Antarctica ice melt gives ~0.25 mm/yr of global sea level rise

[Michaelangelica]

Is the Antarctic ice sheet growing or shrinking?

 

The findings of the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), in 2007, was that the Antarctic Ice Sheet, as a whole, was contributing to sea level rise at a rate 0.2 mm/yr.

 

Ice loss occurred mostly from increased discharge of icebergs by large outlet glacier systems in the Amundsen Sea and Bellingshausen Sea regions of West Antarctica. Loss also occurred by melt along the Antarctic Peninsula, where air temperatures have warmed over the last 50 years.

. . .

. . . new estimates suggest that the total annual loss from Antarctica since 1993 is around 100 Gt/yr (100 billion tonnes of ice per year; equivalent to ~0.25 mm/yr of global sea level rise).

Ice sheets, sea ice and sea level rise: the facts. - Australian Antarctic Division

[TheBigDog]

So the rate moved from 1 meter every 5000 years to 1 meter every 4000 years?

[CraigD]

So the rate moved from 1 meter every 5000 years to 1 meter every 4000 years?

Your arithmetic is flawless, Bill. :)

 

My take on the article is that is describing a revision of the estimated Antarctic net non-floating ice mass loss rate, in term of sea level increase, since 1993, from 0.0002 m/y (1 m/5000 y) to 0.00025 m/y (1 m/4000 y) – that is, not showing that the rate has increased recently, just revising the estimate of what it’s been for the past 17 years

 

The worry, I think, is not that the current rate is very high, but that it might increase. With the Antarctic ice sheet, a concern if that the large, floating ice shelves – the melting of which, as with any floating ice, doesn’t increase sea level – may “uncork” the land-supported glacial ice sheets further inland, allowing them to flow onto the ice shelves and into the sea – which does increase sea level.

 

The dynamics of glaciers are difficult, however, as they depend not only on the blocking action of lower elevation ice, but also on how strongly the ice sheet adheres to the underlying land, which depends on such things as melt water flowing down into the ice sheet, temperature, terrain, etc. Coupled with ancient evidence that in past interglacial peak temperature periods, sea level increases by as much as 6 m/100 y, it’s easy to imagine catastrophe scenarios.

 

The article describes such a scenario as “improbable”, stating that a more acceptable estimate is 0.8 m increase in the next 100 years.

 

This lower estimate makes sense to me, because I imagine ice melt and glacier flow rate depends more on accumulated heat in many mediums – seawater, the ice itself – not just in the air.

 

My opinion is almost entirely due to intuitive guesses and the conclusions of experts, such as contributors to the IPCC. Modeling the atmosphere’s way beyond my skills, modeling glaciers not only beyond my skills, but I suspect beyond the specialists’. Perhaps Moore’s law will improve this situation, allowing very fine-granularity physical simulations to be made without the need for clever insights into the dynamics – but at present, modeling every physical phenomena that can significantly affect future climate appears to me more art than normal science.

[Michaelangelica]

So the rate moved from 1 meter every 5000 years to 1 meter every 4000 years?

Yes a 20% increase on previous estimates, or in total 100 Gt/yr (100 billion tonnes of ice per year).

I won't get my wellies out yet; but do you also want to try to do the sums factoring in Artic ice melt, glacier ice melt, Siberian Ice melt, the rise in sea temperature & mass, and the possible accumulative effects of all this?

 

Most International concern and attention seems to be on N. hemisphere Ice melts. Yet Antarctica seems to be contributing a similar amount of ice to the sea -120 GT/yr =Greenland; 100GT/yr = Antarctica).

What about the Greenland Ice Sheet?

In Greenland the average ice mass loss since 1993 has been about 120 GT/yr (contributing ~0.35 mm/yr to sea level rise). There is evidence that the rate of mass loss may be increasing, with recent values as high as 0.5 mm/yr of sea level rise.

However there can be large variability from year to year in the surface melt in Greenland and the short term changes, from satellite gravity data in particular (which are only available since 2003), may reflect this, rather than a long-term trend.

There has been thickening of the high central ice sheet in Greenland, but this has been more than offset by increased melting near the coast. Flow speed has also increased for some Greenland outlet glaciers.

 

math is after all just another model ,and there are problems with their estimates as they talk about in their pdf document:-

What is happening to ice shelves – and do

they contribute to sea level rise?

A number of floating ice shelves along the

Antarctic Peninsula have disintegrated

dramatically over the last decade. The cause

of their catastrophic collapse is a combination

of melting at the base, which thins and makes

them more vulnerable, and warmer summer

temperatures which cause increased surface

melt that can lead to rapid disintegration. Large

areas of ice shelves (thousands of square

kilometres of ice that is 100 to 200 m thick)

have broken into small pieces and disintegrated

within a few weeks.

The most recent example of this is the Wilkins

Ice Shelf. The Wilkins Ice Shelf has undergone

significant changes since 2008 after two

significant break-up events in February and May

2008 and further losses in June and July 2008.

These changes have been attributed to strong

regional warming, and melting of the ice shelves

from below.

Loss of ice shelves does not contribute to sea

level rise as they are already floating. But where

ice shelves buttress glaciers flowing into the

sea, accelerated glacier flow, due to loss of

the ice shelf, can add to sea level rise. This

is not the case for Wilkins Ice Shelf, but did

occur when the Larsen B Ice Shelf dramatically

collapsed.

What are the gaps in our knowledge that

restrict better estimates of future sea level

rise?

The main gaps are in our understanding are of

some aspects of ice sheet dynamics. There is a

need to improve our mathematical models of ice

streams, ice sheets and ice shelves to be able

to better project future changes. We also need

more detailed measurements of how deep the

bedrock is under the ice sheets, to use in the

models.

Another major gap concerns what is happening

at the bed of the ice sheets – how they react

with liquid water at the base, what role water

may have in sliding processes, and the role of

gravels and slurry at the base. We now know

there is a lot of liquid water under the ice sheets,

but we don’t really know how changes in this

may affect the ice flow.

Schematic showing the relationship between ice sheets, attached to

the continent, ice shelves, attached to the ice sheet but floating in the

ocean, and sea ice, formed when the ocean surface freezes.

MODIS satellite image of Wilkins Ice Shelf break up.

Image: NASA MODIS