2.1.1 What is sea level?
If asked the question in the title of this section, most people would reply, ‘It is the level of the sea surface.’ While this is not incorrect, a more specific definition is required so that this property of the Earth System can be measured and therefore studied in a quantitative fashion. To do this, a reference height or surface must be chosen so that the level, or height, of the sea surface can be measured relative to it.
The most common reference surface is the solid Earth (or sea floor); measurements of the height of the sea surface relative to the ocean floor represent what is known as ‘relative sea level’ (Fig. 2.1). Different methods for measuring relative sea level are outlined in section 2.1.3. Based on this definition, at any location in the ocean, a change in relative sea level can be produced by a vertical shift in either the sea surface or the sea floor, and so a measurement of relative sea-level change is ambiguous in the sense that it provides no information on the contribution to the change from each bounding surface. For example, at coastal locations, a local sea-level rise leading to a landward motion of the coastline, or marine transgression, could be driven by land subsidence and/or sea-surface rise. A retreat of the coast, known as a marine regression, could be driven by land uplift and/or sea-surface fall.