Glaciers worldwide have shrunk to levels not seen in 120 years of record-keeping, with melt-off accelerating in the first decade of the 21st century, according to a study released Monday.
On average, glaciers currently lose between 50 to 150 centimetres (20 to 60 inches) of thickness every year, reported the study, published in the Journal of Glaciology.
"This is two to three times more than the corresponding average of the 20th century," said Michael Zemp, director of the World Glacier Monitoring Service and the study's lead author.
More than a billion people, especially in Asia and South America, get more than half of their drinking water from the seasonal melting of snow melt and glacier ice, previous research has shown.
The current rate of global glacier melt is without precedent for the 120 years covered by scientific observation, and probably for much longer, Zemp added.
Moreover, accelerated ice loss has created a dynamic whereby glaciers in many regions will continue to diminish even if global warming did not continue to boost global temperatures.
Preliminary data from the last five years, not covered in the study, suggest that rapid decline of ice mass is continuing apace.
The 20th-century record ice loss observed in 1998 "has been exceeded in 2003, 2006, 2011, 2013, and probably again in 2014," Kemp said.
The long-term trend of glacier retreat takes into account shorter periods where, in some locations, glaciers have regained some of their lost ice mass.
Many so-called "ice tongues" formed by glacier runoff in Norway, for example, regained a couple hundred metres in length during the 1990s. Overall, though, they have retreated by several kilometres compared to the areas covered in the late 19th century.
The World Glacier Monitoring Service compiles the results of worldwide glacier observations submitted annually from a global network of scientists & observers.
For the study, these observations were compared to all available data gathered on the ground, in the air and via satellite.