Sea levels reflect the state of the climate system. Sea levels change through the addition or removal of mass and through changes in volume. Mass change reflects the exchange between water stored on land in glaciers, ice sheets, dams and ground water and the ocean. Volume change reflects the expansion or contraction of the sea water as it warms or cools.
Over recent decades, improved observations of the different contributing factors have enabled an approximate closure of the global mean sea level budget, i.e. the sum of the individual components sums to the measured sea level increase. Additionally, improved models represent the observed changes, providing confidence in methods for projecting future sea level change from the various contributing factors.
In the IPCC Fifth Assessment Report, global sea level projections are developed for four Representative Concentration Pathways (RCPs). The relationship between the RCPs and the SRES scenarios, which were used by the IPCC in the Fourth Assessment Report in terms of atmospheric CO2 concentrations is illustrated below
The IPCC projections include estimates of contributions from:
Sea levels are not uniform but influenced by various factors including large scale weather patterns and ocean currents, which are represented by the CMIP5 climate models. The melting of the large ice sheets of Greenland and Antarctica cause changes in the Earth’s gravitational field and lead to spatial patterns of sea level rise across the global oceans known as ‘finger prints’. Sea-level rise is larger in the oceans farthest from the ice sheets and smaller immediately adjacent to the ice sheets. Long term movements in the Earth’s crust since the last glacial maximum, referred to as Glacial Isostatic Adjustment (GIA) also leads to differences in relative sea level rise. The IPCC (Church et al, 2013) estimates that while spatial variations in the rates of rise exist, 70% of the global coastlines will experience sea-level rise within ±20% of the global mean sea-level rise (MSLR).
Compilation of paleo sea level data (purple), tide gauge data (blue, red and green), altimeter data (light blue), and central estimates and likely range for projections of global mean sea level rise from the combination of CMIP5 and process-based models for RCP2.6 (blue) and RCP8.5 (red) scenarios, all relative to pre-industrial values. (Source: IPCC (2013) Technical Summary)
Sea level projections for Australia have been updated following the method of the IPCC (McInnes et al, 2015).
The regional distributions of sea level change (four emissions scenarios) for the period centred on 2090 compared to 1986-2005. The projections (shadings) and uncertainties (solid lines) represent the contributions from the changes in terrestrial ice, the gravitational response of the ocean to these changes, and an ongoing GIA.
Sea level projections are also available showing observed changes in tide gauges and satellite altimetry. The large year-to-year variability occurs as a result of natural climate variability due to El Nino/Southern Oscillation (ENSO) to show that in the future sea levels may be above or below the average projected increase for the location depending on the background variability.
Sea level observed and projected for Sydney (left) and Albany (right). The observed sea level records are indicated in black, with the satellite record (since 1993) in mustard and tide gauge reconstruction (which has lower variability) in cyan. Multi-model mean projections (thick purple and olive lines) for the RCP8.5 and RCP2.6 emissions scenarios with likely model ranges are shown by the purple and olive shaded regions from 2006 to 2100. The olive dashed lines represent estimates of interannual variability combined with the range of the projections. Thick dark blue and orange lines represent multi-model mean projections for the RCP 4.5 and 6.0 scenarios, respectively.
For the next few decades, the rate of sea-level rise is partly locked in by past emissions, and will not be strongly dependent on early 21st century greenhouse gas emissions. However, sea level projections closer to and beyond 2100 are critically dependent on future greenhouse gas emissions, with both ocean thermal expansion and the ice sheets potentially contributing metres of sea-level rise over centuries for higher greenhouse gas emissions.