Those who do pay attention to global warming know that Greenland is melting. The world’s second largest ice sheet is melting fast, the rate of melt is accelerating, and it is not going to stop melting anytime soon. In fact, NASA’s twin GRACE satellites measured it gushing 2 trillion tons of ice emptying both fresh melt water, and ice bergs into the North Atlantic over the past decade.
The drainage system on the surface of the ice sheet has been studied for years. The accumulated meltwater forms streams, rivers and lakes called supraglacial lakes. In the case of rivers and streams the water carves a vertical shaft into the ice called a Moulin. This narrow and tubular shaft provides the pathway for surface meltwater to reach all the way to the bottom. In the case of the surface lakes, it is believed that nearby Moulin’s accumulates water at the bottom of the ice sheet which creates a bulge that floats the ice sheet creating tension at the surface underneath the lake. The stress continues until a crack forms below the lake. The volume of water in the lake widens and extends the crack, or hydro-fracture, and the lake water surges to the bedrock.
NASA notes that subglacial lakes form at the “base of an ice sheet or glacier because of either friction or trapped heat from bedrock below. In order for this to happen, ice needs to move quickly or be thick enough to protect the ice sheet base from cold air at the surface and trap heat coming from the bedrock below.” The National Science Foundation team that NASA references, observed that that the “basin floor rose significantly during the next summer at the same time that nearby surface meltwater drained into cracks along the basin’s edge. This led the team to hypothesize that water from surface melting was refilling a lake beneath the ice.
Earth and Space Science news reports on a new report from Roeoesli et al,. on how seismic waves produced by free-falling meltwater could improve understanding of glacial drainage processes.
On most days the seismometers detected tremors that lasted from 4 to 16 hours. According to seismometer data, these events made the moulin’s ice walls resonate in patterns similar to those experienced by an organ pipe with one end plugged. Just as organ pipe resonance depends on the length of the tube, the observed moulin patterns appeared to depend on the level of the collected water.
On the basis of this similarity, the scientists built a model of moulin tremor production. In the model, free-falling meltwater causes tremors when it strikes the surface of the water deep inside a moulin. The impact sends acoustic pressure waves into the walls and base of the shaft and through the ice. Resonance patterns picked up by seismometers can be interpreted to reveal the moulin’s size, water level, and water depth.
Other scientists have previously used seismic data to examine the flow of glacial meltwater, but this marks the first time they have been used to model a glacial moulin. However, this model approximates a moulin as a simple cylinder, and scientists suspect that many moulins actually consist of an alternating series of shafts and wide pools. Future research could refine seismic techniques to detect more complex moulin structures and examine how moulins affect glacial movement. (Journal of Geophysical Research: Solid Earth, doi:10.1002/2015JB012786, 2016)
From the abstract.
Through glacial moulins, meltwater is routed from the glacier surface to its base. Moulins are a main feature feeding subglacial drainage systems and thus influencing basal motion and ice dynamics, but their geometry remains poorly known. Here we show that analysis of the seismic wavefield generated by water falling into a moulin can help constrain its geometry. We present modeling results of hour-long seimic tremors emitted from a vertical moulin shaft, observed with a seismometer array installed at the surface of the Greenland Ice Sheet. The tremor was triggered when the moulin water level exceeded a certain height, which we associate with the threshold for the waterfall to hit directly the surface of the moulin water column. The amplitude of the tremor signal changed over each tremor episode, in close relation to the amount of inflowing water. The tremor spectrum features multiple prominent peaks, whose characteristic frequencies are distributed like the resonant modes of a semiopen organ pipe and were found to depend on the moulin water level, consistent with a source composed of resonant tube waves (water pressure waves coupled to elastic deformation of the moulin walls) along the water-filled moulin pipe. Analysis of surface particle motions lends further support to this interpretation. The seismic wavefield was modeled as a superposition of sustained wave radiation by pressure sources on the side walls and at the bottom of the moulin. The former was found to dominate the wave field at close distance and the latter at large distance to the Moulin.
Glaciologist Mauri Pelto does not entirely agree that larger volumes of meltwater reaching the ice sheet bottom is resulting in overall velocity increase of the ice sheet. In a post in the American Geophysical Union’s blogosphere titled “Moulins: Clarifying Impacts on Glacier Velocity”, Pelto explains.
Having spent considerable time observing moulins and reviewing some excellent studies that indicate their impact, it is worth noting again a more complete picture of the role moulins play. This is a role that warrants considerable further examination. In 2008 and 2011 I wrote a piece indicating why this a generalization that is only sometimes accurate. The key to increasing glacier velocity is high basal water pressure, not simply lots of meltwater. Think of your car, if you have low oil pressure that is an issue for efficient running. If you have high oil pressure that is ideal. If you have high oil pressure and you add more oil that does not further lubricate the engine. Delivering more water to the base of a glacier that already has lots of meltwater drainage will not typically lead to a significant acceleration. A number of studies since 2008 have better illustrated this principle as it applies to ice sheets.
Ahlstrøm et al (2013) examined 17 Greenland glaciers and noted a pattern, “Common to all the observed glacier velocity records is a pronounced seasonal variation, with an early melt season maximum generally followed by a rapid mid-melt season deceleration”. This indicates the Greenland glaciers are more like a typical alpine glacier and are susceptible to the forces that tend to cause alpine glaciers to experience peak flow during spring and early summer. Those forces are the delivery of meltwater to the base of the glacier, when a basal conduit system is poorly developed. This leads to high basal water pressure, which enhances sliding. As the conduit system develops/evolves the basal water pressure declines as does basal sliding, even with more meltwater runoff.
This is what has been reported to be the case by Sundal et al (2011) in Greenland. They found a similar early season velocity in all years, with a reduced velocity late in summer during the warmest years. This suggested that a more efficient melt drainage system had developed, reducing basal water pressure for a longer period of time. The meltwater lubrication mechanism is real, but as observed is limited both in time and area impacted. It is likely that, as on alpine glaciers, the seasonal speedup is offset by a greater slowdown late in the melt season. Most observed acceleration due to high meltwater input has been on the order of several weeks, leading to a 10-20% flow increase for that period.
The media and the Republican party has been able to divert the publics attention away from the very real and grim peril that we find ourselves in as a result of the melting of Greenland ( see Dartagnan’s recommended diary ). Shiny new objects such as Donald Trump’s intolerance and his inept policies, taco trucks, Olympian Ryan Lochte shenanigans in Rio, Hillary Clinton’s emails (but not her plans for immediate climate action), and particularly an act of terrorism. As you recall, terrorism diverted attention from the Paris climate talks when gunmen shot and killed 125 people and injured another 200 people at the Bataclan, Paris restaurants and attacks outside Stade de France. In fact, this particular act of terrorism allowed French President Francois Hollande to ban all demonstrations, including those staged for the climate talks. The media mostly covered these attacks, and not the talks which focused on the very real threat to every individual on the planet. Perhaps they have not received the memo that climate disruption is terrorism on a massive scale. Since we are now in the age of consequences, we will be forced to confront multiple catastrophes occurring simultaneously that we will simply not want to face.