In Europe and North America, glaciers are generally more accessible and there are more trained people to study them. Switzerland's Aletsch glacier, the largest in the Alps, has been retreating for about 150 years. But the glacier, which feeds the River Rhone, still stores an estimated 27 billion tonnes of ice, according to www.swissinfo.ch.
That's about 12 million Olympic-sized swimming pools.In 2008, a total of 79 Swiss glaciers were in retreat, while 5 were advancing, the Swiss Glacier Monitoring network says.
"There are a very small number of glaciers that are monitored," said veteran glaciologist Ian Allison, pointing to less than 100 globally for which there are regular "mass-balance" measurements that reflect how much a glacier grows or shrinks from one year to the next.
Such measurements are the benchmark and several decades of data is regarded as the best way to build up an accurate picture of what's happening to a glacier.
Glaciers originate on land and represent a sizeable accumulation of snow and ice over the years. They tend to carve their way through valleys as more and more ice accumulates until the point where more is lost through melting than is gained.