Why in physics class are we taught that when an ice cube melts in a glass of water it doesn’t raise the level of the water yet the global warming crowd says that icebergs melting will drastically raise the sea level?
Melting ice cubes don’t raise the level in a glass because both the ice and the water are the same fresh water. Sea ice is fresher than sea water, and icebergs become fresher the longer they remain afloat. As should be obvious, floating ice displaces its own weight in the water supporting it—but because salt water is slightly denser than fresh water, once floating ice melts, the fresh water it melts into takes up slightly more space than the salt water that was supporting it—and the sea level rises—a bit.
The “global warming crowd” is well aware, however, that if all the sea ice in the world were to melt, it would only raise sea levels by a few millimeters.
It’s the melting of land based ice sheets that will raise the sea levels, not that of icebergs already in the water melting. I’m not sure there are any decent models for how fast this is likely to occur in this century, since every decade the models get revised to speed up predictions, but it’s easy enough to reference what kind of sea level rise we are talking about.
The short version: if all the ice melted on Greenland alone that would raise sea levels by 7 meters; if all the ice on Antarctica melted that would raise sea levels by 57 meters. Coastal cities are going to face huge problems related to rising sea levels in this century, just not on that scale. How much, no one really knows. Not that it’s likely to happen any time soon.