Antarctica's contribution to sea level

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Antarctica's contribution to sea level

90% of all of the ice on Earth is on Antarctica and its response to climate change is the dominant uncertainty in century-scale sea level projections.

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Observed and projected global sea-level 1900-2100

Observed and projected global sea-level 1900-2100

Observed global mean sea level change (from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Observed and projected Antarctic component of sea-level 1900-2100

Observed and projected Antarctic component of sea-level 1900-2100

Observed global mean contribution of the Antarctic ice sheet to sea level change (from the 1995-2014 average) for 1900-2018 and projected global mean sea level change for 2020-2100 for three future scenarios based on IPCC AR6 WG1. The solid lines are the central estimates for the observations and projections and the shaded area indicates uncertainty.

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Contributions to sea level rise (in mm/year)

Contributions to sea level rise (in mm/year)

Contributions to sea level rise (in mm/year) for two time period, 2006-2018 and 1901-2018. The individual contributions are ocean warming, glaciers, antarctic ice sheet, greenland ice sheet and land water storage. The sum of these is shown (with its uncertainty) and compared to the observed global mean sea level (with its uncertainty). Note that the sum of the contributions and the observed global mean sea level are consistent with one another.

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Why is the Antarctic Ice Sheet important for understanding sea level change?

The Antarctic Ice Sheet is the contributor to future sea level rise with the largest uncertainties, but it is also the largest store of frozen freshwater on Earth. Therefore, quantifying past, present and future changes is vital for planning future response to changing sea levels. If there were to be high emissions and no widespread ice sheet disintegration, global mean sea level could rise by just under 7m by 2300. If there were to be high emissions and widespread ice sheet disintegration, global mean sea level could exceed 15m by 2300.

How has the Antarctic Ice Sheet changed?

The rate of ice loss from the Antarctic Ice Sheet has increased by approximately a factor of three over the past decades, with an average mass loss rate of 148Gt/yr between 2010 and 2019, compared to 49Gt/yr between 1992 and 1999 according to the IPCC. 362.5Gt of ice is approximately equivalent to 1mm of sea level rise. Prior to 1993, the plotted data assume a small sea level contribution of 0.5mm/yr with an uncertainty of 0.4mm/yr. These values are based on the available observational data which suggest that the Antarctic ice sheet was losing mass over this period.

Why has the Antarctic Ice Sheet changed?

Changes in the mass of the Antarctic Ice Sheet are the result of the balance between accumulation of ice - from snow fall and on the underside of the ice shelves - and loss of ice - from melting, icebergs and sublimation (loss of ice as water vapour). The IPCC states that “there is only limited evidence, with medium agreement, of human influence on the Antarctic Ice Sheet mass loss”.

How is the Antarctic Ice Sheet projected to change in the future?

Sea level rise from Antarctic ice sheet mass loss over the coming century is likely, but there is a greater amount of uncertainty in Antarctic Ice Sheet processes than in other contributors to sea level rise. This is reflected in the wider shaded range that can be seen in this projection’s plot relative to the other components (note the y-axis range varies between plots to maximise the details visible in the observed time series as well as the projections). Under low emissions scenarios the uncertainties have little effect on the projected values at 2100, but under high emissions scenarios these uncertain ice processes, lead to a range of possible sea level contributions from 0.5m to 2.3m. This uncertainty stems from the possibility of sudden and/or irreversible loss of parts of the ice sheet, which could lead to a collapse of the West Antarctic Ice Sheet.

It is possible that increased snow-fall over Antarctica, associated with a warmer atmosphere transporting more moisture poleward, will cause an increase of ice mass and associated global sea level fall. However, the possibility of rapid and irreversible ice sheet losses, associated with dynamic ice sheet processes, cannot be ruled out, and these have the potential to contribute to a global mean sea level rise in excess of 15m by 2300 under a very high greenhouse gas emissions scenario.

How are measurements of the Antarctic Ice Sheet made?

Frederikse et al., 2020 make use of two multi-method assessments to determine the ice loss from 1993 to 2003. These are the IMBIE assessment ( IMBIE, 2018) which compares 24 different ice sheet mass balance change estimates for Antarctica and the Bamber et al., 2018 assessment which compares estimates based primarily on literature published since AR5 (2013) along with expert judgement, new analyses, and statistical models. After 2003, the Frederikse et al., 2020 data come from observations from the GRACE satellite missions, which observe redistribution of mass over the Earth’s surface by measuring changes in the Earth gravity field. Other satellite observations which track changes in ice sheets are available from CPOM and these also track changes in sea ice.

Before 1993, a small but steady sea level contribution of 0.05mm/yr (with an uncertainty of 0.4mm/yr) is used, based on observational evidence in certain Antarctic regions ( Cook et al., 2005; Smith et al., 2017) that suggests the Antarctic Ice Sheet mass was declining over this time period.

How are Antarctic Ice Sheet projections made?

Simulations of the Antarctic Ice Sheet from the ISMIP6 and LARMIP-2 ice sheet multi-model intercomparison projects were used as inputs to simple models known as "emulators" to provide a set of projections across scenarios that are consistent with the underlying science assessment of AR6. The projections based on ISMIP6 and LARMIP-2 are then combined to provide estimates of the likely ranges of sea level contribution that incorporate the uncertainties from both. This is done using a ‘p-box’ approach, which uses the mean of the projection medians as the overall median ice sheet contribution, and the outer limits of their likely ranges as the combined likely range – i.e. the lower of the two 17th percentiles and the higher of the two 83rd percentiles of the distributions. Beyond 2100, the projections are extended using emulators based on either a constant rate of mass loss or the AR6 assessment at 2300. A 'p-box' of two high-end estimates, from structured expert judgement and model projections of instability due to ice cliffs collapsing, was also used as an estimate of the "low likelihood, high impact" possibility of rapid mass loss.

Find out more?

Cook, A. J., Fox, A. J., Vaughan, D. G. & Ferrigno, J. G. Retreating glacier fronts on the Antarctic peninsula over the past half-century. Science 308, 541–544 (2005).

Smith, J. A. et al. Sub-ice-shelf sediments record history of twentieth-century retreat of Pine Island Glacier. Nature 541, 77–80 (2017).

References

AR6 Projections

Antarctic Contribution

AR6 SSP1-2.6

AR6 SSP2-4.5

AR6 SSP5-8.5

Frederikse et al.

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Sea Level

Antarctic Contribution