Increasing melt, decreasing flow?

In contrast to the traditional views on the relationship between ice loss and surface melting, there do exist conflicting opinions on the state of glacier acceleration in Greenland. One of the proponents of such a viewpoint are Sundal et al. (2011) who suggest that the Greenland ice sheet response to melting over time is in fact not increased acceleration, but a reduced flow into the fjords.

Figure 1. Ice velocity flows for the study area

The main premise of the paper involves the adaptation of the Greenland ice sheet’s subglacial drainage system to increased melting in warmer years. Thus the ice sheet, which could raise the global sea level by 7 metres if completely melted, could actually be seeing a reduction in the flow of ice into the fjords and oceans. Evidently, the implications for future predictions of global sea level rise are quite large. Indeed, the Greenland ice sheet has shrunk in the last decade due to rising temperatures, but the question of where the meltwater could be transported to remains unresolved.

The orthodox view of ice loss due to increased melting suggests that warmer temperatures cause ice to melt on the surface of the ice sheet. This meltwater is then transported to the base of the glacier, whereby it acts to increase the rate of ice flow across the bedrock and into the fjords. However the authors of the study, using remotely sensed imagery of 6 glaciers in SW Greenland, produced vastly different indications of how ice flow changes across years of variable melting. They instead found that the expected increases in ice sheet melting over the 21^st century (as a result of increased air temperature) may have no impact on ice loss as a product of flow into the ocean. However, it must be noted that the ice sheet isn’t threatened any less, as the studies of Straneo et al. (2011) Rignot et al. (2010) and Holland et al. (2008) revealed in the uncertainty of ocean-glacier interactions.

The study shows that the initial increase of ice flow was the same for both warmer and colder years. However, it also revealed that the expected slowdown in ice flow (as a result of glacier drainage adaptation) actually came quicker in the warmest years. Sundal et al. propose that as a result of much greater meltwater in warmer years, the internal basal drainage switches more quickly, which creates a drop in pressure and therefore slower ice movement across the bedrock. As the paper notes:

“Abundant melt-water can trigger a switch from inefficient (cavity19) to efficient (channelized20) modes of drainage and, consequently, to a reduction in subglacial water pressure and ice speed. Such events have been observed at High Arctic10 and Alaskan valley glaciers11, where summer speed-up is of shorter duration during years of high melting.”

The authors also conducted a numerical simulation of this switching between modes of drainage, which showed that above a critical meltwater flow rate of 1-2 cm per day, switching and thus glacier slowdown would occur.

Whilst displaying these findings, the authors accept that there are many conflicting studies on the topic and that whilst some studies have shown increased ice sheet acceleration from increased melting (Zwally et al. 2002), other studies have identified a long-term decrease in ice flow from the Greenland ice sheet over such a period of increased melting (van de Wal et al. 2008). The authors believe however, that both findings can be combined. Whilst the data shown in this study does identify an increase in the peak rate of flow during years of high melting; the important finding here is that the subsequent faster transition to more efficient subglacial drainage means that the duration and speed of ice flow is much lower than when compared to years of low melting. In cooler years, where the critical meltwater flow rate of 1-2 cm per day is not exceeded, the more efficient ‘channelized20’ mode of drainage does not occur.

With the predicted increase in Greenland ice sheet contribution to global sea levels over the 21^st century (Mernild et al. 2008), the impact of melt-induced ice flow acceleration must not be ignored. Thus, the findings of this study and others are important for the predictions of future climate change impacts. Particularly if the findings of this study are correct, then the observed and predicted rises in air temperature over the next century may not push the Greenland ice sheet over a suggested ‘tipping point’, as a result of melt induced glacier acceleration. However, the melting will of course still occur (increasing basal storage of meltwater) and as the authors admit, their findings do not cover how the switch to more efficient drainage might be adversely affected by more short term spikes in melting. As well as this, the previously discussed studies of warm ocean influences on the outlet glaciers suggest that a much greater melting influence occurs as a result of subtropical waters. Both ocean and melting influences on glacier acceleration remain non-definitive at present; and so it is important for further research to identify how these processes have acted in the past, so that we might better understand the potentially devastating  impacts that acceleration of the Greenland ice sheet could have in the future.