A new paper in Nature Climate Change by Bamber and Aspinall attempts to untangle the thorny problem of how quickly and how much the ice sheets of the world will melt. The rate at which ice sheets melt is difficult to understand, because there are many processes that occur.
Non-linear ice-sheet response
Ice sheets often respond to climate change in non-linear ways, and glacier response to this is discussed in many places on this website. For example, we know that ice sheet melt is primarily driven by the basal melting of ice shelves – see the paper by Pritchard et al. 2012 here. Ice shelves are crucial to the stability of an ice sheet, and ice shelf collapse can lead to the abrupt acceleration, thinning and recession of a glacier that flows into it. Even more concerning is the fact that the West Antarctic Ice Sheet is grounded well beneath sea level, which could make it inherently unstable. Added to all this, you have uncertainty in ice-stream response to climate change. Because ice streams drain the majority of the Antarctic continent, their behaviour is crucial to the stability of the Antarctic Ice Sheet. Ice sheets therefore have inherent thresholds or instabilities, which, once passed, could lead to very rapid and large changes in ice volume, resulting in dramatic sea level rise.
Predicting sea level rise
Because of all this, sea level rise is difficult to predict, and the upper bounds of sea level rise are omitted from the IPCC sea level estimates. The IPCC predicted sea level rise to 2100 of 18-59 cm, and did not include estimates based on the dynamic behaviour of ice sheets. Following this, there have been many attempts to predict Antarctica’s contribution to global sea level rise, with some suggesting it could be 3-5 times faster than the current rate of 3.1 mm per year.
The new paper by Bamber and Aspinall in Nature Climate Change has attempted to quantify the best estimate of global future sea level rise from ice sheets. They have pooled expert assessments from a number of people in order to best understand likely rates of sea level rise to AD 2100.
Sea level rise from ice sheets
Bamber and Aspinall find that the median rate of sea level rise agreed upon by these experts was 5.4 mm per year by 2100 (note – this is just from the ice sheets, and does not include small glacier melting and thermal ocean expansion, which accounts for most modern sea level rise), with variability due to uncertainty in the contributions from the Antarctic and Greenland ice sheets. There is a growing view that the West Antarctic Ice Sheet could become unstable over the next century, contributing as much as 16.9 mm per year by 2100, resulting from a collapse of the West Antarctic Ice Sheet. The threshold for marine ice sheet collapse is therefore a research priority, and much larger rates of sea level rise than 5.4 mm per year by 2100 cannot be ruled out. There is a 5% chance that sea level rise will reach 17.6 mm per year by 2100.
The East Antarctic Ice Sheet is the only ice sheet likely to gain in mass, but even this ice sheet has the potential to rapidly lose ice volume around its marine-grounded sectors. A large contribution from the Greenland Ice Sheet is unlikely, as it is mostly grounded above sea level and so mass loss from calving ice bergs is limited.
Total combined sea level rise
By using a linear measure for ice-sheet contribution of sea level rise from 0.9 mm per year for the last decade to 5.4 mm per year (the median) to 2100, would result in 29 cm of global sea level rise, just from these ice sheets alone. The 90% confidence interval is 10-84 cm. This can be combined with anticipated global sea level rise from thermal expansion of the ocean (14-32 cm). Together, this makes a sea level rise of 55-73 cm by 2100, with 62 cm the most likely, based on a temperature increase to 3.5°C above pre-industrial temperatures.
Combining the upper and lower 5 percentiles for the ice sheets, glaciers and ice caps and thermal expansion gives a range of 33-132 cm for global eustatic sea level rise by 2100. The upper bounds of this would displace 187 million people by the end of this century.
These predictions are limited by a poor understanding of the recent changes observed in the Antarctic and Greenland ice sheets, and a lack of knowledge about the variability of ice sheet behaviour under a warming climate. This sea level rise estimate is larger than that provided by the last IPCC report, but highlights the need for further research on ice sheet variablity and ice sheet response to climate change, both now and in the past.