Publisher Summary This chapter throws light on global dimensions on earth and its atmosphere. The chapter illustrates that the Earth system, as an entity in the cosmos, exists within an envelope of gases that are kept from escaping into the space by gravitational attraction to the planet below. Below 90 kilometers is the thin atmospheric region called “the homosphere”, where gases are well mixed with a highly uniform composition throughout. Moreover, subdivisions of the homosphere are distinguished by temperature zones at different altitudes. Troposphere is the principal layer that connects climate, weather, and life across the planet. Most of the water vapor and other ‘‘greenhouse gases'’ associated with insulating and retaining heat at the Earth's surface exist in the troposphere. Subsequently, the baseline or zero point for determining altitudes in the atmosphere is the sea level. In the Earth system, the largest water reservoir is the ocean. Water is the common environmental feature that makes life possible. It is the major constituent of metabolizing cells in all known life forms. The second largest non-marine reservoir occurs in the ground, with more than 20% of the freshwater on Earth. Finally, each life stage has a unique discrete history unto itself, and at the same time is linked with and often preconditioning future circumstances—as in the Earth system and human civilization.