0 AABW in the news!

It only took 25 years, but my thesis topic is now becoming newsworthy!  Gluttons for punishment can see at least the abstract at A model of the formation of high-salinity shelf water on polar continental shelves.  Which is aimed at one of the important ingredients for AABW (Antarctic Bottom Water).

I've been reluctant to blog about the topic because it is, after all, my baby and I'm sorely tempted to post at excruciating length and detail.  (Not that there aren't other people who have studied the topic before or since, but I'm one of the people who has.)

I'll take this note as opportunity to get in to some detail about the weirdness that is sea water, and come to the climate change, carbon dioxide burial, and heat burial, aspects later.  The story of AABW turns on some odd facts about how sea water behaves in Antarctic conditions.  Not least, it can go below freezing.
The freezing point of sea water, like any material, depends on the pressure it is under.  At the surface, Antarctic waters typically have a freezing point of -1.84 C.  Yet, if you drag this water underneath an ice shelf, which Antarctica has some impressive examples of, you'll get temperatures down to about -2.3 C.  This is 'Ice Shelf Water' (ISW; we're really not very creative).  The 0.4 C difference may seem small, but all that matters is whether the water gets denser than what is formerly below it.  This is 0.4 C drop in freezing point for being at the bottom of an ice shelf something like 1000 meters (yards) thick.  Being this far down in the ocean means the water is under a pressure of about 100 times what you are breathing (100 atmospheres).  The water cools to the local freezing point as it melts the bottom of the ice shelf.  This sort of melting is also turning out to be extremely important for the fate of the Antarctic ice sheet.

The easier way of making water denser in the Antarctic is to freeze some sea ice.  The ice crystals mostly leave behind the ocean's salt.  (Do the experiment, maybe also with some food coloring in the water.)  The sea ice then floats, and the salt makes the sea water denser.  The denser water sinks.  Just how far depends on how dense it is, and how dense water lower down is.  Again, we're looking at small changes.  The so-called 'Winter water' is near freezing and about 34.5 parts per thousand salt.  The High Salinity Shelf Water (HSSW) is what you get after freezing sea ice from Winter Water.  It is also near freezing, but is about 34.7 parts per thousand salt.  Whew, a whole 0.2 parts per thousand (200 parts per million) controlling the process! 

To get AABW, you then mix ISW or HSSW with the waters that are farther away from the Antarctic ice shelves and coast.  AABW isn't quite as dense as the ISW or HSSW, but this mixing means that there is a lot more AABW than ISW or HSSW (call them Andy, Ignatz, and Henry if you like, the acronyms don't matter, just that we're talking about 3 players in the climate system).  The AABW then flows off to the Atlantic ocean (primarily) along the bottom.  Because it is at the bottom, the North Atlantic Deep Water (NADW) flows above it -- and above the mid-ocean ridges, and then in to the rest of the deep and bottom of the ocean. 

But this only happens because of a different strange fact about how sea water behaves, and because of history.  Same as freezing point depends on pressure, so does density.  This isn't too surprising -- if you squeeze a sponge, it gets more compact (denser).  But ... add another character to our story, the very warm, very salty water that flows out from the Mediterranean sea in to the Atlantic.  If we compute the density, at the surface, of this Mediterranean Sea water and the Antarctic Water, we see that it is the Mediterranean which is denser, so 'ought' to be at the bottom of the ocean.

So one last weirdness -- it is easier to compress cold water than warm water.  At and below about 2000 meters (200 atmospheres pressure), it is the Antarctic water which the densest.  The history is this: If the ocean were filled from bottom to 1800 meters depth with the warm salty Mediterranean sea water, Antarctic water would not be able to sink to the bottom.  We'd have a very different ocean.  Among many other differences: sea level would be higher, and much less carbon would be in the ocean.

Detail: AABW in the news!

0 Antarctic Snow and Ice

The Antarctic has long been a favorite area of mine, going back to graduate school days.  This particular note, however, is prompted by a question over in the question place -- regarding Antarctic mass balance and snow.

The question at hand turns on just what is going on with Antarctic mass balance.  The apparent 'conflict' is between a study showing a recent decline in snow melt, and other studies that Antarctic ice mass is decreasing.  This is a particularly simple conflict to resolve, so I'll note that it really is taken as a serious conflict (per the questioner's link) over at WUWT (haven't we heard that name recently?)

The simple reality that the authors of the snowmelt paper are perfectly aware of, but WUWT ignored, is that there is more than one way for the Antarctic to lose mass.  I grant that melting the snow is the most obvious one.  But, when you're dealing with a continent as incredibly dry as the Antarctic is (the driest, and probably largest, desert in the world), you have to pay attention to more subtle processes.  One of them is not at all subtle -- huge icebergs break off of the Antarctic from time to time.  In these cases, you're talking about chunks of ice several hundred meters (call it 1000 feet for simplicity if you're non-metric) thick, and 50-100 km (30-60 miles) on a side.  Chunks large enough to be the size of entire US states and some countries.  (I have an ancient listing of some iceberg sizes and country, state, lake sizes for your comparisons -- additions welcome.)  There's also the very subtle process of evaporation straight from the surface of the ice sheet (sublimation) into the atmosphere.  And there's the not subtle but easy to forget about fact that Antarctica has ice shelves -- ice floating on the ocean that's fed by the continental (sitting on land) ice sheet -- and the bottoms of those ice shelves can and do melt.

Finally, there is the rather bizarre fact that ice is not a solid.  Once you build up to having an ice sheet, the pressure of the ice above a point near the ground is so enormous that the ice flows.  Ok, it's a really, really, thick fluid (think very cold molasses).  But it flows.  This means that the ice sheet move mass out to the edges -- out to the ice shelves where there can be snow melt, ice evaporation, or ice shelf melting, or massive icebergs can break off.

So, just on a fairly cursory consideration -- there's more than one way to skin a cat, or, rather, there's more than one way for an ice sheet to lose mass -- we already know there's a problem with the WUWT article.  In the science, no real conflict.  More below the fold.
The scientific papers involved are ...

First -- a hearty thank you to Jesus for providing the links!  As you can see from my link policy I appreciate substantive links being provided.  That's really the only way I can be sure that I know what science you mean, and only way for you to show what good science (or bad, alas) it is that you've found.  And not only me, since I'm only one reader of the blog, but all my readers (all '6'* of you).  We can all go straight to where the good, substantial, material is, and learn something!

The first paper shows that Antarctica has been losing mass -- Increasing rates of ice mass from the Greenland and Antarctic ice sheets revealed by GRACE (also available from thingsbreak -- I hope he's gotten appropriate permissions.) -- and that the rate of mass loss has been increasing in recent years.  It's not just a simple linear decline.  Rather, the mass loss is not only getting more negative (losing more mass year by year), but the rate it's going more negative is getting even bigger (the increase in mass loss from year to year is getting bigger too).

The second paper shows that in the last couple of years, snowmelt -- only one of the several ways that the Antarctic can lose mass -- has been lower than usual, with the most recent year being the lowest snowmelt year of the last 30.  An updated Antarctic melt record through 2009 and its linkages to high-latitude and tropical climate variability, also at thingsbreak.

So what do we have?  Well, in all seriousness, it's a couple of interesting papers on the science (yay!) and not a whole lot of conflict today.  But we may take a sign of something to keep reading the scientific literature for.  We have on one hand, observations that the total mass lost by the Antarctic ice sheet is going up (over the 6 years that this data source is available).   On the other hand, we have observations that the summer 2008-2009 was a low point for surface melting (of the 30 years this has data for).  But we know that's just one of the many ways the ice sheet can lose mass. 

What we keep reading the science for are:
1) Do either data analysis continue to get support from later observations?  When we're looking at relatively new approaches, which both are, one of the things we have to keep in mind is that the method might be wrong somewhere.  Both look plausible to my non-expert (in these methods) self.  But the real story will be told over the next couple of years as people seriously expert in these methods start doing their own work, and the original authors keep after the issue.  Keep your eyes peeled for more.
2) Only one of the mass sinks for the Antarctic has been examined directly.  Look for (some articles may exist already) or keep your eye out for new articles to come on those other mass loss mechanisms.  It might be that when we add up the individual mass loss mechanisms we don't match what people observe from GRACE.  Such a thing happened in the early 1990s regarding the sinks for CO2 -- the observable amount taken up by the ocean was much too small.  That told us something else (land uptake) was going on.  (In this case, maybe we discover that GRACE isn't accurate about the total mass loss.  Or maybe it's that snowmelt isn't accurately inferred, or iceberg loss, or ....  If we've got many things involved, and we do, then any of them could be the cause of a discrepancy.)

Either way, the serious resolution of a conflict, if there is one, will take place in the scientific literature.  At the moment though, there's no conflict.  Just some interesting science that suggests we have more to be looking for (as the GRACE and the snowmelt methods get more data) and other interesting science to look for, or keep our eyes out for.

* I realize, and appreciate, that I have more than 6 readers.  I'm minded, though, of a local radio person I listen to, who talks of his '13' listeners.  Probably more like 130,000.  (I just wish I were understating as thoroughly as him!).  The thing being, I do realize that this is not one of the higher-traffic blogs around, or even around and on topics somewhat like mine.  I therefore appreciate those of who who read, and who contribute substantive comments. 

Detail: Antarctic Snow and Ice