Prof Christina Hulbe is geophysicist in the School of Surveying at the University of Otago in New Zealand, and Dr Christian Ohneiser is a paleoclimatologist in the Department of Geology at the same university.
In the late 1970s, glaciologist John Mercer was one of the first scientists to warn of “rapid deglaciation” of the West Antarctic Ice Sheet under human-caused warming. Other scientists, studying both modern Antarctica and the geologic record of its past, soon came to a similar conclusion.
Forty years on, with more observations and a better understanding of ice sheet processes, scientists have a clearer idea of how the ice sheet is changing. Yet different models still give different projections of when retreat of the West Antarctic Ice Sheet passes the point of no return.
So, is the eventual collapse of the West Antarctic Ice Sheet already inevitable? Model projections under low emissions scenarios suggest that ice sheet retreat could stabilise, but under medium and high scenarios, collapse is unstoppable.
Ice sheets form where snow accumulates, densifies, and remains stored as ice on the land surface. Antarctica’s massive ice sheets sit astride the cold, windswept continental interior and funnel down towards the (relatively) warm ocean around the coast.
Where the ice meets the water, floating ice shelves form. These are seaward extensions of the ice sheet that connect land-based ice with relatively fast-changing parts of the climate system. Changing winds and warmer oceans bring warm water into contact with the floating ice, increasing the rate at which it melts.
Ocean-driven ice loss is already underway where the West Antarctic Ice Sheet flows into the Amundsen Sea. The best observations available indicate that relatively warm Circumpolar Deep Water has been able to intrude onto the Amundsen Sea continental shelf over the last few decades. The continental shelf is the area of seabed immediately surrounding a land mass, where the sea is relatively shallow compared to the open ocean beyond it.
The warmer-than-usual water increases the melt rate on the underside of floating ice shelves, causing them to thin. For the West Antarctic Ice Sheet, this thinning sets up a chain reaction that scientists think could be unstoppable.
In West Antarctica, most of the ice sheet rests on the seafloor, and the basin that it sits in grows deeper with distance from the coastline. This makes it particularly susceptible to a “self-sustaining retreat”.
The base of the ice sheet is below sea level, which means that the warm ocean can melt and thin the ice at the “grounding line” – the boundary between the grounded ice sheet and the floating ice shelf.
If we were to take only one measure of the well-being of the West Antarctic Ice Sheet, it would probably be the position of the grounding line. The position of the grounding line indicates the balance between water stored in the ice sheet and water returned to the sea. Processes acting at the grounding line can also drive the grounded ice to change.
Melting alone can cause grounding line retreat and sea level rise. But melting can also initiate something called the “marine ice sheet instability”. “Marine” because the base of the ice sheet is below sea level, and “instability” for the fact that once it starts, the retreat is self-sustaining.
Here’s how it works. If changes on the floating side cause ice on the grounded side to lift off from the seafloor and float, the grounding line will retreat. Because the ice flows more rapidly when it is floating than it did when grounded, the rate of ice flow near the grounding line will increase. Faster flow means thinning, which may in turn cause more ice to lift off and float. Because greater thickness also causes the ice to flow faster, grounding line retreat into the deep can also produce faster flow.
You can see this explained in the figure below.
Not whether ice sheets retreat, but how fast
It is not clear yet if this instability has already started along the Amundsen Sea coast. If it hasn’t, and if the ocean warming stops, the grounding line should balance out at a new location. If it has, the retreat will continue no matter what happens next.
Model predictions of the future of the ice sheet can vary by a factor of 10 or more for the same emissions scenarios. These differences depend on how the processes through which climate forces the ice to change are represented in the models. But the question isn’t whether or not climate change will drive ice sheet retreat, the question is how fast it will go, and the extent to which policy decisions to affect our carbon emissions can make a difference to the outcome.
Under medium and high emissions scenarios, the models agree that the West Antarctic Ice Sheet will eventually collapse. At the higher end, marine-based sections of the East Antarctic ice sheet also retreat. However, under low emissions scenarios – lower than our current pathway – retreat may be limited.
If the marine ice sheet instability has not been initiated, then once the warming stops, the rate of retreat declines and grounding lines stabilise. The implication is that wholesale loss of the West Antarctic Ice Sheet may not be inevitable. But the low-end ice sheet scenarios are not well understood and the more scientists study the geologic record of past ice sheet change, the more vulnerable the ice sheets appear to be.
Earth’s climate has been oscillating between relatively warm (interglacial) and relatively cold (glacial) conditions for the last five million years. Even before human influences on climate, those swings appear to have driven grounding line retreat deep into the West Antarctic Ice Sheet interior. Some of the warm swings were into what are called “super-interglacial” conditions. During these times, southern oceans were between 3C and 8C warmer than at present, the ice sheets were smaller, and global sea levels were higher than today.
The causes and timings of super-interglacials are not well understood and even the models most responsive to changing temperatures can’t reproduce the speed of ice loss. But these records make clear how sensitive the Antarctic ice sheets are to climate change, even before the extra kick from global warming. The record of the past never seems to yield less cause for concern.
The motto for early 21st Century cryospheric science should be “that happened faster than I thought it would.” Wherever we look, either in the past or in the present, we are challenged to keep up – in the ways we measure, theorise, project, and prepare for the future.
Hulbe, C (2017) Is ice sheet collapse in West Antarctica unstoppable? Science, doi:10.1126/science.aam9728
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