Don't Icebergs Displace Their Own Mass? [Answered: YES]

[jimmydasaint]

If this has been touched on before, I apologise. However, I have a dumb question to ask. If global warming continues, so the story goes, low lying nations will be swamped by the rising sea levels etc...

 

However I seem to remember an examination question many years back where an ice cube was floating in a glass of water full to the brim and the students were asked to calculate the amount of water that would accumulate under the glass if the ice melted. The answer was that the glass would stay exactly as before because the ice had already displaced its own mass.

 

The question - doesn't this happen on a global scale if large parts of the ice cap melt? Why are we told that there will be a significant increase in the levels of the Oceans etc...? I would appreciate any light shed on this matter.

[iNow]

So, yes. While ice is slightly less dense than water, icebergs and polar ice still displaces water from the ocean. The part you are forgetting, however, is just how much of the polar ice is actually on top of land... it's not all just sitting in ocean waters. So, when that ice from on top of the land melts, it adds to the total volume of liquid water in the ocean, and hence coastal cities go under water the way of Atlantis as sea levels rise.

 

There's also that small bit of ice at the top of icebergs (roughly 10%) which is above the ocean... as that melts, it adds to the volume, and it was not displacing anything whatsoever when it was above water.

[SkepticLance]

iNow

 

You forget that water expands as it freezes. That makes ice less dense which is the reason it floats. An iceberg including the bit on top, displaces its own weight in water, and when it melts it occupies less space, and there is not change in sea level.

 

Sea level rise can come from ice on land melting, as you said. Not from sea ice melting. To date, there is very little sea level rise, if any, from ice on the big Arctic and Antarctic land ice masses, since warming increases precipitation, meaning more snow falling inland, and increasing the total mass of ice on Greenland and Antarctica.

 

However, there has been land ice melt from mountain glaciers, such as the Himalayas and Andes, which have added to net sea level, albeit to a small amount.

[iNow]

iNow

 

You forget that water expands as it freezes. That makes ice less dense which is the reason it floats.

 

Really? That's interesting. Why did I say this then?

 

 

So, yes. While ice is slightly less dense than water, icebergs and polar ice still displaces water from the ocean.

 

 

 

 

 

Since I know how quickly exchanges between you and me get silly and annoying, Lance, I'm going to post the information below so I can be done with this thread and walk away from you.

 

 

http://en.wikipedia.org/wiki/Sea_level_rise

 

The Sea level has been rising at a rate of around 1.8 mm per year for the past century, mainly as a result of human-induced global warming. This rate is increasing; measurements from the period 1993–2000 indicated a mean rate of 3.1 mm/year. Global warming will continue to increase sea level over at least the coming century. The contribution from thermal expansion is well understood; substantial changes to the rate and magnitude of increase are largely dependent on how rapidly ice caps disintegrate with increasing temperatures. The thermal expansion of sea water is currently the dominant contributor to sea level rise, and to the predicted rise over the next century, which is 90 to 880 mm (with a central value of 480 mm). Only if glacial melt substantially increases will it become the larger term. Ice can have a huge effect; the melting of the ice caps during the last ice age resulted in a 120 meters rise in sea level.

 

 

Longer term changes

 

Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice. Over much longer geological timescales, changes in the shape of the oceanic basins and in land/sea distribution will affect sea level.

 

Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.

 

 

Glaciers and ice caps

Each year about 8 mm (0.3 inch) of water from the entire surface of the oceans falls into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm every year. To a first approximation, the same amount of water appeared to return to the ocean in icebergs and from ice melting at the edges. Scientists previously had estimated which is greater, ice going in or coming out, called the mass balance, important because it causes changes in global sea level. High-precision gravimetry from satellites in low-noise flight has since determined Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement.

 

Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. Likewise, the melting of the northern polar ice cap which is composed of floating pack ice would not significantly contribute to rising sea levels. Because they are fresh, however, their melting would cause a very small increase in sea levels, so small that it is generally neglected. It can however be argued that if ice shelves melt it is a precursor to the melting of ice sheets on Greenland and Antarctica.

 

• Scientists previously lacked knowledge of changes in terrestrial storage of water. Surveying of water retention by soil absorption and by reservoirs outright ("impoundment") at just under the volume of Lake Superior agreed with a dam-building peak in the 1930s-1970s timespan. Such impoundment masked tens of millimeters of sea level rise in that span. (Impact of Artificial Reservoir Water Impoundment on Global Sea Level B. F. Chao,* Y. H. Wu, Y. S. Li).
  
  
• If small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m. Melting of the Greenland ice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise. The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5-6 m.
  
  
• The snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about 5,500 metres above sea-level at the equator down to sea level at about 70° N&S latitude, depending on regional temperature amelioration effects. Permafrost then appears at sea level and extends deeper below sea level polewards.
  
  
• As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of the permafrost zone, they cannot melt in a timeframe much less than several millennia; therefore it is likely that they will not, through melting, contribute significantly to sea level rise in the coming century. They can, however, do so through acceleration in flow and enhanced iceberg calving.
  
  
• Climate changes during the 20th century are estimated from modelling studies to have led to contributions of between –0.2 and 0.0 mm/yr from Antarctica (the results of increasing precipitation) and 0.0 to 0.1 mm/yr from Greenland (from changes in both precipitation and runoff).
  
  
• Estimates suggest that Greenland and Antarctica have contributed 0.0 to 0.5 mm/yr over the 20th century as a result of long-term adjustment to the end of the last ice age.
  
  

The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above, but active research continues in this field. The terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.

 

Since 1992 a number of satellites have been recording the change in sea level; they display an acceleration in the rate of sea level change, but they have not been operating for long enough to work out whether this is a real signal, or just an artefact of short-term variation.

 

Edited November 17, 2008 by iNow
multiple post merged

[SkepticLance]

iNow

 

I was being very nice in pointing out that a piece of scientific nonsense that you posted ...

 

"There's also that small bit of ice at the top of icebergs (roughly 10%) which is above the ocean... as that melts, it adds to the volume, and it was not displacing anything whatsoever when it was above water."

 

...was in error. And it was a mistake. The little bit at the top of an iceberg is part of the total weight, which is equal to the weight of water displaced. Thus, the melting of an iceberg does not add to sea levels. You implied that it would raise sea level, which it will not.

 

Sea level increase is about 3 mm per year as global average. Most of that is thermal expansion, and most of the rest is melt from mountain glaciers. Anything else is trivial.

[iNow]

Thanks for the correction on the bit about the top 10% which was out of water. I was thinking that the object had to be within the water to displace it, forgetting about weight. Hence, I thought the "tip of the iceburg" displaced air, not water.

 

With that said, your point about triviality seems pretty thoroughly debunked in the post I made immediately prior to yours. Cheers.

[jimmydasaint]

Thank you for the reply guys. I just got thinking again and wondered where the ice on the land came from in the first place. Is it from glaciers melting and re-freezing? Or from mountain waters freezing up? Also, I wonder if it possible that vegetation would also play a role in absorption of water? Just trivial points though - the Wiki article about a 120m rise in sea levels is a pretty frightening scenario. Lance suggests a negative feedback mechanism where snow over the melting poles would reflect more radiation off the ice masses causing a theoretical drop in temperature. Interesting contributions guys.

[insane_alien]

the ice on land got there because of precipitation. if the snow and ice doesn't melt it will stay there and keep building up as more snow and ice falls or gets blown along. given enough time you have an ice sheet a few kilometers thick

[SkepticLance]

To jimmy

 

I didn't actually suggest a drop in temperature. Increased snow precipitation will not increase albedo unless there is an increase in snow covered area, which is not the case here. However, a warming planet means more moisture in the air, which leads to more precipitation as snow over much of Greenland and Antarctica, which to some extent may slow down the increase in sea levels. Eventually, every drop of snow that lands on Greenland makes its way to the sea, but this may take thousands or even millions of years.

 

To iNow

 

who believes that loss of snow from Greenland is causing major sea level rise.

 

You quoted:

"High-precision gravimetry from satellites in low-noise flight has since determined Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement."

 

This statement appears impressive, but is, in fact, deceptive.

 

The oceans of the world contain E18 tonnes of seawater. That is a million times a trillion tonnes. The average depth is just under 4 kms. A simple calculation tells us that a million tonnes of water off Greenland adds one part per trillion to the world's oceans, or 0.004 mm in depth. Since the sea level rise is over 3 mm per year, this addition is trivial.

[iNow]

To iNow

 

who believes that loss of snow from Greenland is causing major sea level rise.

 

You quoted:

"High-precision gravimetry from satellites in low-noise flight has since determined Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement."

 

This statement appears impressive, but is, in fact, deceptive.

 

The oceans of the world contain E18 tonnes of seawater. That is a million times a trillion tonnes. The average depth is just under 4 kms. A simple calculation tells us that a million tonnes of water off Greenland adds one part per trillion to the world's oceans, or 0.004 mm in depth. Since the sea level rise is over 3 mm per year, this addition is trivial.

 

Total volume of the sea is an irrelevant point in this context, so your 4km deep number is an attempt to displace the topic of conversation here. Since we are talking about rise in sea levels, the delta between previous sea levels and new sea levels as a result of ice melt is what actually matters.

 

 

I grant you that, as a percentage of the total volume of the sea, that the ice melt is (how'd you say it?)... trivial. No argument there. What your assertion misses, however, is the impact of even slight raises from one sea level to a higher sea level, and the impact such rise has on coastal and wetland regions.

 

Basically, if you try to frame the argument in terms of total oceanic volume instead of rise from previous sea level to a new higher sea level then you are being disingenous to the topic of the conversation.

 

 

Finally, my post had zero to do with "belief," and everything to do with data, but I don't know why I feel the need to correct so many of your posts and get sucked into your nonsense. I really don't...

[SkepticLance]

iNow

I just showed you the data. You said originally that land ice melt from Greenland made a significant contribution to rising sea level. I demonstrated that the addition was trivial. One million tonnes of Greenland ice melting equals 0.004 mm sea level rise world wide. The data is good.

 

The impact of even slight rises in sea level? Well, against a background of 3 mm per year, the addition of 0.004 mm is not exactly something to run scared of....

 

I know you hate the fact that I am too sceptical to swallow all the catastrophist propaganda about global warming. However, you should not, even by implication, accuse me of ignoring data or using bad data as a result of that prejudice. I have enormous respect for good data and make use of it to form my own belief system.

[iNow]

iNow

I just showed you the data. You said originally that land ice melt from Greenland made a significant contribution to rising sea level. I demonstrated that the addition was trivial.

Right, when viewed in terms of total oceanic volume, which is not appropriate in this context.

 

 

Now, please stop speculating about my motivations and my feelings about you and stay focussed on the data. I would be much obliged if you were to do so.

[SkepticLance]

iNow

 

Are you agreeing or denying my point about Greenland ice melt being a trivial contributor to sea level rise?

 

I used the tonnage (not volume) of the ocean as the basis of my calculation, since that was the data I had handy. The calculation is valid. Do you need me to explain it?

[Sisyphus]

The total ocean volume doesn't matter. Sea level rise is approximately volume of melted land ice divided by total ocean surface area. I say approximately because the more the ocean rises, the more surface area it has, because more land is underwater. When you're talking about millimeter rises that is probably insignificant, though.

[iNow]

Repeated from the data shared in post #4.

 

 

If small glaciers and polar ice caps on the margins of Greenland and the Antarctic Peninsula melt, the projected rise in sea level will be around 0.5 m. Melting of the Greenland ice sheet would produce 7.2 m of sea-level rise, and melting of the Antarctic ice sheet would produce 61.1 m of sea level rise. The collapse of the grounded interior reservoir of the West Antarctic Ice Sheet would raise sea level by 5-6 m.

 

"Trivial" would not be my first choice of adjectives to describe this change, but you are welcome to call it what you wish, Lance. Also, that graphic measured in centimeters didn't strike me as "trivial" either, nor does the fact that the rate of increase is accelerating.

[SkepticLance]

iNow

You have begun talking about something else entirely. As soon as you introduce the word "if" you are in a realm of speculation. I was discussing today's reality - not some hypothetical possibility in the future.

[iNow]

iNow

You have begun talking about something else entirely. As soon as you introduce the word "if" you are in a realm of speculation. I was discussing today's reality - not some hypothetical possibility in the future.

 

Righto, then. I'm about done here, now.

 

 

 

 

Since I know how quickly exchanges between you and me get silly and annoying, Lance, I'm going to post the information below so I can be done with this thread and walk away from you.

 

The Sea level has been rising at a rate of around 1.8 mm per year for the past century, mainly as a result of human-induced global warming. This rate is increasing; measurements from the period 1993–2000 indicated a mean rate of 3.1 mm/year. Global warming will continue to increase sea level over at least the coming century.

 

 

Jimmy - To answer your original question, it's because there is so much ice over land which melts and flows into the sea. Cheers.

[swansont]

To iNow

 

who believes that loss of snow from Greenland is causing major sea level rise.

 

You quoted:

"High-precision gravimetry from satellites in low-noise flight has since determined Greenland is losing millions of tons per year, in accordance with loss estimates from ground measurement."

 

This statement appears impressive, but is, in fact, deceptive.

 

The oceans of the world contain E18 tonnes of seawater. That is a million times a trillion tonnes. The average depth is just under 4 kms. A simple calculation tells us that a million tonnes of water off Greenland adds one part per trillion to the world's oceans, or 0.004 mm in depth. Since the sea level rise is over 3 mm per year, this addition is trivial.

 

 

iNow's number is deceptive, but for a different reason — it's wrong. (well not from a technical standpoint, but from practical one)

 

The annual mass loss is around 200 billion tons (200 km^3 of ice is ~2e14 kg)

 

http://www.sciencedaily.com/releases/2008/09/080930081355.htm

 

The conclusion that Greenland's contribution is trivial should have alerted, well, everyone, that something was amiss. Its contribution is currently about a half-millimeter per year.

[SkepticLance]

Swansont's reference suggests Greenland contribution to sea level rise is 0.5 mm per year. If correct, that is still only one sixth of the total. So my statement that the bulk of the sea level rise is caused by thermal expansion and loss of water from mountains still stands.

[iNow]

This is not in response to SkepticLance, just another piece of data for everyones perusal.

 

 

http://www.agu.org/pubs/crossref/2008/2008GL035725.shtml

 

Human influence on Arctic sea ice detectable from early 1990s onwards

Human influence has previously been identified in the observed loss of Arctic sea ice, but this hypothesis has not yet been tested with a formal optimal detection approach. By comparing observed and multi-model simulated changes in Arctic sea ice extent during 1953–2006 using an optimal fingerprinting method, we find that the anthropogenic signal first emerged in the early 1990s, indicating that human influence could have been detected even prior to the recent dramatic sea ice decline. The anthropogenic signal is also detectable for individual months from May to December, suggesting that human influence, strongest in late summer, now also extends into colder seasons.

[insane_alien]

Swansont's reference suggests Greenland contribution to sea level rise is 0.5 mm per year. If correct, that is still only one sixth of the total. So my statement that the bulk of the sea level rise is caused by thermal expansion and loss of water from mountains still stands.

 

so your saying one 6th comes from greenland and that means that the bulk must be thermal expansion? aren't you forgetting about antartica which is significantly bigger than greenland and also dumping tonnes of water into the ocean.

 

its not just the greenland ice sheets that are melting you know.

[pioneer]

One of the strange effects is the south pole is average cooling yet the glaciers are retreating. One possible way to explain this is the earth under the ice is warming, geologically, and the air above is cooling due to climate. Liquid water is denser so melt water would stay under the ice and mix with the underground water. The ice lowers over time, under its own weight. Thick ice may create an igloo affect.

 

Today, Antarctica seems to be part of a single tectonic plate. But there is clear palaeomagnetic evidence that, in the past, there has been a large relative rotation between different parts of the continent1.

From Nature.

[iNow]

One of the strange effects is the south pole is average cooling yet the glaciers are retreating. One possible way to explain this is the earth under the ice is warming, geologically, and the air above is cooling due to climate.

 

That is one possible way, but not the most likely. It could be a factor in the change, but the most likely explanation has more to do with how weather patterns around other parts of the globe are shifting, but I suppose you're free to make up just about anything up that you want. Facts and reality have never slowed you down before.

[SkepticLance]

There is a lot of nonsense written about Antarctica and global warming. We tend to get those with political agendas talking about East and West Antarctica, as if the continent was divided in two. This is a deliberate attempt to mislead.

 

Antarctica, in the vast bulk of its continental mass, is cooling. The cooling is trivial, but still cooling. This means more ice, rather than less on its continental land mass. Again, the amounts are relatively trivial.

 

However, the Antarctic Peninsular, which is a tiny part of the continent as a percentage, is warming. Sea ice around the peninsular is melting rapidly, and a small amount of the ice on the land portion is also melting.

 

The peninsular is contributing to sea level rise, though to a limited extent since it is a relatively small area of land. The main continent is contributing to the reverse, though also to a minor extent, since the cooling and precipitation both are small.

 

Overall, any contribution Antarctica makes to sea level rise is small or non existent.

[swansont]

References and numbers would be nice. Especially since the previous conjecture was off by 5 orders of magnitude, and nobody else noticed.