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   35145篇
  免费   301篇
  国内免费   337篇
安全科学   897篇
废物处理   1580篇
环保管理   4154篇
综合类   5104篇
基础理论   9887篇
环境理论   8篇
污染及防治   9084篇
评价与监测   2574篇
社会与环境   2291篇
灾害及防治   204篇
  2022年   306篇
  2021年   298篇
  2020年   220篇
  2019年   267篇
  2018年   576篇
  2017年   606篇
  2016年   930篇
  2015年   616篇
  2014年   913篇
  2013年   2682篇
  2012年   1411篇
  2011年   1580篇
  2010年   1288篇
  2009年   1251篇
  2008年   1404篇
  2007年   1560篇
  2006年   1383篇
  2005年   1547篇
  2004年   1686篇
  2003年   1512篇
  2002年   1072篇
  2001年   1423篇
  2000年   992篇
  1999年   596篇
  1998年   412篇
  1997年   412篇
  1996年   446篇
  1995年   470篇
  1994年   438篇
  1993年   384篇
  1992年   398篇
  1991年   361篇
  1990年   379篇
  1989年   374篇
  1988年   319篇
  1987年   296篇
  1986年   257篇
  1985年   271篇
  1984年   293篇
  1983年   276篇
  1982年   294篇
  1981年   238篇
  1980年   210篇
  1979年   260篇
  1978年   198篇
  1977年   204篇
  1976年   167篇
  1975年   161篇
  1974年   158篇
  1973年   166篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Recruitment precision, i.e. the proportion of recruits that reach an advertised food source, is a crucial adaptation of social bees to their environment. Studies with honeybees showed that recruitment precision is not a fixed feature, but it may be enhanced by factors like experience and distance. However, little is known regarding the recruitment precision of stingless bees. Hence, in this study, we examined the effects of experience and spatial distance on the precision of the food communication system of the stingless bee Scaptotrigona mexicana. We conducted the experiments by training bees to a three-dimensional artificial patch at several distances from the colony. We recorded the choices of individual recruited foragers, either being newcomers (foragers without experience with the advertised food source) or experienced (foragers that had previously visited the feeder). We found that the average precision of newcomers (95.6 ± 2.61%) was significantly higher than that of experienced bees (80.2 ± 1.12%). While this might seem counter-intuitive on first sight, this “loss” of precision can be explained by the tendency of experienced recruits to explore nearby areas to find new rewarding food sources after they had initially learned the exact location of the food source. Increasing the distance from the colony had no significant effect on the precision of the foraging bees. Thus, our data show that experience, but not the distance of the food source, affected the patch precision of S. mexicana foragers.  相似文献   
952.
This paper studies the effects of adaptation and mitigation on the impacts of sea level rise. Without adaptation, the impact of sea level rise would be substantial, almost wiping out entire countries by 2100, although the globally aggregate effect is much smaller. Adaptation would reduce potential impacts by a factor 10–100. Adaptation would come at a minor cost compared to the damage avoided. As adaptation depends on socio-economic status, the rank order of most vulnerable countries is different than the rank order of most exposed countries. Because the momentum of sea level rise is so large, mitigation can reduce impacts only to a limited extent. Stabilising carbon dioxide concentrations at 550 ppm would cut impacts in 2100 by about 10%. However, the costs of emission reduction lower the avoided impacts by up to 25% (average 10%). This is partly due to the reduced availability of resources for adaptation, and partly due to the increased sensitivity to wetland loss by adaptation.
Richard S. J. TolEmail:
  相似文献   
953.
Sharks are known to have been ammonoid predators, as indicated by analysis of bite marks or coprolite contents. However, body fossil associations attesting to this predator–prey relationship have never been described so far. Here, I report a unique finding from the Late Jurassic of western France: a complete specimen of the Kimmeridgian ammonite Orthaspidoceras bearing one tooth of the hybodont shark Planohybodus. Some possible tooth puncture marks are also observed. This is the first direct evidence of such a trophic link between these two major Mesozoic groups, allowing an accurate identification of both organisms. Although Planohybodus displays a tearing-type dentition generally assumed to have been especially adapted for large unshelled prey, our discovery clearly shows that this shark was also able to attack robust ammonites such as aspidoceratids. The direct evidence presented here provides new insights into the Mesozoic marine ecosystem food webs.  相似文献   
954.
In this study, landmark-based methods of geometric morphometrics are used for investigating the main aspects of cranial shape transformation in the evolution of the giant panda, Ailuropoda melanoleuca. Specifically, we explore if the highly derived cranial adaptations for bamboo feeding of the living panda were developed early in the panda's lineage. Results obtained show that the overall cranial morphologies of the oldest known panda, the "pygmy" Ailuropoda microta, and the late Pleistocene Ailuropoda baconi are both very similar to that of their closest living relative, A. melanoleuca, which agrees with a previous proposal based on qualitative criteria. However, we also describe several differences between the crania of A. microta, A. baconi, and A. melanoleuca, including the development of the postorbital process, the orientation of the occipital region, and the expansion of the braincase. As a result, the cranial morphology of A. microta shows a less specialized morphology toward a fibrous and durophagous diet compared to the giant panda. These results are confirmed by a comparative analysis of the dimensions of the upper teeth in bears, which has revealed differences in relative tooth size between A. microta and A. melanoleuca, most probably as a result of mosaic evolution. Therefore, we conclude that cranial shape did not remain essentially uniform in the Ailuropoda lineage, as previously thought, but underwent a number of changes during more than 2?Myr.  相似文献   
955.
A comparative experiment was conducted to test the extent to which grazing of rank Molinia caerulea-dominated grassland increases botanical diversity and improves animal performance. The study was carried out on an area of under-grazed Molinia-dominant pasture that had not been stocked for 20 years. Three treatments were compared from 2001 until 2008: (1) no grazing, (2) grazing by Welsh Mountain sheep and (3) grazing by Welsh Black heifers. Patterns of change in sward surface composition across the grazing season were consistent with greater consumption of Molinia by the cattle, but there was little evidence of management regime influencing changes in sward composition year-on-year. There was a significant effect of year on animal performance (P < 0.001). Although the growth rates of the heifers improved over the course of the first three years of the experiment, this performance was not sustained. In contrast, any weight changes for the sheep were positive. The results highlight the slow pace of change within upland swards, and the need to develop site-specific grazing guidelines.  相似文献   
956.
A recent fieldwork in the Ka??zman-Tuzluca Basin in northeastern Turkey led us to the discovery of three vertebrate localities which yielded some limb bones of the giant rhino Paraceratherium, a crocodile tooth, and some small mammals, respectively. These discoveries allowed, for the first time to date some parts of the sedimentary units of this basin. This study also shows that the dispersal area of Paraceratherium is wider than it was known before. Eastern Turkey has several Cenozoic sedimentary basins formed during the collision of the Arabian and Eurasian plates. They are poorly documented for vertebrate paleontology. Consequently, the timing of tectonic activities, which led to the formation of the East Anatolian accretionary complex, is not constrained enough with a solid chronological framework. This study provides the first biostratigraphic evidences for the infill under the control of the compressive tectonic regime, which built the East Anatolian Plateau.  相似文献   
957.
The paper explores the bi-directional relationship between ISO 14001 certification and financial performance with the aim of shedding light on whether better performance is due to the beneficial effects of ISO 14001 or due to selection-effects where better performance precedes accreditation. The study uses a five year longitudinal analysis to compare the financial performance of firms in Spain before and after certification. The results of a multivariate panel data analysis find that firms with better than average performance have a greater propensity to pursue accreditation but there is no evidence that improvements in performance follow certification. This suggests that the inference that environmental variables cause improved financial performance may be unwise in research studies that can only measure association.  相似文献   
958.
Some emerging technologies are expected to be pivotal for solving many of the environmental challenges faced today, especially those related to energy. However, many of these technologies may incur significant environmental impacts over their life cycle, while having environmental benefits during their use. This paper presents results of a Life Cycle Assessment (LCA) of a proposed type of nanophotovoltaic, quantum dot photovoltaic (QDPV) module. The LCA is confined to the stages of raw materials acquisition, manufacturing, and use. The impacts of QDPV are compared with other types of PV modules and energy sources - both renewable and nonrenewable. To provide a comprehensive comparative assessment, QDPV modules were compared with mature as well as emerging PV types for which data are available. Comparative assessment with other types of energy sources includes coal, oil, lignite, natural gas, diesel, nuclear, wind, and hydropower.QDPV modules may have the potential to overcome two current barriers of solar technology: low efficiencies and high manufacturing costs. If higher efficiencies are realized, QDPV modules could pave the way to large scale implementation of solar energy, helping nations move toward greater energy independence. On the other hand, candidate materials as quantum dots for solar cell applications are mostly compound semiconductors such as cadmium selenide, cadmium telluride, and lead sulfide which may be toxic and for which renewable options are limited. Toxic effects of these materials may be exacerbated by their nanoscale features.The LCA was carried out using the software SimaPro, and the Ecoinvent Life Cycle Inventory (LCI) database supplemented with available literature and patent information. Our results indicate that while QDPV modules have shorter Energy PayBack Time (EPBT), lower Global Warming Potential (GWP), SOx and NOx emissions than other types of PV modules, they have higher heavy metal emissions, underscoring the need for investigation of emerging technologies, especially nano-based ones, from a life cycle perspective. QDPV modules are better in all impact categories assessed than carbon-based energy sources but they have longer EPBT than wind and hydropower and higher GWP.  相似文献   
959.
Ash deposition impact on the energy performance of photovoltaic generators   总被引:1,自引:0,他引:1  
A little known side effect of the atmospheric air pollution is the degradation of photovoltaic (PV) cells’ performance due to the deposition of solid particles varying in composition, size and origin. In this context, an experimental-based investigation is conducted in order to compare the energy performance of two identical pairs of PV-panels; the first being clean and the second being artificially polluted with ash, i.e. a by-product of incomplete hydrocarbons’ combustion mainly originating from thermal power stations and vehicular exhausts. A series of systematic measurements of current intensity, voltage output and solar radiation are executed simultaneously for the clean and the polluted PV-panel, so that the effect of several mass depositions on the PVs’ power output, energy yield and conversion efficiency may be determined. According to the results, a considerable deterioration of the PV-panels’ performance is obtained, i.e. almost 30% energy reduction per hour or 1.5% efficiency decrease (in absolute terms) for ash accumulation on the panels’ surface reaching up to 0.4 mg/cm2.  相似文献   
960.
Environmental radon exposure of residents of domestic premises in the United Kingdom (UK) and elsewhere in Europe is estimated on the basis of the measured radon concentrations in, and the relative occupancies of, the principal living room and bedroom. While studies on radon concentration variability in the individual units in apartment blocks in various countries have been described, little data has been reported on variability in two-storey single-family dwellings, and the majority of extant studies consolidate living room and bedroom data early in the analysis. To investigate this further, detailed analysis was made of radon concentration data from a set of thirty-four homes situated in areas of Northamptonshire known to exhibit high radon levels. All homes were of typical UK construction of brick/block/stone walls under a pitched tile/slate roof. Approximately 50% of the sample were detached houses, the remainder being semi-detached (duplex) or terraced (row-house). Around 25% of the sample possessed cellars, while 12% were single-storey dwellings (bungalows), reflecting the typical incidence of this type of dwelling in England. In the two-storey homes, all monitored bedrooms were on the upper floor. Distribution of the ratios of bedroom/living room radon concentrations (BR/LR ratio) in individual properties was left-skewed (mean 0.67, median 0.73, range 0.05-1.05) with a tail extending to just above 1.0. The mean is consistent with the outcome of earlier extensive studies in England, while the variability depends principally on the characteristics of the property, and not on seasonal factors. In a small set of homes, the BR/LR ratio was anomalously low, (mean 0.3). BR/LR ratios in single-storey homes clustered around a value of 1.0, indicating that house design, rather than lifestyle, is the dominant factor in determining bedroom radon concentrations. Homes with higher mean annual radon concentrations showed lower BR/LR ratios, supporting our proposal that, in some homes, radon emanation from building materials may comprise a significant component of the overall radon level.  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