A piece of ice, the size of four Manhattans, is heading our way. It broke free from northern Greenland last summer and has become home to seals and sea life off Labrador and Newfoundland, Canada. Ocean currents continue to propel it towards Hibernia oil fields and the rich fishery grounds of the Grand Banks. It is a tourist attraction as well.
This largest break-up from Greenland for at least 80-years has raised fears, that a warming climate will raise global sea level. While melting all of Greenland’s ice sheet would increase sea level in Delaware and Bangladesh by over 20 feet, this is unlikely to happen for the next 500 years. But how much does Greenland melt now? How much will our local sea level change the next few years as a result? Will it be inches or feet by the end of this century?
In order to answer these questions, we need to understand how the melting of Greenland’s ice works, if it melts all the time, if it melts everywhere, and if its melting is accelerating. We all know that glaciers grow when snow accumulates atop and shrink when icebergs break off. As big as the ice island from Petermann was, it contributes only a seventh to Petermann’s normal overall loss. Ocean warming and circulation cause most of the rest. During both the cold darkness of winter and bright coolness of summer, the ocean melts the most ice below the surface where it is thickest.
Furthermore, this melting can accelerate ice streams discharging ice hundreds of miles inland when thrown off-balance. Presently, these ice streams are held in place by a delicate balance of forces at the point where ocean, glacier, and the bottom meet. If this triple intersection of water, ice, and rock retreats into an existing landward cavity, then ocean water will rush in, enhance ice-ocean contact, increase the rate of melting, collapse the ice shelf, and thus raise global sea level. That’s bad for Delaware and Bangladesh, because it increases coastal erosion, flooding, and loss of wetlands that are nurseries for fish, crab, and shrimp.
Ice islands breaking off Greenland are visible and dramatic, but the cost of them breaking oil rigs off Newfoundland are small compared with the costs of rising sea level due to accelerating ice streams and disintegrating ice shelves. These sucker punches will be costly for us in Delaware and Bangladesh. An ice island or two … pocket change.
Added today’s particular clear image as the ice island continues to move south. Despite its edging closer to shore, it is still in water about 200-m deep. With the low-pressure weather system offshore the ice island will, I predict, move along the eastern side of Newfoundland towards the Grand Banks.
If so, we may then get to see how fast the Gulf Stream can melt something that size.
Sailing into St. Johns, Newfoundland on an incebreaker in 2003 from tropical Curacao, we suddenly hit a wall of fog and all sensor systems aboard the ship went wild … we had just crossed the so-called “North Wall” of the Gulfstream. Waters on the Grand Banks and the continental shelf are relative cold and fresh as they come from the Arctic. In contrast the Gulf Stream waters are very warm and salty. There is very little mixing between the two, the Gulf Stream is more than 1000-m deep and thus does not penetrate the 200-m deep shelf. Furthermore, shelf waters have a tendency to follow the topography, even as it twists and turns. The ice island extends perhaps 100-m down and thus is dragged by ocean currents over that depth range.
So, based on this, I think it unlikely that the ice island will stray into the Gulfstream. After all, this ice island has for the last year followed a path with water less than 1000-m deep. The fun in making these predictions … of course, is that they can very easily be checked. Bets can be placed, confirmed, and collected.
Good update on Petermann iceberg. Neven also added material. I would like you to expand on this sentence. ” If this triple intersection of water, ice, and rock retreats into an existing landward cavity, then ocean water will rush in, enhance ice-ocean contact, increase the rate of melting, collapse the ice shelf, and thus raise global sea level.”. I do not see the ice rush into this cavity approach as I picture your statement, this would be under the ice. This would require quite thin ice to generate ice shelves. As it is now there are lots of conduits into the ice sheet well below sea level. However, the thick glacier ice leads to these being conduits only for ice streams and subglacial drainage. Am I misinterpreting your point.
Sorry for using non-standard phrases such as “triple intersection of water, ice, and rock” for hinge line. I was thinking vertical, not lateral, perhaps this caused the confusion? Petermann Glacier is currently anchored at a local minimum in bottom depth, about 600 m below sealevel. If the hinge line retreats, it will encounter bedrock sloping downward in the landward direction. The geometry along the glacier is somewhat similar to Pine Island Glacier in Antarctica (not across, though) where the hinge line did move off a local minimum some decades ago with subsequent enhanced subsurface melting and attendant ice streaming. The rather flat section (say less than 200-m thick) of the ice shelf is not much involved in this.
Ah, and there indeed is more to the story yet … a pesky little hydrostatic-looking formulae that connects basal slopes, surface slopes, and hinge line migration to thining of the glacier. It is applied to Petermann Gletscher by Rignot (1998) who references Thomas and Bentley (1978) in the Journal of Glaciology and Quartinary Research, respectively. Learning continues … and I have to be more careful with the “rushing in of ocean water.” Thank you so much, Mauri.
Each glacier is different it is interesting to compare the Petermann and Pine Island the main difference is the depth upstream of the grounding line, which is quite deep, over 1000 meters at Pine Island and is declining toward sea level on Petermann.