INTO THE ICE: The role of increasing meltwater on the future stability of the Greenland ice sheet
The Greenland ice sheet is a massive frozen reservoir of freshwater – one that currently stores an equivalent to 7.4m of global sea-level rise should it completely collapse. The ice sheet is in constant motion – ice flows from the interior out towards the margin by deforming and sliding where it melts or discharges massive icebergs – and ends up in the ocean. Since the 1990s, the ice sheet shifted from a state of equilibrium with climate – where snowfall inputs balanced melting and iceberg calving outputs - to where warming has led to more and more ice being lost to the ocean. Greenland is now the largest icy contributor to global sea levels, raising it at up to 1.5 mm per year.
One project I’m working on “Into the Ice” aims to monitor and explore the role and impact of the meltwater - and the 1000s of moulins that form across the ice sheet each summer melt season – on ice-sheet flow. Moulins are huge shafts – natural wells – that collect trillions of tonnes of meltwater from across the ice surface and drain it to the ice sheet bed each summer. How much meltwater, where it goes and how it behaves once at the bed impacts the future flow response and stability of the ice sheet to climate change.
Where we have a good idea of what happens to that meltwater near (within 10s of kms) to the ice sheet margin - we have virtually no data on the impact of the meltwater on ice flow further inland where the ice sheet is over 1000 m thick. Current ideas suggest that the meltwater melts, and when it drains erodes large conduits into the base of the ice sheet - forming efficient drainage networks much like an urban storm-water system. This allows the water to drain freely beneath the ice sheet to the ice margin. But within the bulk of Greenland’s interior – where the ice is very thick – such conduits cannot form and are crushed by the weight of the overlying ice column – so that water can only be stored in pockets and cavities at the bed – poorly linked through inefficient drainage networks between bumps in the underlying bedrock or through sediments.
The difference in these two drainage systems beneath the ice sheet is critical to how it responds to current and future warming with increased meltwater runoff. Efficient drainage yields low basal water pressures - high friction at the ice and bed interface, whereas inefficient drainage (like a blocked water pipe) leads to high back-pressures which lubricate the bed to yield higher rates of ice movement (like an ice cube across a wet surface).
The meltwater also brings with it energy in the form of latent heat – which acts to warm up the basal layers of ice and unstick parts of the ice sheet where it is frozen to its bed. All of these processes work in consort to accelerate ice flow with ongoing climate warming - and just like an opened sluice gate – the ice sheet discharges faster to the ocean with an increased drawdown of its interior reservoir to raise global sea levels.
The work I’ve been doing in the interior zones of the ice sheet reveals that the basal drainage system, contrary to current thinking – remains at very high pressure throughout the melt season and into the autumn/winter. We have explored and mapped some of the largest and deepest moulins ever on the ice sheet - to 180 m deep - and discovered that they are still actively connected to the basal drainage system even in late October when all surface meltwater has ceased due to the onset of cold, winter conditions down to -25°C. We mapped these moulin networks using a laser scanner and installed real-time passive-seismic, thermal and pressure probes - as well as methane sensors – to find out how active they remain, and how the water level reacts as winter proceeds. Our results indicate that basal water pressure and ice flow have remained high – even though prevailing theory suggests that they should drop with the onset of winter. Furthermore, we also found evidence of methane degassing from the waters deep inside the ice sheet – indicating that there is organic carbon and microbial life present down there – despite the frigid and inhospitable conditions.
I will be back next spring, to continue our “Into the Ice” project and furthermore – to drill through the ice sheet to monitor thermal and hydrological conditions at the bed and furthermore, to retrieve sediment and water samples to analyse them for carbon, microbes and to help understand exactly how the drainage conditions are changing, where the meltwater has come from and how it is impacting on the basal ice down there.