To the best of our knowledge, Earth is unique among the planets of the solar system in its abundance of water in all three phases–gas, liquid, and solid–a consequence of the Earth’s radiative balance. In turn, this balance is strongly affected by global water cycles and biogeochemistry. Another unique feature of the Earth is that oxygen and reduced gases, such as methane, coexist in the atmosphere. Atmospheric composition and climate are intimately tied to biological processes like photosynthesis and decomposition. Therefore, understanding even the simplest aspects of the Earth system requires knowledge of geophysics, geochemistry, and biology.

We regard the Earth system as two subsystems–physical climate and biogeochemical cycles–linked by the global hydrologic cycle. Examination of these subsystems and their linkages defines the critical questions that the Earth Observing System (EOS) addresses.  To learn more about NASA’s Earth Science Enterprise and EOS, click on the forward arrow below.