全文获取类型
收费全文 | 5722篇 |
免费 | 972篇 |
国内免费 | 1205篇 |
专业分类
测绘学 | 151篇 |
大气科学 | 404篇 |
地球物理 | 1587篇 |
地质学 | 2834篇 |
海洋学 | 1184篇 |
天文学 | 9篇 |
综合类 | 304篇 |
自然地理 | 1426篇 |
出版年
2024年 | 34篇 |
2023年 | 137篇 |
2022年 | 251篇 |
2021年 | 231篇 |
2020年 | 256篇 |
2019年 | 323篇 |
2018年 | 276篇 |
2017年 | 234篇 |
2016年 | 261篇 |
2015年 | 221篇 |
2014年 | 356篇 |
2013年 | 360篇 |
2012年 | 312篇 |
2011年 | 296篇 |
2010年 | 296篇 |
2009年 | 344篇 |
2008年 | 338篇 |
2007年 | 369篇 |
2006年 | 401篇 |
2005年 | 340篇 |
2004年 | 304篇 |
2003年 | 299篇 |
2002年 | 285篇 |
2001年 | 260篇 |
2000年 | 202篇 |
1999年 | 163篇 |
1998年 | 158篇 |
1997年 | 126篇 |
1996年 | 115篇 |
1995年 | 79篇 |
1994年 | 65篇 |
1993年 | 48篇 |
1992年 | 42篇 |
1991年 | 31篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 7篇 |
1985年 | 7篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1978年 | 1篇 |
排序方式: 共有7899条查询结果,搜索用时 15 毫秒
991.
构建早稻雨洗花灾害指标及适于早稻产量估算的灾损评估模型,对开展早稻雨洗花灾害监测、损失评估、灾害保险等具有重要意义。该文以江西省早稻为研究对象,利用1981-2015年14个水稻气象观测站逐日气象资料和农业气象观测资料,筛选出基于早稻抽穗扬花期间过程降水量、最大降水量、降水日数及实际产量的雨洗花灾害样本78个,在此基础上,利用相关分析、正态分布以及主成分回归法,建立了雨洗花灾害指标和灾损评估模型,并对其进行验证。结果表明:抽穗扬花期降水对雨洗花灾害形成有显著影响,其主要影响时段为抽穗扬花普遍期前后5 d内,关键时段为抽穗扬花普遍期前后3 d内。日降水量40 mm可作为早稻抽穗扬花期雨洗花灾害临界指标。以该指标为基础,统计日降水量不低于40 mm的降水日数及其对应的累积降水量,当累积降水量为40~170 mm时,为轻度雨洗花灾害,早稻一般减产小于15%,平均减产10%;当累积降水量不小于170 mm时,为重度雨洗花灾害,早稻一般减产不低于15%,平均减产22%。指标验证结果与历史实际灾害发生情况有较好的一致性。雨洗花灾损评估模型检验结果表明:雨洗花年模拟产量与实际产量吻合度较高,平均相对模拟误差为4.3%,78.0%的资料相对误差在5%以内,可利用该模型对雨洗花年的早稻减产率进行模拟和预测。 相似文献
992.
Zeynep Clulow 《Climate Policy》2018,18(2):221-234
Is economic development compatible with mitigation? On the one hand, development should promote effective climate policy by enhancing states’ capacities for mitigation. On the other hand, economic growth creates more demand for production, thereby inhibiting emissions reduction. These arguments are often reconciled in the environmental Kuznets curve (EKC) thesis. According to this approach, development initially increases emissions in poor economies, but begins to lower emissions after a country has attained a certain level of development.The aim of this article is to determine empirically whether the EKC hypothesis seems plausible in light of emissions trends over the birth and implementation of the Kyoto Protocol. Drawing on data from the World Bank World Development Indicators and World Resources Institute Climate Data Explorer, it conducts a large-N investigation of the emissions behaviour of 120 countries from 1990 to 2012. While several quantitative studies have found that economic factors influence emissions activity, this article goes beyond existing research by employing a more sophisticated – multilevel – research design to determine whether economic development: (a) continues to be a significant driver once country-level clustering is accounted for and (b) has different effects on different countries. The results of this article indicate that, even after we account for country-level clustering and hold constant the other main putative drivers of emissions activity, economic development tends to inhibit emissions reduction. They also provide strong evidence that emissions trends resemble the EKC, with development significantly constraining emissions reduction in the South and promoting it in the North.POLICY RELEVANCEThis article contributes to the understanding of the (changing) role of economic development in shaping emissions activity. It demonstrates the need for a contextualized, country-specific approach for evaluating the effectiveness of economic development in promoting emissions reduction and uncovers new evidence in support of the EKC hypothesis. 相似文献
993.
Jonathan Gewirtzman Sujay Natson Julie-Anne Richards Victoria Hoffmeister Alexis Durand Romain Weikmans 《Climate Policy》2018,18(8):1076-1086
After decades of pressure from vulnerable developing countries, the Warsaw International Mechanism on Loss and Damage (the WIM) was established at the nineteenth Conference of the Parties (COP 19) in 2013 to address costly damages from climate change. However, little progress has been made towards establishing a mechanism to fund loss and damage. The WIM's Executive Committee issued its first two-year workplan the following year at COP 20 which offered, among other things, a range of approaches to financing loss and damage programmes, which we review here. We provide brief overviews of each mechanism proposed by the WIM ExCom, describe their current applications, their statuses under the United Nations Framework Convention on Climate Change (UNFCCC), some of their advantages and disadvantages, and their current or potential application to loss and damage. We find that several of these mechanisms may be useful in supporting loss and damage programmes, but identify some key gaps. First, most of the mechanisms identified by the WIM ExCom are insurance schemes subsidized with voluntary contributions, which may not be adequate or reliable over time. Second, none were devised to apply to slow-onset events, or to non-economic losses and damages. That is, if harms are inflicted on parts of a society or its ecosystems that have no price, or if they occur gradually, they would probably not be covered by these mechanisms. Finally, the lack of a dedicated and adequate flow of finance to address the real loss and damage being experienced by vulnerable nations will require the use of innovative financial tools beyond those mentioned in the WIM ExCom workplan.
Key policy insights
Despite a full article of the 2015 Paris Agreement devoted to loss and damage, there is little international agreement on the scope of loss and damage programmes, and especially how they would be funded and by whom.
Most of the loss and damage funding mechanisms identified by the WIM ExCom are insurance schemes subsidized with voluntary contributions, which may burden the most vulnerable countries and may not be reliable over time.
None of the mechanisms were devised to apply to slow-onset events, or to non-economic losses and damages.
994.
Due to the uplift of Qinghai-Tibet Plateau(QTP), the cryosphere gradually developed on the higher mountain summits after the Neocene, becoming widespread during the Late Quaternary. During this time, permafrost on the QTP experienced repeated expansion and degradation. Based on the remains and cross-correlation with other proxy records such as those from glacial landforms, ice-core and paleogeography, the evolution and changes of permafrost and environmental changes on the QTP during the past 150,000 years were deduced and are presented in this paper. At least four obvious cycles of the extensive and intensive development, expansion and decay of permafrost occurred during the periods of 150–130, 80–50, 30–14 and after 10.8 ka B.P.. During the Holocene, fluctuating climatic environments affected the permafrost on the QTP, and the peripheral mountains experienced six periods of discernible permafrost changes:(1) Stable development of permafrost in the early Holocene(10.8 to 8.5–7.0 ka B.P.);(2) Intensive permafrost degradation during the Holocene Megathermal Period(HMP, from 8.5–7.0 to 4.0–3.0 ka B.P.);(3) Permafrost expansion during the early Neoglacial period(ca. 4,000–3,000 to 1,000 a B.P.);(4) Relative degradation during the Medieval Warm Period(MWP, from 1,000 to 500 a B.P.);(5) Expansion of permafrost during the Little Ice Age(LIA, from 500 to 100 a B.P.);(6) Observed and predicted degradation of permafrost during the 20 th and 21 st century. Each period differed greatly in paleoclimate, paleoenvironment, and permafrost distribution, thickness, areal extent, and ground temperatures, as well as in the development of periglacial phenomena. Statistically, closer dating of the onset permafrost formation, more identification of permafrost remains with richer proxy information about paleoenvironment, and more dating information enable higher resolution for paleo-permafrost reconstruction. Based on the scenarios of persistent climate warming of 2.2~2.6 °C in the next 50 years, and in combination of the monitored trends of climate and permafrost changes, and model predictions suggest an accelerated regional degradation of plateau permafrost. Therefore, during the first half of the 21 st century, profound changes in the stability of alpine ecosystems and hydro(geo)logical environments in the source regions of the Yangtze and Yellow rivers may occur. The foundation stability of key engineering infrastructures and sustainable economic development in cold regions on the QTP may be affected. 相似文献
995.
资源环境承载能力预警评价中,除基础评价外,还需针对城市化地区存在的特殊问题开展专项评价。但现有相关研究大多较为复杂、难以操作,且较少考虑目前中国城市化地区之间存在的发展水平差异,难以满足实际应用需求。本文根据近年来中国城市化地区资源环境凸显的主要问题,从城市人居环境入手,选取城市黑臭水体和PM2.5超标天数2个要素设计城市水气环境黑灰指数,作为城市化地区专项评价的主要指标;同时考虑到优化开发区域和重点开发区域的主体功能导向差异,以京津冀城市化地区为例开展试评价分析,结果显示:京津冀城市化地区的104个区县中,47个区县为重警,46个中警,仅11个为轻警,整体形势不容乐观。其中,重警区县主要集中在京广高铁沿线,与地区主要交通线路分布及钢铁工业聚集格局一致;轻警区县主要分布在北部的张承地区及东部沿海,工业相对较少,且地理位置有利于污染物迅速扩散。城市化地区专项评价作为基础评价的补充,能更清晰地反映现阶段影响城市人居环境的主要制约因子以及不同发展水平地区间的差异,评价结果可为城市化地区开展国土空间规划、城镇发展战略研究和产业结构调整等提供科学依据。 相似文献
996.
基于曹娥江下游星一村钻孔(XYC)1717个沉积物样品的粒度分析,探讨钱塘江南岸地区全新世沉积环境记录。初步结论如下:①基于粒度组成,XYC孔沉积物分为粉砂质砂、砂质粉砂、粉砂、粘土质粉砂等4种岩性,自下而上分为9个单元,组成多个粒度粗细变化旋回。其中,第1、4两层为粉砂质砂,砂含量高于粉砂,粘粒含量10%~15%,分选差,动力条件复杂;第3、5两层属砂质粉砂,粉砂为主但含量不超过70%,砂含量20%以上,粘粒含量低;第2、7、9三层为粉砂,粉砂含量超过70%,粘粒、砂含量均低于20%;第6、8两层为粘土质粉砂,粉砂含量高于70%,粘粒含量20%以上。②基于C-M图最大搬运动能和平均搬运动能强弱及其变化幅度分析,初步识别出4种沉积相,自下而上依次为河流相、潮坪相、浅海相、湖沼相。第1、2两层为高能且不稳定的水动力搬运环境,属河流相沉积;第3~7层,最大搬运动能减弱,搬运动力与上虞近海滩涂样品SYTT相似,为潮坪相沉积;第8~9层,搬运动能低且变幅小,属低能静水沉积环境;第8层下段为浅海相,该层上段至第9层为湖沼相。③粒度纵向变化表明,粘粒含量自下而上波动增加;砂含量自下而上波动减少,平均粒径向上变细、分选性向上变好。XYC孔记录了该地区9690~5131 cal.a BP时段海面上升,达到高海面继而又海退的过程。研究结果有利于进一步探索钱塘江流域的古环境演变特征。 相似文献
997.
建立资源环境承载能力监测预警机制和对超载区域实行限制性措施是中央全面深化改革的一项重大任务,而超载成因分析既是资源环境承载能力监测预警技术方法的重要组成内容,也是制定超载区域限制性政策的基础。本文在搭建超载成因分析总体框架、阐述关键因素识别和成因分析方法的基础上,梳理总结了陆域/海域基础和专项评价结果以及综合预警超载成因分析的要点,进而在京津冀地区进行了实证应用。研究结果表明,导致京津冀地区资源环境超载的因素涉及自然、发展和管理各个类别,但关键因素的作用程度及其组合叠加状况在基础评价结果、专项评价结果和综合预警超载成因方面存在较大差异;水资源严重短缺、地形和气象等自然条件是导致超载的基础原因;人口和产业过度集聚以及低端产业、能源消费结构等发展类因素是超载的首要原因;内海净化能力差和海域生态退化、海岸开发强度大综合导致了海域生态环境超载;现行的资源环境管理政策尚不具备有效遏制和解决资源环境超载的能力。既有的超载成因分析框架、方法及要点可供各省、市、自治区开展资源环境承载能力监测预警借鉴。 相似文献
998.
基于环境因素约束的农户土地利用效率及影响因素分析 ——以河南省粮食生产核心区为例 总被引:2,自引:1,他引:1
在农户土地利用引起的环境问题日益严重的背景下,如果农户土地利用效率的测度只考虑“好”产出,而忽略“非意欲”的环境产出,可能造成测度结果不准确。因此,将“非意欲”产出引入农户土地利用效率评价模型,利用河南省粮食生产核心区农户抽样调查数据测度环境因素约束下的农户土地利用效率,并探讨其效率状况及影响因素。主要的结论如下:① 调查区域环境因素约束下的农户土地利用效率平均值仅为0.612,效率值总体不高,有较大的提升空间。不同规模农户的土地利用效率差异较大,二者呈现“U型”变化趋势。② 环境因素约束下的农户土地利用效率的影响因素来自多个方面。其中,农户生计方式兼业化在一定程度上降低了环境约束下的农户土地利用效率;种植结构调整虽然能够增加农户农业收入,但不利于环境因素约束下的农户土地利用效率的提升;农地经营规模扩大对环境约束下的农户土地利用效率产生正面影响,但现实中普遍存在的小规模化经营和土地细碎化消减了正面影响。土地产权的稳定性能够提升环境约束下的农户土地利用效率,但无论是集体土地产权还是承包经营权以及农户间非正式的土地流转产权关系稳定性较差,不利于环境约束下的农户土地利用效率的提高。 相似文献
999.
西南山地滑坡灾害生态风险评价——以大理白族自治州为例 总被引:6,自引:1,他引:5
本文以云南省大理白族自治州为例,综合考量生态风险源、受体、暴露响应过程及生态终点,采用信息量模型评估滑坡灾害危险性,基于景观格局指数表征生态脆弱性,并将生态系统服务纳入风险损失的定量表征,定量评估流域滑坡灾害生态风险。结果表明:① 低于1800 m高程、15°~25°坡度、小于0.31植被覆盖指数等10方面因素构成了诱发大理州滑坡灾害的最佳信息量组合,全州普遍处于滑坡灾害危险性中高水平,且西北低、东南高;② 生态脆弱性高值区主要集中在红河流域南部、金沙江流域东南部、澜沧江流域中部;③ 低生态损失流域的水源涵养、粮食供给服务相对较差,生态损失中等流域的净初级生产、土壤保持服务优势明显,高生态损失流域则具有较强的粮食供给和水源涵养服务;④ 基于高中低3种生态风险等级和“高危险—低脆弱—低损失”等8种风险结构,全州367个小流域可划分出避让监测预警区、生态保护恢复区、避让保护兼顾区、自然适应调控区等4种风险防范类型区。 相似文献
1000.
“全球—国家—地方”尺度下的国际贸易环境效应研究进展 总被引:1,自引:1,他引:0
贸易与环境关系是理解人地关系的重要方面,是“人—地”相互作用复杂性与不确定性的集中体现。国际贸易环境效应研究旨在探究自由贸易和环境保护之间的互补或互斥关系,由此催生了一系列具有关联性和竞争性的理论假说。本文结合全球化过程中全球联系强化、地方力量崛起和国家力量变革等特征,分别从全球联系、国家力量和地方发展3个层面梳理了现有贸易环境效应理论假说和实证研究,指出贸易环境效应是贸易过程(流量变化)与贸易影响(存量累积)共同作用下的结果。建立在比较优势基础上的存量因素包括经济增长、环境规制与地理区位,而贸易与投资一体化的特征则揭示了产业内贸易的重要性。由此,本文发现既有研究更多地建立在比较优势的基础上探讨全球与国家层面的贸易环境效应,表现出“自上而下”的特征,强调环境规制的作用。同时,也存在局限于南北国家关系、对地方化特征关注较少、对贸易政策与环境规制协调机制的理解有待深化等不足。据此,本文提出全球地方化进程中的区域发展分析将为贸易环境效应研究提供有益补充,其作用为:①实现对贸易引致的区域不均衡性的考量;②满足产业内与产业间贸易并存的分析需要;③为协调环境规制与贸易政策提供合宜的尺度。 相似文献