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   242篇
  免费   24篇
  国内免费   5篇
测绘学   5篇
大气科学   19篇
地球物理   64篇
地质学   99篇
海洋学   18篇
天文学   30篇
自然地理   36篇
  2023年   3篇
  2022年   2篇
  2021年   3篇
  2020年   20篇
  2019年   11篇
  2018年   16篇
  2017年   18篇
  2016年   15篇
  2015年   14篇
  2014年   17篇
  2013年   16篇
  2012年   17篇
  2011年   21篇
  2010年   16篇
  2009年   17篇
  2008年   9篇
  2007年   12篇
  2006年   7篇
  2005年   6篇
  2004年   11篇
  2003年   6篇
  2002年   5篇
  2001年   2篇
  2000年   1篇
  1999年   2篇
  1998年   1篇
  1997年   1篇
  1996年   1篇
  1985年   1篇
排序方式: 共有271条查询结果,搜索用时 375 毫秒
231.
How watershed collaborations adapt to changing conditions is an enduring question in collaborative resource management. Our research examined the goals and structure of collaborations within watersheds subject to the same federal and state regulations, but at different restoration states. We employed semistructured interviews, document analysis, and participant observation to complete our qualitative research. Our results suggest that policy stakeholders’ understandings of ecological and social attributes influence watershed goals. Within the impaired system, collaborations invested in shifting public perception to engender support for longer term biophysical restoration. Within the restored watershed, collaborations achieved dramatic biophysical improvements but were challenged in addressing social benefits of restoration. Redefining restoration to incorporate social and ecological systems at the outset provides the opportunity to set benchmarks for measuring success that incorporate social and ecological factors, and to create institutions adept at responding to shifting restoration states.  相似文献   
232.
This study provides a unique examination of Amish farmers’ awareness of water quality issues and their attitudes toward and use of agricultural conservation practices compared with small non-Amish farmers in Northeastern Indiana. There is minimal research about the conservation beliefs and behaviors of this growing and highly diverse ethnoreligious minority, who have faith-based technological restrictions and are often hesitant to work with community outsiders, especially the government. We find that, compared with non-Amish farmers, Amish farmers have limited awareness of the linkages between common agricultural practices and broader water quality problems and lower levels of adoption of conservation practices, with the exception of cover crops. This information is essential for conservation practitioners who need to work with a diverse range of agricultural producers to increase the uptake of conservation practices that address critical water quality concerns.  相似文献   
233.
Past research has characterized countries as displaying the traits of urban or rural bias. Neither concept fits the United States well. We propose, as a hypothesis for research, that it may better be understood as displaying a suburban bias vis-à-vis both urban and rural populations. Drawing on the urban and rural bias literatures, we discuss two forms that suburban bias might take, allocational and dispositional, and the ways in which they might be identified. We offer initial evidence of a prevailing suburban bias in the United States in two spheres, those of judicial interpretation and American planning history, and conclude with suggestions for further research on the hypothesis.  相似文献   
234.
Collaborative online research offers opportunities and constraints for geographers. This article critically appraises a collaborative research process that we used to illuminate spatial and political dynamics of feminism contained within the online group ‘Destroy the Joint’ (DTJ). A mostly Australian initiative of over 74 000 Facebook members, DTJ aims to end sexism and misogyny in multiple ways. It operates as a meeting place, discussion forum, and umbrella organisation for numerous micro-campaigns that change in response to broader social, cultural and political contexts, and occur in online and offline spaces. We formed a collaborative research agreement with the moderators of DTJ to reflect on its work, asking participants to put themselves on a map and complete a survey. Participatory GIS and survey-based research operated in the real and the more-than-real spaces that contain paradoxical possibilities. We use the term more-than-real to highlight the excesses of digital spaces: the affect that social media generates, and is generated by, characterises the more-than-real, where extremes in productive and corrosive relations can permeate. Survey results showed diverse appreciations of DTJ's multiple activist tactics (across seemingly ‘superficial’ and ‘meaningful’ interventions), and creative ideas for future campaigns. Mapping revealed both dispersed networks and urban activist concentrations. Nevertheless, after an initial peak of interest and enthusiasm for reflecting on DTJ, the reporting back of research findings to DTJ followers did not garner significant interest. This article tracks that collaborative research trajectory. Intentions to build a hybrid research collective were not realised because of how the more-than-real affords possibilities and limitations. The politics of ‘giving-back’ within the more-than-real are critically engaged with in this appraisal of an experimental online collaboration.  相似文献   
235.
Kelman  I.  Gaillard  J. C.  Lewis  James  Mercer  Jessica 《Natural Hazards》2016,83(1):129-147
This paper calculated the embodied carbon in China export and its distribution in each industrial sector. The calculation results showed the total carbon emission of export experienced an increase before 2007 and then a decrease till 2010. The ratio of export embodied carbon accounting for the total carbon emission in China also increased from 31 % in 2002 to 52 % in 2007 and then declined to 40 % in 2010. As for distribution, the export embodied carbon emission of the following industries such as machinery and equipment manufacturing, metal products manufacturing industry, chemical industry, textile, clothing and leather products manufacturing industry ranked the highest. According to the calculation and analysis of the main driving factors of embodied carbon growth, we analyzed the structure effect, scale effect and technological effect’s influence on embodied carbon growth, respectively. We also calculated the trade competitiveness index of 17 export industries. Through research, we found that the products with strong international competitiveness belong to high-carbon-emission categories, which was the main reason of high carbon emission in China export. Finally, we proposed related policy suggestions to promote China’s export trade structural readjustment and optimization and China’s low carbon development in export.  相似文献   
236.

De Beers kimberlite mine operations in South Africa (Venetia and Voorspoed) and Canada (Gahcho Kué, Victor, and Snap Lake) have the potential to sequester carbon dioxide (CO2) through weathering of kimberlite mine tailings, which can store carbon in secondary carbonate minerals (mineral carbonation). Carbonation of ca. 4.7 to 24.0 wt% (average = 13.8 wt%) of annual processed kimberlite production could offset 100% of each mine site’s carbon dioxide equivalent (CO2e) emissions. Minerals of particular interest for reactivity with atmospheric or waste CO2 from energy production include serpentine minerals, olivine (forsterite), brucite, and smectite. The most abundant minerals, such as serpentine polymorphs, provide the bulk of the carbonation potential. However, the detection of minor amounts of highly reactive brucite in tailings from Victor, as well as the likely presence of brucite at Venetia, Gahcho Kué, and Snap Lake, is also important for the mineral carbonation potential of the mine sites.

  相似文献   
237.
GWML2 is an international standard for the online exchange of groundwater data that addresses the problem of data heterogeneity. This problem makes groundwater data hard to find and use because the data are diversely structured and fragmented into numerous data silos. Overcoming data heterogeneity requires a common data format; however, until the development of GWML2, an appropriate international standard has been lacking. GWML2 represents key hydrogeological entities such as aquifers and water wells, as well as related measurements and groundwater flows. It is developed and tested by an international consortium of groundwater data providers from North America, Europe, and Australasia, and facilitates many forms of data exchange, information representation, and the development of online web portals and tools.  相似文献   
238.
The importance of key taxonomic groups of eukaryotic phytoplankton as contributors to downward particle flux was evaluated near the European time-series station (ESTOC) in the eastern subtropical North Atlantic in March 2005. For the first time, molecular (cloning and sequencing) techniques were used to compare plankton communities from various depths in the euphotic zone with clone libraries from surface-tethered particle traps. Analyses of 18S rDNA clone libraries revealed compositional differences between the phytoplankton assemblages in the photic zone and those retrieved from shallow particle traps below, suggesting that not all phytoplankton contribute equally to particle flux. Contrary to expectations, our study also showed that it is not diatoms, despite their high abundance in the water column, but rather small phytoplankton taxa that dominated sequences recovered from trap material. We provide here first observational evidence that large taxa with mineral tests may not necessarily contribute more to export production than do smaller taxa even if the former are abundant in the water column.  相似文献   
239.
Both Atlantic and Indian Ocean climate dynamics exert influence over tropical African hydroclimate, producing complex patterns of convergence and precipitation. To isolate the Indian Ocean influence on African paleohydrology, we analyzed the deuterium/hydrogen ratio of higher plant leaf waxes (δDwax) in a 25 000-year sediment record from Lake Challa (3° S, 38° E) in the easternmost area of the African tropics. Whereas both the seismic record of inferred lake level fluctuations and the Branched and Isoprenoidal Tetraether (BIT) index proxy record changes in hydrology within the Challa basin, δDwax, as a proxy for the isotopic composition of precipitation (δDP) is interpreted as a tracer of large-scale atmospheric circulation that integrates the history of the moisture transported to the Lake Challa area. Specifically, based on modern-day isotope–rainfall relationships, we argue that Lake Challa δDwax reflects the intensity of East African monsoon circulation. The three hydrological proxy records show generally similar trends for the last 25 000 years, but there are important differences between them, primarily during the middle Holocene. We interpret this deviation of δDwax from local hydrological history as a decoupling of East African monsoon intensity – which heavily influences the isotopes of precipitation in East Africa today – from rainfall amount in the Challa basin. In combination, the hydrological proxy data from Lake Challa singularly highlight zonal gradients in tropical African climate that occur over a variety of timescales, suggesting that the Congo Air Boundary plays a fundamental role in controlling hydroclimate in the African tropics.  相似文献   
240.
Mapping past vegetation dynamics from heterogeneous databases of fossil-pollen records must face the challenge of temporal uncertainty. The growing collection of densely sampled fossil-pollen records with accurate and precise chronologies allows us to develop new methods to assess and reduce this uncertainty. Here, we test our methods in the context of vegetation changes in eastern North America during the abrupt climate changes of the last deglaciation. We use the network of fossil-pollen records in the Neotoma Paleoecology Database (www.neotomadb.org) and data contributed by individual investigators. Because many of these records were collected decades before the current generation of 14C and age-model technologies, we first developed a framework to assess the overall reliability of 14C chronologies by systematically evaluating individual 14C ages and associated chronologies. We developed a qualitative ranking scheme for individual 14C ages that combines information about their accuracy and precision. ‘Benchmark’ pollen records were defined to have at least one 14C age with an accuracy within 250 years and a precision less than 500 years that is within 1000 years of the time interval of interest, and at least five pollen samples per 1000 years across this time period. Only 22 of >350 late-Pleistocene pollen cores in eastern North America met the benchmark criteria.We then used Bayesian change-point analysis to identify widespread ecological events (Picea decline, Quercus rise, and Alnus decline), and interpolated the ages of these events from the benchmark sites to non-benchmark sites. Leave-one-out cross-validation analyses with the benchmark sites indicated that the spatial error associated with interpolation was less for inverse distance-weighting (IDW) than thin-plate splines (TPS) and was about 500 years for the three biotic events. By comparison, the difference between the original ages of events at poorly constrained sites and the biostratigraphic ages interpolated from the benchmark sites was close to 1000 years, suggesting that the use of biostratigraphic ages can significantly improve the age models for poorly constrained sites. Overall, these analyses suggest that the temporal resolution of multi-site syntheses of late-Pleistocene fossil-pollen data in eastern North America is about 500 years, a resolution that allows analysis of ecological responses to millennial-scale climate change during the last deglaciation.  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