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   763篇
  免费   26篇
  国内免费   9篇
安全科学   32篇
废物处理   17篇
环保管理   146篇
综合类   167篇
基础理论   198篇
环境理论   3篇
污染及防治   159篇
评价与监测   43篇
社会与环境   25篇
灾害及防治   8篇
  2023年   12篇
  2022年   8篇
  2021年   24篇
  2020年   28篇
  2019年   26篇
  2018年   32篇
  2017年   30篇
  2016年   35篇
  2015年   38篇
  2014年   30篇
  2013年   58篇
  2012年   42篇
  2011年   70篇
  2010年   34篇
  2009年   27篇
  2008年   36篇
  2007年   41篇
  2006年   37篇
  2005年   26篇
  2004年   20篇
  2003年   23篇
  2002年   19篇
  2001年   7篇
  2000年   7篇
  1999年   11篇
  1998年   6篇
  1997年   6篇
  1996年   4篇
  1995年   4篇
  1994年   9篇
  1993年   5篇
  1992年   6篇
  1991年   5篇
  1989年   1篇
  1988年   3篇
  1987年   3篇
  1986年   2篇
  1985年   6篇
  1984年   2篇
  1983年   4篇
  1982年   2篇
  1981年   3篇
  1979年   2篇
  1978年   1篇
  1977年   2篇
  1967年   1篇
排序方式: 共有798条查询结果,搜索用时 62 毫秒
101.
Of 2882 women allocated to either transabdominal CVS (TA) or transcervical CVS (TC) at two large obstetric centres in Denmark, 2707 had blood samples drawn before and 30 min after CVS for maternal serum-alpha-fetoprotein (MSAFP) measurement. 2535 of these women had cytogenetically normal pregnancies and 2091 of them went on to have samples drawn at the 18–20 week follow-up. Post-procedure MSAFP values were correlated to the biopsy method used, with mean MSAFP values significantly higher after TA than TC, 33 and 15 kU/l, respectively (P<0·001). Following TA procedures, 18 per cent of cases had feto-maternal transfusion higher than 0·1 ml; this occurred in only 5 per cent of TC cases. MSAFP levels were associated with spontaneous fetal loss in the TA group but not in the TC group. TC, however, was followed by more losses than TA. The post-CVS MSAFP value was positively correlated with the amount of villi aspirated. The difference in post-procedure elevation in MSAFP 30 min later (average 18 kU/l higher for TA than for TC) was not reflected in raised levels at the 18–20 week follow-up. Study medians at mid-trimester did not differ from reference group medians established from a group of singleton pregnancies with sonographically determined gestational age who did not experience invasive procedures and delivered normal infants. Our findings suggest that CVS does not compromise mid-trimester MSAFP for screening for neural tube defects (NTDs). Extremely high mid-trimester MSAFP values in the TC group could predict imminent loss.  相似文献   
102.
Genetic variation in honeybee, Apis mellifera, populations can be considerably influenced by breeding and commercial introductions, especially in areas with abundant beekeeping. However, in southern Africa apiculture is based on the capture of wild swarms, and queen rearing is virtually absent. Moreover, the introduction of European subspecies constantly failed in the Cape region. We therefore hypothesize a low human impact on genetic variation in populations of Cape honeybees, Apis mellifera capensis. A novel solution to studying genetic variation in honeybee populations based on thelytokous worker reproduction is applied to test this hypothesis. Environmental effects on metrical morphological characters of the phenotype are separated to obtain a genetic residual component. The genetic residuals are then re-calculated as coefficients of genetic variation. Characters measured included hair length on the abdomen, width and length of wax plate, and three wing angles. The data show for the first time that genetic variation in Cape honeybee populations is independent of beekeeping density and probably reflects naturally occurring processes such as gene flow due to topographic and climatic variation on a microscale.  相似文献   
103.
Gammage S  Benítez M  Machado M 《Ambio》2002,31(4):285-294
This paper summarizes those factors that have contributed to the degradation of the mangroves in El Salvador, and examines the implications of proposed changes in property rights to the mangroves for poor coastal communities. Current patterns of extraction and conversion in the mangrove ecosystem in El Salvador can be viewed through an entitlement lens that confers property right upon some actors, for certain uses, and denies them to others. Unfortunately, the present system of laws and regulations that governs resource use in the mangroves is contradictory and confusing. A number of laws currently are active that give entirely different directives about the use and transformation of the ecosystem. The rational and sustainable management of the mangroves cannot be secured without a redefinition of entitlement rights. This redefinition of entitlements, however, should take into account the needs of those whose livelihoods are intimately connected to the health of the ecosystem.  相似文献   
104.
The equilibration and bioavailability of metals in laboratory-contaminated sediments have been investigated in order to provide better guidance on acceptable procedures for spiking sediments with metals for use in the development of whole-sediment toxicity tests. The equilibration rates of Cd, Cu, Ni and Zn spiked into three estuarine surface sediments with varying properties were investigated. Changes to sediment pH, redox potential, porewater and acid-soluble metals, acid-volatile sulfide and bacterial activity during equilibration, effects of temperature and disturbances following equilibration are reported. The addition of metals to sediments caused major decreases in pH and increases in redox potential as metals displaced iron(II) into the porewaters and added metals and iron (following oxidation) were hydrolyzed. The rates of equilibration of metals in porewaters varied considerably and were dependent on sediment and metal properties. For the oxic/sub-oxic sediments studied, metal-spikes of Cd, Cu, Ni and Zn appeared near equilibrium after 25-45, 10-15, 30-70 and 20-40 days, respectively. Acid-soluble metal concentrations decreased during the equilibration period indicating that the metals become more strongly associated with the sediments with time (less bioavailable). Bacterial activity was greatest in the sediment equilibrated at pH 7 and decreased following the addition of metals. During the equilibration period, bacterial activity increased in sediments equilibrated at pH 6, remained low in sediments at pH 8 and varied erratically in sediments at pH 7. Spiked sediments were shown to equilibrate more slowly at lower temperatures resulting in high porewater metal concentrations. Disturbances to equilibrated sediments because of sample manipulation caused significant iron(II) oxidation and losses of metals from porewaters. The importance of documenting spiking and equilibration procedures and carefully measuring and reporting sediment parameters is highlighted so that contaminant bioavailability and exposure pathways can be interpreted and organism sensitivity accurately determined. Recommendations are given for the preparation of metal-spiked sediments for toxicity testing purposes.  相似文献   
105.
Fisheries catches are known to be widely underreported, and much of their value flows in informal markets. Goods and services that are not directly sold in a market also have a corresponding economic value, here termed ‘shadow value’, which can apply to discarded fish—or those that are consumed but not sold (e.g., subsistence catches). Here, we estimate the monetary value of fisheries catches in Panama that are landed but not reported, or that are discarded at sea; this includes catches from artisanal and industrial fleets, as well as recreational and subsistence fisheries. Based on available data, we estimate that the market and shadow value of unreported catches in Panama in 2010 was around US$92 million, equal to approximately 43% of the total reported landed value. In the case of discarded fish, the shadow value represents the potential but entirely unrealized economic benefit of landing such fish; in the case of unreported landings, unreported market value represents only the first link in the potentially sophisticated informal seafood economy. One must be careful in considering these results for policy. It is possible that, rather than seeking to capture these ‘lost’ benefits, fish that are discarded or unreported should not have been caught at all, for example, if they are juveniles or of threatened species; conversely, unreported subsistence catches are crucial for food security throughout the world. These results help contextualize the scale of unreported fisheries in economic terms, and can inform subsequent policies and strategies to ensure social, ecological, and economic sustainability.  相似文献   
106.
This contribution presents an introduction to a series of papers on integrated scientific approaches presented at the San Feliu Euroconference on transdisciplinary coastal management research. Coastal management is a challenging area for the integration of social and natural sciences. The papers provide a basis for ongoing cross-disciplinary research.  相似文献   
107.
This paper reflects on how sustainability is being interpreted both theoretically and in practice for the primary extraction industries. Two contrasting perspectives can be adopted: one states that continued extraction of non-renewable resources is a necessary part of sustainable development, whilst the other states that extraction of these resources must be greatly reduced or even eliminated. It is shown that these different approaches can be traced back to underlying, often unarticulated, assumptions about the acceptability of trade-offs between different societal goals; attitudes towards uncertainties in scientific analysis and the reliability of management systems; and the perceived appropriateness of system boundaries at different conceptual scales. These assumptions are founded in the self-perception of organisations and individuals about their roles in society, and the cultural context within which these organisations and individuals define their raison d'être.  相似文献   
108.
As part of a large exposure assessment and health-effects panel study, 33 trace elements and light-absorbing carbon were measured on 24-hr fixed-site filter samples for particulate matter with an aerodynamic diameter <2.5 microm (PM2.5) collected between September 26, 2000, and May 25, 2001, at a central outdoor site, immediately outside each subject's residence, inside each residence, and on each subject (personal sample). Both two-way (PMF2) and three-way (PMF3) positive matrix factorization were used to deduce the sources contributing to PM2.5. Five sources contributing to the indoor and outdoor samples were identified: vegetative burning, mobile emissions, secondary sulfate, a source rich in chlorine, and a source of crustal-derived material. Vegetative burning contributed more PM2.5 mass on average than any other source in all microenvironments, with average values estimated by PMF2 and PMF3, respectively, of 7.6 and 8.7 microg/m3 for the outdoor samples, 4 and 5.3 microg/m3 for the indoor samples, and 3.8 and 3.4 microg/m3 for the personal samples. Personal exposure to the combustion-related particles was correlated with outdoor sources, whereas exposure to the crustal and chlorine-rich particles was not. Personal exposures to crustal sources were strongly associated with personal activities, especially time spent at school among the child subjects.  相似文献   
109.
Long-term integrated monitoring is an important approach forinvestigating, detecting and predicing the effects ofenvironmental changes. Currently, European freshwaters,glaciers, forests and other narural and semi-natural ecosystemsand habitats are monitored by a number of networks establishedby different organisations. However, many monitoring programmeshave a narrow focus (e.g. targeting individual ecosystems) andmost have different measurement protocols and sampling design.This has resulted in poor integration of ecosystem monitoring ata European level, leading to some overlapping of efforts and alack of harmonised data to inform policy decisions. The need fora consistent pan-European long-term integrated monitoring ofterrestrial systems programme is recognised in the scientificcommunity. However, the design of such a system can be problematic, not least because of the constraints imposed bythe need to make maximum use of existing sites and networks.Based on the outcomes of the NoLIMITS project (Networking ofLong-term Integrated Monitoring in Terrestrial Systems), thisarticle reviews issues that should be addressed in designing aprogramme based on existing monitoring sites and networks. Fourmajor design issues are considered: (i) users' requirements,(ii) the need to address multiple objectives, (iii) role ofexisting sites and (iv) operational aspects.  相似文献   
110.
The 7.6 magnitude (Richter scale) earthquake that struck northern Pakistan on 8 October 2005 was devastating. This paper gauges success in targeting vulnerable families during the transition from relief to reconstruction through cash assistance provided by the Livelihood Support Cash Grants (LSCG) programme. Families without a male member, with a disabled male member aged between 18 and 60 years or with more than five children, defined as vulnerable, were provided with USD 50 per month for six months via a bank transfer. The LSCG scheme enrolled around 750,000 families and selected 267,402 vulnerable families to whom it disbursed a total of USD 86.95 million. Using a community‐based survey, this paper assesses leakage and under‐coverage (exclusion). Approximately 30 per cent of families received the cash grant. However, only one in two was eligible for the benefit, and one in two deserving families was excluded. This is a matter of grave concern.  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