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What is happening to the world’s largest ice sheet?

calendar May 19, 2022
Frozen glacier

Antarctica contains over 90% of Earth’s glacier ice, with ~58m of sea level equivalent stored in the Antarctic Ice Sheet that covers virtually all of the continent.

Over the last few decades, mass loss from Antarctica (via iceberg calving and melting) has exceeded mass gains (via snowfall) and its contribution to sea level rise has accelerated. The largest imbalances are found in the West Antarctic Ice Sheet, which holds a sea level equivalent of 5.3m. Recent estimates indicate that it lost over 2,000 gigatonnes of ice between 1992 and 2017, contributing ~6mm to global mean sea level over this time period. Rather than atmospheric warming, mass loss is attributed to warm ocean currents melting the underside of its floating portions, causing the ice margins to thin and retreat, and increasing the discharge (flow) of ice into the ocean.

The vulnerability of the West Antarctic Ice Sheet was recognised by scientists as early as the 1970s, prompting much research that continues unabated. In comparison, much less work has focussed on the vulnerability of the East Antarctic Ice Sheet, which is somewhat surprising given that it is ten times larger than West Antarctica and contains a massive 52.2m of sea level equivalent. Its perceived stability perhaps stems from the fact that we know large parts of it have persisted for at least 30 million years, since widespread glaciation of Antarctica in the Oligocene. We also know that parts of it can actually gain mass in a warmer climate, due to enhanced snowfall from a warmer atmosphere. Indeed, some early numerical modelling simulations suggested it was likely to grow under climate warming not exceeding ~5°C above pre-industrial temperatures.

Although the East Antarctic Ice Sheet continues to be viewed as more stable than the West, recent observations are beginning to challenge this paradigm. The latest efforts to measure its mass balance have raised the possibility of overall mass loss since ~2014 but, due to its sheer size, the measurement uncertainties are much larger than for the West Antarctic Ice Sheet. Different methods sometimes give different answers as to whether the ice sheet is even gaining or losing mass.

Importantly, however, nearly all studies detect a clear signal of loss in one particular region of East Antarctica, known as Wilkes Land, bordering the Indian Ocean. This is particularly concerning because the ice sheet in Wilkes Land sits over a deep subglacial basin (where the ice is up to 4.5km) that alone contains 3.5m of sea level equivalent. Moreover, recent measurements show that warm waters appear to be affecting the outlet glaciers in Wilkes Land in a similar manner to West Antarctica, with evidence that the ice sheet is thinning and that glaciers are retreating and contributing to sea level rise. Of further concern is that there is a growing body of evidence that these same glaciers retreated during past warm periods, such as the mid-Pliocene, around 3.2 million years ago, and possibly during some warm interglacials of the Quaternary period (last 2.5 million years). Future simulations of the Antarctic Ice Sheet also predict multi-metre sea-level contributions from East Antarctica over the coming centuries, if the Paris Climate Agreement – to limit climate warming to well below 2° C – is not met. There is, therefore, an urgent need to answer the question in the title – what is happening to the world’s largest ice sheet – for the benefit of both science and society.

GeoLogica Tutor News – Prof Chris Stokes

Over the last decade, Chris has been leading a research group based at Durham University and specifically targeted at improving our understanding of the response of the East Antarctic Ice Sheet to future warming.

One of their first papers on this topic was published in the prestigious journal Nature and demonstrated that East Antarctic outlet glaciers were far more sensitive to ocean-climate forcing than previously thought: https://www.nature.com/articles/nature12382

More recent work used over 5 million km2 of high-resolution satellite imagery to detect a record 65,000 meltwater lakes on the East Antarctic Ice Sheet, attracting considerable media attention and suggesting that the floating parts of the ice sheet may be highly sensitive to future atmospheric warming: https://www.nature.com/articles/s41598-019-50343-5

You can find out more about climate change and the impacts of global warming on glaciers and the oceans on Chris’s course entitled E523: Ocean and Cryosphere Responses to a Changing Climate: Past, Present and Future