I believe part of the concern is that this ice shelf is holding back land ice that would cause sea level rise. Quoting Wikipedia on the Larsen C shelf:

"After breaking away from Antarctica, it will leave the remainder of the shelf vulnerable to future collapse. If all the ice that the Larsen C shelf currently holds back were to enter the sea, it is estimated that global waters would rise by 10 cm (3.9 in)."

https://en.wikipedia.org/wiki/Larsen_Ice_Shelf#Larsen_C

Ice shelves act as a "plug" to slow glaciers running into the sea. So the melting of ice shelves causes glaciers to run into the sea and melt faster. Check out the BBC documentary "Frozen Planet", episode 7 goes into detail.

It's a big symptom, not a big cause of additional problems (though, IIRC, breaking a big piece of the glacier off will increase the speed that the remaining glacier -- including the parts upstream still on land -- flows out into the ocean, which will result in sea level rise.)

Also, melting sea ice itself increases absorption of solar energy and accelerates global warming slightly. Its not a huge factor, IIRC, but you never want a positive feedback loop that reinforces and undesirable trend.

The concern is that ice in general is melting, not that this particular instance is going to cause global sea rise.

The Artic (north pole) is the one floating on water. Antartica (south pole) is a continent just like North America, and its melting contributes to sea level rise just as much as a melting glacier does.

Sort of. The dramatism of melting ice is played up quite a bit in popular media.

Sea ice doesn't contribute to sea level rise when it rises; continental ice sheets and grounded ice shelves do. There are three great ice sheets: West Antarctica (~2.2 million km³), Greenland (~2.8 million km³) and East Antarctica (~20 million km³). By contrast, the largest ice shelf, Ross Ice Shelf, is only ~0.1-0.2 million km³. I don't know how much of that is grounded and thus capable of inducing sea level rise on loss. By comparison, the piece that's breaking off is only 1700 km³ or so.

Our knowledge of the exact state of these ice sheets and shelves has a lot of holes in them, but it's clear that the East Antarctic Ice Sheet is gaining in mass, West Antarctic Ice Sheet about stable to slightly negative, and Greenland is shedding at a fast clip of a few hundred km³/year. The ice shelves, particularly in West Antarctica, do seem to generally be in retreat.

At current rates of loss, the melting of the ice sheets provide only a very modest rate of sea level rise--maybe 0.5mm/yr. By comparison, in, say, Norfolk, the isostatic rebound from the loss of the Laurentide Ice Sheet contributes about 1.5mm/yr in sea level rise. The fear is that there could be some kind of tipping point that causes the ice sheets to very suddenly and dramatically collapse (à la Larsen B), but we don't have much evidence to suggest that this is likely to happen.

I think this is somewhat misleading. Antarctica likely is still gaining ice mass overall, for complex reasons, even as the overall stability and health of the ice shelves and sheets weaken.

To boil it down to one key point, west antarctica has grounding lines which slope inward, such that the ice melting is already causing a positive feedback loop as melt accelerates the process of warm water working its way under the ice sheet.

So while no one is predicting an immediate collapse, there absolutely is reason to think we will and already have been crossing tipping points which will cause irreversible, continuing acceleration in melt.

[1] http://news.nationalgeographic.com/2016/04/160412-ice-sheet-...

[2] https://www.youtube.com/watch?v=YQMtb1Pd07E

[3] https://en.wikipedia.org/wiki/File:West_Antarctic_Collapse.o...

The numbers you give are generally supported by the 2013 consensus summary linked above by @mewo2 (http://www.vliz.be/imisdocs/publications/248038.pdf), although that summary pegs the sea level rise due to ice sheets (worldwide) to be more like 1 mm/year (see also their Table 1), within a total sea level rise of 2-3 mm/yr (all sources):

"The consensus picture emerging: whereas Antarctica as a whole is losing mass slowly (assessed to be contributing 0.2 mm/yr sea-level equivalent by IMBIE2), Greenland, the Antarctic Peninsula and parts of West Antarctica are together losing mass at a moderate (~1 mm/yr sea-level equivalent) rate today (~70% of this mass loss is from Greenland) and rates for each are becoming increasingly negative."

*

That summary is not willing to commit to a + or - sign for mass change in East Antarctica specifically, although some individual studies have shown mass growth there.

>Sea ice doesn't contribute to sea level rise when it rises

Isn't there a small but not insignificant sea level rise due to warming of the water due to less sea ice (less light reflected, greater warming of the sea than with an ice layer)?

I think you need to caveat this only slightly.

1) At the moment when the sea ice breaks off there is no rising in overall level due to fact that the ice displaces the same volume of water as is in it.

2) during the melting phase if the sea water around the ice is greater than 4 degrees, the melting will cause a temporary drop in the local temperature of water causing the water level to in fact drop slightly.

3) in the LONG term, the additional volume of liquid water added by the ice will be warmed and expand causing a slight increase in water level due to thermal expansion (assisted by the change in albedo of the whole system as you mentioned)

Yes, it's called a shelf for a reason.

Most of the sea level rise comes from melting land ice.

Water in the form of a solid takes up less spaces than as a liquid so not sure I agree... Plus there is a large part of this that is above sea level currently so that part is not current being displaced.

I agree with Firebrand (^above) that if the ice is already floating (sea ice) and it melts it will cause no other immediate displacement of volume (even if a large part of that floating ice is above sea level and even if solid water takes up less space than liquid water). That's an application of Archimedes principle and can be explained here: http://physics.stackexchange.com/questions/110645/why-does-i.... Indeed it's land ice melting that causes sea level rise. However, sea ice melting still changes the albedo and does directly contribute to climate change.

True, I was thinking that its not truly free floating sea ice and that much of it may be supported by the remaining ice its attached to (hence the crack).

Water in the form of a solid is less dense than as a liquid

Source: Put some ice cubes in water. Observe that they float.

Of course it floats. But that's not what I was trying to say. Go to your cup, put some ice in it and then mark the water level. Now let the ice melt and remark the water level. They will be at the same level.

What happens when you add an ice-cube? That's what happens when ice slides off of continents.

A break-up this huge may not directly alter sea-levels but it's certainly symptomatic of a larger disruption that's playing out on a massive scale.

Water takes up more space as a solid.