How do accumulation and ablation determine whether a glacier grows or shrinks?

The growth or shrinkage of a glacier is determined by its net mass balance, which is the difference between the total income of mass, known as accumulation, and the total loss of mass, known as ablation.12

• Accumulation: This includes all processes that add mass to a glacier, primarily snowfall, but also contributions from hoarfrost, rime, hail, the freezing of rain or meltwater, and avalanching.1
• Ablation: This includes all processes that remove mass from a glacier, such as surface melting and runoff, the calving of icebergs, evaporation, and ice avalanches.12

For a glacier to maintain a constant size, there must be a perfect balance between these two factors. If the mass balance is positive—meaning accumulation exceeds ablation—the glacier will grow and increase in thickness. Conversely, if the mass balance is negative—meaning ablation exceeds accumulation—the glacier will shrink. These changes in mass balance eventually trigger a dynamic response, causing the glacier's terminus to either advance or retreat over time.123