Wednesday, June 3, 2015

The Question Is: How Much Acceleration Is Involved In SLR - 6?

Arm of Ericsfjord, on which Eric the Red had his farm
(Dale Mackenzie Brown)
It is still a mystery what happened to the culture started by Eric the Red in southern Greenland (The Fate of Greenland's Vikings).

They set up shop near a volcano that, as far as we know, last had a bona fide eruption 2,200 years ago (Operation IceBridge).

That major eruption was before their time, but it could have eventually scared them off if it rumbled a bit during their 500 year stay.

Anyway, it is evidently no longer a mystery what happened to some large lakes on the surface of the ice sheet of Greenland.

Large lakes, some of which generated as much flow as "Niagara Falls."

But, as the saying goes, the lakes disappeared "in a heartbeat" in recent years:
Due to meltwater, lakes form atop the ice sheet in the summer – scientists call them “supraglacial lakes” — and they can grow to be quite large. And in July 2006, one large lake, over 2 square miles in area, suddenly vanished. It lost most of its water in under two hours – researchers calculated that the rate of drainage “exceeded the average flow rate over Niagara Falls.”
(Washington Post, H/T Randy, emphasis added). These events are part of a potential acceleration engine that includes moulins:
"Water-driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the Greenland Ice Sheet. These drainage events drive transient ice-sheet acceleration and establish conduits for additional surface-to-bed meltwater transport for the remainder of the melt season. Although it is well established that cracks must remain water-filled to propagate to the bed the precise mechanisms that initiate hydro-fracture events beneath lakes are unknown. Here we show that, for a lake on the western Greenland Ice Sheet,
The vanishing lakes mystery solved?
drainage events are preceded by a 6–12 hour period of ice-sheet uplift and/or enhanced basal slip. Our observations from a dense Global Positioning System (GPS) network allow us to determine the distribution of meltwater at the ice-sheet bed before, during, and after three rapid drainages in 2011–2013, each of which generates tensile stresses that promote hydro-fracture beneath the lake. We hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems (vertical conduits connecting the surface and base of the ice sheet). Our results imply that as lakes form in less crevassed, interior regions of the ice sheet, where water at the bed is currently less pervasive, the creation of new surface-to-bed conduits caused by lake-draining hydro-fractures may be limited."
(Nature, 6/3/15, emphasis added). Regular readers will remember that I hypothesized the same thing (moulin induced acceleration) in a previous post:
Some of those streams of rapidly flowing water abruptly disappear when they empty into a giant sink hole in the ice sheet, sink holes that are officially called "moulins."

The strange thing is that it is not perfectly certain what happens to the water when it enters those moulins, other than the water disappears deep down into the ice sheet.

There is speculation that a large lake or lakes is the final destination, rather than the ocean.

More study is being done.

The destination determines sea level rise (SLR), because if the water stays in a sub-glacial lake, there will be no SLR at that time.

There will be no SLR related to that event until that lake flows into the sea.

One fear is that such a flow could take place at once en masse, which could cause not only SLR, but also serious damage to the ice sheet itself (similar to this, but larger).

All that could lead to an acceleration in SLR that would at first be slower than expected, but later it could suddenly exceed normal melting and normal flowing into the sea ...
(The Question Is: How Much Acceleration Is Involved In SLR? - 4, 5/1/15). It is looking more and more like the ice will melt to the point where the ice sheet will go the way of Eric the Red.

Joining the sheet concurrently will be the ports and major sections of cities south of Greenland (The 1% May Face The Wrath of Sea Level Rise First).

The next post in this series is here, the previous post in this series is here.

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