Thursday, 27 October 2016

Antarctica's runaway glaciers

Half a Kilometer of Ice Gone in Just 7 Years — West Antarctica’s Smith Glacier Points To Nightmare Melt Scenario

26 October, 2016
The nightmare global warming melt scenario for West Antarctica goes something like this —
First, ocean waters warmed by climate change approach the vast frozen continent. Melt already running out from the continent forms a fresh water lens that pushes these warmer waters toward the ocean bottom. The waters then get caught up in currents surrounding Antarctica that draw them in toward numerous submerged glacial faces. The added ocean heat combines with falling melting points at depth to produce rapid melt along sea fronting glacier bases. Since many of these glaciers sit on below sea level beds that slope downward toward the interior of Antarctica, a small amount of initial melt sets off an inland flood of these warmer waters that then produces a cascade of melt. This glacial melt chain reaction ultimately generates a Heinrich Event in which armadas of icebergs burst out from Antarctica — forcing global sea levels to rapidly rise.

This is Why We Worry So Much About Multi-Meter Sea Level Rise

Ultimately, seas rising by multiple meters this Century are a very real possibility under current warming scenarios in which such a series of cascading melt events occurs in West Antarctica.

(NASA video narrated by Dr. Eric Rignot, a prominent glacial scientist. Concerns about the origin of melt water pulse 1A during the end of the last ice age led to investigation of large Antarctic melt pulses as a potential source. Subsequent investigation identified melt vulnerabilities at the bases of large sea fronting glaciers in West Antarctica to present and predicted levels of ocean warming. At issue was the fact that bottom waters were warming and that because many glaciers rested on sea beds that sloped inland, melt rates had the potential to very rapidly accelerate.)

Though such a nightmare melt scenario was recently theoretical, it represented a very real potential near-future event as global temperatures rose into the 1-2 degrees Celsius above 1880s range during recent years. For times in the geological past around 115,000 years ago also produced large glacial melt pulses and related sea level rises of 15-25 feet during periods of similar warmth.
However, direct evidence of such a powerful melt dynamic had not yet been directly observed in Antarctica’s glaciers. Fresh water lenses were developing, rates of glacial loss were quickening. Basal melt rates looked bad. But the kind of tremendous losses necessary to produce rapid sea level rise were not yet fully in evidence.
Smith Glacier Loses Half a Kilometer of Ice in Sven Years

The studies, entitled Rapid Submarine Ice Melting in the Grounding Zones of Ice Shelves in West Antarctica and Grounding Line Retreat of Pope, Smith and Kohler Glaciers took a comprehensive look at both surface and underside melt of three major west Antarctic glaciers near the Thwaites and Pine Island Glacier systems. These glaciers included Pope, Smith and Kohler — which have seen increasing instability and rates of seaward movement during recent years. Using multiple instruments, the scientists found evidence of massive ice losses and speeding ice flows.

(Surface velocity of Kohler, Smith and Pope Glaciers provided by NASA. More rapid seaward movement of glaciers = faster rates of sea level rise.)

The losses occurred at a time when an influx of warmer water (warming circumpolar deep water) was heating the ice shelves and grounding lines buttressing these three partially submerged glaciers. This warming was found to have produced melt along the grounding zones of these glaciers in the range of 300 to 490 meters from 2002 to 2009. In other words, about 1/3 to 1/2 a kilometer of ice thickness at the grounding line was lost in just seven years. Melted away from below by warming deep ocean conditions at the rate of up to 70 meters or around 230 feet per annum.

The studies found that the Pope and Kohler glaciers, which rested on up-sloping sea beds, produced slower rates of melt. While Smith, which sat on a retrograde (or down-sloping bed)produced very rapid rates of meltAccording to the Nature study:
We attribute the different evolution of Smith Glacier to the retreat of its grounding line deeper allowing warmer waters to flood its grounding zone, and increasing ocean thermal forcing due to the lowering of the in situ melting point; as well as to the exposure of the glacier bottom to ocean water as the grounding line retreated rapidly.

A Context of Worsening Risk

Unfortunately, numerous glaciers in the Amundsen Sea region including parts of the Thwaites system and the massive Pine Island Glacier also sit on retrograde slopes. These glaciers are seeing increasing fluxes of warm, deep water. By themselves they represent multiple feet of sea level rise (4-7 feet). Furthermore, Thwaites and Pine Island Glacier currently buttress a number of massive inland glaciers that become vulnerable to melt if inland-running retrograde slopes become flooded with warming ocean waters.
The very real concern is that Smith Glacier serves as a harbinger for near future events to come. As a result, coastal regions around the world are now under a heightened risk of swiftly rising seas and rapid coastal inundation over the coming years and decades.

Hat tip to Zack Labe
Hat tip to Miles h

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