hull Sieg 《Restoration Ecology》2010,18(Z1):113-124
Increases in revegetation activities have created a large demand for locally adapted native plant materials (NPM) in the southwestern United States. Currently, there is a minimal supply of local genotypes to meet this demand. We investigated the potential for the initiation of a native plant market in the southern Colorado Plateau. Through a literature search, interviews, and site visits, we identified existing native plant markets outside of the region as useful models to help initiate a regional market. We used web‐based surveys to identify and analyze current and future NPM needs and concerns. Survey results indicate that management policy strongly drives decisions regarding the use and purchase of NPM. From a demand perspective , lack of availability and cost of NPM has kept purchasing minimal, despite policy changes favoring the use of natives. For suppliers, further development of NPM is limited by inconsistent and unreliable demand and lack of production knowledge. The knowledge and tools necessary to initiate an NPM market are available, but inadequate funding sources and insufficient information sharing hinder its development. Communication among producers, land managers, buyers, and researchers, as well as partnerships with local growers, appear to be vital to initiating a functional market. 相似文献
119.
Recent environmental trends, including (1) an expansion of existing command and control directives, (2) the introduction of market‐based policy instruments, and (3) the adoption of extended producer responsibility, have created a need for new tools to help managerial decision‐making. To address this need, we develop a nonlinear mathematical programming model from a profit‐maximizing firm's perspective, which can be tailored as a decision‐support tool for firms facing environmental goals and constraints. We typify our approach using the specific context of diesel engine manufacturing and remanufacturing. Our model constructs are based on detailed interviews with top managers from two leading competitors in the medium and heavy‐duty diesel engine industry. The approach allows the incorporation of traditional operations‐planning considerations—in particular, capacity, production, and inventory—together with environmental considerations that range from product design through production to product end of life. A current hurdle to implementing such a model is the availability of input data. We therefore highlight the need not only to involve all departments within businesses but also for industrial ecologists and business managers to work together to implement meaningful decision models that are based on accurate and timely data and can have positive economic and environmental impact. 相似文献
120.
Pablo del Río Javier Carrillo‐Hermosilla Totti Könnölä 《Journal of Industrial Ecology》2010,14(4):541-557
This article elaborates an integrated policy framework, including policy features and specific measures that can be implemented to mitigate the barriers to eco‐innovations. We claim that factors inhibiting the development and uptake of eco‐innovations are multifaceted and stem from different sources. This systems view, which highlights the multilayered conception of the obstacles to eco‐innovation, calls for a combination of environmental and technology policies adapted to the different barriers and characteristics of the technologies. Drawing on evolutionary economics, the article sheds light on the appropriate combinations of framework conditions and instruments that are most effective to promote eco‐innovation, considering different barriers and eco‐innovation types (process/product, mature/immature, and radical/incremental). 相似文献