地球作为一个高度复杂的非线性系统，各圈层（大气、海洋、陆地、生物、冰雪圈、固体地球）尤其是人类活动等任何组成成份的变化，都会引起地球系统的变化。人类可持续发展面临的巨大科学挑战之一是认识人类赖以生存的、复杂变化的地球系统，认识地球系统如何变化及主要驱动因素，认识地球系统未来变化趋势及如何提高对全球变化的适应能力。卫星独特的全球覆盖和日尺度的观测改变了地球科学的研究方法，它强调所能探测到的多时空尺度上的物理动力过程，在全球范围应对气候变化、能源和环境挑战具有重要作用，揭开了地球系统多学科交叉的新纪元。以地球系统的视野，抓住驱动地球系统的关键循环过程（如能量、水、生物化学循环），是当前地球系统科学的发展趋势。地球系统科学（全球变化）研究需要长期稳定、准确性较高的卫星观测数据，以水循环为例，卫星遥感具备获取全球范围水循环关键参数能力，但是系统性综合观测能力不足，整体精确性受到综合化的可靠空间数据集的限制。目前中国正在积极研制发展新型水循环卫星WCOM（Water CycleObservation Misssion），并寄希望以此为核心传感器发起全球分布式水循环观测星座系统，进一步提高中国在国际水循环观测与地球系统科学研究方面的话语权与领先能力。

Earth, as a single complex system, is changing; these changes affect every aspect of life on the planet and include profound implications on society. Studying the Earth system is essential to understanding the causes and consequences of climate change and other environmental concerns. Space-based observation systems explore interactions among the atmosphere, oceans, land surface interior, ice sheets, biology, and life itself under global coverage and a daily timescale. The present development trend of Earth system study focuses on the important circulation processes(such as energy cycle, water cycle, and biochemical cycle) that drive the Earth system. The water cycle is the most active cycling process and is thus the core foundation of scientific issues in the Earth system. To improve Earth system science and global change research, accurate observations of various spatial-temporal processes and key parameters of the global water cycle should be derived. Moreover, significantly improving the accuracy and synchronization of key water cycle parameter measurements necessitate the development of innovative theory and techniques for observation and retrieval.