Sea Level Rise

Primary reference(s)

IPCC, 2019. Annex I: Glossary [Weyer, N.M. (ed.)]. In: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Intergovernmental Panel on Climate Change (IPCC). Accessed 21 October 2020.

Additional scientific description

Global mean sea-level change resulting from change in the mass of the ocean is termed barystatic. The amount of barystatic sea-level change due to the addition or removal of a mass of water is referred to as its sea-level equivalent (SLE). Sea-level changes, both globally and locally, resulting from changes in water density are termed steric. Density changes induced by temperature changes only are termed thermosteric, while density changes induced by salinity changes are termed halosteric. Barystatic and steric sea-level changes do not include the effect of changes in the shape of ocean basins induced by the change in the ocean mass and its distribution (IPCC, 2019).

Global mean sea level rose by 0.19 m (0.17–0.21 m) between 1901 and 2010 (Abram et al., 2019).

The rate of sea-level rise increased from 1.4 mm yr–1 over the period 1901–1990, to 2.1 mm yr–1 over the period 1970–2015, to 3.2 mm yr–1 over the period 1993–2015, to 3.6 mm yr–1 over the period 2006–2015 (Oppenheimer et al., 2019).

The Intergovernmental Panel on Climate Change (IPCC) projects future global mean sea level rise to be between 0.43 m (0.29–0.59 m, likely range; RCP2.6) and 0.84 m (0.61–1.10 m, likely range; RCP8.5) by 2100 (medium confidence) relative to 1986–2005 (Oppenheimer et al., 2019). Local or relative seal-level rise will depart from this global mean due to local/regional conditions (e.g., local oceanic water currents or local land uplift/subsidence).

Sea-level rise is projected to increase the frequency of extreme sea-level events, leading to more frequent inundation. In several regions of the world, current inundation with a return period of one century could become annual events as soon as 2050 (Oppenheimer et al., 2019) and flooding frequency could increase exponentially, doubling every five years in the future (Taherkhani et al., 2020).

Global sea level is projected to continue to rise after 2100, up to 2.3–5.4 m by 2300 under the high IPCC emission scenario (RCP8.5).

There are large uncertainties about the level of contribution of the melting ice sheets from Greenland and Antarctica to global mean sea-level rise. Recent observations indicate an acceleration of melting and raise questions about a tipping point being reached that makes the melting irreversible over time (e.g., Lenton et al., 2019; King et al., 2020).

Metrics and numeric limits

According to Oppenheimer et al. (2919), in discussing sea-level rise and implications for low-lying islands, coasts and communities in the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (Oppenheimer et al., 2019), ‘sea level’ means the time-average height of the sea surface, thus eliminating short duration fluctuations like waves, surges and tides. They add:

• Global mean sea level (GMSL) rise refers to an increase in the volume of ocean water caused by warmer water having a lower density, and by the increase in mass caused by loss of land ice or a net loss in terrestrial water reservoirs. Spatial variations in volume changes are related to spatial changes in the climate. In addition, mass changes due to the redistribution of water on the Earth’s surface and deformation of the lithosphere leads to a change in the Earth’s rotation and gravitational field, producing distinct spatial patterns in regional sea-level change. In addition to the regional changes associated with contemporary ice and water redistribution, the solid Earth may cause sea-level changes due to tectonics, mantle dynamics or glacial isostatic adjustment.
• These processes cause vertical land motion (VLM) and sea surface height changes at coastlines.
• Hence, relative sea level rise (RSL) change is defined as the change in the difference in elevation between the land and the sea surface at a specific time and location (Farrell and Clark, 1976). Here, regional sea level refers to spatial scales of around 100 km, while local sea level refers to spatial scales smaller than 10 km. In most places around the world, current annual mean rates of RSL change are typically on the order of a few mm yr–1.
• Risk associated with changing sea level is also related to individual events that have a limited duration, superimposed on the background of these gradual changes. As a result, the gradual changes in time and space have to be assessed together with processes that lead to flooding and erosion events. These processes include storm surges, waves and tides or a combination of these processes and lead to extreme sea-level events (ESL).
• Newly emerging understanding of these different episodic and gradual aspects of sea-level change are assessed, within a context of sea-level changes measured directly over the last century, and those inferred for longer geological time scales. This longer-term perspective is important for contextualising future projections of sea level and providing guidance for process-based models of the individual components of SLR, in particular the ice sheets. In addition, anthropogenic subsidence may affect local sea level substantially in many locations, but this process is not taken into account in values reported here for projected SLR unless specifically noted (Oppenheimer et al., 2019).

Examples of drivers, outcomes and risk management

Sea-level rise is controlled by different drivers that act at different timeframes: for example, storms and tsunamis generate sea-level rise at the coast on short timeframes while global warming or geo-isostatic adjustment (vertical movement of the continents in response to change in overlying ice mass) generate sea-level rise over longer timeframes.

Sea-level rise generates a variety of impacts on coastal areas: change in coastline (erosion and accretion), coastal inundation and degradation of coastal vegetation.

Sea-level rise will modify sediment transport along the coast through changing water currents, and different sources of sediments and deposition areas. As a result, erosion and accretion can occur at different locations and at different rates. Although erosion is expected to occur more widely and to be an existential threat for the atoll islands, it seems atoll islands are responding to rising sea level by changing their shape (e.g., McLean and Kench 2015; Duvat, 2019). Although this does not result in a reduction in the size of the islands, it has impacts on coastal infrastructures and livelihoods in the erosion areas (see EN0020 on coastal erosion).

Sea-level rise also generates inundation in the coastal area through overwash (Ford et al., 2018). This results in damaged infrastructure, salination of groundwater, salination of soil and decreased crop yields. In the case of atoll islands, the increase in frequency of the inundation events caused by overwash could result in the inhabitability of some islands due to the reduction of fresh groundwater supply (Storlazzi et al., 2018).

Sea-level rise in atoll islands may also cause inundation by pushing up the fresh underground water lens (e.g., Habel et al., 2019). Although the consequences are different since the inundation is caused by freshwater, which is less corrosive to infrastructure and slightly less damaging to soils and crops, it is more difficult to manage since the classical coastal protection approach is ineffective against this type of inundation.

Coastal vegetation is distributed along the coast based on its tolerance to salt. Sea level will push the vegetation more inland and if there is no space to accommodate the retreat of the coastal vegetation, this ecosystem can be reduced or disappear (Oppenheimer et al., 2019).

Risk management for sea-level rise may be achieved through the reduction of greenhouse gas emissions; however, there will be a lag of several decades between the reduction of emissions and a decrease of sea-level rise since the processes controlling sea-level rise (thermal expansion from ocean warming and ice sheet melting) have delayed responses to global warming of the atmosphere (Oppenheimer et al., 2019).

Risk management for the impacts of sea-level rise at the coast are similar to the risk management for coastal erosion (EN0020). They include the design and construction of engineering structures (seawalls, revetments, etc.), conservation and development of healthy coastal ecosystems (e.g., coral reefs and mangrove forests), development of legislations and policies on coastal zoning and associated building codes, and integrated coastal management and monitoring of extreme sea-level rise events (Spalding et al., 2014).