Valley or Alpine glaciers are among the most sensitive indicators of climate change because they are sensitive to temperature fluctuations and because they reflect average changes over space and time.

Glaciers are not just stationary piles of ice. They are living rivers of ice creeping and grinding down frozen valleys.

Glaciers are the result of snow that accumulates and is compressed into gradually more dense ice by the weight of successive snowfalls. When the ice is about 30 feet thick, it becomes plastic and begins to flow under its own weight.

Although we speak of a glacier “;receding,”; the ice never flows uphill. A glacier recedes when melting, calving or excessive flow rate in the lower reaches exceeds snowfall accumulation at higher elevations.

With few exceptions, glaciers have retreated at ever increasing rates over the last century, and the rate of increase is relatively uniform in both the Northern and Southern hemispheres.

Midlatitude mountain glaciers in the Himalayas, Alps, Rocky Mountains, Cascade Range, New Zealand and the southern Andes, as well as isolated tropical summits such as Mount Kilimanjaro in Africa and Puncak Jaya in New Guinea, are all experiencing net losses of ice.

Because glaciers are sensitive to the temperature and precipitation changes that accompany climate change, their rate of growth or decline serves as an indicator of regional and global climate change. There could be other reasons affecting the changes of individual glaciers, but global trends average out local variations.

The World Glacier Monitoring Service has recorded 17 consecutive years of net ice loss worldwide.

In the Alps the surface area of 940 glaciers decreased 18 percent from 1973-99, and the rate of loss is now six times faster than in the period of 1850-1973. Nearly all of the 93 glaciers monitored by the Austrian Alpine Association receded in 2009. Alpine glaciers in Switzerland, France and Italy are all receding as well.

In North America named glaciers have decreased from 37 to 25 in Glacier National Park, and many of the park's white attractions are either gone or not visible. Many glaciers in the North Cascades and Canadian Rockies have receded miles in an adult lifetime.

In the Himalayas the amount of glacial runoff available for water resources has fallen by more than 20 percent in the past 50 years, and the ice sheet of West Antarctica lost 75 percent more ice in 2006 than in 1996.

Whether global glacial retreat is a result of global warming is debatable, but glaciers are shrinking faster than at any other time in human existence and at an increasing rate.

In the grand scheme of nature, glaciers are much more than casual players. Their presence or absence affects water resources, crop potential, animal habitat, distribution of vegetation and other factors that influence human and nonhuman ecosystems.