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.