Sea-level change is an important consequence of climate change, whether natural or anthropogenic. Global mean sea level has been rising. For the 20th century, the average rate was 1.7 ± 0.5 mm yr–1 (IPCC AR5), and in some locations (such as the east coast of the USA) there is some evidence that the rate of sea-level change appears to have increased from the 19th to 20th centuries. However, sea-level change is not a global process but in fact the sea level at any location and at any time is result of a complex combination of a range of factors. Changes in sea level are a result of any one or more of these different factors altering over time and we term this ‘relative sea-level change’.
Causes of sea-level change
Some of the mechanisms that can cause sea-level change include:
- Changes in the total volume of water in the ocean: for example, the melting of ice sheets adds more water directly into the oceans;
- Changes in the level of the land: the Earth’s crust sits on the mantle, which behaves as a viscous solid. Therefore, if a large mass such as an ice sheet, which has been pressing down on the mantle, disappears, the mantle rebounds by slowly moving upward to fill the space (this is termed isostasy). As this effect causes the land to move upward, the sea level drops relative to the position of the land. More recently, land-level change can also be caused by groundwater extraction.
- Deformation of the ocean surface by gravitational attraction: large bodies (for example the Greenland or Antarctic ice sheets) generate their own gravitational field, in a similar way that the moon does. When ice melts, not only is the meltwater added to the ocean, but the ice sheet also causes a ‘tide’ which causes locations furthest from the ice sheet to experience greater sea-level rise than locations closest too it, in fact locations close to the ice sheet will experience sea-level fall compared to the average sea-level change;
- Thermal expansion: as water warms, it expands and vice-versa when it cools;
- Seismic activity: in seismic zones, sea-level change can be caused by large subduction-zone earthquakes. In the case of the 2011 Japan earthquake the land subsided by up to 2 m, causing a 2 m relative sea-level rise in some locations (in addition to the large tsunami itself).
- Long term tectonic uplift or subsidence (over tens of thousands of years): for example erosion or deposition of large amounts of sediment can either take weight off, or push down the crust, affecting regions which are currently thought to be relatively seismically stable.
Of particular interest to earth scientists is the first of these mechanisms; what happens to the total volume of the ocean as ice sheets grow or melt? And how does this change over time? During recent decades, it has been possible to use satellites to measure the melting and growth of ice sheets, and changes in the ocean surface through time. This makes it possible to produce estimates as to the amount and rate of sea-level change at different locations. But how can we do this for times prior to instrumental measurements?
What happened in the past?
The Earth’s recent history is made up of glacial (cold) and interglacial (warm) periods. During the peak of the warm periods, the ice sheets are thought to be at their smallest and therefore sea level at its highest. It is difficult to work out how small the ice sheets were, as the evidence is now under the ice sheet itself! Instead, if we are able to estimate the height of sea level in the past we can understand how small the ice sheets became. This helps us to understand how much ice may melt in the future under different climate scenarios. To do this we have to use geological evidence of past sea level. This is best done in coastal locations far from former ice sheets where material which marks the location of former sea levels is preserved. As a result, much of the current evidence comes from low latitude regions – areas close to the equator – such as Haiti and Barbados.
Evidence for past sea-level change
The best evidence of past sea level comes from features that have a direct relationship to the height of the sea. For example, modern salt marshes are typically found in temperate latitudes (between the Tropics and the Arctic), where they develop in the zone between mean sea level and the limit of the highest tides. Therefore, finding a fossil salt marsh above present day sea level is a clear indicator that the sea used to be at a higher elevation in the past. If we can understand when the salt marsh was growing, we can then determine not only how high the sea was, but also when it was that high.
In low latitude regions such as the Caribbean, corals are often used instead of salt marshes. Corals live beneath the tide, but typically within 5-20 m of mean sea level. Identifying fossil coral reefs onshore along the coasts of Australia, Barbados and Papua New Guinea, amongst other locations, have allowed researchers to establish that sea level during the previous interglacial (~125,000 years ago) was approximately 2-6 m above present, though this varies with the location. Research is ongoing to find fossil salt marshes in temperate latitudes to complement the data from corals at low latitudes.
Other records of past sea level from previous interglacials include speleothems from coastal caves. In these settings, speleothems will not be able to grow when sea level rises and sea water inundates the cave. If this happens, it causes a hiatus (or ‘gap’) in the growth of the speleothem. We can find the age of this hiatus to establish when the cave was inundated. We can also determine the height of the speleothem (and its hiatus) within the cave to constrain the height of sea level at that particular time in the past. These records (corals and speleothems) provide individual snapshots of past sea level. There are very few continuous direct records of sea-level change. One example is a core from the Red Sea. Measuring isotopes through this core provides continuous information on the level of the Red Sea in the past. Using these data, it can be shown that sea level was 4-6 m above present during the peak warm period of the last interglacial.
Though there are many records of sea-level change from around the world, the challenge is in understanding what the archives are recording. Because the relative sea level at any time is a result of many, if not all, of the processes outline above, the height of a geological indicator cannot simply be used to indicate change in total ocean volume. Instead it is important to understand how all the other process may impact the location over time (e.g. uplift of the land close to the geological indicator). This means that understanding past sea level, and its response to climate change, is a complex challenge! Future research will rely on interdisciplinary investigations of the many parts of the earth system so that we can better understand how sea level may respond to global environmental change in the future.