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. 

Strong and Stable: The East Antarctic Ice Sheet

Discussing climatic changes across the Antarctic continent as a whole can be misleading as the nature of the effects are spatially heterogeneous and complex. Last week, I examined the evidence for climate change in the Antarctic Peninsula and the associated effects. 

The East Antarctic Ice Sheet

Today, I will look more closely at the East Antarctic Ice Sheet (EAIS), which has received  less attention than the West Antarctic Ice Sheet (WAIS) in recent decades.  The EAIS is very very cold (-94.7°C has been recorded) and very very large (if it were to melt completely global sea level would rise by 53m).

The WAIS has thinned in recent decades. On the other hand, the EAIS has remained relatively stable and is growing in thickness, especially in interior areas, at a rate of 0.3cm a year. It is more stable because it rests on bedrock and has experienced less dramatic change in temperatures than the WAIS. 

A warm climate increases the moisture-carrying capacity of the air. Thus, higher temperatures have enhanced snow fall in East Antarctic and this trend matches the thickening throughout the interior. In fact, between 1992 and 2003 modelled data indicates a linear correlation of 0.41 (p<0.01) between snowfall and elevation change in East Antarctica. The size of East Antarctica means that even small increases in accumulation could have a huge effect on global sea level change, and could even balance out losses from the WAIS.

Elevation changes in Antarctica between 1992 and 2003, positive (red) and negative (blue) (Image: Davis et al., 2005)

However, Winkelmann et al., 2012 postulate that as the ice sheet grows under a warmer climate, dynamic ice losses could increase. This is perhaps counter intuitive, but the authors argue that increases in elevation (caused as snow accumulates) could cause more ice to be lost. Simply, as snow falls and ice sheets become thicker, the surface gradient near the grounding line becomes steeper, which increases driving stress on the ice flow. 

Enhanced snowfall under global warming (blue) leads to comparatively higher elevation and driving stresses on ice compared to a warming only scenario (red). Elevations remain constant on the ice shelf, as it floats, but increase on the ice sheet (Image: Winkelmann et al., 2012)

Across the EAIS many glaciers are in equilibrium or growing. However, in coastal area there are cases where glaciers are shrinking and accelerating. Pritchard et al., 2012 present evidence that ice shelf glaciers, including Totten, West Shackleton and the Holmes glacier, are shrinking through basal and surface melt. The Totten Glacier reportedly thinned by 10m between 1996 and 2012. 

Not that strong and stable then (sounds like someone else we know!)

Compared to the WAIS and Antarctic Peninsula the EAIS is relatively stable, yet it is not immune to climatic changes. Coastal areas in particular are experiencing the effects of our changing climate, but these are complex and difficult to predict.