On our ever-changing Earth, conditions may change quickly or slowly. Some changes come from natural processes; some from human activity. Satellites allow us to see conditions and track changes over time — in land use, forest health, land/water interface, and so on. Since 1972, Landsat satellites have been collecting data using various portions of the visible and invisible electromagnetic spectrum, at a scale close enough to see highways, but not individual buildings on a city block.
What is sea ice? It is simply frozen ocean water. Why is sea ice important? While it occurs mainly in polar regions, sea ice influences our global climate. Changing amounts of sea ice can affect ocean circulations, weather patterns, and temperatures around the world. Sea ice insulates the relatively warm ocean water from the cold polar atmosphere, except where cracks in the ice allow for the exchange of heat and moisture. The exchange of salt between sea ice and the ocean alters the density of ocean waters, thus influencing ocean circulation. Many animals, such as polar bears, seals, and walruses, depend on sea ice for their habitat. These species hunt, feed, and breed on the ice. Satellites provide the best way to observe sea ice, the factors that affect sea ice, and the ways sea ice affects global climate. Scheduled to launch in 2010, NASA’s Aquarius mission will measure global sea surface salinity with unprecedented resolution. Even small variations in sea surface salinity — the amount of salt present near the ocean’s surface — can have dramatic effects on sea ice, the water cycle, and ocean circulation.
Water on earth is used over and over. The water cycle, the continuous movement of water from ocean to air and land then back to the ocean in a cyclic pattern, is a central concept in meteorology.