By: Jonathan Gregory
Sea-level rise is one of the most serious consequences of global warming. By the end of this century, if emissions of greenhouse gases continue to increase (mostly carbon dioxide, from burning oil, natural gas and coal), global mean sea level could be more than a metre higher than now. About a quarter of a billion people currently occupy land less than a metre above present sea level (Kulp and Strauss, 2019). By the end of this century, most coastal locations around the world will annually experience extreme sea levels which have historically occurred as a result of violent storms only about once in 100 years (Ocean, cryosphere and climate change, Royal Society briefing, 2019).
Whereas global warming itself and some consequences of climate change could be mostly halted within in a few decades by ceasing greenhouse-gas emissions (although that would be hard enough to achieve), sea level would continue to rise for centuries or millennia under any climate as warm as or warmer than present. By 2300 sea level is projected to rise by 0.6-1.1 metres even if climate is stabilised in coming decades, and by 2.3-5.4 metres if emissions of greenhouse gases are large. It is hard to envisage the seriousness of this for some areas of the world. For instance, two-thirds of Bangladesh is less than 5 metres above sea level, and the highest point in the Maldives is 2.4 metres above sea level.
Large and irreversible future decline of the Greenland ice-sheet 
Figure 1: An image of the Greenland ice-sheet (CPOM/UCL/ESA).
There are several contributions to sea-level rise. In recent decades, a third to a half of the total has been due to the expansion of sea water as it gets warmer (Chambers et al., 2017), and this will continue to be an important effect. The future of the Antarctic ice-sheet is the largest uncertainty in projections of the current century. Although it is smaller, the Greenland ice-sheet (figure 1) is presently contributing more than the Antarctic, and more than all the world’s mountain glaciers together (Special report on the ocean and cryosphere in a changing climate, IPCC i.e. Intergovernmental Panel on Climate Change, 2019). As the climate gets warmer, both surface melting and snowfall on Greenland increase. (Precipitation generally increases in a warmer climate, and on Greenland most precipitation is snow.) The melting increases more rapidly with global warming, so there is a net loss of ice, which ends up as water in the ocean. If the ice-sheet was completely eliminated, global mean sea level would be 7.4 metres higher.
My colleagues Steve George, Robin Smith and I have recently studied the future of the Greenland ice-sheet under a range of climates (work under open review), illustrative of those expected in the late 21st century under various scenarios. In our experiments, the climates were constant. We wanted to see what happens if you maintain a warm climate indefinitely. We used an atmosphere general circulation climate model and an ice-sheet model coupled together. The climate model is like those used for IPCC projections but with less geographical detail because it has to run faster, since we needed to carry out experiments simulating tens of millennia. We ran about 50 experiments, at about 2000 simulated years per day of computer time. This is the first time the future of Greenland has been investigated with a model of such complexity; it has many unavoidable approximations and inaccuracies, but it’s more physically realistic and complete than previous models.
Large and irreversible future decline of the Greenland ice-sheet
Figure 2: Contribution of the Greenland ice-sheet to global-mean sea-level rise in our experiments. The coloured lines are the results of the first set of experiments under constant climates. The colours indicate the global warming with respect to the climate of the late 20th century, from blue (little warming) to red (5 degrees Celsius warmer). The black lines show the second set of experiments. The ice-sheet at the point along a coloured line where each black line begins was instantaneously transplanted into a late 20th-century climate. The solid black lines are from states below the threshold, in which the ice-sheet regrows to around its present size; the dashed black lines are those which began above the threshold, in which it never fully regrows.
Under all climates like the present or warmer the ice-sheet loses mass and contributes positively to sea level (figure 2). It takes tens of thousands of years to reach a new constant state: the warmer the climate, the smaller the final ice-sheet, and the larger the sea-level rise. Unlike in some previous studies, there is no sharp threshold dividing scenarios in which the ice-sheet suffers little reduction from those in which it is mostly lost. Rather, there is a broad range of outcomes. In the warmest climate we consider (about 5 degrees Celsius warmer than recent, which is similar to the most extreme scenarios for 2100), the ice-sheet is reduced over about 10,000 years to a small ice-cap with 1.5% of its present volume. Initially it contributes about 3 millimetres per year to sea level, which is similar to the current observed rate of rise due to all effects. On the other hand, in climates resulting from strongly mitigated emissions during this century (roughly consistent with the Paris target), the final contribution to sea-level rise is less than 1.5 metres.
In a second set of experiments, we took some of the reduced states of the ice-sheet and put them back in a steady climate like the late 20th century, to see if it would regrow, meaning that sea level would consequently fall. The ice-sheet gained mass in all cases, taking even longer than it did in a warm climate to reach a constant state, because snowfall adds mass more slowly than melting can remove it. We found that the final states constitute two groups. If the sea-level rise under the warm climate remained below about 3.5 metres, the ice-sheet eventually regrew to around its present size. If sea level passed this threshold, the ice-sheet did not fully regrow. In this case, about 2 metres of the sea-level rise was irreversible under recent climate. (The full ice-sheet could probably be regenerated in an ice-age climate.) The reason for the irreversibility is that the ice-sheet is a large object which affects its own local climate, like a high cold mountain. Without the ice-sheet, but with present-day temperatures of the surrounding seas, Greenland would a warmer place. In our model, the ice-sheet cannot readvance into the northern part of the island once ice-free, because the snowfall is less than at present.
While our result requires corroboration by other workers with their own models, it illustrates the importance of the coupling of the ice-sheet and its climate. Each affects the other and modelling them independently may lead to unrealistic projections. The experiments also underline the global practical importance of mitigating global warming. Precautionary action to mitigate the threat of irreversible damage is a principle of the Framework Convention of Climate Change, even when there is not full scientific certainty. According to our results, in order to avoid partially irreversible loss of the ice-sheet, climate change must be reversed (not just stabilised in a warmer state, but put back to how it was) before the ice-sheet has declined to the threshold mass, which would be reached in about 600 years at the highest rate of mass-loss for this century in the IPCC assessment.
References:
Kulp, S. A. and B. H. Strauss, 2019, New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding, Nat. Commun, 10, 4844, https://doi.org/10.1038/s41467-019-12808-z
Chambers, D. P, A. Cazenave, N. Champollion, H. Dieng, W. Llovel, R. Forsberg, K. V. Schuckmann and Y. Wada, 2017, Evaluation of the global mean sea level budget between 1993 and 2014, Surv. Geophys. 38, 309-327, https://doi.org/10.1007/s10712-016-9381-3