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   182篇
  免费   11篇
测绘学   7篇
大气科学   10篇
地球物理   58篇
地质学   52篇
海洋学   16篇
天文学   25篇
自然地理   25篇
  2022年   2篇
  2021年   8篇
  2020年   3篇
  2019年   4篇
  2018年   7篇
  2017年   9篇
  2016年   10篇
  2015年   5篇
  2014年   8篇
  2013年   11篇
  2012年   5篇
  2011年   11篇
  2010年   6篇
  2009年   11篇
  2008年   7篇
  2007年   10篇
  2006年   8篇
  2005年   5篇
  2004年   8篇
  2003年   8篇
  2002年   8篇
  2001年   2篇
  2000年   2篇
  1999年   1篇
  1998年   4篇
  1997年   1篇
  1995年   2篇
  1994年   2篇
  1988年   2篇
  1987年   2篇
  1986年   2篇
  1985年   1篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1981年   2篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1976年   2篇
  1974年   1篇
  1968年   1篇
  1956年   1篇
  1949年   2篇
排序方式: 共有193条查询结果,搜索用时 15 毫秒
101.
As climate change has driven dramatic changes in Northern sea ice regimes, marine mammals have gained iconic status around the world reflecting the perils of global warming. There is a tension between policies that have international support like a ban on seal hunting or whaling, and the adoption of adaptive, flexible rules that are likely to work in Northern places. Whereas most wildlife policy focuses on biological information to inform policy strategy, this analysis focuses on the “human dimensions” of Northern marine mammal management. This research examines ways in which human relationships and modes of governance affect conservation success. Standard analyses of risk to animal populations focused on direct sources of take are inadequate to address multi-causal, complex problems such as climate-induced habitat loss or increased industrialization of the Arctic Ocean. Early conservation policy strategies focusing on the moratorium of take have eliminated or reduced such practices as commercialized hunting and high levels of fisheries bycatch, but may be less relevant in an era in which habitats and climate changes are key drivers of population dynamics. This paper argues that effective adaptive policy requires new ways of learning about and governing human interactions with marine mammals. Through an exploration of marine mammal management in three Northern regions (Alaska, Nunavut, and the Finnish Baltic Sea coast), the paper analyzes the extent to which these marine mammal management regimes are practicing adaptive governance, that is, building cross-scale (local to international) understanding while allowing actors at the local scale the flexibility to direct the creation of rules that are ecologically robust and likely to succeed. Lessons are taken from these examples and used to propose selected policy and research recommendations for the marine mammal policy community.  相似文献   
102.
Climate change and land degradation result in decreasing yields and crop failures in Northern Ghana and have caused further impoverishment of Ghana’s poorest region. Farmers have diversified their livelihoods to adapt to uncertain environmental conditions in various ways. While traditionally a diversification of the production and migration were the prime means of adaptation, many farmers have started to intensify their production by adopting shallow groundwater irrigation for vegetable gardening for Ghana’s urban markets. This has helped to cope with a changing environment, ameliorated poverty and reversed rural–urban migration, while the local hydrology curbed an over-exploitation of groundwater resources, commonly associated with an uncontrolled farmer-driven expansion of groundwater irrigation. This research confirms that farmer-driven small-scale irrigation can play an important role in the process of climate change adaptation. However, while farmers tried to integrate in the larger economy, they have become subject to market failures that in their essence are caused by unfair and unpredictable patterns of global trade. It is this double exposure to global environmental change and economic globalization that need to be taken into consideration when local adaptive capacities are discussed. Many convincing arguments call for the revision of some of the most unfair and devastating economic practices; however, the need to enhance adaptive capacity towards global climate change for poor parts of the population in the south should be added to the discussion.  相似文献   
103.
The problem of oblique water wave diffraction by small undulation of the bottom of a laterally unbounded ocean is considered using linear water wave theory. A perturbation analysis is employed to obtain the velocity potential, the reflection and the transmission coefficients up to the first order in terms of integrals involving the shape functions c(x) representing the bottom undulation. Finite cosine transform is used to find the first order potential, and this potential is utilised in obtaining the first order reflection and transmission coefficients. Some particular forms of the shape function representing an exponentially damped undulation, a single hump and a patch of sinusoidal ripples are considered and the integrals for the reflection and transmission coefficients are evaluated. For the exponentially damped undulation, it is observed that the reflection ceases much before transmission while for the single hump, reflection and transmission go hand in hand up to a certain value of the wavenumber, after which they vanish. For the patch of sinusoidal ripples having the same wavenumber, the reflection coefficient up to the first order is found to be an oscillatory function in the quotient of twice the component of the wavenumber along x-axis and the ripple wavenumber. When this quotient becomes one, the theory predicts a resonant interaction between the bed and free surface, and the reflection coefficient becomes a multiple of the number of ripples. High reflection of the incident wave energy occurs if this number is large. Also, when a patch of ripples having different wavenumbers is considered the same result follows. Known results for the normal incidence are recovered as special cases for the patch of sinusoidal ripples. The theoretical observations are shown computationally.  相似文献   
104.
105.
Comet Bowell (1982I) was observed at JHK (1.25, 1.68, 2.25 μm) with the NASA Infrared Telescope Facility on 10 March 1981, 3–5 June 1981, and 22 February 1982. The inner coma was no brighter near perihelion in February 1982 than it was in 1981. Total are × albedo of the grains within 104 km of the nucleus was ~5 × 1012cm2.  相似文献   
106.
Martha S. Hanner 《Icarus》1981,47(3):342-350
Evaporation of icy grains over the distance scale of the visible cometary coma sets very specific limits on their temperature. Unless the grains are very pure water ice, the maximum size of an icy grain halo will be limited to a few hundred kilometers at heliocentric distances ?2.5 AU. It is unlikely that the 1.5- or 2-μm ice band could be detected in the scattering by icy grains. Detection of the 3?μm ice band might be possible in comets which display a coma at large heliocentric distances.  相似文献   
107.
108.
Alaska is among the fastest warming places on Earth, and the Interior region is warming the most statewide. Significant regional-scale ecosystem services disruptions are affecting Alaska Natives’ subsistence hunting and harvest success. The well-being of rural native communities is still highly dependent on access and ability to harvest wild foods such as salmon and moose (Alces alces gigas) among many others. Over the last decade communities in the Koyukuk-Middle Yukon (KMY) region of Interior Alaska report an inability to satisfy their needs for harvesting moose before the hunting season closes, citing warmer falls, changing precipitation and water levels, and the regulatory framework as primary causes. Through the integration of ethnographic methods to record indigenous observations and understanding of climate (IC) with analysis of meteorological data, we provide a comprehensive picture of vulnerability to recent warming trends in the Koyukuk-Middle Yukon region of Interior Alaska, one that captures more than statistical analysis of “norms” can provide. We will demonstrate how low exposure resulting in a small shift in seasonality has truly socially significant effects to people “on the ground” when community sensitivity is high because of the convergence of multiple social-ecological stressors. In this case, a seemingly small climatic exposure when combined with high social-ecological system sensitivity results in vulnerability to this climate change-related seasonality shift because of: (a) the effects on moose and the social-ecological dynamics of the system, and (b) the importance of this time of the year to meeting annual subsistence needs.  相似文献   
109.
We review the results of HI line surveys of extragalactic sources in the local Universe. In the last two decades major efforts have been made in establishing on firm statistical grounds the properties of the HI source population, the two most prominent being the HI Parkes All Sky Survey and the Arecibo Legacy Fast ALFA survey. We review the choices of technical parameters in the design and optimization of spectro-photometric “blind” HI surveys, which for the first time produced extensive HI-selected data sets. Particular attention is given to the relationship between optical and HI populations, the differences in their clustering properties and the importance of HI-selected samples in contributing to the understanding of apparent conflicts between observation and theory on the abundance of low mass halos. The last section of this paper provides an overview of currently ongoing and planned surveys which will explore the cosmic evolution of properties of the HI population.  相似文献   
110.
A vast, ground-water-supported sedge fen in the Upper Peninsula of Michigan, USA was ditched in the early 1900 s in a failed attempt to promote agriculture. Dikes were later constructed to impound seasonal sheet surface flows for waterfowl management. The US Fish and Wildlife Service, which now manages the wetland as part of Seney National Wildlife Refuge, sought to redirect water flows from impounded C-3 Pool to reduce erosion in downstream Walsh Ditch, reduce ground-water losses into the ditch, and restore sheet flows of surface water to the peatland. A water budget was developed for C-3 Pool, which serves as the central receiving and distribution body for water in the affected wetland. Surface-water inflows and outflows were measured in associated ditches and natural creeks, ground-water flows were estimated using a network of wells and piezometers, and precipitation and evaporation/evapotranspiration components were estimated using local meteorological data. Water budgets for the 1999 springtime peak flow period and the 1999 water year were used to estimate required releases of water from C-3 Pool via outlets other than Walsh Ditch and to guide other restoration activities. Refuge managers subsequently used these results to guide restoration efforts, including construction of earthen dams in Walsh Ditch upslope from the pool to stop surface flow, installation of new water-control structures to redirect surface water to sheet flow and natural creek channels, planning seasonal releases from C-3 Pool to avoid erosion in natural channels, stopping flow in downslope Walsh Ditch to reduce erosion, and using constructed earthen dams and natural beaver dams to flood the ditch channel below C-3 Pool. Interactions between ground water and surface water are critical for maintaining ecosystem processes in many wetlands, and management actions directed at restoring either ground- or surface-water flow patterns often affect both of these components of the water budget. This approach could thus prove useful in guiding restoration efforts in many hydrologically altered and managed wetlands worldwide.  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