Re: [asa] Subglacial Water System Moving Faster Than Previously Thought

The seems to be some confusion as to what was reported in AR4 versus
TAR as it pertains to sea level rise due to melting ice.

As I understand it, TAR's estimate included ice dynamics uncertainty
in their range of estimates, while AR4 removed them because of too
much uncertainty and the potential to be much larger than originally
estimated.

From the AR4 SPM

<quote>
Models used to date do not include uncertainties in climate-carbon
cycle feedback nor do they include the full effects of changes in ice
sheet flow, because a basis in published literature is lacking. The
projections include a contribution due to increased ice flow from
Greenland and Antarctica at the rates observed for 1993-2003, but
these flow rates could increase or decrease in the future. For
example, if this contribution were to grow linearly with global
average temperature change, the upper ranges of sea level rise for
SRES scenarios shown in Table SPM-2 would increase by 0.1 m to 0.2 m.
Larger values cannot be excluded, but understanding of these effects
is too limited to assess their likelihood or provide a best estimate
or an upper bound for sea level rise.
</quote>

When looking at the effects of warming on sea level, there are various
relevant components. The first one is expansion because of higher
temperatures, then there is the increase in melt of sea-ice and the
increase in melt of ice-sheets

<quote>
Sea ice shrinks in both the arctic and antarctic under all scenarios,
the reduction is amplified in the arctic where some models predict the
sea ice to disappear by the latter part of the 21 century.

...

Models suggest that a global average warming of 3 C above present
levels would cause widespread mass loss of the greenland ice sheet if
sustained for several centuries, initially contributing up to 0.4 m
sea level rise per century.
The melting rate would increase if dynamical processes increase the
rate of ice flow, as suggested by some recent scenarios.
</quote>

In other words, the largely uncertain effects of dynamical processes
could add significant increases.

<quote>
The antarctic ice sheet is projected to behave differently from
Greenland because it is too cold for widespread surface melting. It is
expected to gain ice through increase snow fall in the 21 century,
acting to reduce global sea level by 0.1m /century. However, in
response to weakening of ice shelves by ocean warming or surface
melting at the margins, ice flow could accelerate. Such effects could
offset our outweigh increased snowfall but ae uncertain.
</quote>

The antarctic in AR4 is described to contribute negatively to sea
level increases due to increases in precipitation. However at the
margins, ice flow could accelerate but such effects are still too
uncertain.

However, Arctic sea ice may very well disappear sooner than later
(http://www.realclimate.org/index.php/archives/2007/01/arctic-sea-ice-decline-in-the-21st-century/)

From AR4
During the last interglacial period, about 125,000 years ago, it is
likely that large scale retreat of the Greenland ice sheet and other
Arctic ice fields contributed between 2 and 3.5 meters to a sea level
rise above current levels.

The issues surrounding sea level changes related to ice melt are quite
interesting and far more complex than initially estimated. However,
even excluding the dynamic effects, the sea level effects in AR4 are
similar to TAR with a potential for much larger changes than
originally expected.

On 2/16/07, Janice Matchett <janmatch@earthlink.net> wrote:
>
> At 10:14 AM 2/16/2007, Rich Blinne wrote:
>

> "This is nice to know, but it has less importance due to the IPCC report
> downgrading the likeliness of Antarctic ice melting. The current IPCC report
> (only the executive report (PDF) has been released) notes that Antarctica
> ice loss should be minimal until after 2100 because the ice is too cold and
> the models predict increased precipitation in Antarctica. Due to this (and
> the Greenland ice loss downgrade) most models now only predict a 20 to 50 cm
> rise in ocean levels due to the seawater heating up and glaciers melting. Al
> Gore is going to feel sad since his 20 ft sea level rise now looks
> incredibly unlikely even by the UN. The IPCC report does note that dynamic
> ice loss events could change this forecast but since they haven't
> demonstrated any likely candidates yet (perhaps they are in the full report)
> I would have to assume that they are classifying those types of events in
> the "what if" category (like what if someone decided to nuke Antarctica 1000
> times).
>

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Received on Sun, 18 Feb 2007 10:27:24 -0800