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   4101篇
  免费   651篇
  国内免费   996篇
测绘学   105篇
大气科学   727篇
地球物理   809篇
地质学   2059篇
海洋学   870篇
天文学   168篇
综合类   177篇
自然地理   833篇
  2024年   16篇
  2023年   80篇
  2022年   203篇
  2021年   179篇
  2020年   171篇
  2019年   199篇
  2018年   161篇
  2017年   160篇
  2016年   194篇
  2015年   211篇
  2014年   290篇
  2013年   328篇
  2012年   289篇
  2011年   339篇
  2010年   206篇
  2009年   312篇
  2008年   314篇
  2007年   249篇
  2006年   280篇
  2005年   219篇
  2004年   199篇
  2003年   158篇
  2002年   138篇
  2001年   129篇
  2000年   130篇
  1999年   105篇
  1998年   97篇
  1997年   78篇
  1996年   52篇
  1995年   48篇
  1994年   30篇
  1993年   35篇
  1992年   15篇
  1991年   17篇
  1990年   14篇
  1989年   10篇
  1988年   21篇
  1987年   6篇
  1986年   18篇
  1985年   15篇
  1984年   9篇
  1983年   4篇
  1982年   8篇
  1981年   9篇
  1980年   1篇
  1954年   2篇
排序方式: 共有5748条查询结果,搜索用时 31 毫秒
61.
62.
63.
In principle, many climate policymakers have accepted that large-scale carbon dioxide removal (CDR) is necessary to meet the Paris Agreement’s mitigation targets, but they have avoided proposing by whom CDR might be delivered. Given its role in international climate policy, the European Union (EU) might be expected to lead the way. But among EU climate policymakers so far there is little talk on CDR, let alone action. Here we assess how best to ‘target’ CDR to motivate EU policymakers exploring which CDR target strategy may work best to start dealing with CDR on a meaningful scale. A comprehensive CDR approach would focus on delivering the CDR volumes required from the EU by 2100, approximately at least 50 Gigatonnes (Gt) CO2, according to global model simulations aiming to keep warming below 2°C. A limited CDR approach would focus on an intermediate target to deliver the CDR needed to reach ‘net zero emissions’ (i.e. the gross negative emissions needed to offset residual positive emissions that are too expensive or even impossible to mitigate). We argue that a comprehensive CDR approach may be too intimidating for EU policymakers. A limited CDR approach that only addresses the necessary steps to reach the (intermediate) target of ‘net zero emissions’ is arguably more achievable, since it is a better match to the existing policy paradigm and would allow for a pragmatic phase-in of CDR while avoiding outright resistance by environmental NGOs and the broader public.

Key policy insights

  • Making CDR an integral part of EU climate policy has the potential to significantly reshape the policy landscape.

  • Burden sharing considerations would probably play a major role, with comprehensive CDR prolonging the disparity and tensions between progressives and laggards.

  • Introducing limited CDR in the context of ‘net zero’ pathways would retain a visible primary focus on decarbonization but acknowledge the need for a significant enhancement of removals via ‘natural’ and/or ‘engineered’ sinks.

  • A decarbonization approach that intends to lead to a low level of ‘residual emissions’ (to be tackled by a pragmatic phase-in of CDR) should be the priority of EU climate policy.

  相似文献   
64.
Energy-intensive industries play an important role in low-carbon development, being particularly exposed to climate policies. Concern over possible carbon leakage in this sector poses a major challenge for designing effective carbon pricing instruments (CPI). Different methodologies for assessing carbon leakage exposure are currently used by different jurisdictions, each of them based on different approaches and indicators. This paper aims to analyse the extent to which the use of different methodologies leads to different results in terms of exposure to the risk of carbon leakage, using the Brazilian industry sector as a case study. Results indicate that carbon leakage exposure is an expected outcome of eventual CPI implementation in Brazilian industry. However, results vary according to the chosen methodology, so the definition of the criteria is paramount for assessing sectoral exposure to the risk of carbon leakage.

Key policy insights

  • Despite increasing discussion about the implementation of carbon pricing on the Brazilian industrial sector, the evaluation of carbon leakage risks is still neglected.

  • Assessments of the risk of carbon leakage are directly related to the indicators and criteria used by each methodology. Thus, a given subsector may present different levels of exposure to carbon leakage depending on the methodological choice.

  • More than a purely technical discussion, the methodological definition of carbon leakage risk is a political discussion – it can be well-conducted, leading to the success of a CPI, or even sabotaged, by implicitly subsidizing energy-intensive industries.

  相似文献   
65.
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.

  相似文献   
66.
Upon completion, China’s national emissions trading scheme (C-ETS) will be the largest carbon market in the world. Recent research has evaluated China’s seven pilot ETSs launched from 2013 on, and academic literature on design aspects of the C-ETS abounds. Yet little is known about the specific details of the upcoming C-ETS. This article combines currently understood details of China’s national carbon market with lessons learned in the pilot schemes as well as from the academic literature. Our review follows the taxonomy of Emissions Trading in Practice: A Handbook on Design and Implementation (Partnership for Market Readiness & International Carbon Action Partnership. (2016). Retrieved from www.worldbank.org): The 10 categories are: scope, cap, distribution of allowances, use of offsets, temporal flexibility, price predictability, compliance and oversight, stakeholder engagement and capacity building, linking, implementation and improvements.

Key policy insights

  • Accurate emissions data is paramount for both design and implementation, and its availability dictates the scope of the C-ETS.

  • The stakeholder consultative process is critical for effective design, and China is able to build on its extensive experience through the pilot ETSs.

  • Current policies and positions on intensity targets and Clean Development Mechanism (CDM) credits constrain the market design of the C-ETS.

  • Most critical is the nature of the cap. The currently discussed rate-based cap with ex post adjustment is risky. Instead, an absolute, mass-based emissions cap coupled with the conditional use of permits would allow China to maintain flexibility in the carbon market while ensuring a limit on CO2 emissions.

  相似文献   
67.
泥炭是一种具有多种用途的宝贵自然资源,泥炭中的有机碳储量是研究全球碳库变化及碳循环过程的重要参数。由于泥炭多分布于沼泽中,且泥炭松软含水量大的特性,对于泥炭深度的调查、获取用于测试泥炭原状样品的质量不高,导致泥炭碳储量评价仍存在不确定性和偏差。笔者等在典型泥炭形成地若尔盖沼泽湿地地区,开展了浅钻泥炭调查取样工作,通过在泥炭斑块边缘及中心两种不同沼泽湿地地层开展的试验,验证了轻便钻机在难进入的沼泽湿地的适应性,配套的振动冲击工艺可高质量的获得无扰动的泥炭样品,同时查明了泥炭层的厚度。通过试验初步探索了采用轻便钻机配套振动冲击钻进工艺进行泥炭调查取样的有效性。浅层取样钻探作为泥炭调查取样的一种直接有效的技术手段,可以有效地提高泥炭调查效率和精度,为准确评价泥炭储量数据提供可靠的钻探技术服务支撑。  相似文献   
68.
Natural gas, consisting primarily of methane(CH4), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep-sourced abiotic CH4 in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH4 flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiot...  相似文献   
69.
土壤碳库研究及碳汇问题是近年来土壤碳循环与全球变化研究的热点领域,土壤是陆地生态系统的核心,研究土壤有机碳储量和碳密度的影响因素对正确评价本区碳循环有重要意义,本文利用土壤类型、土地利用类型、地貌类型、生态系统等影响因素对研究区表层土壤有机碳密度和碳储量进行了评价,分析了兰州—白银地区1987年和2014年土壤有机碳储量、碳密度的变化,其中2014年土壤有机碳储量增加了3.58×106 t,说明这些年土壤固碳效果明显,土地利用方式更合理。认为研究区有机碳碳密度、碳储量在空间上的分布极不均匀,与成土母质、土壤自身理化特性、土地利用方式及自然景观条件、人类活动等因素密切相关,是多因素综合影响的结果。  相似文献   
70.
Simultaneous analysis of carbon and nitrogen isotope ratios by SIMS was applied for the first-time to a natural diamond from the Kelsey Lake kimberlite, State Line Distinct, Colorado (UWD-1). This in situ procedure is faster, reduces sample size for analysis, and measures both isotope ratios from a single ~ 10 μm diameter pit, a critical advantage for zoned diamonds. The carbon isotope ratio (expressed as δ13CVPDB) of the bulk UWD-1 crystal, determined by the conventional combustion method in the present study, is -5.9‰ ± 0.2‰ (VPDB, 2s). Nitrogen mass fraction ([N]) and isotope ratio (expressed as δ15NAir) were determined by stepwise combustion and gas-source mass-spectrometry, resulting in 553 ± 64 μg g-1 and -6.7‰ ± 1.1‰ (Air, 2s), respectively. Secondary ions of 12C2-, 12C13C-, 12C14N-, and 12C15N- were simultaneously measured by SIMS using three Faraday cups and one electron multiplier. The spot-to-spot reproducibility of δ13C and δ15N values for the UWD-1 (178 spots on sixteen chips, 10 μm spots), were 0.3‰ and 1.6‰, respectively (2s). While 12C14N-/12C2- ratios, which are an indicator for [N], varied up to 12% among these sixteen chips, such variation did not correlate with either δ13C or δ15N values. We propose that UWD-1 is a suitable reference sample for microscale in situ analysis of δ13C and δ15N values in diamond samples.  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