A new model developed by researchers at the University of Cambridge has shown that despite its apparent stability, the massive ice sheet covering most of Greenland is more sensitive to climate change than earlier estimates have suggested, which would accelerate the rising sea levels that threaten coastal communities worldwide.
In addition to assessing the impact of the increasing levels of meltwater created and spilled into the ocean each year as the climate continues to warm, the new model also takes into account the role that the soft, spongy ground beneath the ice sheet plays in its changing dynamics. Details are published today (29 September) in the journal Nature Communications.
The Greenland Ice Sheet, which is the second-largest ice sheet in the world, covers 1.7 million square kilometres - an area roughly eight times the size of the United Kingdom - and contains enough ice to raise sea levels by more than seven metres if it were to be lost altogether.
Currently, due to surface melting alone, it is losing ice at a net annual rate of 200 gigatonnes, equating to 0.6 millimetres of sea level rise. A similarly large, but ultimately more uncertain source of sea level rise is tied to a net annual ice loss caused by increased movement of the ice sheet, which results in more ice being discharged into the ocean. Globally, sea levels are rising at three millimetres annually.
Large ice sheets such as in Greenland are far from stationary. Different parts of the ice often move at different speeds, causing ice to shear, a phenomenon known as ice flow.
"When these large ice sheets melt, whether that's due to seasonal change or a warming climate, they don't melt like an ice cube," said Dr Marion Bougamont of Cambridge's Scott Polar Research Institute, who led the research. "Instead, there are two sources of net ice loss: melting on the surface and increased flow of the ice itself, and there is a connection between these two mechanisms which we don't fully understand and isn't taken into account by standard ice sheet models."
Whereas other models of the Greenland Ice Sheet typically assume the ice slides over hard and impermeable bedrock - an assumption which is largely practical and based on lack of constraints - this study incorporates new evidence from ground-based surveys, which show soft and porous sediments at the bed of the ice sheet, more like the soft and muddy bottom of a lake than a sheet of solid rock. The new study specifically identifies the intake and temporal storage of water by weak sediment beneath the ice sheet as a crucial process in governing the ice flow.
Using a three-dimensional ice sheet model, together with an observational record of surface melting produced by collaborators at Aberystwyth University, Dr Bougamont and Dr Poul Christoffersen were able to accurately reproduce how the ice sheet's seasonal movement changes in response to the amount of surface meltwater being delivered to the ground below.
Lakes which form on the surfaces of glaciers, known as supraglacial lakes, are often created during the melt season, and typically last from early June to late August. Co-author Professor Alun Hubbard of Aberystwyth University studied these lakes and found that many empty in just a matter of hours, when hydrofracturing opens up water-filled crevasses, resulting in huge amounts of water entering and flooding the subglacial environment. In warmer years, these high-discharge drainage events are expected to become even more frequent.
"Not only is the ice sheet sensitive to a changing climate, but extreme meteorological events, such as heavy rainfall and heat waves, can also have a large effect on the rate of ice loss," said Dr Christoffersen. "The soft sediment gets weaker as it tries to soak up more water, making it less resistant, so that the ice above moves faster. The Greenland Ice Sheet is not nearly as stable as we think."
While complete loss of all ice in Greenland is judged to be extremely unlikely during this century, the record extent of surface melting in the past decade clearly shows that the ice sheet is responding to Earth's changing climate.
In this study, the researchers used two different approaches. First, they used the total amount of surface runoff as a means to drive their model, but the outcome from this experiment was inconsistent with observations. They then used only water stored temporarily in supraglacial lakes on the ice sheet's surface. They found that although only a small fraction of the total amount of meltwater produced on the surface is stored in supraglacial lakes, the high magnitude and frequency of lake drainage events causes the ice sheet to immediately accelerate as observed.
Having accurately reproduced the hydrological response of ice flow along the western margin of the ice sheet, the authors were able to subsequently evaluate the sensitivity of flow to warmer climatic conditions, resulting in more meltwater on the surface. This showed stable annual flow under present-day conditions, but a more vulnerable ice sheet in warmer years when more meltwater reaches the bed via frequent high-discharge drainage events, not only because of the emptying of supraglacial lakes such as the ones currently observed, but also because daily variations in melt volume will become equally large. The study concludes that there is a limit on how much water can be stored in the soft ground beneath the Greenland Ice Sheet. This makes it sensitive to climate change as well as to increased frequency of short-lived, but extreme, meteorological events including rainfall and heat waves.
The work was funded by the Natural Environment Research Council (NERC).
For additional information, or high-resolution images, please contact:
Sarah Collins, Office of Communications
University of Cambridge
Tel: +44 (0)1223 765542, Mob: +44 (0)7525 337458
Notes for editors:
1. The paper, "Sensitive response of the Greenland Ice Sheet to surface melt drainage over a soft bed" is published on 29 September in the journal Nature Communications: doi:10.1038/ncomms6052
Sarah Collins | Eurek Alert!
Greenland ice flow likely to speed up: New data assert glaciers move over sediment, which gets more slippery as it gets wetter
17.08.2017 | Swansea University
Climate change: In their old age, trees still accumulate large quantities of carbon
17.08.2017 | Universität Hamburg
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
18.08.2017 | Life Sciences
18.08.2017 | Physics and Astronomy
18.08.2017 | Materials Sciences