Although the climate has been relatively warm and stable over the last 6,000 years, there have been cool periods when glaciers expanded in their upper reaches. The most recent of these, sometimes called the ‘little ice age’, began about the 13th century and continued until the 1890s. Since then, warmer temperatures have caused the glaciers to retreat.

Tasman Glacier (foreground) has been gradually melted and thinned, and its surface has become covered in moraines – piles of rocks and sediment deposited by the glacier. A lake began to form at the front of the glacier in the mid-1980s, and the ice is now thin enough to break off and float away, accelerating the rate of retreat. Most of the large glaciers in the Mt Cook area are now retreating.