Dive In: Oceans melting ice shelves

This is the second post in a new series called Dive Into Science.  Here I’ll be explaining results from recent scientific papers.  Dive Into Science gives you a glimpse of current research in an easy to read format that anyone can understand.  To read more, just use the Dive Into Science tag.

Today’s article is “Ocean variability contributing to basal melt rate near the ice front of Ross Ice Shelf, Antarctica” by Isabella B. Arzeno et al, published in Journal of Geophysical Research: Oceans, 2014.

If you think about sea level rise and future climate predictions, and all that, one of the biggest unknowns scientists are trying to understand is the rate at which the ice sheets are melting.  See, it’s not exactly a straight-forward process to figure this out.  You would think that if you knew the temperature of the air, you could calculate how fast the ice melts, and how much melted water goes into the ocean.  But, you’d be wrong.

When we talk about melting ice sheets, we really mean the change in the ice sheet mass balance.  This is exactly what it sounds like.  To figure out how much ice is lost or gained from the ice sheet, we balance out all the sources and sinks.  In Antarctica, the major source of ice to the ice sheet comes in the form of snow over the continent.  This snow eventually compacts to form ice, and adds mass to the ice sheet.

Now, there are several ways an ice sheet can lose mass.  I’ve already mentioned melting ice from the top of the ice sheet – this does not have a large effect (true for Antarctica, not quite true for Greenland).  It is rarely warm enough to melt significant amounts of ice, and the water must also evaporate, or it will just refreeze at a later time.  More important are the outlets of the ice sheet, or where the ice meets the ocean.  Here, ice loss can occur through pieces of ice falling off as icebergs, or through the ocean melting the ice it touches.

Cartoon illustrating ice sheet mass balance.  Here, we’re concerned with the Antarctica side.  Image by Hugo Ahlenius, UNEP/GRID-Arendal

This brings us to the topic of ice shelves.  Ice shelves are the part of the ice sheet that floats in the ocean.  Trick question: How much would sea level change if all the ice shelves melted?  Answer: No change, because they are floating.  BUT, it’s even more of a trick question!  Even though the ice melted from the ice shelf doesn’t contribute to sea level rise, it causes more ice to flow in from the ice sheet and increases the ice loss term of our mass balance.  And, if the balance shows that ice is being lost, well, that does contribute to sea level rise.

So these ice shelves then, act as plugs to the much larger ice sheet.  We need to understand how they melt, by how much, and how that affects the ice sheet.  In a best case scenario, the ice shelves melt a little bit, lose some icebergs off the edges, and are replaced by ice from the ice sheet to form a stable configuration.  In a worst case scenario, the ice shelves melt enough that they no longer act as stable plugs, and the ice sheet becomes unstable and eventually drains out through the open hole.  Even for the worst case, the process would take several hundred years for all the ice in Antarctica to drain out.  But, what is happening now with the ice shelves may determine the eventual fate of the ice sheet.  And, wouldn’t it be nice to know what that is?

Figure taken from Arzeno, et al. 2014.  Data from the mooring under the iceshelf.  A) is current speed, b) is temperature difference from freezing, c) is calculated melt rate from temp, salinity, and current, d) is melt rate from tide velocities only, and e) is the tide velocities.  Notice how a higher temperature leads to a higher melt rate (b vs. c), and how peaks in the tide-only melt rate correspond to high tidal velocities (d vs. e).

This paper is a significant contribution to our understanding of one ice shelf in particular, in the Ross Sea.  (All the ice shelves behave differently, based on size, shape, what water they are in contact with, etc…)  It is the first time scientists have been able to simultaneously collect data on temperature, salinity, and currents under the Ross Ice Shelf.  These three variables are what you need to directly predict ice shelf melt.  The temperature tells you how much heat is available for use in the ocean water, the salinity helps determine what the temperature of ice melt actually is, and the current speeds give you an idea of how long it takes to replace this water.  As the ice melts, the underlying water grows colder and fresher, and the ice melt slows down.  In order to keep melting rates up, the water needs to be mixed around or replaced.

The authors use results from their mooring underneath the ice shelf, and from ocean models to better understand how the melt rate of the ice changes.  They find that tides play a large role, both in the data, and in the ocean model.  As the measurements were taken near the edge of the ice shelf, the movement of the tides back and forth under the ice shelf helps bring the cold melt water out, and replace it with warmer ocean water.  The authors also notice longer time signals in the currents that affect the melt rate, but don’t have enough information to identify them.

Overall, this paper presents rare data on ice shelf melt rates for the Ross Sea, and also investigates the processes that contribute to the melt rate.  This information, along with research from other groups, can be used to better understand, and eventually predict, the fate of the ice shelves and how that affects the Antarctic Ice Sheet.