In the News: Is the East Antarctic Ice Sheet less stable than we previously thought?

Image: National Geographic 

In a previous post I examined the stability of the East Antarctic Ice Sheet (EAIS). This week I will touch again on the topic as some interesting new research has just been published. 

The new study, published in Nature, suggests that the EAIS is less stable than previously thought. The subsequent headline in The Independent reads: "Global sea levels could rise by 'up to 15 feet' if East Antarctic ice sheet melts". Today we will delve deeper into the study behind this news. 

Looking back 

As I have discussed previously, the Antarctic has grown and decayed over the last 50 million years as the climate has varied. During this time it was thought that the East remained relatively stable compared to the West. However, Gulick et al., 2017 present new geological evidence that EAIS was more sensitive to changes in temperature in the past than previously thought. 

Paleo-records from EAIS margins indicate how the ice sheet has evolved over millions of years: the EAIS stabilised around 6 million years ago, after a period of instability. These records reveal that the ice sheet was dynamic and sensitive to atmospheric temperature changes, growing and shrinking accordingly. 

This new research focuses on the Aurora subglacial basin (ASB), located in the south-east of the EAIS, and presents records of glacial evolution which document changes in ice extent.  It provides evidence that past changes in the EAIS are likely to have contributed to significant sea level rise. 

Image: The Jackson School of Geoscience 

Looking forward

Understanding how ice sheets respond to a changing climate is key when making projections about future sea level rise. This new evidence suggests that as temperatures continue to rise the ASB glaciers may shift from the relative stability of the last 7 million years to an unstable period, driven by melt. 

If present warming persists, the ice sheet may become unstable and contribute to global sea level rise. According to the research, if the ice sheet in the ASB melted it would result in 3 to 5m of rise. However, the authors note that although the melting of the EAIS is 'not inevitable', it should be considered a possibility in the context of global climatic changes. 

What Does the Future Hold?

The Antarctic's response to recent climate change is complex: the West has warmed and experienced significant ice shelf melting,  affecting its overall stability. On the other hand, the East has cooled in places and remained comparatively stable. What does the future hold for the icy continent?

Tipping points

Before we examine future projections let's touch briefly on a key concept for ice sheet stability: tipping points.

A tipping point describes the critical threshold at which the system switches from one stable state to another; it may be irreversible. Current knowledge about climatic tipping points is poor and estimations of Antarctic collapse uncertain. However, if certain thresholds are met they could trigger extensive sea level rise and ecological change. 

It's getting hot in here!

Evidence from paleoclimatic data indicates that there is a linear relationship between global temperatures and Antarctic ice accumulation. As temperatures continue to rise, it is likely that Antarctica will shrink. However there are uncertainties attached to this estimation.

At present the East Antarctic Ice Sheet (EAIS) is relatively stable and is expanding in some places. Current evidence shows that the warmer air can hold more moisture, contributing to ice sheet growth. However, Fogwill et al., 2014 suggest that climate change may threaten weaker sections of the ice sheet, which would trigger accelerated mass loss. 

A recent study modelled the effects of continued climatic changes on the stability of the West Antarctic Ice Sheet (WAIS). The findings suggest that the current retreat of peripheral ice shelves would continue under a warming climate. In fact, Gollege et al., 2015 predict that the stability of West Antarctic ice shelves is dependent on two critical temperature thresholds: 0.5°C  of prolonged ocean warming coupled with atmospheric warming of 2°C would result in 80-85% losses of the floating ice around Antarctica. These ice shelves are key for WAIS stability, so a crossing of these thresholds could lead to runaway melt and eventual ice sheet collapse. Arguably, the WAIS is poised to reach a tipping point. 

Modelled changes of Antarctica under the RCP warming scenarios for 2100 (a, d, g, j) , 2300 (b, e, h, k) and 5000 (c, f, i, l). Grey areas show fast flowing ice and light blue areas show high grounded ice loss. The magnitudes and rates of sea level rise are shown in each box (Image: Gollege et al., 2015). 

Why does it matter?

Sea levels were between 6 and 9.3m higher during the Last Interglacial (which occurred between 130 and 115 kyr) and global temperatures were only 0-2 °C warmer than they are today. These high sea levels have been partly attributed to Antarctic melt. 

Using the IPCC's Representative Carbon Pathways (RCP) DeConto and Pollard, 2016 modelled the Antarctic's contribution to global mean sea levels (GMSL) under different emission scenarios:  

1.  RCP2.6 produces a negligible contribution to GMSLs by 2100 and 20cm by 2500. 
2. RCP4.5 would lead to 32cm of rise by 2100 but, worryingly, would cause the WAIS to collapse by 2500 and result in a scary 5m of GMSL rise!

Antarctic contribution to GMSL under RCP scenarios. GMSL by 2100 (a) and by 2500 (b) (Image: Pollard and DeConto, 2015)

Similarly, Bamber et al., 2009 estimated that collapse of the WAIS alone could cause global eustatic sea level rise of 3.3m, with regional variations. Specifically, projections suggest that peak increases in sea level would be focused around the Pacific and Atlantic sides of America. However, projections about sea level rise from the collapse of the WAIS are inherently surrounded by uncertainty in the input data and system behaviours.

The future is uncertain 

Projecting Antarctica's response to future climate change and modelling GMSL is difficult, and inherently uncertain. Firstly, it is not known how global emissions may change in the future. Consequently, studies that use the RCP pathways are particularly useful as they give a range of projections. Secondly, there is parametric uncertainty in the models: complex processes are simplified (including ice and ocean dynamics) so outputs should be interpreted as ranges of possible responses rather than certain projections. Finally, the equilibrium response for ice sheets is longer than that of the atmosphere or oceans, making it difficult to provide a timescale for GMSL change. 

The Plastic Problem

A quick  post for you this week. So far, I have examined the implications of climate change in Antarctica. However greenhouse gases are not the only anthropogenic pollutants affecting the continent. Antarctica is remote, but a recent UCL seminar about plastics in our oceans made me wonder: is Antarctica really immune to plastic pollutants?

Microplastics

Microplastics are plastic pieces which are less than 5mm long and come from many sources: for example, from larger plastic debris, clothing and beauty products. Until recently, scientists believed that The Southern Ocean was relatively free of microplastics, as it is far from pollution sources and the Antarctic Circumpolar Current acts as a barrier. However new research suggests otherwise. 

A study recently reported that microplastic levels in The Southern Ocean were five times higher than you would expect to find from water around ships and research stations. This suggests that microplastics from outside the region are reaching Antarctic waters. 

The National Antarctic Programmes has recorded findings of macroplastics, defined as plastic pieces larger than 5mm (yellow crosses) and microplastics (green crosses) around Antarctica (Image: Waller et al., 2017)

The implications of microplastics on Antarctic ecosystems are unknown. In the Northern hemisphere, evidence suggests that microplastics are entering pelagic ecosystems at the base of the food chain and have been found in seals, seabirds and fish. It is likely that Antarctic organisms could be affected in similar ways but further research is key to understand the extent of the problem. 


Next week I will be returning to climate change and examining projections for climate induced changes to Antarctica as whole.