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The Earth is a very complicated system. For simplicity we divide the Earth system in three major parts: atmosphere, ocean and land, but we also have to include the Earth interior, the cryosphere (ice sheets) and even the rest of the universe (solar wind etc.). 
Observing the Earth is one of the most important activities of today's society and will benefit many areas such as disaster, health, energy, climate, water, weather, ecosystems, agriculture and biodiversity. Global earth observations depend on geodetic products like a global reference frame (coordinate system for positioning) and measurements of the changes of gravity and the earth rotation. This article will give you some background information. I will start to look at the Earth system from a geodetic perspective. The ambition is to start out simple and lead the interested reader to more complicated material - so please, stay tuned as I update this article! Earth Observation We observe Earth from the surface of the planet with various instruments as well as from space with modern satellite technology. Both perspectives are necessary in order to understand our home planet to the fullest.  
Geodesy Geodesy is the science of determining the geometry, gravity field, and rotation of the Earth and their evolution in time. Traditionally, geodesy has been serving other sciences and have had many societal applications, including mapping. With the advent of satellite geodesy it developed into a science making unique contributions to the study of the Earth system, its inherit dynamics, and its response to climate change, as well as a tool underpinning a wide variety of other remote sensing techniques. Facilitated by the Global Navigation Satellite Systems such as GPS, a wide and growing set of applications associated with positioning and navigation is opening up. Geodesy provides the foundation on which all Earth observation systems are built. In this function, geodesy is essential for Earth observation just like the foundation and frame of a house are necessary to keep it stable over time. But modern geodesy does more: with its three pillars of geokinematics, Earth gravity field, and Earth rotation, it also provides comprehensive observations of changes in the Earths shape, gravity field and rotation. Climate Change Geodesy is of fundamental importance in observing climate change. Tide gages measure changes in the sea level, and when combined with GPS measurements at the site, the change in absolute sea level may be determined. Changes in the large ice caps of Antarctica, Greenland may be determined with the help of space geodesy (satellite altimetry). For these ice sheets and large regions like Tibet/Himalay with a large number of glaciers these changes may also be observed from satellites measuring the gravity field. To determine and monitor the ocean circulation and transport and possible changes in these due to climate change, geodesy needs to provide a high precision geoid . When applied to the Arctic this may also contribute in determining the sea ice thickness. Geohazards Geodetic techniques are crucial in the assessment of geohazards and anthropogenic hazards, and they will play a pivotal role in early warning systems of such hazards and disasters. The prospect of these changes is increased security, a better use of resources, and progress towards sustainable development. Geohazards such as earthquakes, volcanic eruptions, landslides, subsidence, and precarious rocks are intimately connected to displacements and deformations of the Earth's surface. Geodetic techniques are capable of observing surface displacements on local to global scales at the highest possible accuracy. The gravitational effect of such changes may also be observed Water Cycle |
