Man is changing the environment, and I think there is quite a lot to worry about. As to the physics of your question:

Let's assume you have a floating ice-block, made from a million kg. (= a million liters) of water. As floating ice, it will still have a mass of a million kg., and therefore, it will displace exactly a million kg. (liters) of water. Therefore, it makes no difference whatsoever, in the water level, if it is floating as ice, or melted.

>The factors to consider are:

> 1. What proportion of an iceberg is below the surface? Figures differ on this but I seem to recall that it's generally 5/6ths?

That makes no difference, as explained above.

> 2. What is the ratio of the volumne of ice to that of very cold water?

Actually, water will expand if it gets heated. The effect should be insignificant.

>4. for the purposes of this exercise we need to discount the ice locked up ON LAND (such as in Greenland's glaciers), that might slip into the sea (how much of the polar ice-cap is effectively one big iceberg anyway)

I think this is the real problem. The real problem is not huge amounts of ice that is already floating on water, and that may melt. The fact is that large amounts of ice that are land-locked, which may either: 1) break off and fall into the sea (making the sea-level rise) and then melt (no effect on the sea-level at this point), or 2) melt directly on the continent and then add to the water in the sea.

Note also that the global warming has lots of other possible effects, many of them damaging.

>I suppose the exercise can be approached simply by considering, say, 1 cc of ice floating in 1 cubic litre of water? Are there any other factors not considered above?

You could do a simple experiment. Put the largest possible amount of ice into a glass or pot of water (make sure that the ice is free to move, i.e., not supported by the borders of the pot), and try to accurately measure the change (or not) of the water level.