Surprises part 2: The collapse of the Antarctic Ice Sheet

In contrast to Greenland ice sheet (GIS), potential melting of the Antarctic Ice Sheet (AIS) may have implications for ocean circulation. If you may recall from the posts on millennial-scale climatic changes,  potential freshwater outburst  (in particular from ice shelves around the Ross and Weddell seas) may affect the production of Antarctic Bottom Water (AABW) which in turn may level deep-water formation and in turn potentially affect global climates (Swingedouw et al., 2008). Swingedouw et al. (2008) attempted to quantify future AIS melting on climate by running 5 different experiments using the model LOVECLIM, a three-dimensional earth system model of intermediate complexity (Figure 1). These included a control simulation (CTRL) whereby forcing was set constant to pre-industrial conditions, notably with the CO₂ concentration in the atmosphere set to 277.6 ppm, where CO₂ concentration increased by 1 % per year until it reaches 4 times initial values, remaining unchanged till the year 3000. Scenario iAiG has fully interactive ice sheets over Antarctica and Greenland, whilst scenario fAfG, components are forced with a fixed ice-sheet component with simulated warming. Scenarios fAiG and iAfG show fixed and interactive GIS respectively.

Figure 1. Time series of annual mean value of the minimum of oceanic global meridional overturning streamfunction at 30°S ( 1 Sv= 10⁶ m³/s) emphasising the export of Antarctic and Circumpolar Deep Water (AABW and CDW) at 30°S and the maximum of Atlantic meridional overturning streamfunction at 30 °S, representing the export of North Atlantic Deep Water (NADW) at 30°S.

Source: Swingedouw et al. (2008).

The results are of particular interest. Without AIS melting, AABW export at 30 °S weakens during first 300 years and recovers, enhanced by changes compared to CTRL scenario after 1000 years. This can be related to sea ice changes due to freshwater forcing relating to the retreat of sea ice cover. Net annual mean sea ice melting in Weddell and Ross seas are lower in fAfG compared to CTRL. This in turn increases sea surface salinity and sea surface density, counter to density loss from projected temperature loss, increasing AABW formation in seas in fAfG compared to CTRL after 3000 years. However I would like to focus on particular observation in the above, that with AIS melting, this affects NADW export in all the scenarios but then this recovers after 1000 years in iAfg in contrast to the fAfg, indicating a potential stabilizing effect of AIS melting on the weakening of the NADW cell. AIS melting cause NADW cell weakening by 1.2 SV in iAiG compared to fAiG. This stabilization effect of the AIS, the authors note, is similar to that of the bipolar seesaw; a mechanism for explaining millennial-scale climate change (explained in blog post 17). This can be explained by the fact that a reduction in AABW density appears to enable NADW to penetrate further and deeper south in the Atlantic with the associated cell.

The findings of this study are important for two reasons. Firstly, the findings of the study highlight the importance of using palaeoscience to understand environmental problems; through the application of mechanisms used to explain millennial-scale climate change to help understand and try to predict future impacts of changes projected with  anthropogenic climate change (such as AIS melting) on changes in ocean circulation. Finally, the study highlights the importance of using models in addition to palaeoscience, to not forecast but to offer an insight into the feedbacks between ice sheets, ocean processes under a warming scenario. This point is important to communicate (to climate skeptics!) the manner in which science of climate change is being developed and understood; in this case through understanding the impact of the Antarctic Ice Sheet on changes in ocean circulation.  

Interhemispheric comparisons

Returning to the conundrum in post 13, one of the most important finds in the study of millennial-scale climate variability is that the response of the North and South hemispheres are out of phase.

The Southern hemisphere has not received much attention so far in the blog post, but as we shall we see in this post and the next post, it holds vital importance for our understanding of global ocean circulation) (Blunier et al., 1998). Blunier et al. (1998) showed that Greenland warming around 36 and 45 kyr before present (BP) lagged Antarctica by 1 kyr. Therefore, in order to synchronize both records, the authors use methane (CH₄) records from the last glaciation from 2 Antarctic ice cores (Byrd station 80°S, 120°W, Vostok 78.47°S, 106.80°E ) and 1 Greenland ice core (GRIP ice core summit, 72.58 °N, 37.64 °W). CH₄ has an approximate residence times of c. 10 years in the atmosphere. The advantage of using CH₄ as a tool for synchronisation is that it has a residence time that is long enough to become globally homogenous but is also short enough to react quickly to budget imbalance linked to climate changes. Therefore any changes in concentration should be synchronous between Antarctica and Greenland (Figure 1).

Figure 1: GRIP, Byrd and Vostok isotopic and CH₄ records on the common timescale (GRIP timescale in years before 1989).Antarctic warming as indicated by A1 and A2, with vertical dashed lines  indicating the location of Greenland warmings 1,8 and 12 in the Antarctic cores.

Source: Blunier et al. (1998)

On average, the authors found that the change in Antarctic climate leads Greenland on an average of 1 1±2.5 kyr across the period 47±23kyr BP. One of the greatest findings of Blunier et al. (1998)’s observation is that the records dispelled any notion of a coupling between Northern and Southern hemispheres via the atmosphere, an idea postulated by Bond et al. (1993). Crucially, the findings potentially favour an ocean connection in order to conceptualise this behaviour.

One of the greatest challenges in climate science is the ability to derive a mechanism that accounts for the observed behaviour of high-latitude north and south hemispheres.

Any such mechanism would need to address the following conundrum (forgive my use of intertextuality with reference to arguably one of the most famous authors in English literature):

To be in Phase or To be in Antiphase?

That is the question…