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Comment by scottw | Hacker News Reader

Wouldn't less ice at the poles (= more water at the equator = higher rotational inertia) mean a slower rotation? Maybe we can get that second back.

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Even if that's so, we are not getting the second "back". This leap second is a second added to UTC, to adjust for the slowed-down earth rotation. There was never a leap second that removes a second from our UTC clock so far (it may happen in the future).

Replacing water with ice at the poles would mean more mass at the poles, not less, since ice is displacing water and is less dense. So he may have a plausible argument that it's the other way round, but I haven't done the calculations.

It doesn't work like that. The mass of water displaced is equal to the mass of the floating ice itself. Melting a piece of ice has no change on water level.

so yeah, nothing changes.

Both the southern polar cap and the Greenland ice mass rest on ground rather than float in the ocean, which means any melting will cause the ocean levels to rise.

I was talking about the displacement only (which the parent thread discusses). I of course acknowledge the common sense that not all ice is floating on water.

Doh, of course you're right. The mass of water displaced is only greater if the ice is submerged.

I don't know that that works - the south pole at least has ground, and a LOT of permafrost that isn't displacing any water.

I meant the global warming thing mostly as a joke. It was inspired by the Wikipedia article http://en.wikipedia.org/wiki/%CE%94T, which says: "the melting of continental ice sheets ... removes their tremendous weight, allowing the land under them to begin to rebound upward in the polar regions, which ... brings mass closer to the rotation axis of the Earth, which makes the Earth spin faster". If it's on Wikipedia it must be true, right?

It's not melting at the poles, but melting of glaciers. The meltwater flows downhill, causing a net movement of mass closer to the center of the planet.

The parent post is correct about the slowing. The land ice is (mostly) at high latitudes, and the meltwater tends to raise sea level, which moves the mass to lower latitudes, hence farther from the axis of rotation. Thus warming slows the Earth's rotation as its moment of inertia increases.

The parent post is incorrect about getting it "back" however; leap seconds are added because the rotation is slowing (a "day" is getting longer). More ice caps melting will lead to more leap seconds, not fewer (or negative).

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