The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (2007) found that global average sea level rose at an average rate of 1.7 millimetres per year over the 20th century. This rate has accelerated to around 3.2 millimetres per year from 1993 to 2012.
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.
Thermal expansion of the oceans since 1993 has produced about half of the current 3.2 millimetres per year increase in global sea level.
Observations have shown that widespread decreases in glaciers and ice caps (excluding the Greenland and Antarctic ice sheets) have contributed significantly to sea level rise. Since 1993 the contribution of sea level rise from mountain melting glaciers and ice caps is estimated to be around 28 per cent.
The contribution from the major ice sheets of Greenland and Antarctica is not currently well understood, and is an active area of research. There is increasing concern about the potential instability of both the Greenland and West Antarctic ice sheets leading to a more rapid rise in sea levels than current model projections.
The IPCC Fourth Assessment Report projects sea level rise of 18 to 59 cm in 2095 (compared with the 1980 to 2000 average), plus an allowance of another 10 to 20 centimetres for a potential dynamic response of the major ice sheets.
Observed sea level since 1993 is currently tracking near the upper limit of the IPCC projections, pointing towards significant risks of sea level related impacts in the 21st century.
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.
Research conducted since the Fourth Assessment Report suggests that a plausible estimate of the amount of sea level rise by 2100 compared to 1990 is 0.5-1.0 metres.
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 centimetres – which will almost certainly be attained during this century – 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 after a sea level rise of 50 centimetres.
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