LOOK at any Admiralty chart or decent shoreline map and you find a line wriggling between land and sea marked ‘MHWS’.

Those living along the coast know the tides don’t just come in and go out twice a day, but the amount varies between ‘neap’ (small) and ‘spring’ (large) tides.

Along East Lothian’s coast, tides vary between a 2m and 6m difference between extremes. MHWS (mean high water springs) shows how high up the beach the sea gets at high tide. Not to alarm anyone, but that line is creeping inland.

The cause is global warming. As the planet warms, seas expand, glaciers melt and ice sheets high out of the water on Greenland and Antarctica add to sea level rise.

It took the 19th and 20th centuries to add 20cm to sea level. It took the last 20 years to add another 5cm. The next three decades are expected to add 10cm, 20cm and 30cm respectively as warming accelerates.

The total of less than 1m may not sound like much but other factors are involved.

NASA’s Sea Level Change Science Team at the University of Hawaii have highlighted a factor liable to make things worse before 2040. The Moon, which causes tides, has an 18.6-year cycle during which it ‘wobbles’. This suppresses the height of high tides for half that cycle (which we are now entering) and amplifies them for the other. The next amplification, reaching perhaps an additional 10cm, will be during the 2030s.

Almost all of the East Lothian coast will see exceptionally high tides but very little will be flooded.

Even with a storm surge adding to unusually high spring tides, even an extra metre will not reach places like Port Seton, Gullane, most of Dunbar or Prestonpans because of slight elevation.

But there are some vulnerable points: Aberlady’s Gullane Road between the village hall and Luffness; Dunbar’s Cromwell Harbour, Lamer Street and part of Belhaven; North Berwick’s Low Quay (33-45 Victoria Rd), any seafront property with a cellar and the sewage works. The largest area that may be threatened is Musselburgh’s Fisherrow and Goose Green.

None of this is certain because several factors need to combine to boost unusually high tides so that they cause major flooding and damage, even in areas listed. But properties do not need to be flooded more than a few times for them to be abandoned.

Before this happens is the time to decide what is worth protecting – and what isn’t.