Many aspects of the Earth’s land, oceans, and atmosphere are continually observed from Earth-orbiting satellites, which are playing an increasingly important role in observing both the natural variability as well as significant modifications to our planet as a direct consequence of human activity. Land and ocean surfaces, including the semi-permanent ice caps and seasonally varying sea ice of the Polar Regions, often change dynamically, with sometimes strikingly large short-term changes as well as seasonal and interannual variations. Glaciers and other land ice generally undergo slow changes over time, but sometimes experience quite large and unexpected changes, including collapse. The advent of Earth-observing satellites in the late 1960s and early 1970s began the field of remote sensing in which instruments on Earth-oriented spacecraft observed our planet from above, without regard to political boundaries and manmade structures, and opened the public’s eyes to our precious planet as well as to its vulnerability and susceptibility to natural and anthropogenic development.

The vantage point of space, coupled with the extraordinary technological developments of the last decade, now permits the observation of many aspects of the Earth’s environment that are simply not possible, or at least not practical, from ground-based, shipborne, or airborne observations. The Earth-observing satellites of today have evolved in capability from the earliest Earth-orbiting satellites, and are providing both monitoring and increased understanding of the many changes that are occurring throughout our planet’s climate system.