# Mass balance of the Antarctic ice sheet from 1992 to 2017

A new paper with a whole host of authors has just been published in Nature (IMBIE Team, 2018). It provides a new estimate of mass balance of the entire Antarctic Ice Sheet over the last 25 years, the longest and most thorough estimate of this to date.

This article argues that the Antarctic Peninsula, the smallest ice sheet in Antarctica, has lost an average of 20 Gigatonnes (Gt) of ice per year over the 25 year study. This increased during the study and especially since the year 2000.  The West Antarctic Ice Sheet lost 53±29 Gt yr-1 from 1992-1997, but this accelerated to 159±26 Gt yr-1 from 2012-2017. The East Antarctic Ice Sheet is more stable, with small gains (with large errors) over the study period.

## Mass balance of the Antarctic Ice Sheet

The “mass balance” of an ice sheet is the product of all the mass gained by the ice sheet (through processes such as snowfall, windblown snow, and other precipitation), and all the mass lost by an ice sheet (through processes such as snow melt, ice melt and calving icebergs).

The figure below, from IMBIE et al 2018, shows the mass balance of the Antarctic Ice Sheet. By using a range of methods, the authors are able to provide a mean estimate and a range of uncertainty for each of the different regions of Antarctica.

The y-axis of the plots is the rate of change (change in mass through time) for the West Antarctic Ice Sheet (WAIS), East Antarctic Ice Sheet (EAIS), and the Antarctic Peninsula Ice Sheet (APIS), and the x-axis is time (1992-2018). The number of collated mass balance estimates is shown for each bar.

## What does this mean for sea level rise?

We can calculate what this ice loss means in terms of sea level rise. The West Antarctic Ice Sheet is now contributing 0.4 mm of water to sea level rise every year. That’s 4.4 mm per decade, and the rate at which it is contributing to sea level rise is increasing.

The figure below shows cumulative sea-level contribution from the different Antarctic Ice Sheets from 1992-2018. The dashed lines are an earlier estimate from this team (Shepherd et al. 2012).

These large losses have been driven and accelerated by thinning and receding floating ice shelves. As ice shelves thin, they are less able to buttress their tributary glaciers, which can result in ice upstream of the ice shelf thinning and accelerating, contributing directly to sea level rise.

## Differences between different estimates of ice sheet mass balance

The paper, like many others, disputes the findings of Zwally et al (2015), who argued that mass gains in East Antarctica were large enough to outweigh losses in the West Antarctic Ice Sheet (see also Medley et al., 2018).

The authors note that the least agreement between measurements occurs in East Antarctica, with the poor temporal resolution of some measurements impeding accuracy. The closest agreement between datasets occurs over the Antarctic Peninsula, where trends tend to agree within 30 Gt yr-1, but trends depart by up to 100 Gt yr-1 in East Antarctica (IMBIE Team, 2018).

## Techniques used

The estimate is robust as it uses a large number of independent measurements, including satellite altimetry, gravitmetry and the input-output method. Satellite measurements of changes in ice sheets and glaciers have become common place since the early 1990s, and can provide excellent details of changes in snow and ice over entire continents, including areas where access is challenging, like Antarctica.

### Gravimetry

Gravimetry is the measurement of the strength of a gravitational field (Pritchard et al. 2010). Measuring the time-varying gravity field by satellite of an ice mass shows mass change in glacial environments. GRACE was launched in 2002, and it calculates the changes of the Earth’s geopotential due to surface and atmospheric mass-transport processes, including glacier accumulation and ablation (Pritchard et al. 2010).

Gravimetric estimates of mass change must be corrected for glacial isostatic adjustment. The IMBIE Team (2018) used 6 different glacial isostatic adjustment (GIA) models, 9 continent-wide simulations and 2 regional simulations. The net gravitational effect of GIA across Antarctica is positive, with the greatest uplift in areas where GIA is a large component of regional mass change (e.g. the Amundsen, Ross and Filchner-Ronne sectors of West Antarctica) (IMBIE Team, 2018).

### Satellite Altimetry

Satellite altimetry uses radars on satellites to measure the surface height of ice sheets. The surface height of 70% of the Antarctic Ice Sheet has been measured continuously since 1992 (Pritchard et al. 2010). This technique shows where ice is thinning, which has largely been attributed to the acceleration of ice.

### Input-Output method

The input-output method subtracts solid-ice discharge from net snow accumulation (IMBIE Team 2018). This method helps the interpretation of mass trends derived from gravimetry and altimetry. Snowfall is the main driver of variability in Antarctic Ice Sheet surface mass change. Antarctic Ice Sheet surface mass balance is taken from atmospheric models, evaluated with in situ weather stations and remote observations from satellites. Accumulation rates on the Antarctic Peninsula are the highest, and are 3x lower in the WAIS and 7x lower in the EAIS.

The variability in all the different products used is similar, and they all indicate an absence of a surface-wide trend in changes in surface mass balance from 1979-2017, which indicates that the mass loss from the Antarctic Ice Sheet is dominated by solid-ice discharge into the ocean.

## The take-home message

This paper agrees with recent studies that have mapped grounding line recession of major ice streams in the WAIS, associated with warm ocean currents melting ice sheets and ice shelves from below. This body of research indicates that it is not changes in snowfall or surface air temperatures that are driving major ice-sheet recession in West Antarctica; rather, it is grounding-line recession and increased calving, associated with decreased ice-shelf buttressing from thinner and smaller ice shelves, that is driving large-scale ice losses.

## References

The IMBIE Team, 2018. Mass balance of the Antarctic Ice Sheet from 1992-2017. Nature 558, 219–222.

Medley, B., McConnell, J.R., Neumann, T.A., Reijmer, C.H., Chellman, N., Sigl, M. and Kipfstuhl, S., 2018. Temperature and snowfall in Western Queen Maud Land increasing faster than climate model projections. Geophysical Research Letters 45, 1472-1480.

Pritchard, H.D., Luthcke, S.B., and Fleming, A.H., 2010. Understanding ice-sheet mass balance: progress in satellite altimetry and gravitmetry. Journal of Glaciology 56 (200), 1151-1161).

Shepherd, A. et al., 2012. A reconciled estimate of ice-sheet mass balance. Science 338, 1183–1189.

Zwally, H.J., Li, J., Robbins, J.W., Saba, J., Yi, D., and Brenner, A.C., 2015. Mass gains of the Antarctic Ice Sheet exceed losses. Journal of Glaciology 61 (230), 1019-1036.