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   175篇
  免费   0篇
安全科学   5篇
环保管理   29篇
基础理论   124篇
污染及防治   6篇
评价与监测   2篇
灾害及防治   9篇
  2015年   1篇
  2014年   9篇
  2013年   11篇
  2012年   1篇
  2011年   6篇
  2010年   12篇
  2009年   13篇
  2008年   5篇
  2007年   13篇
  2006年   8篇
  2005年   14篇
  2004年   17篇
  2003年   6篇
  2002年   2篇
  2001年   7篇
  1999年   1篇
  1997年   9篇
  1996年   4篇
  1995年   6篇
  1993年   5篇
  1992年   1篇
  1991年   4篇
  1990年   3篇
  1989年   2篇
  1988年   4篇
  1987年   1篇
  1986年   1篇
  1983年   1篇
  1981年   3篇
  1980年   2篇
  1978年   1篇
  1977年   2篇
排序方式: 共有175条查询结果,搜索用时 15 毫秒
71.
72.
73.
74.
75.
This article examines several deficiencies in the standard procedure for calculating the intensity of metal use. It then investigates the possibility that the long-term decline in the intensity of copper use in the USA, which has persisted for more than four decades, has in recent years come to an end, even though the standard measures of intensity of use indicate a continuation of the downward trend, albeit at a slower rate. If true, the implications are significant for the copper industry worldwide, because the USA is the largest national market for copper. In addition, consumption trends in many other countries in some respects follow those in the USA. The results do provide some support for the hypothesis that intensity of copper use in the USA is no longer falling. They also indicate that the standard procedure significantly underestimates USA copper consumption and in turn intensity of use.  相似文献   
76.
We evaluated the amounts and implications of changes in habitat connectivity on rural landscapes by modeling the colonization success and subsequent habitat colonization of a model edge organism within real landscapes. We first inventoried the changes in the fencerow and forest-edge network of two contiguous Ohio (U.S.A.) landscapes, an agriculturally dominated till plain and a more diverse and dynamic moraine landscape, from 1940 to 1988. On the moraine the number of fencerows changed little from 1940 to 1971. The number increased by 86 during 1971–1988 as marginal farms were subdivided. The total length of fencerows on the moraine increased 2.5-fold over 48 years. On the till plain the number and total length of fencerows remained relatively constant through the study period. The sum of fencerows and forest edges was used as a measure of total ecotonal edge. On the moraine total edge increased through the study period, whereas on the till plain it decreased. We selected two levels of landscape connectivity, low and high, to model animal habitat colonization success. As connectivity increased the earliest successful colonists preempted an increasingly large proportion of the total suitable habitat, and the probability of successful colonization by later-arriving individuals decreased. The changes in connectivity that resulted from changes in both the fencerow network and the proportion of forested land have resulted in contiguous landscapes that present very different colonization potentials to organisms with long-distance dispersal capability. Given the current uncertainty of the effects of corridors on species-preservation efforts, we suggest that further modeling of this type prior to field testing will add useful insights, especially if conducted using specific species and landscape types.  相似文献   
77.
Strategies to mitigate climate change can protect different types of cool environments. Two are receiving much attention: protection of ephemeral refuges (i.e., places with low maximum temperatures) and of stable refugia (i.e., places that are cool, have a stable environment, and are isolated). Problematically, they are often treated as equivalents. Careful delineation of their qualities is needed to prevent misdirected conservation initiatives; yet, no one has determined whether protecting one protects the other. We mapped both types of cool environments across a large (~3.4M ha) mixed‐use landscape with a geographic information system and conducted a patch analysis to compare their spatial distributions; examine relations between land use and their size and shape; and assess their current protection status. With a modest, but arbitrary, threshold for demarcating both types of cool environments (i.e., values below the 0.025 quantile) there were 146,523 ha of ephemeral refuge (62,208 ha) and stable refugia (62,319 ha). Ephemeral refuges were generally aggregated at high elevation, and more refuge area occurred in protected areas (55,184 ha) than in unprotected areas (7,024 ha). In contrast, stable refugia were scattered across the landscape, and more stable‐refugium area occurred on unprotected (40,135 ha) than on protected land (22,184 ha). Although sensitivity analysis showed that varying the thresholds that define cool environments affected outcomes, it also exposed the challenge of choosing a threshold for strategies to address climate change; there is no single value that is appropriate for all of biodiversity. The degree of overlap between ephemeral refuges and stable refugia revealed that targeting only the former for protection on currently unprotected land would capture ~17% of stable refugia. Targeting only stable refugia would capture ~54% of ephemeral refuges. Thus, targeting one type of cool environment did not fully protect the other. Evaluación de la Distribución y Estado de Protección de Dos Tipos de Ambientes Fríos para Facilitar su Conservación bajo el Cambio Climático  相似文献   
78.
79.
This note discusses environmental aspects of the planning and consultation process undertaken for the UK aviation White Paper. The process as a whole has involved some three years of forecasting and assessment of the operational, economic and environmental implications of some 28 options for airport expansion at 14 UK locations. Unconstrained passenger demand forecasts have been used as a basis for the planning and consultation, and a mitigation approach to environment has predominated. This is inadequate, given the climate impacts of the forecast aviation expansion. Greenhouse gas emissions reduction should be a high priority in transport infrastructure planning, not the subject of post-hoc analysis.  相似文献   
80.
Abstract:  In the 1990s the federal forests in the Pacific Northwest underwent the largest shift in management focus since their creation, from providing a sustained yield of timber to conserving biodiversity, with an emphasis on endangered species. Triggered by a legal challenge to the federal protection strategy for the Northern Spotted Owl ( Strix occidentalis caurina ), this shift was facilitated by a sequence of science assessments that culminated in the development of the Northwest Forest Plan. The plan, adopted in 1994, called for an extensive system of late-successional and riparian reserves along with some timber harvest on the intervening lands under a set of controls and safeguards. It has proven more successful in stopping actions harmful to conservation of old-growth forests and aquatic systems than in achieving restoration goals and economic and social goals. We make three suggestions that will allow the plan to achieve its goals: (1) recognize that the Northwest Forest Plan has evolved into an integrative conservation strategy, (2) conserve old-growth trees and forests wherever they occur, and (3) manage federal forests as dynamic ecosystems.  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