Between 1983 and 2008 while most of the world's glaciers were retreating many New Zealand glaciers advanced. Franz Josef regained nearly half the ice it had lost over the previous century. This anomalous growth has been attributed not to increased rainfall but to a series of cool years caused by increased southerly air flow in the Tasman sea. After 2008 the glacier entered a very rapid phase of retreat, shrinking by 1.5 km between 2008 and 2017. It is now once again 3 km shorter than it was 100 years ago. Based on these patterns, Franz Josef Glacier is predicted to retreat and lose 38% of its mass by 2100 in a mid-range scenario of warming, although it may retreat as much as .