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Why will the sea level then rise when the ice-cap melts?

In my limited understanding of science, I thought that the volume/weight of ice displaced an equal volume/weight of water. Applying this to the polar ice cap, why will the sea level then rise when the cap melts? Won’t it stay the same?
Jocelyn Baines from Worcestershire (Age 45-54)

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3 Responses

  1. There is some truth in what your asking. If the sea ice sections of the polar caps melt then there will be very little effect on the over all worldwide sea level. This applies in particular to recent changes in the extent of the ice pack around northern Canada and Russia. In these areas the sea ice is thicker than it is around Antartica. In Antartica the rougher sea conditions (due to less protection from land masses) do not allow the sea ice to grow to the same extent.

    However, the major future concern is when the land bound ice cap starts to melt (ie the ice on the Greenland Continent and the Antartic Continent). This ice is not in the sea currently. Some areas of the Antartic ice cap is in the region of 2500m to 4000m thick(that is a lot of fresh water).

    The Antartic ice cap volume is estimated at 30M cubic kilometres (not in the sea) and this volume of water would raise sea levels by 73 metres. The Greenland ice cap is 2.6M cubic kilometres and this would raise sea levels by 6.5metres.

  2. You’re quite right about the ice-cap at the north pole: it is floating, so if it melts the sea level will stay the same. The problem comes if the land-based Antarctic (or Greenland) ice caps melt. Since that ice is sitting on the land, if it slides in or runs off as water it will increase the sea level.

    Actually there’s a small twist to this, since if the north polar ice-cap melts it is likely that average sea temperature will also be rising. That would result in an expanded volume of the sea and hence a sea level rise, but I think much less than the melting of significant land-based ice caps.

  3. You are right that floating ice sheets will not cause an increase in sea-level when they melt. So, in terms of sea-level rise, we are not concerned with the loss of the floating ice in the Arctic. However, the ice sheets that cause most concern are the Greenland Ice Sheet and the West Antarctic Ice Sheet, both of which are grounded on land (some of the West Antarctic Ice Sheet is grounded on submerged land and there are concerns about its stability).

    It is estimated that the loss of the Greenland Ice Sheet would add 6-7 metres to global sea-level, while the loss of the West Antarctic Ice Sheet would add 5-6 metres. Many climate scientists are of the opinion that the loss of the Greenland Ice Sheet is probably inevitable, given the relatively small increase in global temperature that is thought to be required to melt it.

    It is entirely plausible that we will lose both these ice sheets, and that sea-levels will rise by significantly more than 10 metres as a result of the melting of these ice sheets, the loss of other terrestrial ice sheets and glaciers, and the thermal expansion of the oceans as the world warms. However, it is likely to take many centuries for this to happen: the highest reasonable, scientifically derived estimates of possible sea-level rise by 2100 are around 1.4 metres. We could easily see a rise in global average sea level of something around a metre or more per century for the next millennium or so. This is quite small compared with the 120 metre or more increase in sea level since the peak of the last ice age, but will still represent a huge challenge for coastal societies that will need to adapt to higher sea levels or migrate away from low-lying areas.

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