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Tuesday, November 28, 2017

The ice pump

It has been a bit of a mystery why the floating ice around Antarctica has been increasing in area over the last few decades despite global warming.  After quite a bit of research and some reference to some well known physics, there is a pretty plausible theory/story to explain this.  It is called the ice pump.  First we need a fact or two before we tie it all together.

1.  Sea level is rising but only some of this rise is  due to the melting of land ice.  The remainder is due to the expansion of the water of the oceans as the water heats up.  The heat is being gradually stirred into deeper and deeper water.  The salty deep 'circumpolar water' around the Antarctic is  a case in point.

2.  H2O expands when it freezes, contracts when it melts.  It makes intuitive sense that as you apply pressure to ice, it will melt at a temperature below zero degrees centigrade.  Indeed this is observed experimentally. 

 Image result for table melting point of ice under pressure
 100MPa equals about 9950m so one interval across the horizontal axis is about 2480m.  At this depth the melting point of ice is depressed about 2.4 degrees C.  As you can see from the following illustration, the depth of the bottom below sea level in West Antarctica is well below 2000m

3.  A few glaciers on East Antarctica and most on West Antarctica are on a retrograde slope.  The ice is so heavy that it has depressed the land and the land  below the ice gets deeper and deeper as you go inland. In East Antarctica some out-flowing glaciers have carved deep channels well below sea level.   Most of West Antarctica land is way below sea level.

So let's put all this together.

The deep circumpolar water over-tops the sill at the outlet of some of the glaciers.  It is salty which keeps it below the surface despite being warmer than the water above it.

Being heavier, it flows down the sloping sea bottom under the floating ice until it comes to the grounding line.  There it comes into contacts with ice.  Not only is it salty and slightly warmer than zero degrees C but ice melts at below zero under pressure so this salty bottom water melts the ice at the grounding line making the grounding line retreat landward.  It is also salty and salt depresses the freezing point of ice. (the reason we put salt on icy roads)

The glacier is moving seaward under the pressure of ice from the interior but grounding lines have been observed to be retreating so clearly the melting is  faster than the flow of ice seaward.

Moreover, as surface ice disappears, the glaciers are flowing more quickly seaward as the plug that helped hold them back disappears.  If we want to talk about the speed that the grounding line is being melted back landward, we must combine the observed retreat with the increased seaward flow.

As the grounding line retreats it is at greater and greater depth and hence at a higher pressure where ice melts at lower and lower temperatures.  The melting becomes greater for a given quantity and temperature of circumpolar deep water flowing down the slope.

When the water from the melting ice mixes with the   salty deep polar water, the mix is fresher and hence lighter than the circumpolar water.  It flows up the slope of the ice ceiling in a sort of up side down river and flows out on to the surface of the ocean.

The deep water is often described as seeping under the ice or some such gentle term.  We can see that as the light super cooled water flows out on to the surface of the ocean, deep water is being sucked in under the ice.   The more water flowing out on the surface the greater the 'suck'.

As the lighter water flows upward into a zone of reduced pressure, it is below the freezing point of ice at that depth.  It begins to freeze and for some reason freezes in thin sheets called platelets which form a sort of mushy layer below the sea ice ceiling.    This is the ice pump.  It is in effect taking ice from the grounding line and depositing it in shallower water under the ice ceiling.  The deeper the grounding line, the more effective the pump.

The sea ice around the Antarctic continent disappears every year or two so this ice from the grounding line is lost to the continent.  ie contributes to sea level rise.

The water which flows out on to the surface of the ocean, either at the edge of the ice shelf or into a lead is still super cooled and freezes readily, especially as it comes into contact with Arctic air which is well below freezing.  Here is one small part of  the explanation of the increasing ice around Antarctica.  Any leads which open up due to wind and currents, fill rapidly with ice  and hence can not close up again if the wind changes.

In addition, this water is a little fresher than open ocean water and hence freezes more readily (at a higher temperature).  The freezing point of water is depressed by 1.86oC per mole* of solute per liter of water.  Salt dissociated completely so contributes twice as many particles as a solute such as alcohol. One mole of salt has twice the effect on the melting/freezing point as one mole of alcohol.  The salinity of open ocean water is approximately 35ppm or 35kg of salt per cube of water.  Hence in a liter of water there is  0.035kg of salt or 35g.  Using Sodium Cloride, which is the major part of the salt as a rough approximation, the molar weight of NaCl is close to 60.  Sea water is therefore a half molar solution. Since you have to double the figure for compounds that dissociate completely, we can approximately say that Sea water, with respect to it's melting/freezing point is a one molar solution and hence will freeze/melt at about minus 1.8oC.  If the water is fresher than open ocean water, it will freeze at closer to 0oC.

* Mole - a gram molecular weight.  ie, the molecular weight of a compound expressed in grams.

As the ice is eroded from underneath the glacier, the floating part of the glacier deflates and increases the slope of ice from the interior, seaward.  The glacier speeds up, pushing more ice seaward.  This is another part of the expansion of the floating ice. As the sea ice melts back, you come to the land ice that is flowing seaward and the further back this melts, the higher the ice cliff.  The higher the ice cliff, the faster it breaks down and collapses into the ocean.

The increased flow of ice seaward should push the grounding line seaward but apparently, at present,  melting trumps glacier flow.  In addition as the glacier deflates it floats up off the ground.  This also contributes to moving the grounding line landward.

There are a couple of further wrinkles to this story.

The  water flowing up the ice ceiling apparently, in at least some locations, carves out up side down valleys in the ice and the light water collects in these and flows seaward.  This will, of course, reduce the surface area where this light up-flowing water is in contact with the surrounding water.  It is not quite a pipe but will reduce mixing compared to a sheet flow.

In addition, these up side down valleys have reduced buoyancy compared to the surrounding ice so will weaken the ice shelf, contributing to it's break up.  If, for instance, you had one valley running along the middle of an ice shelf, the surrounding ice would have a force on it trying to make the ice tip toward the valley from both sides.

Another factor in the expansion of the surface area of floating ice is that the air flowing off the Antarctic continent is apparently getting stronger and this will tend to push ice outward (North).  As mentioned, leads opened up will rapidly freeze, stopping the ice from moving back south.

The winds flowing clockwise (looking down on the continent) around Antarctica are apparently also increasing in velocity.  They push on the ice.  Anything moving in the southern hemisphere and especially if it is near the pole, is veered to the left by Coriolis.  'To the left' is away from the continent.  Again we have ice moving North and leads freezing over, stopping the ice from returning south.

The bottom line of all this is that for a while, we would expect the floating ice to increase in area around the Antarctic due, ultimately, to the warming of the deep salty circumpolar water.  At the same time, we should expect to see coastal glacier deflating and the floating ice shelves breaking up.  Already the two northerly Larson Ice shelves along the Arctic peninsula have disintegrated.  .  The third Larson Ice Shelf may be on its way and the rest should follow in time.  This will remove the plugs and allow inland glaciers to flow more quickly and we will see if this movement can reverse the retreat of the grounding line.  This is unlikely as the glacier deflate and float upward.

What is interesting is that we have probably passed a tipping point in the break down of glaciers which are grounded way below sea level.  When the salty deep circumpolar water contacts ice at relatively shallow depths, it will erode the ice but the flow of ice seaward may be able to balance the melting.  However, when this circumpolar water is contacting ice at greater depth, its erosion ability is greatly increased due to the suppression of the melting temperature of the ice at greater depth and hence pressure.  The removed ice is transfered to the underside of the ice shelf at shallower depth and this ice is lost each summer as it floats off into the ocean.

 Even if the deep circumpolar water cooled to its previous temperature, the depth effect has so increased the ability of this water to melt ice that the process would likely continue.  Since there is no prospect that such a cooling will occur, it is doubly likely that the ice sheets which are grounded well below sea level will collapse.

The disintegration of the Antarctic ice which is grounded below sea level is now probably inevitable, even if we were to stop all green house gases tomorrow.

I wouldn't be buying any coastal property

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