Antarctic Ice Sheet mass balance

How does mass balance vary over Antarctica? | Surface mass balance in the past | Surface mass balance in the future | References | Comments |

How does mass balance vary over Antarctica?

Is Antarctica currently losing or gaining mass? Will this massive ice sheet grow or shrink in the future? And what effect will increased snowfall have over coming centuries? In order to answer these questions, we must analyse the surface mass balance of the Antarctic Ice Sheet.

First, let’s introduce some definitions.

• Mass balance is the sum of all processes of accumulation and ablation, including those at the ice surface and at the bed, but does not include mass changes due to ice flow¹. See this page (Introduction to Glacier Mass Balance) for more information.
• Surface mass balance is the net balance between the processes of accumulation and ablation on a glacier’s surface (it does not include dynamic mass loss and basal melting)¹.
• Climatic mass balance includes surface mass balance and internal accumulation¹.
• Ice dynamical changes may include changes to ice discharge and acceleration or deceleration of flow, which can lead to dynamic thinning or thickening, ice-shelf collapse, marine ice sheet instability, and other factors resulting in changes in the glacier beyond surface mass balance.

Surface mass balance

Surface mass balance varies extensively over Antarctica. The Antarctic Peninsula has the highest accumulation rates (up to 1500 mm per year), followed by coastal West Antarctica, which has around 1000 mm accumulation per year². Compare this with the interior of the Antarctic Ice Sheet, where it is dry and cold; here accumulation can be less than 25 mm per year.

Surface mass balance estimates are constantly improving as scientists gain better understandings of glacio-isostatic adjustment, improve glacier modelling techniques and gain access to higher resolution satellite datasets over longer timescales³. Surface mass balance estimates therefore tend to improve over time, but are subject to large uncertainties⁴. For this reason, there tends to be differences between the results of different techniques used to measure surface mass balance. Surface mass balance of the grounded Antarctic Ice Sheet is currently estimated at ~2000 gigatonnes per year^{2, 5, 6}, and it is subject to large variability across the ice sheet and through time.

Total mass balance

The figure below shows some recent estimates for total mass balance (including basal processes) over Antarctica⁷. Each box is bounded by the time interval studied and the uncertainties identified.

Overall, a recent estimate puts Antarctic net mass balance at -71 ± 53 gigatonnes per year⁸, so just negative over the 19 year survey. Mass losses are increasing in West Antarctica and the Antarctic Peninsula. The mass balance of West Antarctica is dominated by dynamic losses from the Amundsen Sea sector, and dynamic gains from the Kamb Ice Stream⁸. From the period 2005-2010, Shepherd et al. (2012) estimate the mass balance of the entire Antarctic Ice Sheet to be -81 ± 37 gigatonnes per year⁸.

An unweighted average of recent estimates suggests that Antarctica moved from a weakly negative mass balance in the 1990s to a faster rate of mass loss at a rate of between -45 and -120 gigatonnes per year⁷. Larger dynamic losses in West Antarctica are being partially offset by increases in accumulation over East Antarctica.

The total mass balance of Antarctica was recently updated here.

Accelerating total mass losses from Antarctica

The GRACE (Gravity Recovery and Climate Experiment) satellite gravity mission shows that total mass loss in Antarctica is accelerating over time. They found that total mass loss increased by 26 ± 14 gigatonnes per year from 2002 to 2009⁹. Rignot et al. (2011) found a smaller acceleration of 14.5±2 gigatonnes per year from 1993-2011⁵, but this change is still three times larger than that found for mountain glaciers and ice caps.

Surface mass balance of Antarctica in the past

How has the surface mass balance of Antarctica changed in the past? Firn and ice-core records can hold the key to providing a longer perspective on surface mass balance than is currently available from satellite records. Frezzotti et al. used 67  of these cores to reconstruct surface mass balance over the last 800 years. They found that current surface mass balance is not exceptionally high compared with the last 800 years¹⁰. Periods of high accumulation occurred in the past, in the 1370s and 1610s AD, but there has been an increase of 10% in snow accumulation in some coastal regions since 1850 – a fact that agrees with independent work on the Antarctic Peninsula¹¹.

Surface mass balance of Antarctica in the future

Climate models predict that, for a generally warmer climate, snowfall will increase over Antarctica⁷. Surface melt will increase around the more northerly Antarctic Peninsula, and dynamic changes such as increased ice discharge¹², ice-shelf collapse and grounding line recession¹³, and marine ice-sheet instability are likely to offset any increases in precipitation⁷. However, if no dynamical ice response is assumed, then increases in snowfall over the entire continent of 6-16% to 2100 AD and 8-25% to 2200 AD are likely to result in a drop in sea level of 20-43 mm in 2100 and 73-163 in 2200, compared with today¹⁴. However, it is more likely that the Greenland and Antarctic ice sheets will lose mass over the next century, with rapid coastal changes, increases in ice flow and ice-shelf collapse all likely⁴. As a result of these complex expected changes, there are a number of uncertainties in past, present and future ice sheet mass balance.

Further reading

• Glacier mass balance
• Dealing with uncertainty when predicting sea level rise
• The East Antarctic Ice Sheet
• The Antarctic Peninsula Ice Sheet
• Marine ice sheet instability
• Ice sheets and sea level: thinking outside the box (Van den Broeke et al., 2011)
• Glossary of Mass Balance and related terms