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.
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.