Sea level rise is caused by two processes: thermal expansion (ocean water expanding as it heats up) and additional water flows into the oceans from ice that melts on land. Both these processes are currently being observed.
There is robust evidence that sea levels have risen as a result of climate change based on observations from tide gauges, paleo indicators and satellite measurements. The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report finds that sea level has risen by 0.19 m since the beginning of the 20th century.
Over the last century global average sea level rose by 1.7 [1.5 to 1.9] mm per year, in recent years (between 1993 and 2010) this rate has increased to 3.2 [2.8 to 3.6] mm per year. The IPCC report finds that the rate of sea level rise over the last century is unusually high in the context of the last 2,000 years.
The IPCC finds that recent observations of global average sea level rise at a rate of 3.2 [2.8 to 3.6] mm per year is consistent with the sum of contributions from:
- observed thermal ocean expansion due to warming (1.1 [0.8 to 1.4] mm per year);
- glaciers (0.76 [0.39 to 1.13] mm per year) ;
- the Greenland ice sheet (0.33 [0.25 to 0.41] mm per year);
- the Antarctic ice sheet (0.27 [0.16 to 0.38] mm per year); and
- changes to land water storage (0.38 [0.26 to 0.49] mm per year).
The IPCC report projects reflects stronger scientific understanding of sea level rise and projections for future sea level rise high are higher than in previous IPCC reports. If emissions continue to track at the top of IPCC scenarios global average sea level could rise by nearly 1 m by 2100 (0.52−0.98 m from a 1986-2005 baseline). If emissions follow the lowest emissions scenario, then global average sea level could rise by between 0.28−0.6 m by 2100 (compared to a 1986−2005 baseline).
Sea level rise will continue for centuries to thousands of years after greenhouse gas concentrations are stabilised due to the long lag times involved in warming of the oceans and the response of ice sheets.
For the first time the IPCC provides projections for 2300 in its latest report. Sea level rise by 2300 could be kept to less than 1 m if concentrations of carbon dioxide in the atmosphere are stabilised below 500 ppm, or could reach up to 3 m if concentrations of carbon dioxide rise above 700 ppm.
Figure: Past and projected global average sea level. The graph is split in to three sections (estimates of the past, instrumental record and projections of the future). The first section shows estimates of sea level change from 1800 to 1870 for which direct measurements are not available, but indicates a steady sea level varying between 100-200 mm below present day levels. The second section shows the instrumental record (1870-2007). A reconstruction of sea level change measured by tide gauges is shown with a narrow shaded area depicting the uncertainty. Satellite observations of sea level are also shown from 1990. Sea level rises approximately 100-200 mm during this period. The third section represents the range of model projections from 2000-2100 with a range of 220-500 mm sea level rise from 2007 observations by the end of the century. The projections come from an international modelling effort for a medium growth emissions scenario (IPCC SRES A1B) and excludes the additional rise in sea level if Greenland or Antarctica contribute more ice to the oceans in the future. Source: IPCC (2007).
Risks of sea level rise
Australia is a coastal society. 85 per cent of the population lives in the coastal region and it is of high economic, social and environmental value to the nation. Nearly 39,000 residential properties are located within 110 metres of soft, erodible shorelines. Exposure will increase as Australia’s population grows.
The impacts of sea level rise will be experienced mainly through its effect on extreme sea level events such as high tides and storm surges. Rising sea levels will increase the frequency or likelihood of extreme sea level events and resultant flooding.
The risks from sea level rise are not confined to the coast itself. In many cases flooding may impact areas some distance from the sea for example along estuaries, rivers, lakes and lagoons.
A study of 29 locations in Australia (see figure below) found that for a mid-range sea level rise of 50 cm extreme sea level events that happened every few years now, are likely to occur every few days in 2100.
On average, Australia will experience a roughly 300-fold increase in flooding events, meaning that infrastructure that is presently flooded once in 100 years will be flooded several times per year with a sea level rise of 50 cm.
Figure: Estimated increases in the frequency of extreme sea level events (indicated by the diameters of the circles), caused by sea level rises of 10 centimetres (left) and 50 centimetres (right). With 10 centimetres of sea level rise, the frequency of extreme sea level events is estimated to increase by 2 to 6 times in Australia. With 50 centimetres of sea level rise, the frequency of extreme sea level events is estimated to increase by 100 to 10,000 times.
- Sea level rise - CSIRO and Antarctic Climate and the Ecosystems Cooperative Research Centre