ABSTRACT
Sea-level change is one of the main factors in stimulating coastal change, and in the long term it has been controlled by changes in the volume of landlocked ice sheets. If sea level rises, then coasts must be able to change dynamically to keep up, or drown. On the scale of a human lifetime we may consider sea level to be unchanging, but even through the twentieth and early twenty-first centuries it appears that sea level has been rising due to human activity, and may continue to rise well into the future. This chapter covers:
The level of the sea is not constant, it is always rising and falling, whether through the passing of waves (including tides), meteorological influences, or gravitational effects in the form of the earth’s geoid. Long-term and significant sea-level changes, however, reflect changing levels of both land and sea; eustasy refers to absolute changes in global sea level, and isostasy refers to the vertical movement of land due to local geological factors. It is the balance between these two processes at a given coastline that produces observed changes in sea level, referred to as relative sea-level change, because an absolute (eustatic) rise in sea level may not be required to allow the sea to rise relative to the land. Figure 5.1 explores this balance in a simplified way, and a number of cases can be put forward to illustrate the relationship:
l Relative sea-level rise will occur if: (a) eustatic sea level rises whilst the land is isostatically subsiding, static or uplifting at a slower rate than eustatic rise; (b) eustatic sea level is static whilst the land is isostatically subsiding; and (c) eustatic sea level falls whilst the land is isostatically subsiding at a faster rate than eustatic fall.
