Research Paper Review: Schannwell et al. 2016

One of the most critical impacts of climate change in the polar regions is Sea Level Rise (SLR) . SLR is one of the most frequently discussed and poorly understood issues in polar science. Loss of sea ice does not contribute to SLR as the melting ice replaces the volume of water it displaced whilst floating. Instead, a significant contribution to SLR from these regions comes from the loss of ice from ice sheets (i.e. Greenland and Antarctica) via discharge from outlet glaciers which carry ice to the ocean. In light of warming polar ocean and air temperatures, the contribution of polar regions to future SLR has become a leading research area. A recent (November 2016) paper by Schannwell et al. has provided an update on the potential contribution of a key region – the Antarctic Peninsula.  Results from this modelling study indicate that the future contribution of this region may have been underestimated – with previously unconsidered processes playing a large role.

Aim and Methodology: The aim of this recent paper is to model ice loss, and subsequent contribution to SLR, from the Antarctic Peninsula. This is a region of the Antarctic which contains a large number of outlet glaciers which terminate either at directly at the ocean, or into ice shelves.  Previous studies have included predictions of SLR from glaciers which terminate at ice shelves, but not those which terminate directly into the Southern Ocean.

Schannwell et al. have sought to address this by including processes from both types of glaciers to model SLR from the Antarctic Peninsula. The British Antarctic Survey Antarctic Peninsula Ice Sheet Model (BAS – APISM) is the primary model which is used. This ice sheet model is forced with climate model output from 13 General Circulation Models (GCMs) from the Climate Model Intercomparison Project Phase 5 (CMIP5) . This results in 300 yearlong transient simulations of climate and corresponding Antarctic Peninsula ice loss from the year 2000 to 2300. These simulations are carried out for two of the IPCC prospective Representative ConcentrationPathways (RCPs) (pp. 1461); RCP4.5 (a ‘stabilization pathway’) and RCP8.5 (a ‘high emissions pathway’).

Results: Model output shows that previously unconsidered glaciers, (i.e. those which terminate directly at the ocean) could contribute to a significant volume of SLR from the Antarctic Peninsula. For RCP4.5 these glaciers alone are responsible for 79% – 89% of total projected SLR by 2300. The magnitude of these contributions can be seen by the pink (maximum value) and blue (minimum value) bars on Figure 1 (the aforementioned percentage is represented in panel ‘a’ by the scale of the pink and blue bars in comparison to the green bar representing SLR from the other processes).  These glaciers contribute to SLR on a similar scale in RCP8.5 (Figure 1, panel b).

In the same Figure, total modelled SLR from both types of glacier is shown, with maximum and minimum projections shown by the solid red and blue lines respectively. This study therefore shows a range of potential total SLR contribution from the Antarctic Peninsula from between 11 ± 2 and 32 ± 16 mm (minimum RCP4.5 value to maximum RCP8.5 value) by the year 2300. Results from the study indicate notable SLR from both types of glaciers in the Antarctic Peninsula, although the relative importance of these newly considered glaciers is greater in the RCP4.5 scenario.

Figure 1, source: Schannwell et al. 2016. Caption as quoted from the paper itself (pp. 167): ‘Combined SLR for RCP4.5 (a) and RCP8.5 (b) scenarios. Solid red upper line and solid blue lower line correspond to combined maximum and combined projection, respectively. Dashed blue lines approximate timing of ice-shelf collapse. Error bars are displayed where available’.

Discussion and Evaluation: Key findings of the study have shown that previously unconsidered processes in the Antarctic Peninsula have led to potentially greatly underestimated projections of SLR from this region. These previously unconsidered factors are particularly relevant in the stabilized RCP4.5 scenario. In context of the future Anthropocene, results from the study indicate a revised annual SLR of between 0.04 ± 0.01 mm a^-1 and 0.11 ± 0.05 mm a^-1 from the Antarctic Peninsula alone. The author notes the significance of these values in terms of recently (2003-2013) observed SLR from the entire Antarctic region of 0.25 ± 0.07 mm a^-1. Although there is a large degree of uncertainty in the new techniques utilised in this research, this paper offers valuable insight into the extent of SLR underestimation, even in a lower scale emissions scenario (RCP4.5).