Nasa Study: Mass Gains of Antarctic Ice Sheet Greater Than Losses (2015) | Hacker News Reader

Nasa Study: Mass Gains of Antarctic Ice Sheet Greater Than Losses (2015)

nasa.gov

52 pointsiamcurious35 comments

A few notes before the denial camp arrives:

- They point out that the Antarctic ice sheet is gaining in mass due to the melting/compaction of snow from the past 10,000 years, not because of anything having to do with global temperatures.

- Ice losses to calving (ice breaking off and falling/drifting into the ocean and ultimately melting) are still massive, increasing, and "will catch up with the long-term gain in East Antarctica in 20 or 30 years -- I don’t think there will be enough snowfall increase to offset these losses.”

- Also, this article is from 2015. Here's a more recent analysis from 2018: https://www.nasa.gov/press-release/ramp-up-in-antarctic-ice-.... You can even see the East Antarctica trend that the 2015 article talks about in this plot from the 2018 article: https://www.nasa.gov/sites/default/files/thumbnails/image/su...

Can you please elaborate for a non-scientist?

>ice sheet is gaining in mass due to the melting/compaction of snow

How does melting/compaction affect the mass?

That really isn't how mass works.

I assume it means snow melts, turns into water, then freezes again turning into ice thereby increasing the mass of the ice sheet.

the density increases not the mass. the concentration of mass is increased density. it is further evidence of melting.

It reads to me like there's a distinction between ice and snow. Like the snow isn't considered part of the ice or isn't able to be measured as part of the ice until some compaction has happened, and the "snow" layer might be hundreds of years worth of snow. That's the only way I can resolve the definitions with the statements, and I'm hoping someone can more authoritatively describe the paradox because I'm feeling unsatisfied.

there is also a distincion between the coastal ice sheets and the continental interior ice and snow cap. the apparent paradox is caused by misrepresentative interpretation of a report, by the author of the article.

the coastal ice shelf and continental interior are two distinct physical systems.

isostatic rebound is a confound to the measurement of snow/ice accumulation when change of altitude is corelated to snow deposition.

http://rses.anu.edu.au/geodynamics/tregoning/7.pdf [PDF]

https://www.worldatlas.com/articles/what-is-post-glacial-reb...

BTW Re: " and I'm hoping someone can more authoritatively describe the paradox because I'm feeling unsatisfied."

you wont get anymore of an authoritative take on it than discussing it with a scientist, as you are currently doing right now.

You have a layer of ice covered with a layer of snow. Each year, that snow melts a little and then re-freezes on top of the ice. Then, more snow falls on top of that snow. This happens year over year for a long time. Each layer of snow puts more and more pressure on the previous layers of snow and ice, compacting them. Eventually, these layers of snow become so compacted that they form new layers of ice and fuse into the existing layers of ice. Does that make sense?

It doesn't. The only way an ice sheet gains mass under any normal circumstance is new snow falls on top. As more and more snow accumulates over time, the ice sheet grows thicker.

What offsets that is that ice flows under pressure, so the entire sheet has a tendency to head for the ocean. Things are a bit complicated in Antarctica since some of the landscape where the sheet is grounded is below sea level. There are concerns that the seawater is getting warm enough to cause melting there, increasing the flow rate.

If, in fact, ice sheet mass is increasing, that's great in terms of sea level rise. There's no other way to spin it.

I notice in your second link that West Antarctica is the biggest contributor to rising global sea level.

Yet according to JPL, the main cause of this is believed to be volcanic activity under the western side of the continent. [0]

The best quote from the article was this: "I didn't see how we could have that amount of heat and still have ice on top of it."

[0] - https://www.jpl.nasa.gov/news/news.php?feature=6996

I was under the impression that the biggest contributor is actually just the rising temperature of the water, because water expands as it heats.

While there is probably a slight amount of expansion in liquid water from heat, water expands most when frozen solid. this is the entire reason ice floats

Half of all recent sea level rises have been from thermal expansion of water: https://www.nationalgeographic.com/environment/global-warmin...

The average depth of the ocean is about 4km, so a thermal expansion of 1x10^(-4) per degree C (right order of magnitude), would result in about a 4m rise per degree increase in average ocean temperature. Which is obviously significant.

One of the issues with this theory is that the average temperature of the ocean is below 4 deg C, and in this regime the coefficient of expansion is negative. So we might actually see a small sea level drop due to ocean warming.

You might be right and you certainly have a point regarding the "deniers". But your first bullet point is blatantly wrong. Melting and compaction cannot increase mass. You might have simply made a mistake here, but at first glance it seems like you throw any scientific accuracy overboard just to prove your point that the sea level will raise. That way you weaken your argument of "scientific people against deniers" considerably.

The idea is that the ice mass is increasing, because the millenia-old snow cover over the ice is melting and compacting into the ice sheet itself.

Took me a few reads to reconcile that myself.

these are the caveats of data presentation that consume an incredible number of hours compared to actually designing an experimental protocol and generating data sets.

this is one of the major activites during thesis defense, tearing apart semantics to be sure that verbal descriptors are representative of observations, and free of qualifiers or modulations.

Also defense of scientific thesis involves a discourse between supporters and deniers as a matter of course. This is the way science works, two opposing positions meet somewhere in between. usually a third external will weaken or reinforce one side over the other. The ultimate goal is not to proclaim who is right or wrong but to refine interpretations into representations of real physical processes

I must be tired and reading this incorrectly.

1992-2001: +112 b tons / year 2003-2008: +82 b tons / year Prior to 2012: -84 b tons / year 2012-2018: -241 b tons / year

It flipped from positive to negative somewhere between 2008 and 2012, correct?

2015 article: the Antarctic ice sheet showed a net gain of 112 billion tons of ice a year from 1992 to 2001. That net gain slowed to 82 billion tons of ice per year between 2003 and 2008.

2018 article: Prior to 2012, ice was lost at a steady rate of about 83.8 billion tons (76 billion metric tons) per year... Since 2012, the amount of ice loss per year has tripled to 241.4 billion tons (219 billion metric tonnes)

This is a several-years old study that has been controversial. Other results using independent observations from mass-observing missions have come to very different conclusions [1] [2].

One thing people should understand is that the method used in the linked article is altimetry from radar, so you get a very accurate surface height, but the height is not a direct indicator of ice mass because of compression of snow/ice, and rebound of the underlying continent.

It's really important to have some appreciation of the limitations of measurement techniques, before jumping to conclusions from a single study.

[1] https://grace.jpl.nasa.gov/resources/31/antarctic-ice-loss-2...

[2] https://news.ycombinator.com/item?id=19683895

Some clarification on Antarctica:

There are roughly three regions in Antarctica. The Transantarctic Mountains divide Antarctica into a really big East Antarctica and a smaller West Antarctica. The two large ice sheets in Antarctica (Ross and Ronne) are in West Antarctica. The tip edge of West Antarctica is the Antarctic Peninsula.

Everyone agrees that West Antarctica is losing ice. This is especially true of the Antarctic Peninsula, which is basically under going complete collapse (e.g., the Larsen Ice Shelf). It's unclear if the West Antarctic Ice Sheet is merely declining or undergoing a more catastrophic collapse. The complete collapse of this ice sheet is something that is included in the worst-case scenarios for sea level rise.

East Antarctica is more difficult. The East Antarctic Ice Sheet contains far more mass than West Antarctica--about 90% of the Antarctic Ice Sheet in total is here. This ice sheet is roughly in balance, although the succession of studies I've seen have ping-ponged between growing, stable, or declining, which means you can find a study to support your viewpoint, whatever it may be. Due to the sheer size of the ice sheet compared to everything else, small uncertainties in the mass balance here basically dictates whether or not you think Antarctica as a whole is gaining or losing ice mass.

I thought this was well understood, and even expected by the models, since a warming planet brings more precipitation in the air which then falls as snow on the poles.

So the Antarctic is actually gaining mass on the higher ground, and the Arctic during the winter months also.

The contrast is that during the warmer summer months, the Arctic is losing a lot more than it gains over the year, so the global balance is negative - which adds to sea level rises.

The other odd thing I never considered, but was (I think here?) mentioned, is that as the mass of the Arctic decreases, it also presses down less on the mantle, and so lifts higher out of the ocean, also displacing more sea.

I'm pretty sure there's a good explanation of this by Potholer54 on Youtube.

its called isostatic rebound

Loss of ice in the Arctic doesn't contribute to sea level rise because the ice in the Arctic floats, and melting an ice cube floating in water does not change the water level.

(Ice on Greenland is a different matter, but Greenland is a lot smaller than Antarctica.)

this is correct however its not all sea ice. permafrost is melting and running off into the sea. the permafrost remnants, mostly silt subside but the lithostrat[bedrock] rebounds like a slow trampoline. this causes rate of permafrost melt and effluence to increase.

Mass is not density.

"They point out that the Antarctic ice sheet is gaining in mass due to the melting/compaction of snow from the past 10,000 years"

  - i just copy pasta this from other posters comment for dialectic,
 this statement ^ indicates there is a change in density not mass.

If you watch a snow bank melt you will see an increased density, as individual snow flakes [crystals] melt and lose air pockets making contiguous ice mass.

that means density increases.

Its still melting, Everybody!

I interpreted that as meaning that the ice sheet is only defined as that water which is in an icy state, not the snow that sits on top of it. If snow is melting, refreezing, and becoming part of the ice sheet (not sitting above), then the mass of the ice sheet is increasing.

yes the mass of ice sheet is increasing, on the continental interior.

this should not however be interpreted as a change in mass of coastal icesheet. these are two different systems. separated by a mountain range.

Dunno what this means overall, just another reminder that this stuff is complicated.

If climate change in Antarctica causes more snow/percipitation, ice will accumulate there, even if average temperatures increase.

If I understand correctly, this isn't something new. It isn't clear if the trend is accelerating or reversing. Ice always accumulates in Antarctica, because whatever snow falls in the interior just sits there.

> Ice always accumulates in Antarctica, because whatever snow falls in the interior just sits there.

Not exactly. In most cases, the ice accumulates into glaciers and slowly flows towards the sea.

As you can see from this relief map: https://commons.wikimedia.org/wiki/Category:Topographic_maps... , Antarctica's interior is very mountinous and consistently a higher elevation than it's edges. Gravity forces the compacted ice downhill. As the ice on the mountains moves, so does all of the ice in it's way.

Does most of the continent "drain?" My understanding was that most/much of the ice is not glacial.. which I understood as not moving.

> My understanding was that most/much of the ice is not glacial.. which I understood as not moving.

I haven't heard that. Could you link where you found that information?

Newer 2018 article: https://www.nasa.gov/press-release/ramp-up-in-antarctic-ice-...

Thank you for this link. The accompanying Nature article is [1].

As I mentioned elsewhere in this thread, the study linked in the OP has been in conflict with other measurements. The article you reference is one attempt to synthesize the three measurements available (altimetry from SAR, surface flow rate from imagery, and mass change from gravimetric observations) into a complete picture.

[1] https://www.nature.com/articles/s41586-018-0179-y.epdf?autho...

  Im looking at the terms and the grammer being used,

we have to put our bullshit detector hats on for this. its a controversial and political subject, thus vulnerable to spin.

the considerations required for a realistic view are not impossible, sophisticated, yes, but humanly understandable.

there are phrases and terms [ie ICE accumulation] that mask the real description of the situation, and an appropriate model is not expounded upon.

ICE != snow , mass != density

ice accumulation != snow accumulation vice.versa.

The previous HN discussion on this article:

https://news.ycombinator.com/item?id=10487680

Reply on news.ycombinator.com