Most mountain glaciers have been retreating since the latter part of the 19th century, causing sea levels to rise. Tidewater glaciers go through a cycle: the glacier will occupy a stable position with the terminus at the head of a fjord, experience a slow advance that may last 1000 years, remain relatively stable and then experience a rapid retreat that may last less than a century. This cycle is not directly related to short-term climate change. Natural or culturally induced processes can cause global climatic cooling or warming. Glaciers respond to such warming or cooling periods by decreasing or increasing in size, thereby causing sea level to rise or fall. The general shrinkage of mountain glaciers during the 20th century is a major reflection of the fact that rapid secular change in the energy balance of the earth's surface is taking place on a global scale. The characteristic rate of this change (a few decimeters ice depth per year) as deduced from glacier mass losses is broadly consistent with the estimated anthropogenic greenhouse effects. The beginning of this rapid secular glacier retreat tendency was probably little affected by human activity. The observed evolution may, however, contain an increasing part of anthropogenic influence: recent shrinking of glaciers for the first time now coincides with a man-induced radiative effects which could be responsible for a major part of the additional energy flux causing the observed melt rate.
Glaciers can be classified according to their size and underlying topography. Various types of glaciers exist: ice sheets, ice shelves, ice caps, outlet glaciers, piedmont glaciers, and valley glaciers. Ice sheets exist only in Antarctica and Greenland and are nearly continuous masses of ice. Ice shelves represent the ungrounded or floating part of an ice sheet that extends into the ocean. Ice caps, far more numerous than ice sheets, are dome-shaped glaciers usually covering a highland area. Outlet glaciers flow outward from ice sheets and ice caps through valleys with distinct boundaries consisting of moraines and mountains. A piedmont glacier terminates onto flat land and its terminus spreads out into a broad lobate sheet. A valley glacier is a stream of ice flowing down a valley from an accumulation area. A glacier may be classified as cold (polar) or warm (temperate). Most glaciers move slowly, a few centimetres to less than a meter a day. However, some glaciers move very rapidly. For example, surge- type and tidewater glaciers can move very rapidly for short periods of time. A surge-type glacier experiences a relatively short-lived episode of greatly-accelerated flow followed by a relatively longer period of stagnation and retreat. The present volume of the Earth's glacier ice, if totally melted, represents about 80 meters in sea-level rise.
NASA reports that ice extent in the Arctic Ocean declined by about 2% between 1978 and 1987. Mountain glaciers are retreating almost everywhere in the world. Ice cores in glaciers show that temperatures between 1937 and 1987 were higher than for any 50 year period for 12,000 years.