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.
21 October 2009
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20 comments:
Thanks - well put. Indeed, to me, the combination of these two papers is concerning, in that if we're seeing the reported levels of mass loss at a time when (due to ENSO/SAM maxima) snowmelt is very low, the prognosis for accelerated mass loss once ENSO and/or SAM move on is worrying.
Comment from Marco Tedesco, one of the authors of the 'snowmelt' paper:
You are right about the 'mass balance' of an ice sheet: there are several factors contributing to the loss of ice and melting is only one of these. Also, as the term itself states, this is ultimately a 'balance' so the amount of precipitation should also be counted in. Some of the melting snow will refreeze without contributing to the mass loss. What GRACE 'observes' is the change in the 'total' mass: in this sense if the snow melts and will refreeze, GRACE will see a zero gain/loss value but if it melts away GRACE will see the negative balance (to which the loss due to other factors such as icebergs, sublimation must be added).
Let me also catch the opportunity to mention that the results of our work have been extremely simplified and not all results have been reported on many blogs. In the paper we do report the lowest melt in the 30-year satellite record this past austral summer but we also examine the causality of the low melt and find that it is related to the belt of westerly winds that encircles Antarctica. These winds are expected to weaken as the ozone hole is projected to recover significantly during the next 25 years and, as a consequence, we expect that temperature increases over Antarctica will become stronger and more widespread.
We also note that our results do not contradict recently published results on surface temperature trends over Antarctica: the time period used for those studies extends back to the 1950's, well beyond 1980, and the largest temperature increases are found during winter and spring rather than summer, and are generally limited to West Antarctica and the Antarctic Peninsula.
sincerely,
Marco Tedesco
Matt:
As you see from Marco's response, you've picked up on a point that he would like to stress. I'll probably take it up in a separate note. It's interesting in its own right, unrelated to the original question.
I'm quite intrigued. To what extent does the ozone hole influence southern hemisphere climate, and to what extent can we expect the recovery of the ozone hole to restore said climate to what was previously "normal"? And how is this related to ENSO?
Is it possible (some isotope measure?) to say how much of the sea ice has been formed from contemporary salt ocean water, and how much has been formed from glacial meltwater coming out from under the continental ice shelves where they're floating?
Cite and quote (rather than retyping)
http://tamino.wordpress.com/2009/08/31/open-thread-16/#comment-36424
@quasarpulse
Ozone depletion and to a lesser extent GHGs are thought to be contributing to the SAM's increasingly positive mode, which appears to have teamed up with ENSO in producing the snowmelt minimum. As ozone levels recover, the anthropogenic influence on the SAM is suspected to lessen, which Tedesco and coauthors think will lead to a resumed increase in snowmelt. Others have posited that a similar dynamic is at work with SH sea ice.
Accounting for isostatic rebound? I see there have been studies for quite a while.
http://jspc-www.colorado.edu/~isabella/2001JB000708.pdf
http://wwwrses.anu.edu.au/geodynamics/tregoning/36.pdf
Thanks a lot for the comprehensive post, very interesting!
Now I wonder about the relatinship between these findings and temperature. As Marco Tedesco points out, temperatures have increased in winter and spring, rather than summer, so that seems compatible with some decrease in summer snowmelt. However, under warmer conditions, shouldn’t it snow more (and so more ice would be formed in winter)?… Then it might point to basal melting in the ice shelves and icebergs due to a warmer ocean as the main cause (also mentioned in Pritchard et al).
Hank Roberts, I guess you are referring to this recent paper (press).
Cheers.
OK, you can make that seven readers... I read the RSS feed, so I don't tend to read or contribute to the comments, but I definitely like the topics you cover and the level of detail you use. I'm an Applied Physics graduate (over 20 years ago). Keep it up.
GPS work also blogged here:
http://www.nicholas.duke.edu/thegreengrok/iceupdate10222009
I don't see where at WUWT they are discussing any conflict at all. They just appear to be celebrating over the snowmelt data. I'm not entirely sure what they think it means. Where do they mention the mass data?
One person seems to think Tedesco should be worried about his job, which cracked me up.
carrot eater,
They find a contradiction inasmuch as they are suggesting that this study points to more ice in Antarctica and nothing to worry about sea level rise. That's just the opposite of what the mass balance study says about the Antarctic ice. But they still want all newspaper and media to report that there's no reason for concern.
You can also see comments like "Its directly contradictory to the vast majority of stories out there – what’s more sensational than that? That they haven’t tells us clearly that its a very one sided and biased agenda in the general media out there – and for many years now its all been aimed at pushing the whole AGW spie".
Well, just the usual "conspiranoid" and illinforming stuff at WUWT.
Seems Arctic is refreezing a bit slowly this year. Any chance it will become a record low for at least a short time?
http://www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm
Thomas:
Sure, there's a chance. It could also turn out to be the lowest winter maximum. Then again, neither might happen. I don't put a lot of store in these weather record sorts of numbers. As least not until it blows away the previous record by 3 standard deviations or so; then it gets my attention. The longer we keep a low ice extent, the longer we avoid seriously cold temperatures over North America, and the less chance ice has to grow thick over the winter. So there's something to watch for at least in the weather sense.
Thanks Adrian. I realize there are more than 6, or even 7, readers out there. Still, since I'm a little quiet myself in person, it doesn't hurt to envision a group I could be chatting with around a table.
Topic suggestions always welcome. Though they may scroll off the main page, the 'question place' posts are always open.
Hank:
The paper you cite in your comment over at OpenMind is looking only at ice shelf/ice tongue ice. They see a big signal of re-freezing of ice shelf meltwaters occurring as ice melts from the deepest parts of the shelf and then freezes back to shallower parts.
For sea ice ... I'm blanking on the reference, but some work has indeed been done on looking at how much ice is from which source.
The thing involved is the different isotopes of Oxygen (as a rule, sometimes Hydrogen). Snow in the polar regions has very little of the heavy isotope Oxygen-18 (18O). So ice sheets and ice shelves have very little of it. The ocean has very nearly the reference amount. In freezing, the ice gets more of the 18O than there is in the ocean, so sea ice frozen from the ocean is somewhat enriched in 18O. Sea ice that forms when snow freezes on to the ice floe is depleted. And, if there were any notable amount of it, sea ice forming from melting of ice shelves would also be depleted in 18O.
The study I'm thinking of was Arctic, though, and there aren't any big ice shelves up there, so no test of your thought.
Penguindreams, I'm not saying it is important, just fun for people who like to bet on stuff like this. (And as a rebuttal to people who claim the Arctic ice is increasing again)
Since my last post the likelihood of a record low has increased a lot too.
So there it goes my summary on Antarctic land ice (since GRACE, i.e., 2003):
Antarctica is losing ice at an accelerated rate (Velicogna 2009), though in Velicogna 2006 the central trend was 152 ± 80 km3/year and in Velicogna 2009 (full pdf) it is 143 ± 73 Gt/yr (lower than previously, in spite of the acceleration(?)). More recently, Chen et al 2009 reported 190 ± 77 Gt/yr.
This seems to be related to ocean melting of ice shelves (Pritchard et al 2009, Zhang 2007) or ice discharge from outlet glaciers, rather tan surface mass balance (Tedesco and Monagan 2009).
A first caveat is that models may overestimate post-glacial rebound (Bevis et al 2009 - press) and this may substract around 33 Gt/yr to the estimates by GRACE. If that's correct, this would leave around 110-157 Gt/yr.
Currently, this is not a significant contribution to see level rise, as the upper part would mean around 4 cm/century. Unless the acceleration continues ,of course.
The main contribution comes from the West Antarctica, that the IPCC 2007 blamed on ongoing grounding line retreat since the Last Glacial Maximum:
"They found [...] a trend in antarctic shrinkage of about 90 Gt yr–1, primarily because of retreat of the West Antarctic grounding line in response to the end of the last ice age"
http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter4.pdf (p 366)
The references are Huybrechts 2002 (abstract) and Huybrechts et al 2004 (full pdf): "quite large thinning rates up to 30 cm/year over the West Antarctic ice sheet, related to ongoing grounding line retreat since the Last Glacial Maximum".
I'm not really sure they are speaking about the same thing, but this would leave just around 20-67 Gt/yr that might be AGW-related (110-157 (PGR-corrected) minus 90 Gt/yr related to the grounding line retrat).
Please don't take this summary as authoritative, as I'm not a scientist at all (check the sources). Any corrections would be very welcome.
Cheers!
Jesús:
I'll delve in to the papers themselves later. But I did want to say, this is an excellent sort of thing to do. Namely, read a number of papers on a particular topic that interests you -- and then try to make a summary of it all in your own words.
Watching other people summarize is one thing. It has value, and is a good starting point. But the next level is to become an active reader. That includes writing your own summaries. It's amazing how many things that were obvious when you were just reading what someone else said become pretty murky when you try to pull it together with other peoples' writing on the same topic.
Oops.
http://www.scientificamerican.com/article.cfm?id=antarctica-andrill-ice-sheets
"... emerging evidence from an Antarctic geological research drilling program known as ANDRILL suggests that the southernmost continent has had a much more dynamic history than previously suspected—one that could signal an abrupt shrinkage of its ice sheets at some unknown greenhouse gas threshold, possibly starting in this century. Especially troubling, scientists see evidence in the geological data that could mean the vast East Antarctic Ice Sheet, which holds at least four-fifths of the continent's ice, is less resistant to melting than previously thought.
...
... Modeler and geologist Robert DeConto of the University of Massachusetts, Amherst, says the policy implications are grim. "Our models may be dramatically underestimating how much worse it's going to get," he says, noting that many population centers worldwide are within a few meters of sea level. Looking at signs of meltwater in the early Miocene, DeConto says, "we're seeing ice retreat faster and more dramatically than any model predicts."
Dang that physical reality.
It kills more lovely theories ...
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