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Monday, November 2, 2015

Antarctic ice - growing or shrinking? NASA vs Princeton and Leeds etc

Sou | 11:26 PM Go to the first of 43 comments. Add a comment
There's a new paper out about Antarctic ice, from H. Jay Zwally and colleagues at NASA. They report that over the period from 2003 to 2008, there was a net increase in ice over all Antarctica of 82±25 Gt/year. This paper looks to be based on a conference paper at a SCAR workshop back in July 2012 (though that doesn't explain why there wasn't data from the past six years in the final published paper).

These findings are different to the results of work reported earlier this year from two scientists at Princeton, Christopher Harig and Frederik J. Simons. The Princeton team found that over the period January 2003 to 2014, there was a loss of ice overall. the overall mass loss from Antarctica since January 2003 at 92 ±10 Gt/yr.

It's also different from the results reported in a paper by Malcolm McMillan and colleagues last year. They estimated the current mass loss over all Antarctica at 159 ± 48 Gt/year. ".

So one group of scientists find that ice has been on balance increasing, while others find that ice has been on balance decreasing.


Note that the periods differ. The new paper stops in 2008. Harig15 is for 2003-2014 inclusive, and McMillan14 doesn't to an average number, but reports "the contemperaneous loss" - based on from 2010 to 2013.

In 2012, a paper was published in Science mag by a whole bunch of people (h/t ATTP). That paper combined data sets of different types: satellite altimetry, interferometry, and gravimetry. They found there was good agreement between the different satellite methods, and concluded:
Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

I'm obviously not in a position to "pick sides" and say who's right and who's not. The scientists involved are all experts, and experts don't always agree. What is agreed is that Western Antarctica is losing ice. And it's losing ice very quickly.

There's a simplified diagram in Zwally15, which shows the different processes that affect mass balance in Antarctica:

Fig. 1 | The principal processes affecting the mass balance and dynamics of the ice sheets are ice mass input from snowfall with losses from sublimation and drifting. Surface melting on the grounded ice of Antarctica is very small, and subject to refreezing in the firn. Interaction with the ocean occurs at the undersides of the floating ice shelves and glacier tongues, and consequent changes in thickness affect the rate of ice flow from the grounded ice. Source: Zwally15

Antarctica is huge at around 14 million sq kilometres it's almost twice as big as Australia; and almost twice as big as the contiguous USA (not including Alaska). Most of it has never been visited by people, so satellites are relied on a lot to figure out what is happening there.

In Zwally15, the authors divided up Antarctica into 27 different regions. They separated dynamic-driven mass changes and accumulation-driven mass changes. The accumulation-driven changes are from snowfall. Unlike Greenland, there's not much surface melting in Antarctica. It's too cold. There are losses from warm water getting under the ice and melting it - particularly in Western Antarctica, the Peninsula and some parts east. These processes are shown in the diagram above.

In Zwally15 the authors discussed changes for the periods 1992 to 2001, based on ERS radar altimetry, and from 2003 to 2008 using ICESat data.  They explained how they calibrated the two so they could be compared.

As Jay Zwally explained in the press release, scientists are pretty much in agreement about the increasing ice losses in parts of Western Antarctica and on the Peninsula:
"We're essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica," said Jay Zwally, a glaciologist with NASA Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study, which was published on Oct. 30 in the Journal of Glaciology. "Our main disagreement is for East Antarctica and the interior of West Antarctica -- there, we see an ice gain that exceeds the losses in the other areas." Zwally added that his team "measured small height changes over large areas, as well as the large changes observed over smaller areas."

And they may not be in a heap of disagreement up to 2008 either. For example, here is a chart from Harig and Simons. I've animated it, blocking out the period after 2008, and then showing the entire period:

Fig.2. Ice mass changes (mass corrected using the GIA model by Ivins et al., 2013) in gigatons (Gt). The black lines are monthly GRACE observations with 2σ error bars determined from our analysis. The solid blue lines are the best-fit quadratic curves. Source: Harig and Simons 2015


So you can see that most of the loss since 2000 occurred after 2008. It would have been more obvious without the blue line - just focus on the monthly data. The figure in the paper has more panels, and shows that West Antarctica is losing a lot of ice, while Dronning Maud Land region was reported to be gaining ice.

I expect there will be a lot more research to come, because Antarctica is the biggest concern in regard to rising seas, together with Greenland.



Where is the water coming from?


Sea levels are rising from both the expansion of water as it warms, and from melting ice. So if Antarctica is reducing sea level by 0.23 mm a year, and sea level is rising by at least 3.3 mm a year, then where is the extra water coming from?

Other papers have reported variously that Antarctica is adding to sea level rise, for example:



I was going to write a lot more, and there's probably much more that could be said. One is that the melting of Antarctica and Greenland will cause big problems sooner or later this century.

This article might not be all that satisfactory, in that it leaves unresolved questions. Is Antarctica really gaining mass? If it was gaining mass from 2003 to 2008 is it still gaining mass or is it now losing mass? However, it does cover more ground than is covered at WUWT (archived here), and shows where there are differences and where the findings are not so different.

Update: Be sure to read the comments below. There's a wealth of information there. [Sou 3 November 2015, a day that will go down in horse racing history as the day the first woman rode the winner of the Melbourne Cup :)]


References and Further Reading


Zwally, H. Jay, ; Li, Jun; Robbins, John W.; Saba, Jack L.; Yi, Donghui; Brenner, Anita C. "Mass gains of the Antarctic ice sheet exceed losses". Journal of Glaciology, 2015 DOI: 10.3189/2015JoG15J071 (open access)

Harig, Christopher, and Frederik J. Simons. "Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains." Earth and Planetary Science Letters 415 (2015): 134-141. doi:10.1016/j.epsl.2015.01.029 (pdf here)

McMillan, Malcolm, Andrew Shepherd, Aud Sundal, Kate Briggs, Alan Muir, Andrew Ridout, Anna Hogg, and Duncan Wingham. "Increased ice losses from Antarctica detected by CryoSat‐2." Geophysical Research Letters 41, no. 11 (2014): 3899-3905. DOI: 10.1002/2014GL060111 (open access)

Shepherd, Andrew, Erik R. Ivins, A. Geruo, Valentina R. Barletta, Mike J. Bentley, Srinivas Bettadpur, Kate H. Briggs et al. "A reconciled estimate of ice-sheet mass balance." Science 338, no. 6111 (2012): 1183-1189. DOI: 10.1126/science.1228102 (pdf here)

43 comments:

...and Then There's Physics said...

(though that doesn't explain why there wasn't data from the past six years in the final published paper).
The satellite stopped working, I think.

Sou said...

Thanks, ATTP. Yet they used data from a different source for the period from 1992 to 2003, so I'd have thought they could've got data for the period since, and done a calibration, like they did before. (And like the Science authors did.)

Though I expect it's not that simple.

I figured it just took a while to finish the work, and they decided to publish anyway and not try to add the latest data.

Magma said...

ATTP is correct. ICESat's primary instrument failed in February 2010, with data collected over periods from Feb 2003 to October 2009. Appropriately enough the remnants of the decommissioned satellite crashed to Earth on August 30, 2010 over the Barents Sea.

http://www.nasa.gov/mission_pages/icesat/icesat-end.html

Magma said...

Its improved successor ICESat-2 is under construction but the launch date has progressively slipped from late 2015 to the current June 2018.

izen said...

The ICEsat satellite saga is another sorry story of a NASA snafu in which partial success is snatched from the jaws of failure. Cf Hubble and Apollo 13.

When Launched in Jan 2003 it had 3 Lasers each of which was expected to last for 2 years and enable it to measure surface elevation and material globally every 8 days.

The first Laser failed at the end of March 2003. It became clear that a fault with the lasers would yield less than a years worth of data before the remaining 2 failed if the original mission profile was followed.
So the orbit and operation of the system was changed so that it was switched on only 3 times a year and could scan the ice sheets once every 91 days at best.

Ice dynamics in both polar regions and the Greenland icecap proved to be particularly interesting during the period of operation from 2003-2009 that the reduced observing pattern kept ICEsat in operation. No doubt a near weekly record ofr chnages would have been preferred than the once every 3-4 months that ICEsat could provide, but science has to work with the data available.

Because of the dynamic changes revealed from the limited ICEsat data and other sources, when the satellite failed in 2009 the Icebridge program started. this uses aircraft flying with LIDAR and GPS to duplicate the elevation information that ICEsat had provided. Unfortunately it is of limited coverage compared to the satellite data, which is why the Zwally et al paper probably did not try and extend their analysis based on it. Its Antarctic coverage is particularly thin. Inevitable given the scale of the continent.

A more reliable replacement for ICEsat is in the works. NASA claims the launch date is 2017. Presumably this assumes that funding for Earth sciences will not be cut from the NASA budget as has been suggested, "because NASA is a SPACE agency, not an EARTH agency."
(sorry about the space-flight geek mode!)

None of the ICEsat problems should imply that there are problems with the Zwally paper. But it does show that ANY estimated measurement of the ice mass balance of the major ice-sheets is at the uncertain cutting edge of scientific measurement.
Our expectations derived from known paleoclimate history would be that the icecaps and glaciers should be expanding as we cool from the rapid melt at the end of the last ice age. The last 7000 years have seen massive accumulations of ice on Greenland and in the polar regions, and in all past ice-age to interstadial cycles that is what is seen. The icecaps grow during the cooling phase from the peak melt, and sea levels fall.

That most of the measured land ice seems to be melting and sea levels are rising at this point in a glacial cycle is exceptional in the paleoclimate record. That the Antarctic is at best showing a slowing gain in ice mass, and given the uncertainty and increasing peripheral loss, may already be loosing mass is a significant sign that the Holocene/Anthropocene is very different to any previous ice-age warm period.

andreas dobbertin said...

I think by now the case is already settled. ESA's far superior CRYOSAT 2 made accurate measurement of Antarctica from January 2011 until January 2014. ESA Data to be found here : http://www.esa.int/Our_Activities/Observing_the_Earth/CryoSat/Ice_sheet_highs_lows_and_loss .
This is consistant with the GRACE measurements, and CRYOSATs SIRAL is the first altimeter to overcome the limitations of the instruments used before.

JCH said...

As I understand this, observations from tidal gauges, satellite altimetry, etc. show a large volume of water has been added to the oceans. Those are the observations. Zwally finds Antarctica is taking .23mm a year out of the oceans versus the accepted .27mm contribution to the oceans other scientists have concluded is being added. Which means Zwally put the sea level budget, which was considered balanced, back out of balance by .50mm per year. So if Zwaly is right, from where did the water come?

0^0 said...

So in principle Zwally et al could have extended their time range by using CRYOSAT 2 data as well? It will be quite interesting to see how this real scientific process pans out.. Unfortunately wuwt crowd will not have a clue and will use this as one of their"science is not settled" arguments..

Everett F Sargent said...

And icesAT-2 ain't going anywhere's if Lamar Smith has any say.

I say frack going to Mars with century old chemical rocket technologies.

Frack even going back to the Moon.

I say we 10X Earth science missions.

Mars and the Moon ain't going anywhere. Heck let China go to Mars. We (USA) did our big science in the 60's.

All I know is that we need some major House cleaning come next fall, otherwise you all need to vote America off this fracking island called Earth.

PL said...

There are three different types of satellite systems used to measure changes in the ice sheets: gravity (GRACE), radar altimeter (ERS-1, ERS-2, Envisat, and Cryosat-2), and laser altimeter (ICESat). They each have strengths and weaknesses. Combining them is hard, so the Shepherd et al. 2012 paper "A reconciled estimate ..." is the paper to pay most attention to. Zwally is an author on that.

The new Zwally et al paper is strange because it combines radar with laser, which is hard to do because the laser sees the absolute top of the snow while the radar penetrates some distance into the snow and firn (compressed snow) layer. It is also odd because there is another radar satellite (Envisat) that continued the ERS-1 and ERS-2 record.

For mass, GRACE is best (Shepherd et al., and Harig and Simons), because it doesn't care if the surface height changes because of snow compaction and other processes; it just cares about the mass changing. The Cryosat-2 record published by McMillan and others is still fairly short, and radar requires a lot of corrections to turn it's heights into ice mass.

MarkR said...

Cryosat-2 is awesome, unfortunately it's a -2 because the first one crashed on launch in 2005 when it would have overlapped with ICEsat.

The Zwally paper raises very good questions though. GRACE shows ice mass loss over the same period, and if I understand it correctly they generally have different sources of uncertainty (snow and firn densification versus underlying geology) so they're independent techniques.

This needs to be resolved. There are many possible combinations but if Zwally is right and Cryosat-2 results are right then that means GRACE is right now but wrong in the past, so something must have changed in the gravity field or gravity field corrections. Alternatively, Zwally and the Cryosat-2 teams could have undetected drifts or different densification calculations and we need more work to get at why.

Robert Way said...

Helm et al (2014) also shows losses from cryosat-2

PL said...

All these satellites are awesome. Even ICESat with its post-launch limitations completely changed what we know about ice sheets, ice shelves and sea ice. Zwally was a major player in making the ICESat program a success.

Like MarkR says, all the satellites have different sources of uncertainty, and working out how they relate to each other is critical (that's what Shepherd et al., 2012 did). In general though, it's safer to stick with a single system (GRACE) than try to combine completely different systems that depend on different corrections, especially when there is no overlap period for calibration. Would have been great to have Cryosat-1 overlap with ICESat, but at least Cryosat-2 overlapped with Envisat.

MikeH said...

“I would pin more weight to the GRACE data than to this latest paper,” Schmidt told VICE News.

https://news.vice.com/article/just-because-antarctica-might-be-gaining-ice-doesnt-mean-climate-change-isnt-happening

KR said...

GRACE data has its own uncertainties, discussed here, so it's not without its own issues - geocenter motion, modeling of Earth oblateness, and glacial isostatic adjustments (GIA), and so on. But the data from GRACE looks awfully clean, and the contradictions between gravity data and this paper cannot be shrugged off.

Keep in mind that Zwally et al is but a single paper with results anomalous to what others have calculated, and we'll have to consider it in reference to other works and after seeing how it holds up to review by the rest of the scientific community. It's all too easy to fall into the pattern of single-study syndrome - where one recent result is trumpeted as overturning all others.

Neven said...

'm obviously not in a position to "pick sides" and say who's right and who's not.

Neither is Anthony Watts, but he is picking sides with Jay Zwally, and then next year during the Arctic melting season he will yet again promote the lie that back in 2007 Zwally had said that Arctic sea ice would be gone by 2012.

PL said...

@KR: Thanks for the link to the IAG book. Yes, all measurements have their uncertainties, which is why group syntheses like Shepherd et al. 2012 are so important. Ultimately the real test will be that the global water budget can be balanced between ice and sea-level rise above expansion from warming, but that will be a while.

Cam said...

Can I ask why you say it's a lie?

This week, after reviewing his own new data, NASA climate scientist Jay Zwally said: "At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions."

http://news.nationalgeographic.com/news/2007/12/071212-AP-arctic-melt.html

Tokodave said...

The Washington Post adds this to the mix:

https://www.washingtonpost.com/news/energy-environment/wp/2015/11/02/scientists-confirm-their-fears-about-west-antarctica-that-its-inherently-unstable/?hpid=hp_hp-more-top-stories_ee-antarctica-425pm%3Ahomepage%2Fstory

PL said...

@Cam: You even cited the important words "At this rate". Zwally was correct in what he said: the loss from 2006 to 2007, *if maintained*,would have led to complete loss by 2012. But Zwally (and Serreze) did not say that the ice *would* be gone. Watts and Co don't like to use exact quotes.

Windchasers said...

Basically, Zwally never said that the current rate would or should continue. It wasn't a prediction, just a "hey, this is kinda interesting" sort of topic.

Cugel said...

That should be Dronning Maud Land, not Droning Maud, for those who are wondering who the hell she was.

Sou said...

Thnx. Fixed spelling.

numerobis said...

From press reports I can't figure out what's new in this article. We heard this last year, that WAIS was going down -- it kind of catapulted Rignot to the limelight as an articulate popularizer (on top of being a good scientist). What new know leg does this article provide?

Rattus Norvegicus said...

They really need to name some part of Antarctica "Dunning-Kruger Land".

Sou said...

It's about a new PNAS paper:

The Antarctic Ice Sheet is losing mass at an accelerating rate, and playing a more important role in terms of global sea-level rise. The Amundsen Sea sector of West Antarctica has most likely been destabilized. Although previous numerical modeling studies examined the short-term future evolution of this region, here we take the next step and simulate the long-term evolution of the whole West Antarctic Ice Sheet. Our results show that if the Amundsen Sea sector is destabilized, then the entire marine ice sheet will discharge into the ocean, causing a global sea-level rise of about 3 m. We thus might be witnessing the beginning of a period of self-sustained ice discharge from West Antarctica that requires long-term global adaptation of coastal protection.

----

There was another paper a couple of weeks ago, in Nature, where scientists modeled Antarctic ice loss under different emissions pathways. From the ABC article about it:

---

The model showed that Antarctic ice would remain "incredibly stable" over 5,000 years at the lowest IPCC emission scenario, but ice shelves would collapse under all the other scenarios.

"In those higher concentration pathways we destabilise those ice shelves," said Dr Fogwill.

Collapse of the ice shelves under such scenarios would lead to Antarctic ice sheets contributing around 40 centimetres rather than 4 centimetres to sea level by 2100, said Dr Fogwill.

But, he said, it would not be until 2300 that the Antarctic ice melt rate would peak. By that stage the not-so-frozen continent would be contributing as much as 3 metres to sea level rise.

---

Joshua said...

Sou -

==> ...So if Antarctica is reducing sea level by 0.23 mm a year, and sea level is rising by at least 3.3 mm a year, then where is the extra water coming from?"

Could you explain that *reducing* part? Did you mean increasing?

Sou said...

No, Joshua. It means what it says. The Zwally paper indicates that ice is accumulating on Antarctica, which if nothing else contributed, would mean that sea level would fall by around 0,23 mm a year.

(As described above, his paper is the odd one out. Other scientists have found that ice mass is falling in Antarctica, which would be contributing to the rise in sea level.)

Millicent said...

The quote our friend provides is an example of how Anthony Watts either struggles to understand the English language or else deliberately misrepresents scientists.

But deniers do this, are corrected, and yet continue to make the same 'mistake' with the exact same material. Its one of those days when the influence of all that fossil fuel money in deniers' trousers is obvious.

So is Cam merely a dupe who has been misinformed by others, or is he himself a liar? Let's see how he reacts to being put straight. Will he apologise?

marke said...

..where is the extra water coming from?:

....a team of Dutch scientists led by hydrologist Yoshihide Wada, a Ph.D. researcher at Utrecht University:

... he and his colleagues have found, groundwater depletion is adding about 0.6 millimeters per year (about one-fortieth of an inch) to the Earth's sea level.

http://news.nationalgeographic.com/news/2012/05/120531-groundwater-depletion-may-accelerate-sea-level-rise/

Nature Geoscience | Letter Model estimates of sea-level change due to anthropogenic impacts on terrestrial water storage.
Yadu N. Pokhrel,

We find that, together, unsustainable groundwater use, artificial reservoir water impoundment, climate-driven changes in terrestrial water storage and the loss of water from closed basins have contributed a sea-level rise of about 0.77 mm yr−1 between 1961 and 2003, about 42% of the observed sea-level rise. We note that, of these components, the unsustainable use of groundwater represents the largest contribution.

http://www.nature.com/ngeo/journal/v5/n6/full/ngeo1476.html

Anonymous said...

Thank you for your link - very interesting and demonstrates how current info being discussed, from the NASA dead satellite, is so misleading.

Sou said...

Marke, that's good, but it's something of a red herring. Groundwater depletion does add to sea level (and these days is taken into account in reconciliing the sea level budget). Estimating the contribution is not straightforward. A review paper in Nature Climate Change published subsequently (still in 2012) discusses this aspect in some detail, and indicates the contribution is more likely 0.40 ± 0.11 mm/year (from 2001-2008). It says in part:

The different estimates of global groundwater depletion produce variable estimates of its current contribution to SLR (34% or 0.57 ± 0.09 mm yr−1 versus 23% or 0.4 ± 0.1 mm yr−1). Direct observations of groundwater depletion continue to be hampered by a dearth of ground-based observations, which not only limits our understanding of localized groundwater storage changes but also our ability to constrain evidence from GRACE satellite observations at larger scales (≥150,000 km2).

For an easier read, there's an article by Scott K. Johnson at Ars Technica - which discusses the different studies and how you also have to allow for dams etc.

The latest IPCC report says:

Model-based estimates of climate-related changes in water storage on land (as snow cover, surface water, soil moisture and ground water) do not show significant long-term contributions to sea level change for recent decades. However, human-induced changes (reservoir impoundment and groundwater depletion) have each contributed at least several tenths of mm yr–1 to sea level change. Reservoir impoundment exceeded groundwater depletion for the majority of the 20th century but the rate of groundwater depletion has increased and now exceeds the rate of impoundment. Their combined net contribution for the 20th century is estimated to be small. {13.3}

Regardless of this, Antarctica is going to add a whole heap more to sea level in the future.

P said...

Surely all this reveals is that snowfall has increased in some areas of Antartica otherwise where does this increase in ice mass come from? Increasing snowfall may just be another symptom of climate change so using this as any sort of argument against the reality of global warming is clutching at straws.

Johnny Vector said...

It's in the big EMI (Electromagnetic Interference) test chamber at Goddard now, so I am inclined to believe the current launch date. (Modulo a couple months. spacecraft are difficult beasts, especially when high power lasers are involved.)

cRR Kampen said...

Noooo!

Joshua said...

Sou -

What's the direct link between ice accumulation and sea level decrease?

Couldn't ice be accumulating (say from increased precipitation due to increased humidity) in one area (say the interior - with no direct impact of reducing sea level) even as ice is melting in others (increasing sea level)? What is the mechanism by which ice mass increases? Above, it says that accumulation is snowfall driven.

Sou said...

The press release I linked to is a bit confusing I suppose because Zwally says there's been a decline in snowfall in recent years..

http://www.sciencedaily.com/releases/2015/10/151030220523.htm

In the paper they say that the increase in mass in much of east Antarctica is from reduced ice flow rather than increased snowfall..

But leaving that aside, to answer your question, if ice is building up in the interior then if no ice melted (and leaving aside groundwater elsewhere) it would lead to a decline in sea level. The snow mainly comes from water in the ocean, not from evaporation of solid water on Antarctica. There's some loss through sublimation (going from solid water to water vapour) but AFAIK most comes from the atmosphere bringing water from the ocean.

So for sea level to rise from changes in Antarctica, you need an overall decrease in ice mass (which is what most other papers show)..

Millicent said...

Its a shock to me that groundwater extraction could be having so large an effect as 0.77mm: so for once I have learnt something from a climate change denier.

But I am surprised that Marke has the time to post here. It is a fairly frequent denier meme that the world possesses an infinite capacity to soak up whatever damage humanity inflicts upon it. Or that God will not allow us to harm his 'perfect' creation. So I'd have thought he would have his hands full correcting his fellow deniers on this point.

chrisd said...

Shockingly, Watts doesn't include his standard "Claim:" in front of the headline on this:

"Ooops! New NASA study: Antarctica isn’t losing ice mass after all!"

"Claim:" has been replaced by "Oops!". Apparently this study is Not To Be Trifled With. Only the ones that yield results he doesn't like get the "Claim:" rubber stamp.

Sou said...

From other papers that looks to be an overestimate. (See my comment above. Still not inconsequential at around 0.4 mm/year.)

numerobis said...

here we take the next step and simulate the long-term evolution of the whole West Antarctic Ice Sheet

Aha! There's the new knowledge (or 'know leg' as my phone preferred). Thanks!

Everett F Sargent said...
This comment has been removed by the author.
Everett F Sargent said...

"Utter nonsense. Scientists know exactly what is causing sea level rise. Groundwater mining."

From one Steve Godtard of Really Sciency which references the 1st cited paper.

The other paper cited above is mentioned at WUWT, as well as the Wada paper, again wrt groundwater.extraction. Also can't forget about silting (e. g. erosion which must have only started recently).

I'm kind of thinking they forgot about recent dam building though. Don't know why.