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   794篇
  免费   12篇
  国内免费   11篇
测绘学   12篇
大气科学   99篇
地球物理   187篇
地质学   229篇
海洋学   69篇
天文学   147篇
综合类   6篇
自然地理   68篇
  2021年   6篇
  2020年   7篇
  2019年   6篇
  2018年   7篇
  2017年   8篇
  2016年   14篇
  2015年   15篇
  2014年   17篇
  2013年   43篇
  2012年   28篇
  2011年   32篇
  2010年   28篇
  2009年   37篇
  2008年   60篇
  2007年   30篇
  2006年   26篇
  2005年   30篇
  2004年   31篇
  2003年   26篇
  2002年   32篇
  2001年   23篇
  2000年   17篇
  1999年   22篇
  1998年   20篇
  1997年   15篇
  1996年   15篇
  1995年   7篇
  1994年   14篇
  1993年   10篇
  1992年   8篇
  1991年   11篇
  1990年   9篇
  1989年   8篇
  1988年   8篇
  1987年   6篇
  1986年   9篇
  1985年   13篇
  1984年   7篇
  1983年   17篇
  1982年   13篇
  1981年   11篇
  1980年   9篇
  1979年   8篇
  1978年   4篇
  1977年   8篇
  1976年   6篇
  1975年   5篇
  1974年   5篇
  1973年   5篇
  1970年   3篇
排序方式: 共有817条查询结果,搜索用时 15 毫秒
31.
32.
Four policies might close the gap between the global GHG emissions expected for 2020 on the basis of current (2013) policies and the reduced emissions that will be needed if the long-term global temperature increase can be kept below the 2 °C internationally agreed limit. The four policies are (1) specific energy efficiency measures, (2) closure of the least-efficient coal-fired power plants, (3) minimizing methane emissions from upstream oil and gas production, and (4) accelerating the (partial) phase-out of subsidies to fossil-fuel consumption. In this article we test the hypothesis of the International Energy Agency (IEA) that these policies will not result in a loss of gross domestic product (GDP) and we estimate their employment effects using the E3MG global macro-econometric model. Using a set of scenarios we assess each policy individually and then consider the outcomes if all four policies were implemented simultaneously. We find that the policies are insufficient to close the emissions gap, with an overall emission reduction that is 30% less than that found by the IEA. World GDP is 0.5% higher in 2020, with about 6 million net jobs created by 2020 and unemployment reduced.

Policy relevance

The gap between GHG emissions expected under the Copenhagen and Cancun Agreements and that needed for emissions trajectories to have a reasonable chance of reaching the 2 °C target requires additional policies if it is to be closed. This article uses a global simulation model E3MG to analyse a set of policies proposed by the IEA to close the gap and assesses their macroeconomic effects as well as their feasibility in closing the gap. It complements the IEA assessment by estimating the GDP and employment implications separately by the different policies year by year to 2020, by major industries, and by 21 world regions.  相似文献   

33.
The concentrations of polycyclic aromatic hydrocarbons (PAHs) and trace elements were determined for surface (top 2 cm) sediment samples collected during the deep Gulf of Mexico benthos (DGoMB) study .These elements and compounds are known to be toxic to organisms at high concentrations and may affect biological communities. There is no indication of major anthropogenic input of the elements Be, Co, Cr, Fe, Si, Tl, V, K, Mg, Ca, Sr and Zn, based on normalization to Al. The concentrations of these metals in the sediment are a function of the relative amounts of trace-metal-rich Mississippi River-derived silicate material and trace-metal-poor plankton-derived carbonate. This is not true for the elements Ba, Ni, Pb, Cd, As, Cu and Mn, whose concentrations show considerable scatter when normalized to Al and a general enrichment. On a normalized basis, Mn is enriched 5–10 fold, Cu and Ni 2–3 fold and Pb 2 fold over Mississippi River-derived material. These enrichments are likely the result of remobilization of metals from depths in the sediment column where reducing conditions exist. The Ba concentrations at selected sites are higher than those of average clay-rich sediments, but are typical of sediments from near oil well platforms in the northern Gulf of Mexico. In the case of Ba, it seems likely that the enrichments, as high as a factor of 10, are due to disposal of oil well drilling mud. The Ba-enriched samples are from the three shallowest water sites in the Mississippi Trough (sites MT1, 2 and 3) and from site C1 and WC5. All are in an area of intense petroleum exploration and development. PAH concentrations are also elevated at MT1, MT3 and C1. The total PAH concentration ranged from not detected (ND) to 1033 ng/g with a mean of 140 ng/g. Even at the sites most enriched in PAHs and trace elements, the concentrations are not at the levels expected to adversely affect the biota. However, these predicted non-effects are based on research using mostly near-shore estuarine species, not on the indigenous species at the sampling sites.  相似文献   
34.
A series of enclosed ecosystem experiments were conducted in a land-based tank near the seaside of West Xiamen Harbor. The results of experiments conducted in different seasons and years showed a repeatable phytoplankton succession. In this relatively stable ecosystem with added nutrients and trace metals, diatoms dominated initially, dinoflagellates dominated in the later stage, and dinoflagellate red tides eventually occurred. Vitamin B12 enrichment may speed up this succession process. Stirring the water column could stop this process. Soluble Mn at a level of 3–4 μg/L in seawater, which also is the existing concentration of soluble Mn in Xiamen Harbor seawater, is sufficient for the multiplication of algae and occurrence of red tide. The present study showed that excessive soluble Mn in Xiamen Harbor cannot cause red tide, and that Fe was one of the important factors causing diatiom red tide in this present study. Project 39570145 supported by NSFC.  相似文献   
35.
1 Introduction The Gaojiacun intrusive complex is one of the numerous ultramafic-mafic intrusions in Sichuan Province of China. It was mapped during the 1970s and studied mainly by Chinese scientists (e.g. Geological Team 106, 1975; Shen et al., 1986, 1989; CGGJC, 1986; Yang et al., 1993; Li et al., 1995; Shen et al., 2003; Zhu et al., 2004a). Since the year 2000, China has become one of the largest PGE consumers. While the country can produce only less than 1 ton PGE/year, the Chin…  相似文献   
36.
The importance of the nitrogen source for phytoplankton growth in a highly eutrophic embayment, Dokai Bay, was investigated. The DIN concentration often exceeded 100 μM of which 40–70% was NH4 +. During two incubation experiments, the natural assemblage of mainly diatoms took up NH4 + instead of NO3 . The growth of two Skeletonema species isolated in Dokai Bay were significantly faster on NH4 + (1.86 and 1.27 div. d−1 respectively) than on NO3 (1.55 and 1.04 div. d−1 respectively). Our results indicated that these diatoms could grow faster by using NH4 + compared to NO3 in this eutrophic bay.  相似文献   
37.
全新世初期气候的数值模拟研究   总被引:2,自引:1,他引:1       下载免费PDF全文
本文尝试用一个完全耦合的大气-海洋-植被模式(Atmosphere-Ocean-Vegetation General Circulation Model, AOVGCM)模拟全新世初期的地球气候.模拟结果表明,耦合模式成功地再现了全新世初期复杂气候条件下的基本特征,为研究这一时期的气候状况提供了重要的模型支持.就全球平均而言,11ka BP冬季地表气温比现代约低1.6 K,夏季比现代低约0.3 K;大气温度从低层到高层有不同的表现,高层反映了太阳辐射的重要作用,而低层气候对下垫面(如冰川、植被和海洋等)的影响比较敏感.从区域分布来看,11 ka BP冬季大部地区比现在偏干,但热带太平洋和南半球少部地区降水偏多;夏季大部地区比现在偏湿,亚洲和非洲季风偏强,主要的季风区降水偏多.  相似文献   
38.
Changes in lake levels during the last 12000 years in eastern North America show spatially coherent patterns, implying climatic control. Conditions were generally wetter than today during the late glacial, becoming more arid towards 6000 years BP when most lakes were low. Lakes rose after 6000 years BP, reaching modern levels by about 3000 years BP. These palaeohydrological changes broadly agree with simulated changes in moisture balance derived from experiments with the NCAR Community Climate Model (Kutzbach and Guetter 1986) with changing orbital parameters and lower boundary conditions (sea-surface temperature and ice extent). However, the model simulates maximum aridity at 9000 years BP. Data and model show broadly similar spatial patterns, implying that the lake-level changes can be explained by the changing boundary conditions and their effects on atmospheric circulation. At 12000 years BP most lakes were high because of increased precipitation along the jet-stream storm-track south of the ice sheet. By 9000 years BP, with the much reduced ice sheet, many lakes along the eastern seaboard and in the southeast were lower than present because of greater evaporation due to high summer insolation. The warming of the continental interior generated an enhanced monsoon low in the southwest, causing increased southerly flow which helped to maintain higher lakes in the Midwest. Dry conditions spread eastwards across the Midwest between 9000 and 6000 years BP. This effect is not shown by the model, which continues to bring monsoonal precipitation into the Midwest while simulating enhanced westerly flow and drier conditions further to the west. Such displacements of circulation features are unimportant at the continental scale, but could be significant if general circulation models are used for regionalscale predictions of changes in the moisture balance.  相似文献   
39.
Argon diffusion domains in K-feldspar II: kinetic properties of MH-10   总被引:2,自引:0,他引:2  
Viewing K-feldspars as containing a discrete distribution of diffusion domain sizes reconciles otherwise disconsonant features common in their 40Ar/39Ar age spectra and Arrhenius plots but raises a fundamental question. What are the features in K-feldspar that endow it with this behavior? We report here the results of two different kinds of experiments that help isolate the nature of the responsible diffusion properties. To assess the thermal stability of the diffusion domains during laboratory treatment, MH-10 K-feldspar was step-heated to 850°C, removed from the furnace and split. One split was reirradiated and the other returned to the furnace and completely degassed. Following re-irradiation, the original heating schedule was used to degas the second aliquot. Apart from the first 5% of gas released, the diffusion properties show little change relative to the original result but, it appears, the physical character of a portion of the smallest domain has been altered. Results of duplicate step-heating experiments of samples treated at 750°C, 950°C and 1100°C prior to irradiation are consistent with the conclusions of the double irradiation experiment. In a second series of experiments, sized aggregates of MH-10 K-feldspar were analyzed by the 40Ar/39Ar step-heating method. The resultant log(r/r o) plots reveal that the largest domain is annihilated when the particle size is reduced to about 50 μm. From this result we infer that the largest diffusion domain size is between 60 and about 130 μm in diameter. This estimate, together with knowledge of the relative domain size distribution obtained from modeling the log(r/r o) plot, sets the size of the smallest domain to be less than about 1 μm. Microstructural examination of MH-10 K-feldspar identifies sub-grain features that correspond in size to our independent estimates for the largest and smallest diffusion domains. These results strongly support the view that low-temperature K-feldspars contain a distribution of diffusion length scales that are well approximated as discrete domain sizes and that laboratory heating below the onset of melting does not significatly affect the ability to obtain thermal reconstructions from the 40Ar/39Ar systematics.  相似文献   
40.
Laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults. We have conducted friction experiments on antigorite and lizardite serpentinites, rocks common to both continental and oceanic crustal faults. We conducted both velocity-step tests and timed-hold tests on bare surfaces and gouge layers of serpentinite at room temperature. We find that the coefficient of friction of lizardite serpentinite is quite low (0.15–0.35) and could explain the apparent low stresses observed on crustal transform faults, while that of antigorite serpentinite is comparable to other crustal rocks (0.50–0.85). The frictional behavior of both types of serpentinite is well described by a two-mechanism model combining state-variable-dominated behavior at high slip velocities and flow-dominated behavior at low velocities. The two-mechanism model is supported by data from velocity-step tests and timed-hold tests. The low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite.  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