<P> Following the Little Ice Age's end around 1850, glaciers around the Earth have retreated substantially . A slight cooling led to the advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous . </P> <P> Glaciers move, or flow, downhill due to gravity and the internal deformation of ice . Ice behaves like a brittle solid until its thickness exceeds about 50 m (160 ft). The pressure on ice deeper than 50 m causes plastic flow . At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers . When the stress on the layer above exceeds the inter-layer binding strength, it moves faster than the layer below . </P> <P> Glaciers also move through basal sliding . In this process, a glacier slides over the terrain on which it sits, lubricated by the presence of liquid water . The water is created from ice that melts under high pressure from frictional heating . Basal sliding is dominant in temperate, or warm - based glaciers . </P> <P> Although evidence in favour of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as the idea that melt water, refreezing inside glaciers, caused the glacier to dilate and extend its length . As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow . James Forbes came up with the essentially correct explanation in the 1840s, although it was several decades before it was fully accepted . </P>

Which is the longest alpine glacier on earth