全文获取类型
收费全文 | 307篇 |
免费 | 10篇 |
国内免费 | 9篇 |
专业分类
测绘学 | 7篇 |
大气科学 | 18篇 |
地球物理 | 62篇 |
地质学 | 144篇 |
海洋学 | 15篇 |
天文学 | 54篇 |
综合类 | 2篇 |
自然地理 | 24篇 |
出版年
2021年 | 4篇 |
2020年 | 3篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 16篇 |
2013年 | 9篇 |
2012年 | 10篇 |
2011年 | 7篇 |
2010年 | 8篇 |
2009年 | 15篇 |
2008年 | 13篇 |
2007年 | 10篇 |
2006年 | 7篇 |
2005年 | 7篇 |
2004年 | 16篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 11篇 |
2000年 | 4篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 5篇 |
1996年 | 6篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1989年 | 3篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 6篇 |
1981年 | 8篇 |
1980年 | 9篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1976年 | 4篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1972年 | 1篇 |
1971年 | 3篇 |
1969年 | 3篇 |
1968年 | 1篇 |
排序方式: 共有326条查询结果,搜索用时 31 毫秒
51.
52.
The importance of sulfur dioxide emissions for climate change is now established, although substantial uncertainties remain.
This paper presents projections for future sulfur dioxide emissions using the MiniCAM integrated assessment model. A new income-based
parameterization for future sulfur dioxide emissions controls is developed based on purchasing power parity (PPP) income estimates
and historical trends related to the implementation of sulfur emissions limitations. This parameterization is then used to
produce sulfur dioxide emissions trajectories for the set of scenarios developed for the Special Report on Emission Scenarios
(SRES). We use the SRES methodology to produce harmonized SRES scenarios using the latest version of the MiniCAM model. The
implications, and requirements, for integrated assessment modeling of sulfur dioxide emissions are discussed. We find that
sulfur emissions eventually decline over the next century under a wide set of assumptions. These emission reductions result
from a combination of emission controls, the adoption of advanced electric technologies, and a shift away from the direct
end use of coal with increasing income levels. Only under a scenario where incomes in developing regions increase slowly do
global emission levels remain at close to present levels over the next century. Under a climate policy that limits emissions
of carbon dioxide, sulfur dioxide emissions fall in a relatively narrow range. In most cases, the relative climatic effect
of sulfur dioxide emissions decreases dramatically to a point where sulfur dioxide is only a minor component of climate forcing
by the end of the century. Ecological effects of sulfur dioxide, however, could be significant in some developing regions
for many decades to come. 相似文献
53.
Obituary
Obituary Robert W. Pennak 相似文献54.
Background
Forest fuel treatments have been proposed as tools to stabilize carbon stocks in fire-prone forests in the Western U.S.A. Although fuel treatments such as thinning and burning are known to immediately reduce forest carbon stocks, there are suggestions that these losses may be paid back over the long-term if treatments sufficiently reduce future wildfire severity, or prevent deforestation. Although fire severity and post-fire tree regeneration have been indicated as important influences on long-term carbon dynamics, it remains unclear how natural variability in these processes might affect the ability of fuel treatments to protect forest carbon resources. We surveyed a wildfire where fuel treatments were put in place before fire and estimated the short-term impact of treatment and wildfire on aboveground carbon stocks at our study site. We then used a common vegetation growth simulator in conjunction with sensitivity analysis techniques to assess how predicted timescales of carbon recovery after fire are sensitive to variation in rates of fire-related tree mortality, and post-fire tree regeneration.Results
We found that fuel reduction treatments were successful at ameliorating fire severity at our study site by removing an estimated 36% of aboveground biomass. Treated and untreated stands stored similar amounts of carbon three years after wildfire, but differences in fire severity were such that untreated stands maintained only 7% of aboveground carbon as live trees, versus 51% in treated stands. Over the long-term, our simulations suggest that treated stands in our study area will recover baseline carbon storage 10?C35?years more quickly than untreated stands. Our sensitivity analysis found that rates of fire-related tree mortality strongly influence estimates of post-fire carbon recovery. Rates of regeneration were less influential on recovery timing, except when fire severity was high.Conclusions
Our ability to predict the response of forest carbon resources to anthropogenic and natural disturbances requires models that incorporate uncertainty in processes important to long-term forest carbon dynamics. To the extent that fuel treatments are able to ameliorate tree mortality rates or prevent deforestation resulting from wildfire, our results suggest that treatments may be a viable strategy to stabilize existing forest carbon stocks. 相似文献55.
Hugh A. L. Henry 《Climatic change》2008,87(3-4):421-434
Changes to soil freezing dynamics with climate change can modify ecosystem carbon and nutrient losses. Soil freezing is influenced
strongly by both air temperature and insulation by the snowpack, and it has been hypothesized that winter climate warming
may lead to increased soil freezing as a result of reduced snowpack thickness. I used weather station data to explore the
relationships between winter air temperature, precipitation and soil freezing for 31 sites in Canada, ranging from the temperate
zone to the high Arctic. Inter-annual climate variation and associated soil temperature variation over the last 40 years were
examined and used to interpolate the effects of projected climate change on soil freezing dynamics within sites using linear
regression models. Annual soil freezing days declined with increasing mean winter air temperature despite decreases in snow
depth and cover, and reduced precipitation only increased annual soil freezing days in the warmest sites. Annual soil freeze–thaw
cycles increased in both warm and dry winters, although the effects of precipitation were strongest in sites that experience
low mean winter precipitation. Overall, it was projected that by 2050, changes in winter temperature will have a much stronger
effect on annual soil freezing days and freeze–thaw cycles than changes in total precipitation, with sites close to but below
freezing experiencing the largest changes in soil freezing days. These results reveal that experimental data relevant to the
effects of climate changes on soil freezing dynamics and changes in associated soil physical and biological processes are
lacking. 相似文献
56.
Hugh M. Pitcher 《Climatic change》2009,97(1-2):49-58
Energy is a key requirement for a healthy, productive life and a major driver of the emissions leading to an increasingly warm planet. The implications of a doubling and redoubling of per capita incomes over the remainder of this century for energy use are a critical input into understanding the magnitude of the carbon management problem. A substantial controversy about how the Special Report on Emissions Scenarios (SRES) measured income and the potential implications of how income was measured for long term levels of energy use is revisited again in the McKibbin, Pearce and Stegman article appearing elsewhere in this issue. The recent release of a new set of purchasing power estimates of national income, and the preparations for creating new scenarios to support the IPCC’s fifth assessment highlight the importance of the issues which have arisen surrounding income and energy use. Comparing the 1993 and 2005 ICP results on Purchasing Power Parity (PPP) based measures of income reveals that not only do the 2005 ICP estimates share the same issue of common growth rates for real income as measured by PPP and US $, but the lack of coherence in the estimates of PPP incomes, especially for developing countries raises yet another obstacle to resolving the best way to measure income. Further, the common use of an income term to mediate energy demand (as in the Kaya identity) obscures an underlying reality about per capita energy demands, leading to unreasonable estimates of the impact of changing income measures and of the recent high GDP growth rates in India and China. Significant new research is required to create both a reasonable set of GDP growth rates and long term levels of energy use. 相似文献
57.
Solubility of Pt in sulphide mattes: Implications for the genesis of PGE-rich horizons in layered intrusions 总被引:1,自引:0,他引:1
Raúl O.C. Fonseca Ian H. Campbell Hugh St. C. ONeill Charlotte M. Allen 《Geochimica et cosmochimica acta》2009,73(19):5764-5777
The partitioning of Pt in sulphide melt (matte) has been studied as a function of fS2 and fO2 at 1200 and 1300 °C. The results show that the solubility of Pt in mattes increases strongly with increasing fS2 and decreases weakly with increasing fO2. The increase in Pt solubility with increasing fS2 is attributed to Pt dissolving in the melt as a sulphide species and the weak inverse dependence of Pt solubility on fO2 to the diluting effect of increasing O in the melt at high fO2. These results, coupled with measurements of Pt solubility in silicate melts taken from the literature, allow the calculation of Pt matte/silicate-melt partition coefficients () for a range of conditions pertinent to the formation of Pt-rich horizons in layered intrusions. The calculated values range between 107 and 1011, depending on fO2 and fS2, several orders of magnitude higher than previously published values. Our preferred value for for conditions appropriate to the Merensky Reef is 107 and for the Stillwater Pt-rich horizon 108. The new results are consistent with the magmatic hypothesis for Pt-rich horizons in layered intrusions. 相似文献
58.
Hugh M. French Susan W. S. Millar 《Boreas: An International Journal of Quaternary Research》2014,43(3):667-677
This paper presents three maps that summarize current knowledge as to the extent of Past permafrost and Relict permafrost in North America at approximately the time of the Last Glacial Maximum (LGM; c. 25–17 ka BP) and during subsequent deglaciation until c. 10 ka BP. Analysis of the post‐1983 literature suggests that the extent of Past permafrost south of the LGM limit was broader in eastern North America and slightly narrower in the Interior Great Plains than previously mapped. The recognition and dating of Relict permafrost in the nonglaciated terrain of the northwestern Arctic suggests that permafrost may be of great antiquity and can persist under changing climatic conditions. The formation of permafrost features during deglaciation suggests that ice‐proximal climatic conditions remained cold at least long enough for short‐lived permafrost aggradation; a latitudinal gradient is evident in the timing of its development as the Laurentide Ice Sheet retreated. 相似文献
59.
Michael I. H. Hartnady Christopher L. Kirkland Chris Clark Catherine V. Spaggiari R. Hugh Smithies Noreen J. Evans Brad J. McDonald 《Journal of Metamorphic Geology》2019,37(6):823-838
Titanite can be found in rocks of wide compositional range, is reactive, growing or regrowing during metamorphic and hydrothermal events, and is generally amenable to U–Pb geochronology. Experimental evidence suggest that titanite has a closure temperature for Pb ranging from 550 to 650°C, and thus titanite dates are commonly interpreted as cooling ages. However, this view has been challenged in recent years by evidence from natural titanite which suggests the closure temperature may be significantly higher (up to 800°C). Here, we investigate titanite in an enclave of migmatitic gneiss included within a granite intrusion. The titanite crystals exhibit textural features characteristic of fluid‐mediated mass transfer processes on length scales of <100 µm. These textural features are associated with variation in both Pb concentrations and distinct U–Pb isotopic compositions. Zr‐in‐titanite thermometry indicates that modification of the titanite occurred at temperatures in excess of 840°C, in the presence of a high‐T silicate melt. The Pb concentration gradients preserved in these titanite crystals are used to determine the diffusivity of Pb in titanite under high‐T conditions. We estimate diffusivities ranging from 2 × 10?22 to 5 × 10?25 m2/s. These results are significantly lower than experimental data predict yet are consistent with other empirical data on natural titanites, suggesting that Pb diffusivity is similar to that of Sr. Thus our data challenge the wide‐held assumption that U–Pb titanite dates only reflect cooling ages. 相似文献
60.
Thomas Bernard Hugh D. Sinclair Mark Naylor Frdric Christophoul Mary Ford 《Basin Research》2021,33(1):118-137
The transition to a post‐orogenic state in mountain ranges has been identified by a change from active subsidence to isostatic rebound of the foreland basin. However, the nature of the interplay between isostatic rebound and sediment supply, and their impact on the topographic evolution of a range and foreland basin during this transition, has not been fully investigated. Here, we use a box model to explore the syn‐ to post‐orogenic evolution of foreland basin/thrust wedge systems. Using a set of parameter values that approximate the northern Pyrenees and the neighbouring Aquitaine foreland basin, we evaluate the controls on sediment drape over the frontal parts of the retro‐wedge following cessation of crustal thickening. Conglomerates preserved at approximately 600‐m elevation, which is ~ 300 m above the present mountain front in the northern Pyrenees are ca. 12 Ma, approximately 10 Myrs younger than the last evidence of crustal thickening in the wedge. Using the model, this post‐orogenic sediment drape is explained by the combination of a sustained, high sediment influx from the range into the basin relative to the efflux out of the basin, combined with cessation of the generation of accommodation space through basin subsidence. Post‐orogenic sediment drape is considered a generic process that is likely to be responsible for elevated low‐gradient surfaces and preserved remnants of continental sedimentation draping the outer margins of the northern Pyrenean thrust wedge. 相似文献