MadSci Network: Astronomy

Re: Is the annual buildup of multi-million tons of ice at both poles sufficient

Date: Sun May 31 19:04:45 1998
Posted By: Nick Hoffman, Physics
Area of science: Astronomy
ID: 896351700.As

Whoa, Craig,

That's quite a handful for one question, so let's take it piece by piece...

1)    Is ice building up at the poles relentlessly, year by year?

The North Pole is floating sea ice, with a cover of snow. It drifts on sea
currents and after a few decades, drifts away from the pole, breaks up and
melts. At the South Pole, there is a permanent ice sheet.....

Sure, it snows every year at the South pole, and never melts, but that snow
doesn't just pack down to ice and sit there. What happens is that once the ice
is thick enough, it starts to slowly flow under the pressure of its own weight,
like a huge glacier. The ice spreads out in all directions like crazy putty, and
flows away from the poles. When these glaciers and ice sheets reach the edge of
the ice shelf, they calve off as icebergs, especially in the summer. In fact,
most people monitoring this situation will tell you that the ice caps are
shrinking due to global warming, not growing.

2)    Even if the ice built up, could it "topple" the Earth's spin?

The Earth is not round, well not quite. Because the rock making up the interior
of the Earth is hot, it is soft (on a geological time scale, anyway, on human
timescales it is still rigid - Earthquake waves are transmitted as fast as
through steel). As the Earth spins on its axis every 24 hours, there is a weak
centrifugal force {Yes, folks, we can argue till we're blue in the face about
the reality of centrifugal forces, but they're easy to understand, and seem

This centrifugal force causes the Earth's equator to bulge out just a little
bit, until the force of gravity acting on the bulge cancels out the centrifugal
force. For the Earth, the equatorial radius is 1+(1/298) times the polar radius.
Given a mean radius of 6371 km, this means that the equator is 21 km higher than
the poles. For toppling to occur, the ice would need to build up at the poles
until it compensated for this height difference.

Now, we wouldn't just need 21 km of ice sat on top of the poles. Ice has density
1, while the solid body of the earth has a density of 3, so over 60 km of ice
would need to build up before the Earth was sufficiently imbalanced to "topple"
on its axis.

Don't forget that the crust can't sustain loads like this without subsiding. The
Antartic ice sheet only rises 3 to 4 km above sea level, but its base has
foundered and subsided to about 2km below sea level due to the weight of ice. To
build up a stack 60 km high, we would need another 30 km of subsidence. Thus we
would need 20 times as much ice as is presently seen at the poles, and we would
need an  equivalent stack at the North Pole too. At the moment it's just
floating sea ice, remember?

If we built up 90km thickness of ice at the poles, three things would happen:

*1* All the world's oceans would be stacked as ice at the poles

*2* The Ice stack would be so high compared to Earth's atmosphere that it would
stop snowing anyway (in fact, at present it doesn't snow much near the South
Pole because the air is already so high, cold, and dry)

*3* The internal heat of the Earth would melt the base of the ice stack.
Remember that as we drill or mine deep into the Earth, the temperature rises by
about 3 degrees Centigrade per kilometer. I think that ice is more conductive
than rock, so perhaps only 1 degree  C per km here, but even so, with a 90 km
stack of ice, the basal temperature will be 90 degrees hotter than the surface.
In other words, the surface would need to be at minus 90 Centigrade to stabilise
the situation. - also, ice melts under pressure, and this stack of ice makes
some pressure...

3)    Planetary alignments

The combined gravity effects of every body in the Solar system is less than 1/10
of one percent of that due to the Sun and Moon. Everything else is too far away
and too small (even Jupiter). The alignment may look pretty, but will have no
effect here on earth.

4)    Shifting of the surface plates of the Earth, independent of its main mass?

This is the only way to escape the difficulty of shifting the entire axis of
rotation, but geological history and what we know of the strength of the Earth
makes this impossible as a sudden event. 

The surface of the Earth does shift as a series of plates or shells (read about
Plate Tectonics), but at a speed of Centimetres per YEAR, and over a time frame
of Tens of Millions of years.

There won't be any tidal waves, or global destruction from huge Earthquakes,
just the same slow grinding of the plates past each other, with the regular dose
of locally dangerous Earthquakes and Volcanoes.

The forces involved in driving the Earth's plates are immense and slow, and they
won't be upset by a temporary planetary alignment. Anyway, these happen again
and again over geological time. The Earth hasn't been catastrophically
resurfaced and had life as we know it obliterated yet!

Even the end of the age of dinosaurs was a slow drawn out process, with life
continuing on, albeit changed. Palaeontologists know of many other mass
extinction events, but current thought sees these as slow events over millions
of years, due to global climate change, perhaps aided by meteor impacts and eras
of enhanced volcanic eruptions, rather than due to catastrophic resurfacing
processes. After all, Geologists can still pick up almost undeformed pieces of
sedimentary rock that are nearly 3 Billion years old. If the processes you have
read about actually worked, all that old stuff would have been ground to dust
long ago!

In short, there's no evidence that what you describe has happened in the last 3
Billion years, so it's unlikely to happen in our lifetimes!


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