How much ice is there in Antarctica? And if it were to melt, how much would global sea levels rise, and how quickly?
These are questions that glaciologists are trying to answer, using a variety of methods. Palaeo-glaciologists look at longer-term records, to assess if modern change is abnormal (see Why Study Glaciers), and to determine what possible glacier dynamical changes are likely to occur with changes in oceanic and atmospheric temperatures. Numerical ice-sheet modellers use reconstructions of past ice sheets to test and train their models, and to assess if they can accurately replicate or repeat the dynamical behaviour observed by glacial geologists. Ice-core scientists look at the past record of atmospheric gasses trapped in bubbles in the ice, and atmospheric scientists and climate scientists look at the changing conditions in our atmosphere and the resulting warmth. Sea level scientists use a variety of methods, including ancient coral reefs and more recently-formed tidal salt marshes, to look at past sea level fluctuations, which will inform future projections. The result of all this work is a vast amount of data, which makes it difficult to discern exactly how much the sea level will rise in the future, and by how much.
How much ice?
Antarctica holds 27 million km3 of ice, which if it all melted, would raise global sea levels by 58 m . It is warming very rapidly, with warming particularly evident around the Antarctic Peninsula[2, 3] but also across the West Antarctic Ice Sheet . This warming is associated with increase in ocean temperatures, changes to atmospheric circulation, and incursions of warm circumpolar deep water onto the continental shelf [5-9].
This warming is likely to increase snowfall over the Antarctic continent. Surface melting over most of Antarctica is minimal, so this increased snow fall could result in mitigation of sea level rise from melting glaciers and ice caps elsewhere and thermal expansion of the ocean.
Ice sheet response
However, glaciers respond in a complex way to changes in atmospheric temperatures. Glacier dynamics, including the amount of ice that is discharged from the snout of a glacier that terminates in the ocean, are important factors in understanding overall ice-sheet behaviour. They are strongly influenced by changes in oceanic currents or winds, and melt from below as well as from above.
The measured warming, for example, has already resulted in numerous observations of change across the Antarctic Peninsula. The Antarctic Peninsula Ice Sheet currently contributes around 0.22 ± 0.16 mm per year of sea level rise, but has the potential to raise sea levels by up to 0.24 m if all the ice melted. Mountain glaciers and ice caps around the Antarctic Peninsula are expected to make a larger contribution to sea level rise in the future, as they are small, relatively northerly, and susceptible to small changes in temperature.
Glaciers are thinning across the Antarctic Peninsula[12, 14, 15], and they are also receding [16, 17] and accelerating. This means that they are discharging more ice to the oceans, resulting in an increased contribution to sea level rise.
In addition to this, ice shelves are shrinking and thinning due to basal melting from below , and from meltwater pooling on their surface during warm summers[19-21]. Ice shelves control the amount of ice entering the ocean from glaciers[22, 23] by “buttressing”, or holding back, the glaciers on land. When ice shelves rapidly disintegrate, as several have now been observed to do around the Antarctic Peninsula[21, 24-32], glaciers accelerate and rapidly recede, calving more and more icebergs into the ocean[26, 27, 30, 33-36]. So, warmer oceanic and atmospheric temperatures are already influencing the amount of sea level rise coming from the Antarctic Peninsula.
The West Antarctic Ice Sheet
However, it is not just the Antarctic Peninsula that is vulnerable to climate change. The West Antarctic Ice Sheet is called a “Marine Ice Sheet”, which means that much of the bedrock on which it rests is below sea level . The West Antarctic Ice Sheet is large, surrounded by ice shelves, and the bedrock slopes downwards towards the interior of the ice sheet. It is also a region of rapid warming. It is considered to be unstable, because ice discharge increases with depth, meaning that as the water gets deeper, more icebergs are calved. It may therefore be inherently susceptible to rapid recession and collapse[39, 40], and pass tipping points that mean rapid sea level rise occurs. Pine Island Glacier is already thinning and receding[41-44], making it susceptible to rapid recession in ever deeper water. The ice streams are also melted rapidly at their base by warm ocean waters, leading to more melting, increased ice discharge, more recession into deeper water, more melting again…
Scientists have called this the “Marine Ice Sheet Instability Hypothesis”. Dynamic glacier behaviour like this, linked to oceanic circulation and the relationship between glaciers, ice shelves, ice streams and their bedrock troughs, could be a significant contributor to future sea level rise.
It doesn’t end there. Although there may be more snow over the Antarctic Ice Sheet under a warmer climate, this too could lead to changes in glacier dynamics. Increased snow will steepen the surface gradient near the grounding line of the Antarctic Ice Sheet, which will increase the driving stress acting on ice flow. In short, the glacier will flow faster, discharging more ice bergs into the ocean.
So, we’ve learned that the Antarctic Ice Sheet could experience more snow fall under a world with a warmer climate, and that this could mitigate against sea level rise from the melting of small glaciers and ice caps. However, this growth is likely to be smaller than increased ice discharge from changes in glacier dynamics. But how much is the sea rising now, and how does that compare to the past?
Global sea level rise
Global sea level rise is currently about 3.1 mm per year. The most recent projections suggest that there is potential for rates 3-5 times faster, which would have significant impacts on coastal zones. Rates such as these are common in the geological record, and Cronin et al. suggest that future rates of 10 mm per year are reasonable, or even conservative. That would equate to 1 metre over the next 100 years.
In addition to this, a collapse of the West Antarctic Ice Sheet would result in a global sea level rise of 3.3 m, which could occur within 500 years. The Antarctic Peninsula Ice Sheet would raise global sea levels by 0.24 m on full melting, over a similar timescale. The sea level record contains large anomalies of this magnitude and exceeding these rates in the past[47, 48]. Dynamical changes in glacier behaviour could therefore result in much faster rates of sea level rise in the future. Uncertainties about glacier dynamical behaviour, however, make it difficult to accurately assess future maximum rates and magnitudes of change. Understanding dynamical changes in glacier behaviour is a major research priority for the glaciological community.
Sea level rise of this magnitude could still be prevented. Although rates of 10 mm per year would have very profound consequences for communities in low-lying areas, particularly in poorer countries, a global disaster could still be prevented.
- Marine Ice Sheet instability
- Ice shelves
- Sea level rise
- Post-Glacial Rebound
- Glacier recession in Patagonia
- Glacier recession on the Antarctic Peninsula
- Glaciers and climate change
Van den Broeke et al., 2011 (open access)
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