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   1586篇
  免费   46篇
  国内免费   49篇
测绘学   22篇
大气科学   140篇
地球物理   428篇
地质学   395篇
海洋学   307篇
天文学   249篇
综合类   31篇
自然地理   109篇
  2022年   13篇
  2021年   19篇
  2020年   30篇
  2019年   28篇
  2018年   47篇
  2017年   51篇
  2016年   68篇
  2015年   46篇
  2014年   76篇
  2013年   105篇
  2012年   64篇
  2011年   94篇
  2010年   85篇
  2009年   82篇
  2008年   93篇
  2007年   81篇
  2006年   53篇
  2005年   50篇
  2004年   35篇
  2003年   37篇
  2002年   46篇
  2001年   19篇
  2000年   35篇
  1999年   34篇
  1998年   20篇
  1997年   20篇
  1996年   13篇
  1995年   20篇
  1994年   11篇
  1993年   15篇
  1992年   15篇
  1991年   11篇
  1990年   8篇
  1989年   12篇
  1988年   10篇
  1987年   10篇
  1986年   8篇
  1985年   14篇
  1984年   10篇
  1983年   14篇
  1981年   10篇
  1980年   15篇
  1979年   12篇
  1978年   11篇
  1977年   15篇
  1976年   12篇
  1975年   10篇
  1974年   8篇
  1973年   9篇
  1907年   7篇
排序方式: 共有1681条查询结果,搜索用时 6 毫秒
101.
Despite remaining uncertainties, Reducing Emissions from Deforestation and forest Degradation in developing countries (REDD) projects are being planned and implemented across the tropics, primarily targeting countries with high forest cover and high deforestation rates. However, there is growing recognition that REDD planning requires a broadened approach; a future REDD mechanism should incentivise emissions reduction in all developing forested countries, and should address critical non-carbon dimensions of REDD implementation—quality of forest governance, conservation priorities, local rights and tenure frameworks, and sub-national project potential. When considering this broader suite of factors, different REDD priorities can emerge, including in countries with low forest cover that would be overlooked by conventional site selection criteria. Using the Philippines as a case study, the paper highlights the importance of an enabling environment to REDD implementation, and presents a more comprehensive and inclusive approach for thinking about what comprises a “REDD country.”  相似文献   
102.
Summary Freeze thaw cycles are examined in Toronto Canada. Using data from 1960 to 1989 for three Toronto area weather stations, trends in freeze thaw activity, the relationship to mean monthly temperature and projections of freeze thaw activity are examined. For downtown Toronto the annual frequency of freeze thaw cycles is decreasing significantly, most notably in the shoulder months of October and April. At the Pearson International Airport and the Toronto Island Airport similar annual trends were not found, however there was evidence of decreased freeze thaw activity in April and October. Polynomial curve fitting provided functional relationships between mean monthly temperature and freeze thaw activity. These relationships enabled the assessment of freeze thaw activity under synthetic warming conditions. The results of this analysis show that the warming of the magnitude typically projected for the rest of this century will not likely generate a significant change in the freeze thaw activity although there are indications that the freeze thaw season will contract.  相似文献   
103.
This paper formally introduces the concept of mitigation as a stochastic control problem. This is illustrated by applying a digital state variable feedback control approach known as Non-Minimum State Space (NMSS) control to the problem of specifying carbon emissions to control atmospheric CO2 concentrations in the presence of uncertainty. It is shown that the control approach naturally lends itself to integrating both anticipatory and reflexive mitigation strategies within a single unified framework. The framework explicitly considers the closed-loop nature of climate mitigation, and employs a policy orientated optimisation procedure to specify the properties of this closed-loop system. The product of this exercise is a control law that is suitably conditioned to regulate atmospheric CO2 concentrations through assimilating online information within a 25-year review cycle framework. It is shown that the optimal control law is also robust when faced with significant levels of uncertainty about the functioning of the global carbon cycle.  相似文献   
104.
We present the first climate prediction of the coming decade made with multiple models, initialized with prior observations. This prediction accrues from an international activity to exchange decadal predictions in near real-time, in order to assess differences and similarities, provide a consensus view to prevent over-confidence in forecasts from any single model, and establish current collective capability. We stress that the forecast is experimental, since the skill of the multi-model system is as yet unknown. Nevertheless, the forecast systems used here are based on models that have undergone rigorous evaluation and individually have been evaluated for forecast skill. Moreover, it is important to publish forecasts to enable open evaluation, and to provide a focus on climate change in the coming decade. Initialized forecasts of the year 2011 agree well with observations, with a pattern correlation of 0.62 compared to 0.31 for uninitialized projections. In particular, the forecast correctly predicted La Niña in the Pacific, and warm conditions in the north Atlantic and USA. A similar pattern is predicted for 2012 but with a weaker La Niña. Indices of Atlantic multi-decadal variability and Pacific decadal variability show no signal beyond climatology after 2015, while temperature in the Niño3 region is predicted to warm slightly by about 0.5 °C over the coming decade. However, uncertainties are large for individual years and initialization has little impact beyond the first 4 years in most regions. Relative to uninitialized forecasts, initialized forecasts are significantly warmer in the north Atlantic sub-polar gyre and cooler in the north Pacific throughout the decade. They are also significantly cooler in the global average and over most land and ocean regions out to several years ahead. However, in the absence of volcanic eruptions, global temperature is predicted to continue to rise, with each year from 2013 onwards having a 50 % chance of exceeding the current observed record. Verification of these forecasts will provide an important opportunity to test the performance of models and our understanding and knowledge of the drivers of climate change.  相似文献   
105.
The effects of the northeastern Eurasian snow cover on the frequency of spring dust storms in northwestern China have been examined for the period 1979–2007. Averaged over all 43 stations in northwestern China, a statistically significant relationship has been found between dust-storm frequency (DSF) and Eurasian snow-water equivalent (SWE) during spring: mean DSF of 7.4 and 3.3 days for years of high- and low SWE, respectively. Further analyses reveal that positive SWE anomalies enhance the meridional gradients of the lower tropospheric temperatures and geopotential heights, thereby strengthening westerly jets and zonal wind shear over northwestern China and western Inner Mongolia of China, the regions of major dust sources. The anomalous atmospheric circulation corresponding to the Eurasian SWE anomalies either reinforces or weakens atmospheric baroclinicity and cyclogenesis, according to the sign of the anomaly, to affect the spring DSF. This study suggests that Eurasian SWE anomalies can be an influential factor of spring DSF in northwestern China and western Inner Mongolia of China.  相似文献   
106.
107.
Field survey of the 1994 Mindoro Island,Philippines tsunami   总被引:2,自引:0,他引:2  
This is a report of the field survey of the November 15, 1994 Mindoro Island, Philippines, tsunami generated by an earthquake (M=7.0) with a strike-slip motion. We will report runup heights from 54 locations on Luzon, Mindoro and other smaller islands in the Cape Verde passage between Mindoro and Luzon. Most of the damage was concentrated along the northern coast of Mindoro. Runup height distribution ranged 3–4 m at the most severely damaged areas and 2–4 in neighboring areas. The tsunami-affected area was limited to within 10 km of the epicenter. The largest recorded runup value of 7.3 m was measured on the southwestern coast of Baco Island while a runup of 6.1 m was detected on its northern coastline. The earthquake and tsunami killed 62 people, injured 248 and destroyed 800 houses. As observed in other recent tsunami disasters, most of the casualties were children. Nearly all eyewitnesses interviewed described the first wave as a leading-depression wave. Eyewitnesses reported that the main direction of tsunami propagation was SW in Subaang Bay, SE in Wawa and Calapan, NE on Baco Island and N on Verde Island, suggesting that the tsunami source area was in the southern Pass of Verde Island and that the wave propagated rapidly in all directions. The fault plane extended offshore to the N of Mindoro Island, with its rupture originating S of Verde Island and propagating almost directly south to the inland of Mindoro, thereby accounting for the relatively limited damage area observed on the N of Mindoro.  相似文献   
108.
K台网简介     
前言K-NET(KyoshinNet)是1995年兵库县南部地震后,科学技术厅防灾科学技术研究所在全国1000个地点(现为1001个地点)布设的强震仪台网。由于是在全日本以约25km的间隔布设同一规格的加速度型强震仪,故能进行非常均匀的观测。该台网是...  相似文献   
109.
Multiple Discriminant Analysis of Marine Sediment Data   总被引:2,自引:0,他引:2  
A multivariate statistical approach was used in the analysis of inter-relationships among marine benthic infaunal structure, physical and chemical characteristics of sediment samples and toxicity data derived from laboratory tests at 16 locations in shallow, inshore waters of Hong Kong. The method involved classification (cluster analysis using the Bray–Curtis similarity index) and ordination (multi-dimensional scaling) of infaunal patterns and the use of multiple discriminant analysis to relate groupings of the locations to the selected environmental and toxicity data. Analysis of the combined sediment physical, biological, chemical and toxicity dataset by stepwise multiple discriminant analysis allowed identification of those variables most sensitive for discriminating location groups. The use of multiple discriminant analysis in sediment quality characterization was evaluated against the Sediment Quality Triad approach and other statistical techniques.  相似文献   
110.
Preferential flow in soils deserves attention due to its potential role in accelerating the movement of contaminants to groundwater. This study investigates the movement of Cd, Cu, and Pb through preferential flow paths under different applied rainfall intensities. Artificial acid rain (pH of 4.1) containing CdCl2, CuCl2, and Pb(NO3)2 was applied to undisturbed soil and repacked sand columns at low and high intensities, and leachate metals and chloride concentrations were measured. Cd was found in the leachate at both low and high rates in all columns, while the increase in Cu concentrations in the leachate was detected only at the high rate of the undisturbed columns. Pb was retained in both columns. For undisturbed columns, the breakthrough curves of Cd and Cu were similar to those of Cl, showing early initial breakthrough by preferential flow and dependency on rainfall intensities. The Cd concentrations were detected in the leachate from repacked columns for high rate rainfall, implying that even homogeneous soil may not be perfectly able to retain metals and the initially wet condition is more harmful for subsurface contaminant transport. In conclusion, the study demonstrated that, despite its highly sorptive nature, the transport of some metals may be as fast as that of a tracer under preferential flow conditions, and the rainfall intensity is a significant factor for the degree of transport.  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