全文获取类型
收费全文 | 6443篇 |
免费 | 282篇 |
国内免费 | 81篇 |
专业分类
测绘学 | 235篇 |
大气科学 | 512篇 |
地球物理 | 1522篇 |
地质学 | 2161篇 |
海洋学 | 525篇 |
天文学 | 1177篇 |
综合类 | 28篇 |
自然地理 | 646篇 |
出版年
2022年 | 30篇 |
2021年 | 76篇 |
2020年 | 87篇 |
2019年 | 122篇 |
2018年 | 178篇 |
2017年 | 161篇 |
2016年 | 231篇 |
2015年 | 173篇 |
2014年 | 196篇 |
2013年 | 407篇 |
2012年 | 257篇 |
2011年 | 342篇 |
2010年 | 291篇 |
2009年 | 381篇 |
2008年 | 336篇 |
2007年 | 291篇 |
2006年 | 269篇 |
2005年 | 260篇 |
2004年 | 261篇 |
2003年 | 205篇 |
2002年 | 214篇 |
2001年 | 115篇 |
2000年 | 144篇 |
1999年 | 102篇 |
1998年 | 114篇 |
1997年 | 81篇 |
1996年 | 85篇 |
1995年 | 83篇 |
1994年 | 90篇 |
1993年 | 74篇 |
1992年 | 88篇 |
1991年 | 67篇 |
1990年 | 55篇 |
1989年 | 50篇 |
1988年 | 58篇 |
1987年 | 52篇 |
1986年 | 56篇 |
1985年 | 67篇 |
1984年 | 64篇 |
1983年 | 76篇 |
1982年 | 57篇 |
1981年 | 65篇 |
1980年 | 55篇 |
1979年 | 65篇 |
1978年 | 51篇 |
1977年 | 34篇 |
1976年 | 28篇 |
1975年 | 26篇 |
1974年 | 26篇 |
1973年 | 30篇 |
排序方式: 共有6806条查询结果,搜索用时 15 毫秒
541.
Geoengineering: Worthy of Cautious Evaluation? 总被引:1,自引:0,他引:1
Michael C. MacCracken 《Climatic change》2006,77(3-4):235-243
542.
Using paleoclimate proxy-data to select optimal realisations in an ensemble of simulations of the climate of the past millennium 总被引:1,自引:0,他引:1
Hugues Goosse Hans Renssen Axel Timmermann Raymond S. Bradley Michael E. Mann 《Climate Dynamics》2006,27(2-3):165-184
We present and describe in detail the advantages and limitations of a technique that combines in an optimal way model results and proxy-data time series in order to obtain states of the climate system consistent with model physics, reconstruction of past radiative forcing and proxy records. To achieve this goal, we select among an ensemble of simulations covering the last millennium performed with a low-resolution 3-D climate model the ones that minimise a cost function. This cost function measures the misfit between model results and proxy records. In the framework of the tests performed here, an ensemble of 30 to 40 simulations appears sufficient to reach reasonable correlations between model results and reconstructions, in configurations for which a small amount of data is available as well as in data-rich areas. Preliminary applications of the technique show that it can be used to provide reconstructions of past large-scale temperature changes, complementary to the ones obtained by statistical methods. Furthermore, as model results include a representation of atmospheric and oceanic circulations, it can be used to provide insights into some amplification mechanisms responsible for past temperature changes. On the other hand, if the number of proxy records is too low, it could not be used to provide reconstructions of past changes at a regional scale. 相似文献
543.
Thara V. Prabha Anandakumar Karipot Michael W. Binford 《Boundary-Layer Meteorology》2007,123(2):239-261
Large-eddy simulation is used to study secondary circulations in the convective boundary layer modulated as a result of horizontally
varying surface properties and surface heat fluxes over flat terrain. The presence of heat flux heterogeneity and its alignment
with respect to geostrophic wind influences the formation, strength and orientation of organized thermals. Results show boundary-attached
roll formation along heat flux maxima in the streamwise direction. The streamwise organization of the updrafts and downdrafts
formed downwind of heterogeneities leads to counter-rotating secondary circulations in the crosswind plane. The distribution
of resolved-scale pressure deviations shows large pressure gradients in the crosswind plane. Spanwise and vertical velocity
variances and heat flux profiles depict considerable spatial variability compared to a homogeneous forest simulation. Secondary
circulations are observed for various ambient wind scenarios parallel and perpendicular to heterogeneities. In the presence
of increased wind speed, thermals emerging from the heat flux heterogeneity are elongated, and organize along and downwind
of large-scale heterogeneity in the streamwise direction. Simulation with a reduced heat flux shows a shallower circulation
with a lower aspect ratio. Point measurements of heat flux inside the roll circulation could be overestimated by up to 15–25%
compared to a homogeneous case. 相似文献
544.
Michael A. Wulder Trisalyn A. Nelson Chris Derksen David Seemann 《Climatic change》2007,82(1-2):113-130
Twenty-four winter seasons (1978–2002) of mean February snow water equivalent (SWE) values were analyzed in an exploration of the spatial pattern of temporal variability in snow cover across the non-mountainous interior of Canada. The SWE data were derived from space-borne passive microwave brightness temperatures processed with a land cover-sensitive suite of algorithms. Spatial patterns in the frequency and amount of variability were investigated on an annual basis through comparisons with average trends over all 24 years. Changes in temporal variability through time were also investigated by comparing three eight year time periods to general trends. Analyses were synthesized at the ecozone scale in order to link results both to potential land cover influences on algorithm performance and climatological variability in SWE. Prairie and northern ecozones were typically found to be the most variable in terms of SWE magnitude. Analyses indicate that non-treed land cover classes are generally more variable than treed classes. The results also indicate that extreme weather events appear to be occurring with increasing consistency in the Prairie and Arctic regions. Discerning climatologically significant variability in the time series, compared to algorithm-related issues can be a challenge, but in an era of eroding surface observing networks the passive microwave time series represents an important resource for monitoring and detecting trends and variability in terrestrial snow cover. 相似文献
545.
Michael C. Murrell James D. Hagy Emile M. Lores Richard M. Greene 《Estuaries and Coasts》2007,30(3):390-402
The relationships between phytoplankton productivity, nutrient distributions, and freshwater flow were examined in a seasonal
study conducted in Escambia Bay, Florida, USA, located in the northeastern Gulf of Mexico. Five sites oriented along the salinity
gradient were sampled 24 times over the 28-mo period from 1999 to 2001. Water column profiles of temperature and salinity
were measured along with surface chlorophyll and surface inorganic nutrient concentrations. Primary productivity was measured
at 2 sites on 11 dates, and estimated for the remaining dates and sites using an empirical regression model relating phytoplankton
net production to the product of chlorophyll, euphotic zone depth, and daily solar insolation. Freshwater flow into the system
varied markedly over the study period with record low flow during 2000, a flood event in March 2001, and subsequent resumption
of normal flow. Flushing times ranged from 1 d during the flood to 20 d during the drought. Freshwater input strongly affected
surface salinity distributions, nutrient flux, chlorophyll, and primary productivity. The flood caused high turbidity and
rapid flushing, severely reducing phytoplankton production and biomass accumulation. Following the flood, phytoplankton biomass
and productivity sharply increased. Analysis of nutrient distributions suggested Escambia Bay phytoplankton alternated between
phosphorus limitation during normal flow and nitrogen limitation during low flow periods. This study found that Escambia Bay
is a moderately productive estuary, with an average annual integrated phytoplankton production rate of 290 g C m−2 yr−1. 相似文献
546.
Ben K. Greenfield Geoffrey S. Siemering Joy C. Andrews Michael Rajan Stephen P. Andrews David F. Spencer 《Estuaries and Coasts》2007,30(4):627-640
Management actions to control invasive aquatic species can have significant ecosystem-scale effects. We evaluated the water
chemistry and nutrient effects of mechanical shredding to control water hyacinth (Eichhornia crassipes) in an agricultural slough and a tidal wetland on the Sacramento-San Joaquin River Delta, California. Shredding was conducted
with two types of shredder boats in fall of 2003 and another boat in spring of 2004. Shredding measurably affected water quality,
but specific effects varied as a function of shredding site and season. Significant increases were observed for total Kjeldahl
nitrogen and total phosphorus for all experiments. Dissolved oxygen effects varied by site, decreasing after shredding at
the agricultural slough but increasing at the tidal wetland. The increase in dissolved oxygen likely resulted from tidal incursions
from the adjacent river. A year-long time series of dissolved oxygen data indicated a negative relationship between hyacinth
abundance and dissolved oxygen concentrations. Hyacinth contained similar tissue concentrations of mercury to underlying sediments,
suggesting that plant harvesting could aid mercury remediation efforts. Simple mass calculations indicated that Delta-wide
shredding operations could cause between 0.1% and 9.6% increases in the overall abundance of carbon, nitrogen, and phosphorus
in the Delta water column. Results suggest that local effects of management actions to control invasive aquatic plants will
vary widely as a function of site-specific hydrology, but that estuary-wide effects would be limited. 相似文献
547.
Jean S. Kane Philip J. Potts Thomas Meisel Michael Wiedenbeck 《Geostandards and Geoanalytical Research》2007,31(3):285-288
The International Association of Geoanalysts (IAG) published a protocol for the certification of reference materials in close accord with the International Organisation for Standardisation (ISO) guidelines (Kane et al. 2003). This article supplements that protocol, providing additional discussion of best approaches for pre-selecting laboratories for participation in certification projects. This discussion also makes a distinction between inter-laboratory certifications, where n = 15 is the general standard, and expert laboratory certifications, where a much smaller number of laboratories will be deemed qualified to provide data of the quality needed for certification. 相似文献
548.
549.
550.
Theo van Leeuwen Charlotte M. Allen Ade Kadarusman Marlina Elburg J. Michael Palin Muhardjo Suwijanto 《Journal of Asian Earth Sciences》2007,29(5-6):751-777
The Malino Metamorphic Complex (MMC) is located at the western end of the north arm of Sulawesi. It consists of mica schists and gneisses (derived from proximal turbidite and granitoid protoliths), with intercalations of greenschist, amphibolite, marble, and quartzite, forming an E-W elongated dome-like structure bounded on all sides by faults. The age of the MMC is constrained between Devonian and Early Carboniferous. This Paleozoic age, the presence of Archean and Proterozoic inherited zircons, and the isotopic signature of the mica schists and gneisses indicate that the terrane was derived from the New Guinea-Australian margin of Gondwana. Similarities with basement rocks in the Bird’s Head suggests a common origin. Greenschists forming a discontinuous selvage (metamorphic carapace) around the complex were derived from adjacent autochthonous Paleogene formations. The rocks of the MMC show a Barrovian-type progression from greenschist through epidote-amphibolite to amphibolite facies. P–T estimations suggest a depth of burial of up to 27–30 km. K/Ar and 40Ar/39Ar cooling ages of 23–11 Ma, and a 7 Ma age for unconformably overlying volcanic rocks, indicate that the complex was exhumed during the Miocene. Two tectonic scenarios are considered: 1. the continental fragment docked with Sulawesi during the Mesozoic and was exhumed as a metamorphic core complex during the Miocene; 2. it was subducted beneath the north arm during the late Oligocene and then rapidly returned back to the surface. 相似文献