Difference between revisions of "Sea level rise"
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[[Category:Climate Change in the ICZM Process]]
[[Category:Climate Change in the ICZM Process]]
Revision as of 15:54, 9 October 2019
The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.
Contributions to sea-level rise
Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. It is almost certain that global warming due to human emissions of greenhouse gases is responsible for steepening this trend since at least a few decades. The most recent projections for future sea-level rise are presented in the Special IPCC Report on the Ocean and Cryosphere in a Changing Climate (2019). This report is an update of the previous IPCC AR5 report (2013) , and includes newer insights in the response of the Greenland and Antarctic ice sheets to global warming. It also provides an estimation of the possible sea-level rise up to the year 2030, see Fig. 1. Two scenarios for greenhouse gas emissions are considered in this figure: (1) a "low" scenario, called RCP2.6, with strong reduction of global greenhouse gas emission, such that global warming will probably not exceed 2 oC; (2) a "high" scenario, called RCP8.5, in which no measures are taken to limit greenhouse gas emissions ('business as usual'). The high scenario can lead to a rise of up to 5 m of the global average sea level in 2300, but with great uncertainty.
Several phenomena contribute to sea-level rise. On a global scale, sea-level rise is mainly due to an increase of the water mass and water volume of the oceans. This global sea-level rise (often termed Eustatic sea-level rise) has three components:
(1) thermal expansion of ocean waters related to decrease of the density (also referred to as thermo-steric component of sea-level rise, related to increasing temperature),
(2) water mass increase, which is mainly due to melting of mountain glaciers and decrease of the Greenland and Antarctic ice sheets, and
(3) decreasing storage of surface water and groundwater on land.
Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall . Most important are:
(4) vertical earth crust motions - in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,
(5) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of soft deltaic soils,
(6) changes in the earth gravitational field, related in particular to decrease of the Greenland and Antarctic ice sheets,
(7) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and
(8) regional sea-level change related to changes in seawater salinity.
Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-8).
Observed sea-level rise
Trends in sea-level from world-wide available tide gauge records and from satellite measurements have been analyzed by Church and White (2011) . The tide gauge data were corrected for vertical land surface motion, by using estimates for glacial isostatic adjustment (assuming that this is the major cause of vertical land surface motion). From these corrected tide gauge data, a linear trend of 1.7 ± 0.2 mm/year sea-level rise was found for the period 1900 to 1990 and a linear trend of 2.8 ± 0.8 mm/year for the period 1990 to 2009.
Trend analyses of regularly updated satellite data can be viewed at the NOAA site  for global and regional sea-level changes around the world.
Even after correcting for the effect of glacial isostatic adjustment, substantial regional differences in sea-level rise occur . Major causes are:
- self-gravitation related to changes in land ice mass, and elastic solid Earth deformation;
- changes in seawater density (mainly related to fresh water input and water temperature), in ocean currents and in the atmospheric pressure distribution.
There is hardly any doubt that the global sea level has risen faster during the past decades than in the past century. While sea-level rise is estimated at 1-2 mm/year during the past century, the estimate of the present sea-level rise (2019) ranges between 3 and 4 mm/year, with an acceleration rate of 0.12±0.07 mm yr-2  . The estimated global sea-level rise for the past decades (1993-2017) is based on satellite altimeter data and estimates of different contributions to the ocean water budget (especially melt of the Greenland and Antarctic ice sheets). A much higher than average sea-level rise is observed in the Indian Ocean–Southern Pacific region. This regional feature has a strong impact on the estimate for the global sea-level rise. However, uncertainties remain in the calibration of the satellite altimeter data. A much lower acceleration rate of 0.018 ± 0.016 mm yr-2 has been found by Kleinherenbrink et al. (2019), based on a reanalysis of calibration drifts in the satellite altimeter data. Fig. 2 shows the sea-level data of 6 tide gauge stations along the Dutch coast, corrected for soil subsidence. No acceleration of sea-level rise is visible in these data. This can be partly due to changes in the gravitational field (the influence of the decreasing ice mass of Greenland may account for a 0.9 mm/year sea-level decrease along the Dutch coast), but the difference with the global 3-4 mm/year sea-level rise cannot be fully explained.
Projections of future sea-level rise
Many model studies have been conducted to predict future sea-levels. Different forecasts of future sea levels display a large spread. This is due to uncertainty regarding future emissions of greenhouse gases, to shortcomings in the present understanding of climate dynamics (including ocean-atmosphere interaction) and to restrictions imposed on model grid scales. All models predict an increase of the rate of sea-level rise. Projections for the main components of sea-level rise according to different scenarios and different models are presented in Table 1.
Sea-level rise lags behind global warming. Even if greenhouse gas emissions would stop today, sea levels will continue rising for at least a century . In the hypothetical case that there will be no greenhouse gas emissions from now on, sea levels will be 0.7-1.2 m higher in 2300 than in 2000 .
|sea-level rise component||SLR range [m]
low IPCC scenario (RCP2.6)
|SLR range [m]
high IPCC scenario (RCP8.5)
|Thermal expansion||0.1 - 0.18||0.21 – 0.33|
|Mountain glaciers||0.07 - 0.12||0.15 - 0.25|
|Greenland ice sheet||0.04 - 0.12||0.08 – 0.27|
|Antarctic ice sheet||0.01 - 0.11||0.03 – 0.28|
|Total SLR||0.29 - 0.59||0.61 – 1.1|
It has been suggested that the contributions from the Antarctic to sea-level rise could be much larger when considering structural collapse of the marine-terminated ice cliffs and disintegration of the West Antarctic ice sheet after removal of the ice shelves. This could contribute to an additional sea-level rise of 1 m in 2100 and up to 15 m in 2500. However, some doubts exist whether marine ice-cliff instability is a realistic scenario .
Hansen et al. (2016)  and Golledge et al.  provide evidence from modelling and paleoclimate records for a feedback process that can enhance melting of the Antarctic ice sheet and produce additional sea-level rise. This feedback process is triggered by increasing amounts of fresh meltwater from the polar ice sheets that strengthen ocean stratification, reduce the sinking of Antarctic cold water and decrease the ocean heat flux to the atmosphere. This results in sequestration of warm deep water and enhanced melting of the Antarctic ice sheets. These authors also predict a slowing of the Atlantic meridional overturning circulation (AMOC) due to increasing meltwater outflow from the Greenland ice sheet, with possibly important consequences for the North Atlantic Gulfstream and the climate of northwestern Europe. A more detailed discussion is presented in the articles Ocean circulation and Thermohaline circulation of the oceans.
Extreme sea levels
Most coastal zones are more vulnerable to extreme sea levels than to the mean sea level. This holds in particular for coasts situated on broad continental shelves (North Sea, East China Sea, for example) where extreme levels are much higher than the mean sea level, due to amplification of the ocean tides and water-level setup by strong winds (storm surges). Nevertheless, rise of the local mean sea level is always the major component of the projected rise of the local extreme sea level (for any given long return period). However, climate-induced change in extreme wind and wave conditions can influence extreme sea levels significantly in some regions. Along the eastern African coast extreme wind and wave conditions will be less frequent, whereas in northern Europe (especially the Baltic region in the RCP8.5 scenario) extreme levels will increase more than the mean sea level.
Sea-level rise does not only affect extreme sea levels, but also the average return period. This will be the case in particular for coasts situated close to the deep ocean, where sea levels are less influenced by storm surges, and for coasts outside the zone of tropical cyclones. For these coasts the average return period of extreme sea levels will strongly decrease; in many cases a reduction of a factor greater than 100 is projected in the IPCC scenario RCP8.5 in 2100: a once in 100 year extreme sea level will become a yearly event. For coasts situated on broad continental shelves where extreme levels are much higher than the mean sea level, the average return period will be reduced by a factor 10 or more . For uplifting coasts the reduction of the average return period will be less, because of a smaller relative mean sea-level rise.
Impact of sea-level rise
Sea-level rise will have a great impact, in particular on low-lying coastal regions, such as river deltas and coral islands. Delta coasts and coral islands are shaped under the influence of marine bio-geomorphological processes; their natural elevation is therefore around the present high water level – not much higher and sometimes lower. Many low-lying coastal zones are densely populated and host large cities; a large number of coastal megacities are located in developing countries . In densely populated coastal zones, sea-level rise is often exacerbated by soil compaction and land subsidence in connection with drainage works and the extraction of groundwater or oil / gas mining. The vulnerability is further enhanced by coastal erosion, because of sediment retention behind upstream dams, hard coastal structures and/or conversion of mangrove forests to aquacultures , see also Human causes of coastal erosion. Considerable investments are required for adapting these vulnerable coastal zones to sea-level rise, in particular to reduce flooding risks .
Sea-level rise enhances shoreline retreat (for retreating coasts) or reduces shoreline progradation (for accreting coasts), see Natural causes of coastal erosion. The influence of sea-level rise on the shoreline position of sandy barrier coasts can be estimated by means of the Bruun rule . Sea-level rise also threatens coastal wetlands, which may not be capable to keep pace with sea level and be partly lost due to so-called coastal squeeze. This can be the case for mudflats and salt marshes in the Wadden Sea  and for mangrove forests in the tropics and subtropics, see Potential Impacts of Sea Level Rise on Mangroves.
Salt intrusion is another major impact of sea-level rise in low-lying river deltas around the world. This impact is compounded by soil subsidence and by reduced fresh water supply to the coastal zone due to upstream diversion of river water for irrigation and other uses. Salt intrusion threatens crucially important fresh groundwater reservoirs in arid regions, for example in the Nile Delta . Relative sea-level rise also causes loss of fertile agricultural land in the coastal hinterland by increased salt seepage to surface waters , with great economic and social consequences. Salt intrusion further affects drinking water availability in densely urbanized coastal regions.
Strategies for dealing with the impacts of sea-level rise depend on local conditions. Different strategies are reviewed in recent IPCC Assessment reports . See also the article Climate adaptation policies for the coastal zone.
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