Re: Melting icecaps

On Nov 26, 5:53 am, patmpow...@xxxxxxxxx wrote:
On Nov 24, 10:13 am, brad <lbjohnson1...@xxxxxxxxx> wrote:





On Nov 23, 6:55 am, patmpow...@xxxxxxxxx wrote:

Assume that the polar ice caps melt.  (You don't have to believe it,
just humor me, OK?)  Then the weight of the water will be
redistributed over the Earth.  Suppose the oceans rise by two meters.
Now my question is, how will this affect the shape of the Earth?  It
seems to me the bulge in the center would grow and move southward
since
most of the water is in the south.  How much could one expect the
shape of the
Earth to change?

Google GRACE . This question is more complicated
and dynamic than it seems .

Brad

I looked up GRACE and it is clear that redistribution of weight on the
Earth changes the shape of the Earth.  No surprise.  The change has
been measured, but none of these articles tells me how much.  The best
they seem to be able to do is that the change is "significant."    It
is possible to find the data showing the change in the shape of the
Earth due to weight distribution and estimate how much weight was
involved.  It is also possible to calculate how much and where the
weight would move if, say, the oceans increase by one meter.  Then one
can make a rough estimate as to how much the shape would change.  It
wouldn't be terribly accurate but it would be much better than
nothing.

http://www.hokudai.ac.jp/bureau/populi/edition34/littera34e_pdf/litte...

Looking at changes in gravity in North America (Fig.
2), the reduction of mountain glaciers in southern
Alaska as well as continental ice sheets in southern
Greenland caused by global warming is manifest in
the decreased levels of gravity. The gravitational force
decreases by an amount equivalent to the attractive
force of the ice that disappears. Conversely, gravity
is increasing in northern Canada, where the last
glacial period was many thousands of years ago. The
ice sheets that blanketed the nation thawed, and the
earth’s crust that had been beneath the ice sheets
has slowly been upheaving. The gravitational force
increases by the increase in the amount of rock.
If all the ice in Greenland thaws in the future, the same
will happen. Surprisingly, gravity does not change
if ice thaws in the Arctic Ocean, a habitat of polar
bears. This is because the ice simply turns to water,
and the quantity of matter remains unchanged. Once
land-based ice thaws and flows into the sea, however,
terrestrial gravity decreases (in turn, ocean gravity
should also increase, but since the change is spread
thinly over a large area, it is difficult to see). These
fluctuations amount to several microgals annually,
representing a change of around one billionth of the
earth’s gravity.

So icecap melting doesn't matter but other melting does.- Hide quoted text -

- Show quoted text -

When glaciers form on continents their weight depresses
the land beneath them. The continent , in turn , depresses
the mantle underlying it . But , mantle material cannot be
compressed by surface mass . So it responds by flowing
away .

In the case of N America during the last glaciation the entire
continent * tilted * to the north and mantle material responded
by flowing to the south .

As the ice on the continent melted that water flowed off and
into the ocean . The loss of ice mass would have been reflected
as a decrease in the pull of gravity . However , the weight removed
allowed the land to rebound and mantle material flowed back in
under the continent and this is reflected as an increase in gravity .

So ice on the oceans is not a problem in terms of sealevel rise .
The big question is continental ice and when it melts. Considering
that N America is still rising after 13000 years mantle flow cannot
protect us from imminent sealevel changes due to melting glaciers.

In addition , mantle movement from beneath ocean basins to compensate
for continental rebound actually makes those basins slightly deeper .
So the answer is complex and the dynamics of mantle movement
provide hints , that In effect , the planet changes shape every
second
of every day .

Does this help ?

Brad .
.