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.
Time Series Lesson 2
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