The recent paper by Zwally et al. in the Journal of Glaciology has been widely reported as evidence that Antarctic is gaining mass, and hence lowering sea level. Is it? Expert Jonathan Bamber weighs in.
Guest post by Jonathan Bamber, University of Bristol
There have been quite few big media stories related to Antarctica recently, including a paper on the irreversible collapse of the marine portion of the West Antarctic Ice Sheet and a NASA-funded study that finds, contrary to numerous previous results, that the Antarctic ice sheet as a whole has been gaining mass between 1992 and 2008. This most recent study received a lot of media attention because it runs counter to what was said in the last IPCC Report. Certain parts of the media hailed this as another sign that the impacts of climate change had somehow been exaggerated a risk that the lead author Jay Zwally was concerned about before the research was published.
So what did Zwally and his colleagues do, what did they find, and why does it contradict a plethora of previous studies that suggest Antarctica has been losing mass over the same time period?
Figure 1. Comparison of various estimates of Antarctic mass balance trends. Vertical dimension of boxes gives the published uncertainty; horizontal gives the time period covered. Figure courtesy of Luke Trusel, Woods Hole Oceanographic Institution.
Zwally and his team measured the changing height of the ice covering Antarctica using two types of instruments — a radar altimeter and a laser altimeter — on two different satellites. The radar provided elevation changes for 1992-2003 and the laser from 2003-2008. They compared the trends in height from the two instruments over a flat part of East Antarctica that covers subglacial Lake Vostok, finding a very good match between the trend for ‘92-’03 and the one for ’03-‘08. So far, so good (though note that they couldn’t compare the two instruments directly because they do not overlap in time.)
Using measurements of elevation change to estimate changes in mass requires knowing the density of the snow. This is the difficult part and one of the key reasons that Zwally’s numbers are so different from previous estimates. The density of snow at the surface of Antarctica is about 1/3 than that of solid ice. Most scientists working with similar data sets agree that over the last ~25 years the surface of the East Antarctic Ice Sheet (EAIS) has been going up very slightly (at about 1-3 cm a year). Previous studies employing altimetry over the EAIS have assumed that the change in elevation is due to a recent increase in snowfall and used a density of snow. They have done this primarily because the interior of the EAIS is moving very slowly and will react very slowly to changes in climate. But there is no evidence for an increase in snowfall from ice cores in East Antarctica and if anything, regional climate models suggest the opposite has been happening over at least the last decade. Zwally and his team argue that instead, mass is actually accumulating in spite of no increase in snowfall. How can this be? The answer is that the ice sheet is still adjusting to the ~doubling snowfall that took place at the end of the last glacial period, between about 18,000 and 12,000 years ago. Because the accumulation rates and temperature in East Antarctica are so low, the ice sheet has a response time to changes in climate of many millennia. So the ice sheet may still be growing even though there has been no increase in accumulation for more than 10 millennia. The difference here is crucial: if the increasing height of 1-3 cm a year is owing to recent increases in snowfall, then we should use the density of snow in the calculation. One cm of snow over the entire East Antarctic Ice Sheet (EAIS) would increase its mass by around 35 Gt (35 billion metric tons). But if the increase in height simply reflects continuing adjustment since the last glacial period, then — Zwally et al. argue — we should be using the density of ice. That would mean an 1 cm increase in height would reflect an increase in mass of ~92 Gt.
But is the density of ice the right one to use? In East Antarctica it takes thousands of years for the snow to form ice. This process is called firn densification (firn is compressed snow) and how fast it occurs depends on snowfall rates and temperature: i.e. climate. If the climate changes, then so does firn densification and the surface of the ice sheet can go up or down with no change in mass but just a change in density of the surface layers. Variations in firn densification rate can be larger than the measured elevation change, so it is essential to correct for this effect and that is usually done with a model. The model requires snowfall and temperature histories to drive it and these aren’t that well known for the EAIS. If we knew the snowfall history perfectly then there wouldn’t be any controversy!
Another problem with this type of measurement is that small biases in the laser and radar data can have a big effect when the signal is only on the order of 1 cm/year. Both instruments suffer from biases and researchers attempt to correct for these in various different ways. Each group has their preferred bias correction approach. In a previous study, published just three years earlier, Zwally and co-workers obtain a volume change from laser altimetry that is only about half the value they get in their latest study. They argue that various improvements to their bias-correction approach and some other subtle differences in processing are responsible for the difference. They also argue that their approach is superior to other studies using altimetry. But it is probably more accurate to say that it is just different, rather than better. Results from the newest satellite that can make this type of elevation measurements, CryoSat 2, which aren’t included in the Zwally et al. study, may yield a different picture yet again (Fig 2). Indeed, comparing results from CryoSat-2 height change for 2011-2014 (Fig 1) with those for 2003-2008 from Zwally and others is not very encouraging. Although inter-annual variability in snowfall is important, the estimate from CryoSat-2 is less than half that from the Zwally laser altimetry result. Such a large difference (about 110%) is more than would be expected from snowfall variations alone. This doesnt show that Zwally et al. results are necessarily wrong, clearly, this is challenging measurement to make, and translating these measurements into mass change adds yet more uncertainty.
Figure 2. Elevation changes from 2011-2014 from Helm et al., 2014.
Let’s go back to the laser/radar comparison over Lake Vostok. At face value, this looks reassuring. But if you compare the trends between laser and radar data for other parts of the EAIS, the agreement is poor and, depending on the time period you chose for Vostok it can be more or less good there too. Thus, the agreement over Vostok appears to be fortuitous rather than evidence that the two instruments are measuring the same thing in the same way. And this leads on to another issue with the new study: if the thickening over the EAIS is due to a long term trend going back more than 10000 years, it should be indistinguishable from constant over the two time periods discussed. For the whole of the EAIS, the trend for 1992-2003 and 2003-2008 are the same, but at a regional scale (and by regional I mean areas that are still pretty big: the same size as a US state) things become less clear. For some regions the trend more than doubles, for others it goes from negative to positive or vice versa. Some of these differences are small and within the uncertainties. but some are not. The signal simply should not change much at all at this scale if it were due simply to long-term ice sheet adjustment.
Comparing results from CryoSat-2 volume change (not mass) for 2011-2014 (Fig 1) with those for 2003-2008 from Zwally and others is also not very encouraging. Although inter-annual variability in snowfall is important, the estimate from CryoSat-2 is less than half that from the Zwally laser altimetry result. Such a large difference (about 110%) is more than would be expected from snowfall variations.
There is another way to measure the changing mass of the ice sheet and that is to weigh it using the Gravity Recovery and Climate Experiment (GRACE) twin satellites. This mission measures gravity anomalies around the Earth every 10 days and has, for example, been able to detect the groundwater depletion in California in recent years. It can also measure the growth of mass-loss of an ice sheet, but the signal from the ice is confounded by another signal from the bedrock beneath, which is responding to changes in ice loading over thousands of years. This response is known as glacio-isostatic adjustment (GIA), and in the interior of East Antarctica we have very limited information on how big the bedrock signal is. Geophysicists have attempted to model how the solid Earth responds to changes in ice loading and, although the physics is fairly well known, some critical variables such as the viscosity of the lower mantle and the history of ice load changes are not. Zwally argues that the GIA estimates that have been used for correcting GRACE data are wrong in East Antarctica because they do not know the full ice loading history over the last ~22,000 years. Maybe. But GIA in the EAIS is poorly constrained and there is, in general, poor agreement among different estimates of GIA over the EAIS.
So what is really happening? One thing that Zwally’s study does highlight is how difficult it is to nail what is happening in East Antarctica because the signal is small and contaminated by unwanted effects that are as large or even larger. Zwally et al get a different result from previous studies because they make a different set of assumptions. Those assumptions are, by their nature, subjective and difficult, without additional evidence, to corroborate. There are, however, other lines of evidence that suggest that Antarctica is unlikely to have been gaining mass in the last few decades. That would, for example, make closing the sea level budget a whole lot harder (that is, making the sum of the sinks and sources match the observed rate of sea level rise). One other thing is certain: West Antarctica has been losing mass at an increasing rate since the 1990s and, irrespective of what is happening further East, that trend looks set to continue. Going to the other end of the Earth, the Greenland ice sheet has also been losing mass at an accelerating rate since around 1995. Greenland is now the single biggest source of mass to the oceans. These trends at both poles are huge signals that are unequivocal and uncontested.
Boening 2012 report a gain along the coast of east Antarctica and attributable to recent snow of “350 Gt from 2009 to 2011”. That observation is higher than the estimated 35 Gt per 1 cm of snowfall, unless snowfall is adding 3 to 4 cm of height per year.
I want to wait and hear what Victor has to say.
Thinking about the computation of column density, versus height changes in
Zwally.
An unkown is the compaction profile (density versus depth) near the
surface. If this compaction profile is time independent, or for our
purposes, the mass deficit of the profile from what the mass it would have
if it were pure gacial ice, is time invariant, then the correct density
to use for height changes would be the density of ice. If because of
changing surface conditions (net snowfall, degree of wind packing, temperature)
the compaction profile is changing, then that would upset this assumption.
If one is interested in the change in the total mass of ice, then any
isostatic changes in height would have to be accounted for as well. I suspect
the recent compaction profile is reasonably known, however past profiles
are probably unavailable.
I saw something recently which indicates that a substantial surface
area of the antarctic ice sheet has been undergoing net wind ablation.
It was claimed that enough of this was due to sublimation, rather than
snow transport, so there are apparently areas of the ice sheet that
are losing ice from the surface -presumably, if these areas don’t change
much from year to year, there is net ice flow into these abalation areas leading to a quasi steady state surface height. In any case, this effect very likely
means that compaction profiles could be highly variable.
If
and for ice:
Then I would think that the mass of 1 cm of snow in Antarctica would be ~61 GT. But we are given:
Something is not right.
[Response: Corrected. That should have been “snow is about 1/3 as dense as ice”, not that snow is 1/3 lower in density. –eric]
Why don’t we stick to GRACE gravity measurements?
Could the increasing height of ice be caused by ground uplift due to volcanic activity?
I’d like to understand this. But I’m trying to put a couple of pieces together. First:
What, I ask myself, will cause the surface to rise–or, equivalently, the ice to becomes less dense? For it would seem that is what must be happening. It’s a tough question (for me, at least) because I would think that a glacial (sorry ’bout that) response to increased snow load would be a very slow densification–not the reverse. So I can imagine a slow response, but I haven’t yet been able to come up with one that has the right sign! (I assume GIA is already factored in, since isostatic adjustment is discussed at some length in the post–but in any case, I’d expect the sign of GIA to be wrong, too: the bedrock should be sinking in response to loading, not rising.)
This doesn’t get much better when I read that:
Seems like ‘negative densification’ is possible?
Or is there a whole other component to this mysterious ‘adjustment’ process that we should know about?
[Response: It’s simpler than Jon made it seem in his write-up. Imagine an ice sheet that gets 10 kg of snowfall each year over each square meter of surface. That is, an accumulation rate of 1 cm/year, ice equivalent (or around 3 cm of snow). Now imagine the ice is flowing at exactly the right rate, such that the same mass gets to the edge of the continent and falls into the sea each year, as icebergs. That ice sheet would be in “steady state”. Now imagine that the snow accumulation rate doubles, to 20 kg per square meter, every year. If the flow of ice does not increase, then the ice sheet will grow at 1 cm per year in height. Eventually, the extra mass will cause the ice sheet to flow faster, and it will eventually come back into balance, but this can takes thousands of years. So the ice sheet could still be increasing in height, even though there has been no increase in snowfall for thousands of years.
Does that help? –eric]
If increasing SSTs are increasing evaporation over broad ocean basins and increasing precipitation where weather systems cause large amounts of rain to fall in small areas, then would it be unusual to suspect that over time Antarctica might also be accumulating deeper snow in places as weather systems sweep the continent? Winds would distribute local events over broad areas. Perhaps Antarctica is gaining ice mass irrespective of isostatic rebound after the last ice age.
Given this is happening but decreases in masses of ice everywhere else are evident, how does a minor Antarctic mass increase armor climate skeptics that we should be complacent? Small subtractions from a growing threat simply means the avalanche we’re surfing down hits the wall
a fraction slower. Does it really make a lot of difference?
This veers off topic a bit but a look at Blanco River Hydrology for Wimberley, Texas clearly indicates that increasing regional precipitation has come with disastrous results.
http://water.weather.gov/ahps2/hydrograph.php?wfo=EWX&gage=WMBT2
See Historical Crests.
This list may be useless. Unfortunately it substantiates Lamar Smith’s horror at government data. I use it with some trepidation. But, I live next to this river and can remember the floods of the last fifteen years. (2) below is the crest before the flash flood broke the gauge. (1) is the measured height of the water. In the narrow canyon at our place the water rose about 60 feet and reduced a beautiful place to a wasteland
that looks like it’s been hit with a tornado.
Historic Crests
(1) 44.90 ft on 05/24/2015
(2) 40.21 ft on 05/24/2015
(3) 33.30 ft on 05/28/1929
(4) 28.89 ft on 11/16/2001
(5) 28.50 ft on 10/17/1998
(6) 26.75 ft on 10/31/2013
(7) 25.71 ft on 07/05/2002
(8) 20.79 ft on 03/12/2007
(9) 20.79 ft on 03/12/2007 ???-T
(10) 20.46 ft on 06/09/1997
(11) 20.12 ft on 11/17/2004
(12) 20.10 ft on 12/21/1991
(13) 19.86 ft on 11/10/2000
(14) 18.25 ft on 03/04/1992
(15) 16.54 ft on 07/17/1987
(16) 14.29 ft on 10/07/1981
(17) 13.63 ft on 06/07/1997
(2) and (9) are duplications for the date… a more recent crest qualifying in the upper range occurred the day before Halloween this past October – about 32 feet at Wimberley.
On the Memorial Day weekend in Wimberley,Texas the area suffered incredible damage including the loss of a highway bridge upstream that crossed 40 ft above the river due to an increase in regional precipitation. Floods are becoming a regular occurrence.
It seems to me that post glacial isostatic rebound in Antarctica would easily overcome a few more centimeters of snow accumulation. Land masses should be rising. Whatever the case we would be fools to be complacent about climate change because it’s snowing in Antarctica.
“Does that help?” –Eric’s inline @ 7.
Yes, it did, after a moment’s thought. Key was the realization that the accumulation rate *remains* elevated after the posited step change.
Duh.
Thanks!
A minor typo: It appears to me that the third paragraph from the end is a repetition of sentences within the fifth paragraph from the end.
Could someone comment on whether GRACE can address this? Seems the right tool for the job, as pointed out in 5.
I confess, I don’t get it. Mass change is just ice in minus ice out. If the rate of precipiation isn’t increasing then the only way for overall mass to increase is that the rate of ice loss must be decreasing. Which doesn’t seem likely.
[Response: You are right that if the rate of precipitation has not increased, then mass couldn’t have increased either (unless loss rate goes down). But precipitation doesn’t have to be increasing now. Precipitation did increase at the end of the last ice age, and if the rate of ice loss hasn’t yet caught up with that increase, then, yes, the ice sheet can still be growing. —eric]
Reply to 11: Can GRACE address this. I do discuss the issues with GRACE data in the blog. The problem with GRACE is that it measures the integrated gravity change over an area and for places like Antarctica that is a combination of GIA and mass change. Around the edges of Antarctica we have GPS observations that help validate our modeled GIA estimates but in the interior we have no validation data so it is really hard to know how good the modeled GIA estimates are and these are essential to separate the solid Earth effects from the ice loss. Does that help?
“Could the increasing height of ice be caused by ground uplift due to volcanic activity?”
No volcanoes in East Antarctica nor does it have tectonic setting for volcanoes to occur.
Chuck Hughes: “I want to wait and hear what Victor has to say.”
Sorry to disappoint you, Chuck, but I have nothing of interest to say on this matter. So I’ll leave it to the experts for a change:
http://www.nasa.gov/jpl/news/antarctic-ice-sheet-20140512/#.VkVcv9KrTs0
http://phys.org/news/2014-06-major-west-antarctic-glacier-geothermal.html
And by the way, isn’t this a climate science blog? No mention of climate in either of the above. No mention of climate in Bamber’s article either. So what gives? :-)
[Response: Holy confusion Batman! That article about geothermal heat sources is okay, but the title implies that the reason that Thwaites glacier is retreating is geothermal. No so. It’s ocean heat. Ocean heat is part of the climate system. So is glacier ice, for that matter. –eric]
Question for 13. Given that the uncertainty with GIA, it is not clear to me why elevation measurement are superior to gravity measurements. Both will be limited by the GIA corrections, right?
OK. I think I get it now. If Zwally and his team are correct, then the ice sheet has been growing for over 10,000 years, due to a step change in precepitaton as long time ago.
There is no suggestion that there is a /recent/ increase in the rate at which the Antarctic gains mass, therefore this finding (even if correct) in no way contridicts the fact that the oceans and land-surface are warming due to human-driven changes in forcing over the last 250 years.
Sorry. Re-reading the article I can see that now, but it wasn’t clear to me previous.
Eric: “But precipitation doesn’t have to be increasing now. Precipitation did increase at the end of the last ice age, and if the rate of ice loss hasn’t yet caught up with that increase, then, yes, the ice sheet can still be growing.”
How on earth can you say that? Please think a bit. Of course the ice sheet cannot be growing today from snow deposited long time ago.
Does your bank balance continue to grow in the days following your monthly salary payment?
[Response: The precipitation rate increased at the end of the last ice age. If I spend $100/month, and I get paid $100 a month, then my account doesn’t grow. If I spend $199/month, but get paid $200/month, then it does. The debate about the *current* balance of the ice sheet is a bit like asking whether I get paid $198 or $203/month. (You may want to revisit your own savings plan.) –eric]
Jonathan Bamber:
Huh. I may be revealing my ignorance here (wouldn’t be the first time), but why not drop GPS instruments that can be remotely queried on the interior ice?
In order to understand the GIA on the interior, it is necessary to calculate the change in height of the bedrock, not the surface. Rock has about 3 times the mass per unit volume than ice, so minor changes in basal surface elevation make a large gravity signal.
So, you need to drill holes and monitor change, not drop instruments on the surface.
http://in.rbth.com/economics/2015/04/24/new_glonass_stations_to_be_deployed_in_antarctica_42823
Eric: “Ocean heat is part of the climate system.”
Not if it’s due to geothermal activity.
From the NASA report:
“The study, led by glaciologist Eric Rignot at NASA’s Jet Propulsion Laboratory, Pasadena, California, and the University of California, Irvine, follows decades of research and theory suggesting the West Antarctic Ice Sheet is inherently vulnerable to change.”
Note the absence of the word “climate” prior to the word “change.” In fact “climate” is never mentioned at all in this report.
From the same document:
“Antarctica is so harsh and remote that scientists only began true investigation of its ice sheet in the 1950s. It didn’t take long for the verdict on the West Antarctic Ice Sheet to come in. “Unstable,” wrote Ohio State University glaciologist John Mercer in 1968. It was identified then and remains today the single largest threat of rapid sea level rise.”
By 1968 there had been no evidence of global warming (for whatever reason) for the previous 27 years — since 1941. And prior to 1941, CO2 emissions from fossil fuels amounted to only a small percentage of what they are today. Given the strong evidence of geothermal activity directly under the Thwaites glacier, it seems likely that this is the primary source of the melting now being observed. Has anyone ever proven otherwise?
Finally, even the Guardian, in a recent, highly disturbing article, acknowledged that it’s too late for us to do anything about this situation, regardless of the cause. Even if all burning of fossil fuels ceased tomorrow, that would have no effect on the fate of the doomed West Antarctic glacier. So what are we talking about here, and why?
[Response: You misunderstand the article, which is written without providing much context. All that it points out is that the melting at the base of the grounded ice sheet may be greater than expected because of greater geothermal heat than previously thought. That makes the ice sheet more sensitive to climate-driven perturbations. The climate driven-perturbations are at the margin, under the floating ice shelves. That’s ocean heat and it has essentially nothing to do with geothermal heat. Whether anthropogenic climate change is important here is another question of course, but it is most certainly climate. Whether we can do anything about it is a different question too. Whether or not we agree on that point, there is no ambiguity about the glacier dynamics, or its relevance to climate.–eric]
Re- Comment by Victor — 13 Nov 2015 @ 5:21 PM, ~#21
Victor, the sun provides the earth with 341 watts per meter squared, while geothermal heat from all sources provide only 0.09 watts per meter squared. Geothermal heat is about 3 times that of the direct energy (sensible heat) derived from burning fossil fuels and nuclear energy, but is around only 1/20th of the energy added by anthropogenic greenhouse gasses. This information takes only a minute or two to find on the web.
Steve
#16:
I’m guessing that the same amount of GIA (bedrock elevation change in cm) has about three times the effect on Grace that it has on Zwally et al’s calc, because of the differing densities of bedrock and ice.
Victor,
I just wanted to say on behalf of all of us who are actually concerned about the future of the planet and its population:
Thank you so much for being on the other side.
Eric, it’s not my intention to challenge your understanding of climate science. You obviously know far more on this topic than I do. And I appreciate your patient and knowledgeable responses. However, I am still left with questions that strike me as reasonable, so I hope you don’t mind if I persist. And I hope you won’t be offended and will continue to respond. Thanks.
First, let me repeat my earlier quote from the NASA article: “It didn’t take long for the verdict on the West Antarctic Ice Sheet to come in. “Unstable,” wrote Ohio State University glaciologist John Mercer in 1968. It was identified then and remains today the single largest threat of rapid sea level rise.”
Also, from the same source:
“Mercer based his statement on geologic evidence that West Antarctica’s ice had changed considerably many, many millennia ago at times when the ice sheets of East Antarctica and Greenland had not.”
If the instability of the Thwaites was identified in 1968 as “the single largest threat of rapid sea level rise,” and this instability can be traced to changes initiated “millennia ago,” then it’s hard to see how its identification as “the single largest threat of rapid sea level rise” could be attributed to modern CO2 emissions. This could certainly be a contributing factor, but the impression one gets from the NASA assessment is that the problem is primarily due to an age-old instability of some sort, predating AGW. The authors have nothing to say about climate change at all.
Secondly:
If the primary cause of instability is AGW, then why aren’t all the Antarctic glaciers similarly affected, not just the Thwaites? If global warming is truly global we should be seeing its effects over the entirety of Antarctica, not just this one relatively small region, no? The localization of the problem, coupled with its ancient origins, strikes me as far more likely to have some sort of underlying geothermal activity as its source, not AGW.
[Response: I think the evidence that the current retreat of Antarctic glaciers is owing to anthropogenic global warming is weak. The literature is mixed on this, about 50% of experts agree with me on this. So you’ll get no argument from me there. Second, the localization in West Antarctica is well understood, and I’ve written about it extensively. It’s obviously time for a post on this. Meanwhile you could start with the two papers below. –eric]
http://www.nature.com/nature/journal/v484/n7395/full/nature10968.html
http://www.sciencemag.org/content/343/6167/174.abstract
We can and should puzzle over this kind of unusual finding, but it does not change some truly basic global warming facts like the increase in temperature and rise of sea level. The studies of global glaciers suggests loss of glacier mass is the baseline in current warming and sea level is rising in response to loss of glacier mass. The influx of cold water associated with glacier loss will be seen as is the case for the Atlantic (south of Greenland) and that cold water will create perturbations in systems like AMOC and sea level rise associated with increasing global sea temperature, but these perturbations should be somewhat local (even AMOC is somewhat local as confined to a single ocean). The big picture remains a warming planet and that warming planet will put stresses on many species and contribute to extinction of species in what will be the sixth great extinction.
On specific species is likely to survive the great extinction: the internet troll. Don’t feed them, but don’t expect them to go away, trolls retain great resilience through some mysterious primitive mechanisms.
despite troll-feeding concerns, I do appreciate Eric’s inline response above. Helpful big picture stuff about the unstable WAIS. All folks who think climate change is not happening should certainly invest in coastal real estate. They will do very well if they are correct that global warming is not happening. I have checked and could not find a single mutual fund dedicated to investment in assets that are devalued by climate change concerns.
What does this mean for sea-level rise? I realize it is extremely difficult to get a highly accurate mean sea level, much less contrast it accurately year over year. But it has been my understanding that over the past half-century, the mean sea level rise has gone from about 1.7 mm/year to about 3.3mm/year. Some of that rise has been attributed to mass loss from Antarctica. If Antarctica is actually gaining instead of losing mass, then there should be other sources of the rise of which we’re unaware, or does the discrepancy evaporate into the error bars when all is looked at closely? Or when Antarctica does go into negative mass balance in a few years, will seal level rise accelerate by ~15% or so, as a w.a.g.? Or am I totally wrong about sea level rise rates?
I have always had trouble with Zwally’s tendency to not discuss the temperature of the ice he was discussing.
In the days when there was enough latent heat in the atmosphere to drop a lot of snow, then the resulting ice was warm and weak. The bottom of the ice would have melted under the pressure of the ice above and the whole thing slid on a film of liquid water.
Now, the atmosphere is too cold and dry to drop a lot of snow, and the ice is colder and stronger. Nevertheless, there are still lakes under the ice, and we can not assume that the flow rates of today are NOT the flow rates of the days when the ice was fresh and warm. Nor should we assume that the flow rates of today, will be the flow rates of tomorrow.
Latent heat in the form of rain could hydro-fracture the ice and again lubricate the base very quickly. The flow process would work the ice from top to bottom, quickly introducing heat into the ice column faster than it could enter by traditional conduction.
[Response: The atmosphere has not changed enough on the relevant timescales for any of this to matter, today. the timescale for surface temperatures affecting the bottom of the ice sheet is tens of thousands of years. –eric]
Prof. Steig, you wrote: “I think the evidence that the current retreat of Antarctic glaciers is owing to anthropogenic global warming is weak.”
What are your thoughts on the role of the southern ozone hole ?
[Response: I don’t think it’s very relevant, at all. It is often implied that it is relevant, but I have seen no evidence. That’s not to say that the winds haven’t been affected by the ozone hole. They have. But the biggest wind changes, which have occurred along the coast of West Antarctica, have occurred in winter and spring when the ozone effect is small to non-existent. It’s those wind changes that have affected the ocean, and hence the West Antarctic glaciers. See: Steig et al., Tropical forcing of circumpolar deep water inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica. Annals of Glaciology 53: 19-28 (2012) and numerous citations therein and following.–eric]
Why would one expect uniform change?
Bottom ice melt from warmer water under some glaciers could happen only for glaciers that are not grounded, and under which the inside seabed slopes downhill, and where the relevant ocean circulation affects the site. That would be both thermohaline circulation and the vertical circulation locally — which ones are those?
Proc Natl Acad Sci U S A. 2014 Jun 17; 111(24): 8753–8758.
Published online 2014 Jun 2. doi: 10.1073/pnas.1323922111
PMCID: PMC4066517
Earth, Atmospheric, and Planetary Sciences
Antarctic sea ice control on ocean circulation in present and glacial climates
I think there’s a certain missing of the point in this, from Victor’s #26:
It’s not that the instability is due to anthropogenic factors; it’s that the pre-existing instability poses a dangerous vulnerability in the context of sustained anthropogenic warming.
[Response: Yes, exactly. –eric]
Without knowing accurately the magnitude of the glacio-isostatic adjustment, any conclusions about Antarctic ice mass change seem premature. Maybe the ice mass, and therefore the ice weight, are decreasing enough to allow rebound. We don’t really know.
I do wish people who reference Mercer would have read his papers.
Mercer(1968) explicitly references anthro influence as a possibility:
“If the apparent warming trend is real and continues until hypsithermal conditions are reached and exceeded, whether because of industrial pollution of the atmosphere …”
Mercer(1978) makes his opinion clear:
” … deglaciation of Western Antarctica would not occur in the foreseeable future without Man’s injection of massive amounts of industrial CO2 into the atmosphere.”
I take it that Prof. Steig disagrees in that the present West Antarctic melt cannot be directly attributed to anthro fossil CO2 emission.
Mercer(1978) goes on:
“A disquieting thought is that … this deglaciation may be part of the price that must be paid to buy enough time for industrial civilisation to make the changeover to other sources of energy. If so, major dislocations in coastal cities, and submergence of low-lying areas … lies ahead. More sophisticated climatic modelling may show that the outlook is less alarming than this, but on the other hand it may show that the situation is even more threatening.”
sidd
Lets weigh the total mass of snow and ice today, and call it x Gt. This x Gt includes any snow that would have fallen 10,000 years ago.
Can you see how to many people it doesnt make sense to say that snow from 10,000 years ago is making the ice sheet grow? Surely that mass would have been there all along?
Doing the science is great, but often the failings come when trying to explain it to decison makers. Don’t let good research fall on deaf ears.
I am another who took the article to be saying the greater snowfalls from 18,000 to 12,000 yrs ago were responsible for recent ice sheet mass accumulation; so more correctly recent snowfall is not as great as back then but it still exceeds ice outflow? And Antarctica’s contribution to ocean volume has been – presuming Zwally is correct – as a long term source of reduction, that is expected to shift to one of increase within a few decades?
And, whilst I can see that different estimates for GIA may be estimated and used, I would have thought GIA is relevant to any estimate of ice sheet mass balance, whether it’s as changes in elevation or as changes in mass.
Off topic but what is the general thinking on the ideas to use East Antarctica as a CO2 landfill by refrigerating air to Martian levels to sequester out the CO2.
It seems to have one problem in that the CO2 is more dense in the Northern hemisphere (is that right?) What are other reasons it would not work if any?
#38–The scheme sounds a bit goofy to me, just off the top of my head–questions occurring to me are:
1) How would this be powered? *Lots* of power would be needed, and providing it would, I suspect, be very, very expensive due to the remoteness and harsh environment. Aren’t there more capital-efficient ways of sequestering CO2?
2) How is the CO2 to be retained in solid form? It’s not cold enough (is it?) to just leave it lying about. So is it permanently refrigerated? Would that imply ever-growing CO2 ‘warehouses’?
But I’m not saying those are thoroughly considered objections, more doubts that immediately spring to mind.
However, the differential in CO2 between hemispheres would *not* be an issue. That differential is quite small, as you can see WRT the South Pole Observatory CO2 data:
http://www.esrl.noaa.gov/gmd/dv/iadv/graph.php?code=SPO&program=traj&type=traj
They call CO2 a ‘well-mixed gas’ for a reason!
#36 & #37:
No, according to Eric’s answer to me, the contemporary rate of snowfall is still elevated; i.e.., there was a step change in rate several millennia back, and that higher rate remains in effect today.
Accumulation is the difference between snowfall and snow loss via whatever mechanism; snowfall changed, increasing accumulation. Thus, ice has been accumulating since the change.
However, the mechanism by which the glacier responds to the increased load–glacial flow rate changes–is very, very slow. That’s why it’s not back in equilibrium yet.
Of course, the loss of ice shelves will ‘help’ the flow rate increase and bring things (perhaps briefly) into equilibrium but that’s not a cheerful story.
#39, Kevin: the idea is to use about 19 GW of wind power (although it seems that 19 nuke plants would be better as having the advantage of being housed out of the elements) to refrigerate the humid air to make the CO2 in it solid particles, and that would be feasible technically: albeit an O&M bonanza for wind turbine maintenance crews to keep operating in sub zero weather.
The landfill would be covered with snow naturally, gradually building up a layer of covering.
As to how
Glaciologist Jan Wuite also put up a nice explainer of how this new study fits in the context of previous studies about the Antarctic: https://ourchangingclimate.wordpress.com/2015/11/25/antarctica-ice-gain-or-loss/
http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-12-0110.1
Agee, Ernest, Andrea Orton, John Rogers, 2013: CO2 Snow Deposition in Antarctica to Curtail Anthropogenic Global Warming. J. Appl. Meteor. Climatol., 52, 281–288.
http://dx.doi.org/10.1175/JAMC-D-12-0110.1
CO2 Snow Deposition in Antarctica to Curtail Anthropogenic Global Warming
Related
Any slump will skew results.
To Hank Roberts #43-44. Right; that was the study I was referring to: CO2 Snow Deposition in Antarctica to Curtail Anthropogenic Global Warming.
So: does this make sense to climate scientists? or to scientists/engineers in general?
Politically it seems to me to have some advantages:
1. Nobody owns Antarctica.
2. Its occupants are interested parties (climate scientists) from every nation.
3. There are no politicians to be voted into power in Antarctica, so no captives to petrostate thinking.
Therefore, Antarctica could be a global blank slate to create and fund a shared global solution, monitored by resident climate scientists.
For Susan K
I’m no expert on thermodynamics but I doubt building big equipment to freeze out CO2 makes sense at all. Refrigerators throw off heat and the heat has to go somewhere, and they’d have to keep running forever. Replace burning fossil carbon as fast as possible instead, and you get far more benefit from the work done.
“Nobody owns Antarctica” as long as international agreements are respected keep it that way but the entire continent is divided up among nations to administer, and as it melts out you can count on a “gold rush” like in Greenland and the Arctic Ocean.
Agreements to leave alone what can’t be reached are easy. Agreement to leave alone what can be stripmined, not so much. Look at seabed mining and deep water trawling — they’re destroying ecosystems planetwide and “nobody owns them” except the future. And the future has no say in protecting them.
PS for Susan K: one clue, that paper has been cited only once, and by a paper discussing a different method of sequestering CO2. When a paper doesn’t attract attention from other scientists, odds are there’s not enough to it to be worth pursuing. If it made sense, someone would have written more on it.
http://arstechnica.com/science/2015/11/inside-literally-wind-turbines-meant-to-work-at-the-south-pole-and-mars/
#48 True, lack of other scientists – except Australia’s former govt climate science advisor Tim Flannery (The Weather Makers), whose new book brought it up again.