Assessing sea levels is a complicated business, as there are a number of factors which can affect sea level relative to a fixed point on land. For a start, land levels rise and fall; in some places land is lifted up by plate movements or volcanism, in other places it may fall. Rising and falling atmospheric pressure makes very temporary local changes. The factor which causes the most concern these days is the rise in mean sea level caused by melting ice on land increasing the amount of water in the ocean. This has been going on since the end of the last ice age some 12,000 years ago, and is caused by global warming. This mean sea level is the one from which the height of mountains is calculated and the altitude of aeroplanes. Melting of sea ice causes no rise in mean sea level. Another factor is that, relieved of the burden of thousands of cubic km of ice at the end of the ice age, places like Scotland gradually rise up, and this rising of the land can persist for many thousands of years after the ice has vanished. In some places there is a seesaw movement where the rising of one part of a plate causes another part hundreds of miles away to descend.