SIR – David Rhodes (T&A, November 29) asks why the water temperature under the polar ice is always minus-2 degrees and has been for a long time, even though the rest of the oceans are warming.

It’s the same reason we use salt to melt the ice on footpaths – salty water only freezes at a minus temperature, and so for there to be a permanent ice cover, the temperature of the water underneath, just about to turn to ice, must be below what we normally think of as freezing.

This has been the case in the Arctic for millions of years, though it may not be so for much longer with global warming.

Keith Thomson, Heights Lane, Bradford