全文获取类型
收费全文 | 153篇 |
免费 | 10篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 41篇 |
地球物理 | 33篇 |
地质学 | 48篇 |
海洋学 | 12篇 |
天文学 | 10篇 |
综合类 | 1篇 |
自然地理 | 16篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 4篇 |
2018年 | 11篇 |
2017年 | 5篇 |
2016年 | 11篇 |
2015年 | 5篇 |
2014年 | 16篇 |
2013年 | 17篇 |
2012年 | 9篇 |
2011年 | 9篇 |
2010年 | 12篇 |
2009年 | 6篇 |
2008年 | 7篇 |
2007年 | 7篇 |
2006年 | 7篇 |
2005年 | 1篇 |
2004年 | 2篇 |
2003年 | 3篇 |
2002年 | 4篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1978年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有164条查询结果,搜索用时 281 毫秒
1.
2.
Jonas Ardö Meelis Mölder Bashir Awad El-Tahir Hatim Abdalla Mohammed Elkhidir 《Carbon balance and management》2008,3(1):1-18
Background
Large spatial, seasonal and annual variability of major drivers of the carbon cycle (precipitation, temperature, fire regime and nutrient availability) are common in the Sahel region. This causes large variability in net ecosystem exchange and in vegetation productivity, the subsistence basis for a major part of the rural population in Sahel. This study compares the 2005 dry and wet season fluxes of CO2 for a grass land/sparse savanna site in semi arid Sudan and relates these fluxes to water availability and incoming photosynthetic photon flux density (PPFD). Data from this site could complement the current sparse observation network in Africa, a continent where climatic change could significantly impact the future and which constitute a weak link in our understanding of the global carbon cycle.Results
The dry season (represented by Julian day 35–46, February 2005) was characterized by low soil moisture availability, low evapotranspiration and a high vapor pressure deficit. The mean daily NEE (net ecosystem exchange, Eq. 1) was -14.7 mmol d-1 for the 12 day period (negative numbers denote sinks, i.e. flux from the atmosphere to the biosphere). The water use efficiency (WUE) was 1.6 mmol CO2 mol H2O-1 and the light use efficiency (LUE) was 0.95 mmol CO2 mol PPFD-1. Photosynthesis is a weak, but linear function of PPFD. The wet season (represented by Julian day 266–273, September 2005) was, compared to the dry season, characterized by slightly higher soil moisture availability, higher evapotranspiration and a slightly lower vapor pressure deficit. The mean daily NEE was -152 mmol d-1 for the 8 day period. The WUE was lower, 0.97 mmol CO2 mol H2O-1 and the LUE was higher, 7.2 μmol CO2 mmol PPFD-1 during the wet season compared to the dry season. During the wet season photosynthesis increases with PPFD to about 1600 μmol m-2s-1 and then levels off.Conclusion
Based on data collected during two short periods, the studied ecosystem was a sink of carbon both during the dry and wet season 2005. The small sink during the dry season is surprising and similar dry season sinks have not to our knowledge been reported from other similar savanna ecosystems and could have potential management implications for agroforestry. A strong response of NEE versus small changes in plant available soil water content was found. Collection and analysis of flux data for several consecutive years including variations in precipitation, available soil moisture and labile soil carbon are needed for understanding the year to year variation of the carbon budget of this grass land/sparse savanna site in semi arid Sudan. 相似文献3.
Samples were collected at stations located in the mesohaline, oligohaline, and tidal fresh regions of the Potomac River, Maryland,
between April 1998 and December 1999 to evaluate the seasonal distribution of bacterioplankton and microbially labile organic
carbon (MLOC) in relation to hydrodynamic parameters (dissolved oxygen, salinity, and temperature). Bacterioplankton abundance
(BA) averaged 13 × 106 cells ml−1 at all stations, a value that is higher than the average observed in many other temperate estuaries around the world, and
were almost exclusively free-living. During the summer of 1998, BA often exceeded 30 × 106 cells ml−1 in the mesohaline region during periods of anoxia in subpycnocline waters. Dissolved MLOC typically accounted for 40% of
total MLOC and on some occasions during summer and autumn accounted for 80%. A significant positive relationship between dissolved
MLOC and BA was evident in the mesohaline Potomac River, the region where anoxia occurs each summer, but the regressions of
particulate MLOC and chla on BA were not significant at this location. In the mesohaline Potomac River, BA regressed negatively and significantly on
dissolved oxygen (r2=0.50, p<0.001). BA may be an important indicator of ecosystem health in this and other eutrophied estuaries, because of the
relationships between BA, dissolved MLOC, and dissolved oxygen in the salinity stratified Potomac River and because free-living
BA was elevated along the length of the river. 相似文献
4.
5.
Jonas Hein Soeryo Adiwibowo Christoph Dittrich Rosyani Endriatmo Soetarto Heiko Faust 《The Professional geographer》2016,68(3):380-389
The recent expansion of protected areas and oil palm plantations in Jambi (Sumatra), Indonesia, has been notably disruptive. This article investigates the scalar dimensions of land conflicts within a privately managed conservation area. We built on qualitative research focusing on struggles related to the formation of two informal settlements within the conservation area. Results indicate that, especially in the context of rapid rescaling (e.g., decentralization), new power constellations emerge, thereby altering preexisting property relations. We argue that competing scales of meaning and regulation are structuring the ability of actors to access land and, consequently, reflect tensions between structure and agency. 相似文献
6.
Torbern Tagesson Jonas Ardö Idrissa Guiro Ford Cropley Cheikh Mbow Stephanie Horion 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2016,116(2):93-109
Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to ?48 μmol CO2 m?2 s?1 and 20 μmol CO2 m?2 s?1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of ?229 ± 7 ± 49 g C m?2 y?1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost .20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change. 相似文献
7.
Aviation constitutes about 2.5% of all energy-related CO2 emissions and in addition there are non-CO2 effects. In 2016, the ICAO decided to implement a Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) and in 2017 the EU decided on faster emission reductions in its Emissions Trading System (EU ETS), which since 2012 includes the aviation sector. The effects of these policies on the expected development of air travel emissions from 2017 to 2030 have been analyzed. For the sample country Sweden, the analysis shows that when emissions reductions in other sectors are attributed to the aviation sector as a result of the EU ETS and CORSIA, carbon emissions are expected to reduce by ?0.8% per year (however if non-CO2 emissions are included in the analysis, then emissions will increase). This is much less than what is needed to achieve the 2°C target. Our analysis of potential national aviation policy instruments shows that there are legally feasible options that could mitigate emissions in addition to the EU ETS and CORSIA. Distance-based air passenger taxes are common among EU Member States and through increased ticket prices these taxes can reduce demand for air travel and thus reduce emissions. Tax on jet fuel is an option for domestic aviation and for international aviation if bilateral agreements are concluded. A quota obligation for biofuels is a third option.Key policy insights
Existing international climate policies for aviation will not deliver any major emission reductions.
Policymakers who want to significantly push the aviation sector to contribute to meeting the 2°C target need to work towards putting in place tougher international policy instruments in the long term, and simultaneously implement temporary national policy instruments in the near-term.
Distance-based air passenger taxes, carbon taxes on jet fuel and quota obligations for biofuels are available national policy options; if they are gradually increased, and harmonized with other countries, they can help to significantly reduce emissions.
8.
In this paper, a literature‐based compilation of the timing and history of salt tectonics in the Southern Permian Basin (Central Europe) is presented. The tectono‐stratigraphic evolution of the Southern Permian Basin is influenced by salt movement and the structural development of various types of salt structures. The compilation presented here was used to characterize the following syndepositional growth stages of the salt structures: (a) “phase of initiation”; (b) phase of fastest growth (“main activity”); and (c) phase of burial’. We have also mapped the spatial pattern of potential mechanisms that triggered the initiation of salt structures over the area studied and summarized them for distinct regions (sub‐basins, platforms, etc.). The data base compiled and the set of maps produced from it provide a detailed overview of the spatial and temporal distribution of salt tectonic activity enabling the correlation of tectonic phases between specific regions of the entire Southern Permian Basin. Accordingly, salt movements were initiated in deeply subsided graben structures and fault zones during the Early and Middle Triassic. In these areas, salt structures reached their phase of main activity already during the Late Triassic or the Jurassic and were mostly buried during the Early Cretaceous. Salt structures in less subsided sub‐basins and platform regions of the Southern Permian Basin mostly started to grow during the Late Triassic. The subsequent phase of main activity of these salt structures took place from the Late Cretaceous to the Cenozoic. The analysis of the trigger mechanisms revealed that most salt structures were initiated by large‐offset normal faults in the sub‐salt basement in the large graben structures and minor normal faulting associated with thin‐skinned extension in the less subsided basin parts. 相似文献
9.
Jessica D. Lundquist Susan Dickerson-Lange Ethan Gutmann Tobias Jonas Cassie Lumbrazo Dylan Reynolds 《水文研究》2021,35(7):e14274
When formulating a hydrologic model, scientists rely on parameterizations of multiple processes based on field data, but literature review suggests that more frequently people select parameterizations that were included in pre-existing models rather than re-evaluating the underlying field experiments. Problems arise when limited field data exist, when “trusted” approaches do not get reevaluated, and when sensitivities fundamentally change in different environments. The physics and dynamics of snow interception by conifers is just such a case, and it is critical to simulation of the water budget and surface albedo. The most commonly used interception parameterization is based on data from four trees from one site, but results from this field study are not directly transferable to locations with relatively warmer winters, where the dominant processes differ dramatically. Here, we combine a literature review with model experiments to demonstrate needed improvements. Our results show that the choice of model form and parameters can vary the fraction of snow lost through interception by as much as 30%. In most simulations, the warming of mean winter temperatures from −7 to 0°C reduces the modelled fraction of snow under the canopy compared to the open, but the magnitude of simulated decrease varies from about 10% to 40%. The range of results is even larger when considering models that neglect the melting of in-canopy snow in higher-humidity environments where canopy sublimation plays less of a role. Thus, we recommend that all models represent canopy snowmelt and include representation of increased loading due to increased adhesion and cohesion when temperatures rise from −3 to 0°C. In addition to model improvements, field experiments across climates and forest types are needed to investigate how to best model the combination of dynamically changing forest cover and snow cover to better understand and predict changes to albedo and water supplies. 相似文献
10.
Jonas Ebbesson 《Global Environmental Change》2010,20(3):414-422
This article discusses the impact of the rule of law on the resilience of societies for governing complex socio-ecological changes. It concludes that the notions of the rule of law and legal certainty have changed, and that they can be compatible with the use of framework laws of a rather open-textured character, provided certain legal safeguards, such as the right to a legal review, are at hand. While legal certainty is an important virtue of law, it does not as such necessarily prevent adequate flexibility in administrative decision-making concerning health, the environment or the use of natural resources. The article also considers to what extent certain established administrative means of control in the field of environment protection and the use of natural resources match the findings and proposals, e.g. on flexibility and adaptability, provided by resilience research. Finally the article discusses the impact of state sovereignty on governance of large-scale socio-ecological changes, with reach across state borders. It concludes that, despite some attempts of softening the impact of state borders in transboundary environmental decision-making and management, state sovereignty still hampers multilevel governance and management of resources in such contexts. 相似文献