Waves Rippled Through Greenland’s Ice. That’s Ominous

by Brian Kahn, Climate Central, May 26, 2017

On its surface, the Greenland ice sheet is a vast expanse of seemingly immovable ice. But beneath the monotonous stretch of white, scientists have discovered evidence of waves rippling through one of its outlet glaciers and roiling its innards.

The waves, observed during the two most intense melt seasons on record, sent an unprecedented cascade of ice and water rushing into the sea and warping the very bedrock upon which the ice sits. As temperatures continue to rise, scientists fear that massive waves of ice could expedite Greenland’s melt even further, pushing sea levels higher.

Rink Glacier from 34,000 feet. Credit: John Sonntag/NASA

It’s the latest piece of bad news about Greenland’s ice. The ice sheet has been pouring roughly 270 megatons of ice a year into the ocean via the glaciers that stretch out from its hulking mass since 2000. That’s a big uptick compared to preceding decades.

The new research, published earlier this week in Geophysical Research Letters shows a new way that climate change is taking a toll. Scientists at the NASA Jet Propulsion Laboratory, led by Surendra Adhikari, were looking at data from a series of GPS stations set up around the various outlet glaciers that tumble from Greenland’s ice sheet to the sea. Ironically, they were looking at the GPS data to see if it was worth maintaining the network of stations that rings Greenland.

They found evidence of a never-before-observed phenomenon affecting Rink Glacier, a glacier on the western flank of Greenland. The glacier usually sends about 11 gigatons of ice into the ocean each summer melt season.

But 2012 was different. A fast-moving (by glacial standards), massive wave rumbled through the glacier’s interior, causing an extra 6.7 gigatons of ice and water to slosh into the sea. That’s the equivalent of 55 million blue whales, the largest animal on earth.

The wave — dubbed a solitary wave because of its singular nature — traveled at 2.5 miles per month in the summer, picking up to 7.5 miles per month in the fall. Rink Glacier typically only moves a mile or two in a normal year.

Scientists picked up on the wave using GPS sensors, which shifted more than half an inch as it rolled by. That might not sound like much, but the sensor is sitting on stable Greenland bedrock, which isn’t exactly susceptible to perturbations.

A similar, though less extreme, wave passed by the sensor in 2010. The behavior is like nothing scientists have ever observed on Greenland.

“If you were to place a piece of plywood onto two sawhorses then place a large rock in the middle, the plywood would bend, sagging in the middle,” said Erik Ivins, a researcher at JPL who co-authored the study. “When the rock is removed, the plywood returns to the original potion.”

An animation showing horizontal bedrock motion in response to the solitary wave passing by.
Credit: Surendra Adhikari/NASA

Though they’re still trying to understand exactly why it happened, there are a few major clues. Those clues don’t bode well for the future of Rink Glacier or the enormous amount of ice it’s holding back.

The melt seasons of 2010 and 2012 were two of the most extreme on record. In 2012, a prolonged summer heat wave coupled with soot from Siberian fires that cast a dark coat over the ice sheet resulted in 95% of the ice sheet going into meltdown.

Scientists suspect that that meltdown is responsible for the wave that tore through Rink Glacier. Meltwater from the interior created new pathways for water to move around and likely lubricated the based of the glacier where it meets the bedrock, priming it for the massive shudder of ice that moved through it.

Robin Bell, an ice researcher at Lamont-Doherty Earth Observatory, called the research a “beautiful study linking how the surface of the ice sheet melts and slides with how the surrounding mountains (solid earth) responds. (It’s) remarkable to see the earth lurch in a year when the ice changes more.”

Temperatures are likely to only keep rising due to climate change. Previous research has suggested that melt seasons like 2012 could become the norm by the end of the century, increasing the risk of more glacial waves in the coming century.

“We suspect that solitary waves may be unique to high melt years,” Adhikari said. “The more warming, the more surface meltwater available to trigger ‘extraordinarily’ dynamic behavior of glacier such as the one we discovered in Rink Glacier.”

Greenland’s melt is currently responsible for roughly 25% of observed sea level rise. That percentage could increase in the coming years if what happened at Rink Glacier spreads to other glaciers.

http://www.climatecentral.org/news/waves-greenland-ice-melt-21488

"Mass transport waves amplified by intense Greenland melt and detected in solid Earth deformation," GRL,

Geophysical Research Letters, (26 May 2017); doi: 10.1002/2017GL073478

Mass transport waves amplified by intense Greenland melt and detected in solid Earth deformation

S. Adhikari, E. R. Ivins and E. Larour

Abstract

The annual cycle and secular trend of Greenland mass loading are well recorded in measurements of solid Earth deformation. Horizontal crustal displacements can potentially track the spatiotemporal detail of mass changes with great fidelity. Our analysis of Greenland crustal motion data reveals that a significant excitation of horizontal amplitudes occurs during the intense melt years. We discover that solitary seasonal waves of substantial mass transport (1.67 ± 0.54 Gt/month) traveled at an average speed of 7.1 km/month through Rink Glacier in 2012. We deduce that intense surface melting enhanced either basal lubrication or softening of shear margins, or both, causing the glacier to thin dynamically in summer. The newly routed upstream subglacial water was likely to be both retarded and inefficient, thus providing a causal mechanism for the prolonged ice transport to continue well into the winter months. As the climate continues to produce increasingly warmer spring and summer, amplified seasonal waves of mass transport may become ever more present with important ramifications for the future sea level rise.

Plain Language Summary

It has become well known that seasonal ice flow variability of Greenland outlet glaciers may often be associated with the drainage of supraglacial lakes that accumulate meltwater during summer. However, tracking the details is inevitably limited due to the fact that the mechanisms and rates of meltwater transfer are hidden from view, and theoretical models are fraught with a number of difficulties. Here we use a previously unrecognized source of data that constrain the mass transport during a season of intense Greenland melting and document the evolution of a mass transport wave as it passes down glacier. The breakthrough is twofold: demonstration of the power of the new technique and the first measurement of the mass amplitude. The technique is effectively using the measurement of the deformed solid Earth elastic response as a filter that uniquely responds to neighboring glacier mass changes. We quantify that the wave through Rink Glacier is enormous in terms of its mass transport, amounting to about half of the average annual discharge during 2000–2005, and travels at an average speed of 7.1 km/month. Our mass transport wave measurement is the first of its kind, on any of the major outlet glaciers of either Greenland or Antarctica.

1. Introduction

Three-dimensional (3-D) crustal motions caused by the redistribution of mass on Earth's surface are captured in bedrock Global Navigational Satellite System (GNSS) data. The Greenland GNSS Network (GNET) consists of more than 50 bedrock stations (Figure 1a) having many geophysical applications. A fundamental goal has been to weigh the contemporary mass changes in the Greenland Ice Sheet (GrIS) and to assess the rate of ongoing viscoelastic solid Earth response to past changes in ice mass [Khan et al., 2007, 2016; Bevis et al., 2012]. This mainly requires an analysis of vertical component of the crustal motion. In this paper, we analyze the horizontal displacement data set and show for the first time that these geodetic stations are capable of mapping the magnitude and location of seasonal mass transport waves that travel downstream through GrIS outlet glaciers [Sharp, 1988; Hewitt and Fowler, 2008]. The breakthrough is twofold: demonstrating the power of the new (geodetic) technique to detect solitary seasonal waves of ice mass transport and the first quantitative measurement of wave amplitude and speed.

Figure 1. Sensitivity of GNET data to land mass changes. (a) Locations of 54 permanent geodetic stations (circles) around the coastal Greenland. The mapped ice surface velocity [Rignot and Mouginot, 2009] highlights GNET locations relative to fast ice discharge. The boundary of a JPL-GRACE mascon encompassing the RINK station is also shown. (b) A sensitivity gradient map of vertical displacement dU1/dH, shown within the mascon boundary, for the RINK station. Units reflect a measure of vertical displacement (up positive) at RINK caused by a unit ice load over a unit area placed anywhere in the region. (Note: 1.0 μm/m/km2≈1.1 mm/Gt) (c) Same as Figure 1b but for the north-south component of horizontal displacement dU2/dH(north positive). The dipole shape of the sensitivity gradient is caused by the opposing signs of the displacements for loads placed on either side of the station. (Sensitivity gradient map for the east-west component dU3/dH is not shown.) (d) The ZOI of ice loading on crustal displacement at RINK station. Red zone quadruples, for example, represent the high sensitivity zones for all three components of at RINK.. These are essentially the derivatives of the displacements at a specified geodetic station with respect to changes in load at any arbitrary location on the Earth's surface: .

Complete, free, open-access article here:  http://onlinelibrary.wiley.com/doi/10.1002/2017GL073478/full

WaPo: So much water pulsed through a melting glacier that it warped the Earth’s crust

by Chris Mooney, The Washington Post, May 25, 2017

Rink Glacier on Greenland’s west coast. (NASA/John Sonntag)

NASA scientists detected a pulse of melting  ice and water traveling through a major glacier in Greenland that was so big that it warped the solid Earth — a surge equivalent in mass to 18,000 Empire State Buildings.

The pulse — which occurred during the 2012 record melt year — traveled nearly 15 miles through the Rink Glacier in western Greenland over four months before reaching the sea, the researchers said.

“It’s a gigantic mass,” said Eric Larour, one of the study’s authors and a researcher at NASA’s Jet Propulsion Laboratory. “It is able to bend the bedrock around it.”

Such a “wave” has never before been detected in a Greenland or Antarctic glacier. The total amount of mass carried in the wave — in the form of either water, ice or some combination of both — was 1.67 billion tons per month, or 6.68 billion tons over four months, according to the study, which was published in Geophysical Research Letters.

The study was led by the lab’s Surendra Adhikari and co-authored by Erik Ivins.

“These solitary waves, they’re fairly well known in rivers,” said Ivins, also a researcher at the Jet Propulsion Laboratory. “Rivers can have inundations upstream where a lot of water is collected, and the water gets bunched up as it’s going downstream and doesn’t ever really flatten out. It just remains as this wave and continues down a river.”

However, the scientists don’t know what the wave actually looked like or precisely what caused it — much of it was occurring below the surface of the glacier. They also don’t know precisely what it was made of. “We are losing a combination of water and ice. We don’t know what fraction,” said Adhikari.

The researchers were able to detect the wave only because a GPS sensor, located in a rocky inland area a little over 12 miles, moved 15 millimeters as the wave went by, pushing down on the Earth’s crust and causing a deep indentation.

“The GPS can sense that,” Larour explained.

Richard Alley, a glaciologist at Penn State University who was not involved in the study, explained it this way:

“Find a bed,” Alley said by email. “Put a little piece of tape on the sheet.  Put your fist right next to the tape and push down, while watching the tape.  The tape will move down as you push down, and also will move horizontally toward your fist just a little. Put your fist farther away, and the tape won’t move as much.  Push harder, and it will move more. While pushing down, slide your fist past the tape, and you’ll see a pattern of vertical and horizontal motions of the tape.”

“A bed isn’t exactly the elastic Earth, but that’s sort of what this team did,” Alley continued. “They saw a ‘fist’ of mass sliding down the glacier past their GPS station, caused by extra meltwater.”

Adhikari provided this animation showing the direction of the GPS device’s movement (and therefore that of the bedrock or solid Earth) as the bulk of mass went by:

An animation showing horizontal bedrock motion in response to a nearby glacier mass change in the form of a wave. Ice mass change is portrayed by ice thinning/thickening (delta H), the centroid or fulcrum of mass anomaly is denoted by the star, and the direction of bedrock motion is represented by the arrow as measured at a GPS station (circle). As the glacier gains (loses) mass, the bedrock moves toward (away from) it. (NASA-JPL/Caltech)

The wave occurred in the wake of a 2012 summer melting event that saw most of the surface of Greenland become covered with liquid water, and that still has not been surpassed by subsequent warm years.  The researchers suspect that some of that meltwater flooded beneath the ice sheet and then pulsed outward through Rink Glacier.

“It’s really related to the deep interior of Greenland that’s full of melt, and it’s trying to get rid of that melt through gravitational processes,” said Ivins.

The study also documented another, smaller “wave” at Rink Glacier in 2010, another major melt year.

Rink is far from the largest glacier in Greenland. It is about 3.4 miles wide at its front where it touches the ocean and a little over half a mile deep in the same location. Researchers have also shown that pulses of meltwater flow out from beneath the glacier in colorful silt-filled plumes, presumably through subterranean channels, which could be how some of this mass exited to the ocean in 2012.

The scale of the pulse, 6.68 billion tons, or gigatons, is still only a fraction of what Greenland contributes to the ocean every year in the form of water and ice. NASA has estimated that Greenland loses 287 billion tons annually at present (though it lost far more than that in the banner melt year of 2012).

Still, the research gives a sense of the tremendous magnitude of the changes occurring on Greenland, which is covered by enough ice to raise sea levels by over 20 feet if it were all to slide into the ocean.

And it pairs with other studies showing that the breaking off of large pieces from Greenland glaciers causes major earthquakes and that enormous lakes atop the Greenland ice sheet can vanish within hours into its depths.

The study also raises questions about whether more huge ice and water pulses will be seen as the Arctic continues to warm and Greenland to melt — and thus whether this is how a melting ice sheet exports its mass to the ocean.

But mostly, it’s just staggering to contemplate.

If the analogy of 18,000 Empire State Buildings isn’t striking enough, the researchers offered another: The mass loss through Rink Glacier from the wave, they say, was equivalent to “150 million fully loaded 18-wheelers.”

https://www.washingtonpost.com/news/energy-environment/wp/2017/05/25/so-much-water-pulsed-through-a-melting-glacier-that-it-warped-the-earths-crust/

WaPo: Scientists find more reasons that Greenland will melt faster

by Chris Mooney, The Washington Post, April 30, 2016

Photograph of Torsukatat Avannarleq, a tidewater glacier in West Greenland, with two visible sediment plumes at its terminus. These plumes are made up of glacier meltwater that has traveled under the glacier, gathering eroded material, and buoyantly floated to the surface after entering the ocean. This photograph was taken in July 2014 by Adam LeWinter, US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory.

So much about the planet’s future will depend on processes that humans today cannot directly observe — because they are occurring hundreds of meters below the sea surface where enormous marine glaciers, in Greenland and Antarctica, simultaneously touch the ocean and the seafloor.

The more we learn about this crucial yet inscrutable place, the more worrying it seems.

The latest exhibit: New research out of Greenland conducted by Dartmouth earth sciences Ph.D. student Kristin Schild and two university colleagues — work that has just been published in the Annals of Glaciology. The study examined the 5.5-kilometer-wide Rink Glacier of West Greenland, with particular focus on how meltwater on the ice sheet’s surface actually finds its way underneath Rink, pours out in the key undersea area described above and speeds up the glacier’s melt.

It’s a feedback process that, if it plays out across many other similarly situated glaciers, could greatly worsen Greenland’s overall ice loss. “These big tidewater outlet glaciers are the ones that are contributing these huge icebergs, they’re the ones that have rapidly, rapidly sped up in the last decade,” Schild said. This makes it critically important to learn “what are the main factors…that are leading to all these fast changes,” she added.

Greenland is an enormous sheet of ice, capable of raising sea levels by some 20 feet if it were somehow to melt entirely and its waters were to pour into the ocean. Fortunately, it can’t just do that all of a sudden — the vast ice sheet only reaches the ocean at relatively narrow, finger-like glaciers that stretch out into fjords, or underwater canyons that lead out to the sea.

There are nearly 200 of these large outlet glaciers overall — and as Greenland goes, Rink is fairly large in size but far from the largest. It’s less than 1 kilometer tall as it extends from the seafloor deep in a west Greenland fjord up above the surface of the water, Schild said.

That’s hardly as massive as the nearby Jakobshavn Glacier, which has a base submerged well over a kilometer below sea level — and which is sending ice out into the ocean faster than any other in Greenland. But Rink, like Jakobshavn, touches the ocean across a wide, icy front, and is grounded deep below the surface of the fjord’s waters. Here is where all the action is — including spectacular calving events, in which enormous icebergs break off, tumble into the water and eventually float out of the fjords.

There’s growing concern that warming ocean waters are snaking into these fjords at depth and lapping at the glacier bases, making such breakups more likely. It doesn’t help matters that scientists studiously mapping the fjords are finding, over and over again, that they’re deeper than previously believed, creating more opportunities for the warm ocean to trigger melting.

But the situation is even more dynamic: Amid warmer atmospheric temperatures, Greenland is also melting on its surface, a process that forms vanishing lakes, ice-banked rivers and downward channels, called moulins, that carry meltwater deep beneath the ice sheet. This water then makes its way to the bases of outlet glaciers and, after traveling through complex passageways and, perhaps, being held up or stored in icy caverns, eventually flows out from beneath them and enters the sea.

It’s the net consequence of all of these processes that will ultimately govern how quickly Greenland loses mass and causes the seas to rise. And that’s what the new study gets at: It attempts to measure the mysterious process by which Greenland’s surface meltwater eventually makes its way beneath the ice sheet and then out into fjords, by flowing to glacier fronts and escaping from underneath them.

To do so, the Dartmouth researchers used satellite imagery, as well as time lapse photography, to observe the seafront in the fjord where water touches Rink Glacier. They were searching for what they call “sediment plumes”: When water rushes out from the glacier base and into the fjord, it’s filled with sediments from the bedrock below. These pulses of water then ascend hundreds of meters to the surface and create an often colorful emergence there, as you can see in the NASA image below:

The image above, acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite on September 9, 2014, shows sediment plumes from meltwater exiting glaciers in southwest Greenland. (NASA Earth Observatory image by Jesse Allen, using data from the Level 1 and Atmospheres Active Distribution System (LAADS).)

The study resulted in three separate new findings about how meltwater from Greenland’s surface is making its way under Rink Glacier and speeding its ice loss — each of which suggests that not only Rink, but other glaciers like it, could lose their ice faster than previously thought.

First of all, the satellite and time-lapse images revealed that meltwater is pouring out from beneath Rink Glacier in not just one but four separate locations. That’s bad news, because it means more overall melting of the glacier is possible. “Previously that has not been observed, to have more than one ocean location for a plume,” Schild said.

Each individual plume could be causing additional melting, Schild said. Here’s how it works: As the cold, fresh water rushes out from beneath the glacier, it cascades into ocean water that is saltier and warmer. So the cold water, being lighter, rises toward the surface hundreds of meters away — pulling the salty, warm water inward to fill the void that it leaves behind as it rises.

This doesn’t just bring more warm water toward the glacier — it does so in a turbulent way. “As it’s going up the front of the glacier, it kind of goes up in a corkscrew fashion,” Schild says. “It kind of creates a tornado as it goes up the front of this glacier, it’s bringing in that warm ocean water that then is hitting the terminus of the glacier.” This creates much more melting than would occur if the warm ocean water simply pressed steadily against the glacier front.

And that’s just one effect. The study also found that these meltwater plumes destabilize glacier fronts in another way. Over the winter in Greenland, the waters in front of glaciers develop a thick covering made up of sea ice and chunks of icebergs. This ice “melange,” as the researchers put it, freezes against the front of the glacier and acts to stabilize it.

But the meltwater plumes, the study showed, rise up early in the Greenland melt season and take chunks out of the ice melange. And no wonder — they mix with warm water as they rise to the surface, and so both their velocity and also their temperature help break up the ice and set the stage for the glacier to start calving new icebergs.

And as if that’s not enough, the plume observations also led to yet another conclusion: There appear to be significant pockets of liquid water stored beneath Rink Glacier — water that does not freeze because of the incredible pressure that it’s under. And these pockets should also speed the glacier’s flow toward the sea — glaciers move much more rapidly atop water than they do when grinding against bedrock.

The scientists were able to infer the existence of these subglacial storage chambers based on the timing of the plumes, which continued to form more than 20 days after Greenland’s surface melting itself had ceased as summer came to a close. “As soon as it stops melting on the surface, we still see plumes up to almost a month later, coming out of the glacier,” Schild said. “And so that water is getting stuck, and it’s getting trapped underneath the glacier.”

The presumption, of course, is that while every glacier is different, similar processes could be playing out at many other glaciers besides Rink — including monsters like Jakobshavn.

If you put all these pieces together, then, you can begin to see why global warming can be so devastating to Greenland. It warms the ocean, allowing warmer seas to come visit marine glaciers — but it also warms the atmosphere, leading to melting high atop Greenland’s surface.

Each of these elements, on its own, is bad enough. But their combination is even more dastardly. In fjords at the base of glaciers, the cold water actually acts in concert with the warm to speed up total glacial loss. They’re kind of a dynamic duo.

And in the future, as climate change proceeds, there will be more of both of them.

https://www.washingtonpost.com/news/energy-environment/wp/2016/04/30/another-typical-day-for-greenland-scientists-find-more-reasons-it-will-melt-faster/