14.3: Glacial Budget

A glacial budget is like a bank account, with the ice being the existing balance. If there is more income (snow accumulating in winter) than expense (snow and ice melting in summer), then the glacial budget shows growth. A positive or negative balance of ice in the overall glacial budget determines whether a glacier advances or retreats, respectively. The area where the ice balance is growing is called the zone of accumulation. The area where ice balance is shrinking is called the zone of ablation.

The diagram shows these two zones and the equilibrium line. In the zone of accumulation, the snow accumulation rate exceeds the snow melting rate and the ice surface is always covered with snow. The equilibrium line, also called the snow line or firn line, marks the boundary between the zones of accumulation and ablation. Below the equilibrium line in the zone of ablation, the melting rate exceeds snow accumulation, leaving the bare ice surface exposed. The position of the firn line changes during the season and from year to year as a reflection of a positive or negative ice balance in the glacial budget. Of the two variables affecting a glacier‘s budget, winter accumulation and summer melt, summer melt matters most to a glacier’s budget. Cool summers promote glacial advance and warm summers promote glacial retreat [5].

Figure \(\PageIndex{1}\): Cross-sectional view of an alpine glacier showing internal flow lines, zone of accumulation, equilibrium line, and zone of ablation.

If warmer summers promote glacial retreat, then global climate warming over decades and centuries will accelerate glacial melting and retreating significantly. Global warming due to human burning of fossil fuels [6] is causing the ice sheets to melt (or lose mass) at an increasing rate over years and decades rather than over centuries [7]. Current glacial melting is contributing to rising sea-levels faster than expected based on the previous history.

As the Antarctica and Greenland ice sheets melt during global warming, they become thinner or deflate [8; 9]. The edges of the ice sheets break off and fall into the ocean, a process called calving, becoming floating icebergs. A fjord is a steep-walled valley flooded with sea water. The narrow shape of a fjord has been carved out by a glacier during a cooler climate period. During a warming trend, glacial meltwater may raise the sea level in fjords and flood formerly dry valleys.

Figure \(\PageIndex{2}\): Fjord in Norway.

Glacial retreat and deflation are well-illustrated in the 2009 TED Talk by James Balog.

References

5. Earle, S. Physical geology OER textbook. (BC Campus OpenEd, 2015).

6. Stocker, T. F. Climate change 2013: the physical science basis: Working Group I contribution to the Fifth assessment report of the Intergovernmental Panel on Climate Change. (Cambridge University Press, 2014).

7. Velicogna, I. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophys. Res. Lett. 36, L19503 (2009).

8. Nick, F. M., Vieli, A., Howat, I. M. & Joughin, I. Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus. Nat. Geosci. 2, 110–114 (2009).

9. Jensen, T. S., Box, J. E. & Hvidberg, C. S. A sensitivity study of annual area change for Greenland ice sheet marine terminating outlet glaciers: 1999–2013. J. Glaciol. 62, 72–81 (2016).