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951.
Kirsten Zickfeld Anders Levermann M. Granger Morgan Till Kuhlbrodt Stefan Rahmstorf David W. Keith 《Climatic change》2007,82(3-4):235-265
We present results from detailed interviews with 12 leading climate scientists about the possible effects of global climate
change on the Atlantic Meridional Overturning Circulation (AMOC). The elicitation sought to examine the range of opinions
within the climatic research community about the physical processes that determine the current strength of the AMOC, its future
evolution in a changing climate and the consequences of potential AMOC changes. Experts assign different relative importance
to physical processes which determine the present-day strength of the AMOC as well as to forcing factors which determine its
future evolution under climate change. Many processes and factors deemed important are assessed as poorly known and insufficiently
represented in state-of-the-art climate models. All experts anticipate a weakening of the AMOC under scenarios of increase
of greenhouse gas concentrations. Two experts expect a permanent collapse of the AMOC as the most likely response under a
4×CO2 scenario. Assuming a global mean temperature increase in the year 2100 of 4 K, eight experts assess the probability of triggering
an AMOC collapse as significantly different from zero, three of them as larger than 40%. Elicited consequences of AMOC reduction
include strong changes in temperature, precipitation distribution and sea level in the North Atlantic area. It is expected
that an appropriately designed research program, with emphasis on long-term observations and coupled climate modeling, would
contribute to substantially reduce uncertainty about the future evolution of the AMOC. 相似文献
952.
953.
A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum. 相似文献
954.
Tourism is largely dependent on climatic and natural resources. For example, “warmer'' climates generally constitute preferred environments for recreation and leisure, and natural resources such as fresh water, biodiversity, beaches or landscapes are essential preconditions for tourism. Global environmental change threatens these foundations of tourism through climate change, modifications of global biogeochemical cycles, land alteration, the loss of non-renewable resources, unsustainable use of renewable resources and loss of biodiversity (Gössling and Hall, 2005). This has raised concerns that tourist flows will change to the advantage or disadvantage of destinations, which is of major concern to local and national economies, as tourism is one of the largest economic sectors of the world, and of great importance for many destinations. In consequence, an increasing number of publications have sought to analyse travel flows in relation to climatic and socio-economic parameters (e.g. Lise and Tol, 2001; Maddison, 2001; Christ et al., 2003; Hamilton et al., 2003; Hamilton and Tol, 2004). The ultimate goal has been to develop scenarios for future travel flows, possibly including “most at risk destinations'', both in economic and in environmental terms. Such scenarios are meant to help the tourist industry in planning future operations, and they are of importance in developing plans for adaptation. 相似文献
955.
Stefan Niggemann Augusto Mangini Detlev K Richter 《Earth and Planetary Science Letters》2003,216(4):539-547
Calcitic stalagmites from caves in the Sauerland, Germany, prove the existence of sub-Milankovitch cycles in precipitation during the last 6000 yr. The δ18O record dated with Th/U is interpreted as an indicator of paleohumidity. Spectral analysis of δ18O from 6000 a BP up to the recent top of a stalagmite from the Atta cave yields statistically significant peaks at 1450, 117, 64 and 57 a. Additionally we find a good correlation of the stalagmite’s δ18O and Δ14C from European tree rings. The 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by Bond et al. [Science 278 (1997) 1257-1266; 294 (2001) 2130-2136] derived from the amount of ice rafted debris in deep sediments from the North Atlantic. Our results suggest that the centennial to millennial shifts observed in the North Atlantic are accompanied by synchronous shifts of the climate in Northern and Central Europe, which most probably can be attributed to solar irradiation variations. 相似文献
956.
Diana Menzel Stefan Schouten Pim F. van Bergen Jaap S. Sinninghe Damst 《Organic Geochemistry》2004,35(11-12):1343
The δ13C values of higher plant wax C27–33 n-alkanes were determined in three, time-equivalent Pliocene (2.943 Ma) sapropels and homogeneous calcareous ooze from three different sites forming an east-west transect in the eastern Mediterranean Basin in order to study the composition of the vegetation on the continents surrounding the Mediterranean Sea. A two-end member mixing model transformed the measured δ13C values into the contribution of C4 plants to the terrestrial vegetation. These calculations indicated a high C4 plant contribution (i.e. 40–50%) in the periods just before and just after sapropel formation. During sapropel deposition the C4 plant contribution increased by up to 20% at all sites. This is interpreted to record the increased overall plant coverage of the Mediterranean borderlands resulting from the change in formerly barren desert areas into C4 grass-dominated savannahs as a response to the wetter climate during sapropel deposition. Enhanced accumulation rates (ARs) of long-chain n-alkanes (C27–33) and n-alkan-1-ols (C26–30) towards the middle of the sapropel in concert with a decrease in the Ti/Al ratio confirm an increased delivery of terrigenous organic matter at all sites. These biomarkers were probably predominantly fluvially transported to the Mediterranean Sea, not only by the Nile but by fossil wadi river systems on the northern African continent. 相似文献
957.
Stefan van Baars Baars 《Geotechnical and Geological Engineering》2004,22(3):391-400
In The Netherlands 70% of the steel sheet piles are installed with vibrators. This is because vibrators have a high production
rate and therefore a low installation cost. This method only works for soft peat and clay layers and saturated sand layers
which are not densely compacted. During pile installation several problems might occur. In this paper the attention is paid
to avoiding jamming of the pile before it has reached its planned depth. In most of these cases this is caused by a design
error which has let to a vibrator which is too light. Over the years different design tools, such as computer models, design
graphs and design equations, have been developed in order to predict the minimum required vibrator force. In this paper a
new design equation is presented, which consists of three parts: the mass of the sheet pile, the friction along the shaft
and the soil resistance at the tip of the pile based on the cone penetration resistance. This equation and several other design
tools for sheet pile installation, have been compared with 18 field tests. It was found that the general design graphs of
the Dutch Union of Foundation Contractors (NVAF), the design equation of the Dutch Civil Engineering Centre for Construction
Research and Design Rules (CUR) and the German design equation of the EAU1996 gave very poor results. The computer model Vibdrive
from Holeyman (1996), the design graphs for specific locations from the NVAF and the new presented design equation from the
author gave reasonable predictions. Since the computer program is, because of its complexity, not regarded as a design tool
for daily design work and the local design graphs only exist for a small number of locations in The Netherlands, the new design
equation is according to the field tests the best method for the calculation of the vibrator needed for sheet pile installation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
958.
Since hydromorphology in about 80% of German streams and rivers is degraded to a high degree, increased efforts in hydromorphological renaturalization are necessary. A measurement of the success of the first realized projects shows that improvement in stream morphology has a remarkably positive influence on aquatic ecology. An example of a restored stretch of a lowland stream in Saxony-Anhalt is used to describe the possibilities of success measurement programs for improvement of poor renaturalization. Therefore, a combined morphological and hydrobiological approach was developed. An integrated ecological assessment is possible by using the multimetric index EQIM (Ecological Quality Index using benthic Macroinvertebrates) and the GFI (German Fauna Index). The latter represents a tolerance measure to evaluate the hydromorphological status of a site by using certain taxa that indicate either positive or negative physical attributes. To consider the special characteristics of the stream in its landscape unit, specific reference conditions (‘Leitbild’) were defined for macroinvertebrate communities by sampling comparable but undisturbed streams in the same landscape unit. Only the combination of biological indices, hydromorphological mapping and comparison to the reference status allows for an expressive evaluation of renaturalization measures and precise conclusions for their improvement. 相似文献
959.
A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Straitwas, for the first time, systematically measured on 12 March 1998 by aircraft in thelowest 3 km over a 300-km long distance. The air mass modification and the processesinvolved are discussed.Over the water, air temperature was lower than water temperature so that a convectiveboundary layer (CBL) was present as initial condition. As soon as the CBL passed theice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s-1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures causedby both different ice thickness and changes in the cloud conditions, and (b) leads in theice deck. At the front side of the on-ice flow, the air mass boundary between the warmair and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted withheight. Observations suggest that the stratified IBL was lifted as a slab on top of thecold air. 相似文献
960.
Stefan Tilmes JØrgen Brandt Frode FlatØy Robert Bergström Johannes Flemming Joakim Langner Jesper H. Christensen Lise M. Frohn Øystein Hov Ingo Jacobsen Eberthard Reimer Rainer Stern Jörg Zimmermann 《Journal of Atmospheric Chemistry》2002,42(1):91-121
Eulerian state-of-the-art air pollution forecasting systems on the European scale are operated routinely by several countries in Europe. DWD and FUB, both Germany, NERI, Denmark, NILU, Norway, and SMHI, Sweden, operate some of these systems. To apply such modeling systems, e.g. for regulatory purposes according to new EU directives, an evaluation and comparison of the model systems is fundamental in order to assess their reliability. One step in this direction is presented in this study: The model forecasts from all five systems have been compared to measurements of ground level ozone in Germany. The outstanding point in this investigation is the availability of a huge amount of data – from forecasts by the different model systems and from observations. This allows for a thorough interpretation of the findings and assures the significance of the observed features. Data from more than 300 measurement stations for a 5-month period (May–September 1999) of the German monitoring networks have been used in this comparison. Different spatial and temporal statistical parameters were applied in the evaluation. Generally, it was found that the most comprehensive models gave the best results. However, the less comprehensive and computational cheaper models also produced good results. The extensive comparison made it possible to point out weak points in the different models and to describe the individual model behavior for a full summer period in a climatological sense. The comparison also gave valuable information for an assessment of individual measurement stations and complete monitoring networks in terms of the representativeness of the observation data. 相似文献