全文获取类型
| 收费全文 | 57864篇 |
| 免费 | 8528篇 |
| 国内免费 | 12653篇 |
专业分类
| 测绘学 | 5625篇 |
| 大气科学 | 7718篇 |
| 地球物理 | 10916篇 |
| 地质学 | 27551篇 |
| 海洋学 | 7381篇 |
| 天文学 | 7205篇 |
| 综合类 | 3797篇 |
| 自然地理 | 8852篇 |
出版年
| 2024年 | 183篇 |
| 2023年 | 672篇 |
| 2022年 | 1712篇 |
| 2021年 | 2027篇 |
| 2020年 | 2416篇 |
| 2019年 | 2511篇 |
| 2018年 | 2050篇 |
| 2017年 | 2546篇 |
| 2016年 | 2501篇 |
| 2015年 | 2763篇 |
| 2014年 | 3466篇 |
| 2013年 | 3882篇 |
| 2012年 | 3710篇 |
| 2011年 | 3926篇 |
| 2010年 | 3255篇 |
| 2009年 | 4089篇 |
| 2008年 | 4104篇 |
| 2007年 | 4329篇 |
| 2006年 | 4145篇 |
| 2005年 | 3484篇 |
| 2004年 | 3085篇 |
| 2003年 | 2638篇 |
| 2002年 | 2156篇 |
| 2001年 | 1847篇 |
| 2000年 | 1818篇 |
| 1999年 | 1721篇 |
| 1998年 | 1441篇 |
| 1997年 | 1144篇 |
| 1996年 | 926篇 |
| 1995年 | 791篇 |
| 1994年 | 691篇 |
| 1993年 | 652篇 |
| 1992年 | 518篇 |
| 1991年 | 367篇 |
| 1990年 | 290篇 |
| 1989年 | 256篇 |
| 1988年 | 200篇 |
| 1987年 | 118篇 |
| 1986年 | 110篇 |
| 1985年 | 103篇 |
| 1984年 | 68篇 |
| 1983年 | 41篇 |
| 1982年 | 47篇 |
| 1981年 | 33篇 |
| 1980年 | 38篇 |
| 1979年 | 19篇 |
| 1978年 | 21篇 |
| 1977年 | 36篇 |
| 1976年 | 26篇 |
| 1973年 | 18篇 |
排序方式： 共有10000条查询结果，搜索用时 31 毫秒
71.
72.
73.
We present results from a new simulation code that accounts for the evolution of the reservoirs of carbon dioxide on Mars, from its early years to the present. We establish a baseline model parameter set that produces results compatible with the present (i.e., Patm?6.5 mbar with permanent CO2 ice cap) for a wide range of initial inventories. We find that the initial inventory of CO2 broadly determines the evolutionary course of the reservoirs of CO2. The reservoirs include the atmosphere, ice cap, adsorbed CO2 in the regolith, and carbonate rocks. We track the evolution of the free inventory: the atmosphere, ice cap and regolith. Simulations begin at 4.53 Gyr before present with a rapid loss of free inventory to space in the early Noachian. Models that assume a relatively small initial inventory (?5 bar) have pronounced minima in the free inventory of CO2 toward the end of the Noachian. Under baseline parameters, initial inventories below ∼4.5 bar result in a catastrophic loss of the free inventory to space. The current free inventory would be then determined by the balance between outgassing, sputtering losses and chemical weathering following the end of the late bombardment. We call these “thin” models. They generically predict small current free inventories in line with expectations of a small present CO2 ice cap. For “thick” models, with initial inventories ?5 bar, a surplus of 300-700 mbar of free CO2 remains during the late-Noachian. The histories of free inventory in time for thick models tend to converge within the last 3.5 Gyr toward a present with an ice cap plus atmospheric inventory of about 100 mbar. For thick models, the convergence is largely due to the effects of chemical weathering, which draws down higher free inventories more rapidly than the low. Thus, thick models have ?450 mbar carbonate reservoirs, while thin models have ?200 mbar. Though both thick and thin scenarios can reproduce the current atmospheric pressure, the thick models imply a relatively large current CO2 ice cap and thin models, little or none. While the sublimation of a massive cap at a high obliquity would create a climate swing of greenhouse warming for thick models, under the thin model, mean temperatures and pressures would be essentially unaffected by increases in obliquity. 相似文献
74.
Characteristics of Spatial and Temporal Variations of Monthly Mean Surface Air Temperature over Qinghai-Tibet Plateau 总被引：1，自引：0，他引：1
ZHANG Qianggong KANG Shichang YAN Yuping 《中国地理科学(英文版)》2006,16(4):351-358
1 Introduction The Qinghai-Tibet Plateau, known as the highest plateau with the most complex topography in the world, covers an area of more than 200km2, with a mean elevation of more than 4000m a.s.l. (Ye and Gao, 1979). Surrounded by the Earth’s highest mountains, such as the Himalayas, Pamir, Kunlun Mountains, the plateau plays a significant role in climate change in China even in the world, thus attracted great attention of researchers. Up to now, many achievements have been gained by… 相似文献
75.
地下水资源可持续利用的一个急待解决的重要问题,是对地下水补给和更新能力的评价.利用环境同位素技术研究地下水的补给和可更新性是当前较为新颖的方法之一.在西北干旱、半干旱的隐伏岩溶地区,地下水埋藏条件复杂,常规的地质勘探方法所能提供的水文地质信息有限,环境同位素方法在研究地下水的补给及可更新能力方面发挥了优势,可对传统方法进行补充和验证.其结果表明,研究区隐伏岩溶水形成较早,且有大量现代水的混入,平均混入量为54%.说明区内隐伏岩溶水的补给和更新能力较好.环境同位素分析结果还显示,大岔河隐伏岩溶水为一相对独立、半开放的水文地质单元,其补给来源部分为流域内大气降水、地表水的补给,部分为东南部三道沟岩溶地下水的补给;根据环境同位素EPM模型计算,地下水的滞留时间为36 a.地下水储存量为1.314×108 m3; 储水系数为7.29×10-3.这一结果与传统勘探方法的计算结果基本吻合,说明环境同位素方法的实用性. 相似文献
76.
77.
78.
79.
80.