Your sea level correspondent lists three causes of sea level rise: (1) undersea volcano building, (2) erosion on land followed by transport to the sea, and (3) increased number and size of ships. I add (4) if I go into the sea for a swim, that will raise sea level too. Listing possible causes is only the first step: what we need is to know by how much the sea will rise. Volcano building and erosion need complex data and calculation, unsuited to a newspaper letter, but we can do something for ships. Here is an outline. Following your correspondent, take 10,000 ships of average displacement 250,000 tons (values rounded up), convert tons to cubic metres, and spread that over the 360,000,000 square km of all oceans. I make that a rise of 0.007 mm – not a worrying problem. Compare that with (5) the thermal expansion for a 1 degree rise in temperature (0.0001) applied over the average depth of the oceans (3,700 m). That would raise the sea by 370 mm: about 53,000 times bigger than the shipping effect. These calculations are very crude, but good enough to show what matters and what does not. Melting ice is probably the biggest potential cause of sea level rise. Please send your letters to rye.battle@jpimedia.co.uk. Letters must be accompanied by a full name and address. Anonymous letters are never printed. We reserve the right to edit letters for any reason.