Some Key Points
- Sea level is increasing by about 4mm a year.
- Sea level changes all the time but is it is currently rising rapidly
- Sea level rise is caused by two main factors related to global warming: the added water from melting ice sheets and glaciers and the expansion of seawater as it warms.
- Sea Level change is "lumpy", the mass of the ocean is increasing but the sea level change varies greatly across the ocean.
Figure 7.1 is derived from coastal tide gauge data and shows how much sea level changed from about 1870 to 2000. Sea levels reflects the climate system. During ice ages a large volume of water is stored on land in the form of ice sheets and glaciers, leading to lower sea levels, while during warm interglacial periods, glaciers and icesheets are reduced and more water is stored in the oceans. Sea levels typically vary by over 100 metres during glacial-interglacial cycles as the major ice sheets wax and wane as a result of changes in summer solar radiation in high northern hemisphere latitudes. The sea level has increased by more than 120 metres since the end of the last ice age but it has stabilised over the last few thousand years, and there was little change between about 1AD and 1800AD. Sea level began to rise again in the 19th century and accelerated again in the early 20th century, in the 21st century the rate has increased to about 4mm each year on average.
Figure 7.1 - Historical Sea Level Change - Coastal tide gauge records