Antarctic Reflections

To summarise all that I have covered in the last few months I thought it would be useful to reflect on the questions I laid out in my first post.

How is Antarctica responding to a changing climate and what are the spatial complexities?

The effects of climate change on Antarctica are complex: regions have been subjected to different processes, resulting in different effects. I have focused on the three main regions in this blog: The Antarctic Peninsula and East and West Antarctic Ice Sheets (EAIS; WAIS). 

Surface temperature anomaly map for 2017 compared to the 1951-1980 average. It is clear that the Earth has warmed, yet there is spatial complexity. (Image: NASA)

The Antarctic Peninsula has recently experienced some of the most rapid warming (2.5°C since the Industrial Revolution). The effects of this rapid warming are stark: ice shelves have thinned and glaciers are retreating . There is a high certainty that this is caused by anthropogenic climate change. Similarly, the WAIS has experienced ice shelf thinning and channelised melt.  It is becoming more unstable as a result.

In contrast the EAIS is relatively stable, and is even growing in some places, as a warmer climate allows the air to hold more moisture. However, there are other mechanisms of ice loss, such as dynamic ice loss, which could cause it to shrink and become unstable in the future. 

How has Antarctica responded to changing climates in the past?

Artists depiction of a warmer Antarctica (Image: Nature) 

Earth's temperatures have fluctuated on geological time scales, as a result of orbital cycles, greenhouse gases and complex feedbacks. Delving into the past provides some insight into current anthropogenic-driven climate change in the context of natural variability; it is clear that the current changes are unusual in the context of natural variability. 

In the past, the mass balance of Antarctica has fluctuated and resulted in dramatic sea-level change during warmer periods. Whilst this is a warning for future climate change, it is not a direct analogue as there are many complex variables which determine climate. 

What does Antarctica's future look like and what are the implications for the rest of the Earth?

If warming continues it is likely that the EAIS will weaken and the WAIS will become even more unstable, eventually collapsing. Models project implications of different emission scenarios, indicating the importance of meeting emission targets to constrain the melting of the continent. Whilst Antarctica may seem far away, its melt could have a catastrophic effect on global sea levels. 

Climate Change and Antarctica

In this blog I have highlighted the implications of climate change in Antarctica. I have been shocked by the statistics and distressed by the projections. Even as one of the most pristine environments on Earth, Antarctica cannot escape human impact and the implications of climate change on it are likely to be catastrophic, for both the continent and globally. In short, the need for drastic action has never been more apparent.

(Image: Huffington Post)

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. 

Delving Into the Past

Previously, I have examined the recent climatic changes in the Antarctic and the associated effects. Today I will be delving into the past to understand: (1) whether the current changes can be attributed to anthropogenic activity or whether it is part of the natural variability, and (2) what the Earth may look like under a warmer climate and the difficulties with providing a direct analogue for future change. 

Fossilised evidence and ice core records indicate that conditions in the Antarctic have not always been the same: temperatures have fluctuated, and the landscape has varied accordingly. This story by The Guardian provides an interesting introduction to Antarctica as 'a tropical paradise'. But what caused these changes in the past? 

An artists impression of tropical Antarctica (Image: British Geological Survey)

External and internal forcings

Milankovitch cycles are orbital and axial variations that occur on time scales of 100,000 (eccentricity), 41,000 (obliquity) and 21,000 (precession of the equinoxes) years. The link that these orbital cycles have to the Earth's global climate is complex (the role of eccentricity in particular is widely debated), but simply they drive changes in summer insolation, which affects the growth and decay of ice sheets. For example, Naish et al., 2009 hypothesise that the obliquity cycle may regulate upwelling of the Circumpolar Deep Water, having consequences for basal melt around Western Antarctica and driving 41,000 year ice sheet oscillations. 

There are other controllers of climate, which we are more familiar with. Analysis of ice cores indicates that CO2 concentrations have fluctuated and are closely linked to surface air temperatures. In fact, a study by Parrenin et al., 2013 conclude that Antarctic temperatures are strongly correlated with atmospheric CO2 over the past 800,000 years. Furthermore, ice cores indicate that interglacial CO2 concentrations were 80ppm higher on average than during glacial periods. 

CO2 concentrations and global temperatures from the past 800,000 years (Image: Discovering Antarctica)

The mechanisms behind these glacial and interglacial fluctuations in CO2 are complex and still not fully understood.  Stephen and Keeling, 2000 have postulated that low CO2 concentrations may be a result of low Antarctic sea ice cover which reduces deep water ventilation. Others have hypothesised that changes were caused by alterations in the biological pump: as iron is added to the oceans, photosynthesis rates increase and CO2 is sequestered from the atmosphere. 

How has Antarctica responded to climatic changes in the past?

Evidence shows that fluctuations between glacial and interglacial periods correspond with changes in Antarctic ice volume.  Pollard and DeConto, 2005 modelled changes in West Antarctic ice volume over the past 5 million years and highlighted periods when ice has retreated, for example in the warm early Pleiocene and Pleistocene interglacial periods. Importantly, deglaciation occurred relatively quickly compared to glaciation; the West Antarctic ice sheet (WAIS) sometimes collapsed in 1,000 to 10,000 years.  

Studies have also shown that the EAIS has shrunk in the past. Evidence from the Pleicoene suggests that the ice sheet is sensitive to warming: simply when temperatures increased the ice sheet got smaller. 

What does the future hold?

Ice cores indicate that greenhouse gas concentrations are higher today than they have been over the past 650,000 years and the forcings from these are "unprecedented in more than 10,000 years" (IPCC, 2007). Analysis of evidence indicates that in the past, the advance and retreat of Antarctic sea ice occurred over thousands of years. Present warming, particularly across the West and Peninsula, suggests that the Antarctica is vulnerable to anthropogenic induced climate change, which is occurring much more quickly than it has in the past. 

Changes in Antarctic ice volume has been closely linked to global sea level for the last 5 million years. Many argue that Marine Isotope Stage 11 (MIS11), which began 400,000 years ago, provides insight into an interglacial without anthropogenic influences. MIS 11 is arguably a good comparison for the present interglacial as orbital configurations are similar. During this interglacial, sea level was around 20m higher than today and studies have suggested that this was caused by the collapse of the West Antarctic Ice Sheet (WAIS). 

Modelled changes in Antarctic volume over the last 5 million years. a) δ18O record b) Antarctic ice volume (red), changes in global sea level are shown on the right. (Image: Pollard and DeConto., 2005) 

Similarly, during the Last Glacial Maximum (LGM), which occurred around 21,500 years ago, the WAIS advanced to the central and eastern Ross Sea.  It has been postulated that the subsequent deglaciation of the WAIS during this period, around 14.5 ka, was responsible for abrupt sea level rise. This suggests that if Antarctica continues to melt sea levels could increase dramatically. 

The past may be the key to the future but it is not the future 

The evidence presented above indicates that both the WAIS and EAIS have responded to climatic changes in the past. During the last interglacial temperatures were similar to those projected for the future, seemingly a good analogue for future changes. However, evidence from the Greenland Ice Sheet (GIS) highlights why this is not the case. 

The difference between temperatures during the last interglacial and 1850. During the last interglacial temperatures were a lot warmer than pre-industrial ones, particularly in the Northern Hemisphere.(Image: Ganopolski and Robinson, 2011)

The mass balance of ice sheets is primarily controlled by air temperatures and absorption of solar radiation. Sensitivity analysis of the GIS during the last interglacial indicates that only half the mass balance is controlled by air temperatures, the rest is controlled by insolation (dictated by the orbital configuration). This means that the same changes in mass balance cannot be inferred for the future given the current difference in insolation. 

Paleoclimatic records provide some context for current anthropogenic induced changes and offer some insight into warmer periods when ice sheets were smaller and global mean sea levels higher. However, the past is not the future and should therefore not be used as a prescriptive guide to projecting future change. 

Polar Opposites

Over the last few weeks I have spent time examining the effects of climate change in Antarctica. However, it is also important to consider the Northern Hemisphere.

The effects of climate change in the Arctic are frequently reported in our news. The Arctic has warmed more than any other region on Earth and it is estimated that between 1979 and 2012 sea ice extent decreased by an average of 3.5 - 4.1% per decade. This is a very different story from that of Antarctica. 

This video from NASA clearly outlines the differences in sea ice extent in the Antarctic and Arctic and provides an interesting insight into how climate change is affecting the poles. 

The West Antarctic Ice Sheet

So far I have examined the recent climatic changes in the East Antarctic and Antarctic Peninsula, and the associated effects. The West Antarctic Ice Sheet (WAIS) tells yet another story.

Ice Shelves Matter!

Temperatures in West Antarctica, particularly in the Amundsen Sea region, have increased and glaciers have retreated in recent decades. The WAIS is inherently less stable than the East and vulnerable to anthropogenic climate change. As a Marine Ice Sheet (MIS) it is located mainly below sea level and its stability rests on the ice shelves which fringe its exterior. Ice shelves are floating tongues which act as a buttress to inland glaciers, but are vulnerable to rising ocean temperatures. 

Ice shelves buttress inland glaciers (Image: AntarcticGlaciers)

In fact the MIS instability hypothesis states that as ice shelves melt, grounding lines retreat and as a result the glacier rests on deeper water and the ice is thicker. This results in sustained ice losses in a positive feedback loop. (This complex process is well explained in the video below, fast forward to 2'30!)

In recent decades many ice shelves have thinned, leading to an increase in ice discharge. In the Amundsen Sea region studies have shown ice shelves are particularly vulnerable to warming oceans, which melt them from below. 

Channelised melt 

A recent study indicated that the Dotson, Crosson and Pine Island ice shelves have experienced rapid basal melting in recent decades, thinning by 2m per year on average. What is even more worrying is a channel which has recently been discovered under the Dotson ice shelf. Formed by Circumpolar Deep Water (CDW), this channel could cause the ice shelf to collapse in just 40 years, 170 years earlier than at current thinning rates. Similar channels are likely to be present beneath other ice shelves in the region, but few have been identified. This will have huge implications for WAIS stability. 

High resolution satellite data reveals a channel of high melt beneath the DIS (Image: Gourmelon et al., 2017)

Shrinking glaciers 

It is not just ice shelves which are vulnerable to climate change. A study of the Thwaites, Haynes, Smith and Kohler glaciers indicate retreat of 14km, 10km and 35km respectively between 1992 and 2011. These dramatic losses indicate that this section of the WAIS is starting to become more unstable. 

The implications of channelised melting, ice shelf collapse and the stability of the WAIS are closely linked. Recent studies suggest that the WAIS is indeed unstable and becoming more so. I will be examining the associated implications in a few weeks.