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   250149篇
  免费   16034篇
  国内免费   35825篇
安全科学   23342篇
废物处理   11552篇
环保管理   28823篇
综合类   102120篇
基础理论   47350篇
环境理论   211篇
污染及防治   50483篇
评价与监测   16431篇
社会与环境   14768篇
灾害及防治   6928篇
  2024年   678篇
  2023年   3279篇
  2022年   9003篇
  2021年   9033篇
  2020年   8639篇
  2019年   6776篇
  2018年   7896篇
  2017年   9214篇
  2016年   9153篇
  2015年   8909篇
  2014年   11277篇
  2013年   20801篇
  2012年   16014篇
  2011年   17932篇
  2010年   15696篇
  2009年   15465篇
  2008年   15869篇
  2007年   15552篇
  2006年   14476篇
  2005年   11188篇
  2004年   8551篇
  2003年   7554篇
  2002年   6653篇
  2001年   6128篇
  2000年   5204篇
  1999年   3558篇
  1998年   2403篇
  1997年   2195篇
  1996年   2214篇
  1995年   2296篇
  1994年   2030篇
  1993年   1505篇
  1992年   1512篇
  1991年   1365篇
  1990年   1251篇
  1989年   1146篇
  1988年   1013篇
  1987年   906篇
  1986年   859篇
  1985年   870篇
  1984年   896篇
  1983年   826篇
  1982年   919篇
  1981年   818篇
  1980年   627篇
  1979年   686篇
  1978年   585篇
  1977年   513篇
  1975年   467篇
  1974年   461篇
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
291.
The Clean Development Mechanism (CDM) under the Kyoto Protocol to the UN Framework Convention on Climate Change (UNFCCC) has its origins in the decade of UNFCCC negotiations. ‘Joint implementation’ and ‘activities implemented jointly pilot’ opened the door for the project-based mechanisms between developed and developing countries. The US proposal of the Joint Implementation in the Kyoto Protocol negotiations was almost identical with CDM approved in Kyoto; however, a detour around the Clean Development Fund (CDF) concept raised by Brazil in the negotiations catalyzed the mutual understanding on the win-win nature of the concept of joint implementation.CDM has been played an important role to bridge the developed and developing countries in its development process initiated as the joint implementation in the UNFCCC, and can lead to the cooperative future in the implementation stage starting from the year 2003, including the development of future commitments beyond 2013. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
292.
This paper demonstrates the potential for induced preference experiments to test previously unverified explanations of observed behavior in contingent valuation surveys. The NOAA Panel on Contingent Valuation called for experimental evidence on potential biases in the double referendum format. We test Carson, Groves, and Machina's (Incentives and informational properties of preference questions, Plenary address to the European Association of Resource and Environmental Economists, Oslo, Norway, June 1999) simple cost uncertainty and weighted averaging explanations of inconsistent responses to follow-up offers in such double referenda against a baseline of certainty and truthful preference revelation. The results find evidence to support the Weighted Average hypothesis. Results regarding the cost uncertainty hypothesis are more ambiguous and merit further investigation.  相似文献   
293.
294.
Because the classification of extant and fossil articulate brachiopods is based largely upon shell characters observable in fossils, it identifies morphotaxa whose biological status can, in practice, best be inferred from estimates of genetic divergence. Allozyme polymorphism and restriction fragment length polymorphism of mitochondrial DNA (mtDNA RFLP) have been used to show that nuclear and mitochondrial genetic divergence between samples of the cancellothyridid brachiopods Terebratulina septentrionalis from Canada and T. retusa from Europe is compatible with biological speciation, but the genetic distances obtained were biased by methodological limitations. Here, we report estimates of divergence in 12S rDNA mitochondrial sequences within and between samples of these brachiopods. The sequence-based genetic distance between these samples (5.98ǂ.07% SE) is at least 10 times greater than within them and, since they also differ in a complex life-history trait, their species status is considered to be securely established. Divergence levels between 12S rDNA genes of three other cancellothyridids, T. unguicula from Alaska, T. crossei from near Japan, and Cancellothyris hedleyi from near Australia are higher than between the two North Atlantic species, and the mean nucleotide distance between all these cancellothyrids is similar to the mean distance between species of Littorina (Mollusca: Gastropoda). Sequences of both 12S and 16S genes from cancellothyridids and other short-looped brachiopod species show neither saturation nor lineage-specific rate differences and, when analysed with different outgroups, either separately or together, yield one unexpected, but well-supported, tree with Alaskan T. unguicula basal and C. hedleyi nested within Terebratulina, i.e. these genera are paraphyletic. A geologically dated divergence between Antarctic and New Zealand species of the short-looped brachiopod Liothyrella is used to calibrate the rate of 12S divergence at ca. 0.1% per million years (MY), and this rate is used to infer that T. septentrionalis and T. retusa have been diverging for ca. 60 MY and that they and T. unguicula have been diverging from their last common ancestor for ca. 100 MY. This indicates a Mesozoic origin for the present-day distribution of cancellothyridids and the basal position of T. unguicula suggests a possible North Pacific centre of origin, with separate Atlantic and Pacific radiations. The inclusion of Cancellothyris within Terebratulina also shows that adult shell characters such as umbo, foramen and symphytium shape, whilst probably indispensible for the practical classification of fossils, are not reliable guides to genealogy.  相似文献   
295.
This paper summarizes studies of carbon mitigation potential (MP) and costs of forestry options in seven developing countries with a focus on the role of agroforestry. A common methodological approach known as comprehensive mitigation assessment process (COMAP) was used in each study to estimate the potential and costs between 2000 and 2030. The approach requires the projection of baseline and mitigation land-use scenarios derived from the demand for forest products and forestland for other uses such as agriculture and pasture. By using data on estimated carbon sequestration, emission avoidance, costs and benefits, the model enables one to estimate cost effectiveness indicators based on monetary benefit per t C, as well as estimates of total mitigation costs and potential when the activities are implemented at equilibrium level. The results show that about half the MP of 6.9 Gt C (an average of 223 Mt C per year) between 2000 and 2030 in the seven countries could be achieved at a negative cost, and the other half at costs not exceeding $100 per t C. Negative cost indicates that non-carbon revenue is sufficient to offset direct costs of about half of the options. The agroforestry options analyzed bear a significant proportion of the potential at medium to low cost per t C when compared to other options. The role of agroforestry in these countries varied between 6% and 21% of the MP, though the options are much more cost effective than most due to the low wage or opportunity cost of rural labor. Agroforestry options are attractive due to the large number of people and potential area currently engaged in agriculture, but they pose unique challenges for carbon and cost accounting due to the dispersed nature of agricultural activities in the tropics, as well as specific difficulties arising from requirements for monitoring, verification, leakage assessment and the establishment of credible baselines.  相似文献   
296.
Daytime sampling of mangrove and seagrass (Halophila/Halodule community) habitats every 7 wk at Alligator Creek, Queensland, Australia, over a period of 13 mo (February 1985–February 1986) using two types of seine net, revealed distinct mangrove and seagrass fish and crustacean faunas. Total abundance of fish and relative abundance of small and large fish also varied between habitats and seasonally. Post-larval, juvenile and small adult fish captured with a small seine-net (3 mm mesh) were significantly more abundant (4 to 10 times) in the mangrove habitat throughout the 13 mo of sampling. Mangrove fish abundance showed significant seasonality, greatest catches being recorded in the warm, wet-season months of the year. Relative abundances of larger fish (captured in a seine net with 18 mm mesh) in the two habitats varied throughout the year, but did not show a seasonal pattern. At the same site, small crustaceans were significantly more abundant in the mangroves in all but one dryseason sample. Similar comparisons for three riverine sites, sampled less frequently, in the dry and wet seasons of 1985 and 1986, respectively, showed that in general mangrove habitats had significantly more fish per sample, although the relative abundance of fish in mangroves and other habitats changed with season. Crustacean catches showed a similar pattern, except that densities among sites changed with season. Fish and crustacean abundance in mangroves varied among sites, indicating that estuaries differ in their nursery-ground value. The juveniles of two commercially important penaeid prawn species (Penaeus merguiensis and Metapenaeus ensis) were amongst the top three species of crustaceans captured in the study, and both were significantly more abundant in the mangrove habitat. By contrast, mangroves could not be considered an important nursery for juveniles of commercially important fish species in northern Australia. However, based on comparisons of fish catches in other regions, the results of the present study indicate the importance of mangroves as nursery sites for commercially exploited fish stocks elsewhere in South-East Asia. Contribution No. 378 from the Australian Institute of Marine Science  相似文献   
297.
ABSTRACT: Two dynamic programming models — one deterministic and one stochastic — that may be used to generate reservoir operating rules are compared. The deterministic model (DPR) consists of an algorithm that cycles through three components: a dynamic program, a regression analysis, and a simulation. In this model, the correlation between the general operating rules, defined by the regression analysis and evaluated in the simulation, and the optimal deterministic operation defined by the dynamic program is increased through an iterative process. The stochastic dynamic program (SDP) describes streamflows with a discrete lag-one Markov process. To test the usefulness of both models in generating reservoir operating rules, real-time reservoir operation simulation models are constructed for three hydrologically different sites. The rules generated by DPR and SDP are then applied in the operation simulation model and their performance is evaluated. For the test cases, the DPR generated rules are more effective in the operation of medium to very large reservoirs and the SDP generated rules are more effective for the operation of small reservoirs.  相似文献   
298.
A significant positive correlation between the lipid content of eight fish species and their bioconcentration factor (BCF) of 1,2,4-trichlorobenzene is demonstrated. The log BCF value on a lipid basis is in good agreement with the log n-octanol/water partition coefficient.  相似文献   
299.
Benthic macroinvertebrate communities in streams adjacent to cornfields, streams where cows had unrestricted access, and reference locations without agriculture were compared to examine the effects of local land use and land use/land cover in the watershed. At each local site, macroinvertebrates and a variety of habitat parameters were measured upstream, adjacent, downstream, and farther downstream of the local land use. A geographic information system (GIS) was used to calculate drainage basin area, land use/land cover percentages in each basin, and the distance from sample sites to the stream source. Three‐way analysis of covariance (ANCOVA) tests with date, site type, and sampling location as main effects were used to explore differences in macroinvertebrate metrics using median substrate size, percent hay/pasture area, and stream depth as covariates. The covariates significantly improved model fit and showed that multiple contributing factors influence community composition. Local impacts were greatest at sites where cows had access, probably because of sedimentation and embeddedness in the substrate. Differences between the upstream and the adjacent and downstream locations were not as great as expected, perhaps because upstream recolonization was reduced by agricultural impacts or because of differences in the intensity or proximity of agriculture to riparian areas in the watershed. The results underscore the importance of both local and watershed factors in controlling stream community composition.  相似文献   
300.
The production of brominated aromatics from combustion was shown to be influenced by the operating conditions. Brominated aromatics also showed high yields compared to their chlorinated analogues.  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