In the series I have noted that there are dynamics involved in sea level rise (SLR) besides CO2 emissions, the resulting warming, and the subsequent ice sheet melting and/or calving.
Today, we will take a close look at one of those dynamics, an ice dynamic on the Greenland Ice Sheet (Fig. 1: Northeast Ice Stream).
Since an ice stream is a quick way that glaciers can move through ice sheets, today we look specifically at a critical ice stream on the Greenland Ice Sheet.
That is because it could determine to a significant degree when SLR acceleration takes place, and significantly affects ports of the Northeast United States (Why Sea Level Rise May Be The Greatest Threat To Civilization, 2).
First, let's take a look at a definition of an ice stream:
A fast-moving ice or ice stream is a region of an ice sheet that moves significantly faster than the surrounding ice. Ice streams are a type of glacier. They are significant features of the Antarctic where they account for 10% of the volume of the ice. They are up to 50 km wide, 2 km thick, can stretch for hundreds of kilometres, and account for most of the ice leaving the ice sheet.(Wikipedia, Ice Stream). We must consider these ice streams in the projection of SLR, because until the ice in a flowing glacier melts or calves into the sea, there is no SLR merely because the ice is moving.
The speed of an ice stream can be over 1,000 metres per year, an order of magnitude faster than the surrounding ice. The shear forces at the edge of the ice stream cause deformation and recrystallization of the ice, making it softer, and concentrating the deformation in narrow bands or shear margins. Crevasses form, particularly around the shear margins.
Most ice streams have some water at their base, which lubricates the flow. The type of bedrock also is significant. Soft, deformable sediments result in faster flow than hard rock.
Another concept discussed in the context of SLR is the notion of "zones."
This applies to Greenland and Antarctica, both of which are areas where acceleration of melt and/or calving depends on the physical characteristics of the four zones:
The basic approach I took was to first establish four melt zones for the three melt locations, which locations are "non-polar", "Greenland", and "Antarctica."(The Question Is: How Much Acceleration Is Involved In SLR?). The concept is that sequential melt in zones is the rule, enhanced by the notion of some concurrent melt and/or calving.
The latter two are the major future sources of water from melting ice.
Those four melt zones were described in earlier posts as "Coastal", "Inland 1", "Inland 2", and "No Melt" (Will This Float Your Boat - 7).
In the evolving model, each melt zone has its own beginning phase, rate of delay, rate of melt, rate of acceleration of melt, volume of ice, and total possible contribution to SLR.
That is, zones can lose ice concurrently, but generally not at the same rate:
That the zones are geographically distinct (height above sea level, distance from ocean, etc.) does not mean that there will be no overlapping melt, it just means that, by and large, the melt will proceed like dominoes sequentially falling as the ocean and air warm up.(The Evolution of Models - 5). It so happens that on the Greenland Ice Sheet there is an ice stream that flows through more than one zone.
III. The Ice Stream Flowing Into The Greenland Sea
The south Wandel Sea and the north Greenland Sea is where the Storestrommen, Zachariae, and Nioghalvfjerdsbræ glaciers empty (Fig. 1).
Those three glaciers are branches of the larger ice stream we are discussing today:
Ice flow in the interior of the NE quadrant the Greenland sheet is focused on the large ice stream draining the north side of the summit dome. The rapid ice flow in the stream is apparent in the surface features in the stream and at the margins, in the broad scale topography that drives ice flow, and in satellite-derived motion information. The patterns of ice flow in the upper half of the stream are remarkable for their level of organization, simple geometry, and effects on local surface topography. The stream begins less than 100 km from the ice divide as a current 15 km wide and then broadens symmetrically downstream by the addition of ice from the sides to a width of more than 60 km. Elevation data and visible-band imagery show that the stream has marginal troughs tens of meters deep in its upper reach which are coincident with regions of high shear strain rate. The topography of the margins and undulating surface of the stream is generated by the ice flow; the surface undulations in the stream are fixed in location and shape over the 5 year period from 1994 to 1999. The enhanced flow presents a challenge for researchers trying to understand the history of ice discharge from a significant area in the interior of the ice sheet.(Journal of Geophysical Research, 2001, PDF). I am focusing on this ice stream because it has become more active in the decade or so since 2001.
IV. Acceleration of Ice Streams Can Mean Acceleration of SLR
A paper in the journal Nature indicates that acceleration is in the cards:
Here, we show that the northeast Greenland ice stream, which extends more than 600 km into the interior of the ice sheet, is now undergoing sustained dynamic thinning, linked to regional warming, after more than a quarter of a century of stability. This sector of the Greenland ice sheet is of particular interest, because the drainage basin area covers 16% of the ice sheet (twice that of Jakobshavn Isbræ) and numerical model predictions suggest no significant mass loss for this sector, leading to an under-estimation of future global sea-level rise. The geometry of the bedrock and monotonic trend in glacier speed-up and mass loss suggests that dynamic drawdown of ice in this region will continue in the near future.(Nature, Climate Change, March 2014, emphasis added). For this reason, I have generated an additional "doubling" graph and updated the data base of the Dredd Blog SLR calculation software (The Evolution of Models, 2, 3, 4, 5, 6, 7, 8, 9, 10).
V. The New Acceleration Graph
The acceleration in SLR added by the Northeast Ice Stream melt / calving acceleration is modeled by the graph in Fig. 2.
|Fig. 2 Two-year Doubling Projection (updated)|
Dr. Hansen's expectations were 5yr doubling = 2045, 7yr doubling = 2055, and 10yr doubling = 2067.
I added a 3yr doubling which resulted in the year 2035 being the 3ft. / 1 m point (The Evolution of Models - 10).
The .csv file on the 2yr doubling data shows the 3ft. / 1 m year as 2031, with an additional 4ft. of SLR (over the 3yr doubling) by 2100 (Fig. 2).
The part of destiny we do not really want, the port authorities really do not want, and only insane people want, is moving closer and closer to us.
The next post in this series is here, the previous post in this series is here.