Global and European sea-level rise (CLIM 012) - Assessment published Nov 2012
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Climate change (Primary topic)
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
- CLIM 012
Key policy question: What is the trend in mean sea level globally and across European seas?
- Tide gauges show that global mean sea level rose at a rate of around 1.7 mm/year over the 20th century, but there has been significant decadal variations around this value.
- Satellite measurements show a rate of global mean sea-level rise of around 3 mm/year over the last 2 decades.
- Sea level is not rising uniformly at all locations, with some locations experiencing much greater than average rise.
- Projections of global mean sea-level rise in the 21st century range between 20 cm and about 2 m. Modelling uncertainty contributes at least as much to the overall uncertainty as uncertainty about future GHG emissions scenarios. It is likely that 21st century sea-level rise will be greater than during the 20th century. It is more likely to be less than 1 m than to be more than 1 m.
- Coastal impacts also depend on the vertical movement of the land, which can either add to or subtract from climate-induced sea-level change, depending on the particular location.
Change in global mean sea level
Note: The figure shows the global mean sea level from 1860 to 2009 as estimated from coastal and island sea-level data (1880 – 2009, blue) and from satellite altimeter data (1993 – 2009, grey).
- Global mean sea level provided by Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Trend in relative sea level at selected European tide gauge stations
Note: The map shows investigation of global mean sea level trends since 1970. These measured trends are not corrected for local land movement. No attempt has been made to assess the validity of any individual fit, so results should not be treated as suitable for use in planning or policymaking
- Relative sea level trends provided by Permanent service for mean sea level (PSMSL)
Range of high-end estimates of global sea-level rise published after the IPCC AR4
Note: This figure shows the range of high-end global sea-level rise (metre per century) estimates published after the IPCC Fourth Assessment Report (AR4). AR4 results are shown for comparison in the three left-most columns.
- Mean Sea Level Trend from satellite altimetry (T/P-Jason-1-Jason-2 merged datasets) provided by Ocean Monitoring and Forecasting (MyOcean)
Sea-level changes can be measured using tide gauges and remotely from space using altimeters. Many tide gauge measurements have long multi-decade time series, with some exceeding more than 100 years. However, the results can be distorted by local effects. Satellite altimeters enable sea level to be measured from space and give much better spatial coverage (except at high latitudes). However, the length of the record is limited. Rates of global mean sea-level (GMSL) rise have been estimated at approximately 3 mm/year since around the mid-1990s [i]. This is greater than the longer term rise during the 20th century of around 1.7 mm/year (Figure 1). There is evidence that the contribution from the melting cryosphere has increased recently [ii]. Both for recent decades and over the longer term historical period, there is some variability evident about the trend. In particular, there are periods during the 20th century before the 1990s where the rate of sea-level rise may have reached the recent rate of 3 mm/year for some years, although the higher rates of sea-level rise were generally sustained for shorter periods than recently. For a very recent time period, the variability in sea level includes a notable dip, starting in 2010. It has been suggested, based on observations from the GRACE satellite, that this observed recent dip in sea level may be related to the switch from El Niño to La Niña conditions in the Pacific and associated changes in precipitation patterns and storage of water on land [iii].
It is not yet clear from observations whether the generally increased rate of sea-level rise observed since the mid-1990s will continue into the future. The many observations of surging outlet glaciers and ice streams (which could lead to high future rates of sea-level rise) must be balanced by recent work showing that some outlet glaciers on the Greenland ice sheet have now either stopped accelerating or even slowed down [iv]. Modelling work of individual ice sheet glaciers also shows the potential for decadal and multi-decadal variability in glacier flow [v]. There is sufficient evidence, based on recent observations, to be concerned about the possibility for an increase in the rate of sea-level rise to 2100 beyond that projected by the models used in the IPCC AR4 (see Figure 5) [vi]. However, a greater understanding of the potential for accelerated ice sheet dynamical processes that could give rise to such rapid sea-level rise is needed from improved physically-based models and from appropriate palaeo observations before more precise and reliable estimates of future sea-level rise can be made.
Deviations in the rate of sea-level rise at individual locations are evident in both tide gauge and satellite studies. Figure 2 shows the rates of change in sea level since 1992 for the European region based on satellite observations. Rates of change in sea level since 1992 for the European region are available from satellite observations. Trends in the North Sea are typically around 2 mm/year, except for some parts of the southern-most North Sea where they are larger. Parts of the English Channel and the Bay of Biscay show a small decrease in sea level over this period. The Baltic Sea shows an increase of between around 2 mm/year and 5 mm/year. In the Mediterranean Sea there are regions with increases of more than 6 mm/year, and with decreases of more than - 4 mm/year. The Black Sea has seen an increase in sea level of between zero and around 5 mm/year.
The reasons for these big differences, even within a particular sea or basin, are due to different physical processes being the dominant cause of sea-level change at different locations. For instance, the Mediterranean Sea is a semi-closed, very deep basin, exchanging water with the Atlantic Ocean through the narrow Gibraltar Strait only. It is a concentration basin where evaporation greatly exceeds precipitation and river run-off. Therefore, salinity is one of the main physical parameters influencing the thermohaline circulation and sea-level variability in the Mediterranean, which may counteract the thermal expansion due to a rise in temperature. The NAO, interannual wind variability, changes in global ocean circulation patterns, and the location of large scale gyres are further factors that can influence local sea level in the European seas.
Figure 3 shows observed trends in sea level from selected tide gauge stations in Europe. These trends can differ from those measured by satellites (see Figure 2) because of the different time periods covered and because tide gauge measurements are influenced by vertical land movement whereas satellite measurements are not. In particular, the lands around the northern Baltic Sea are still rising since the last ice age due to the post-glacial rebound [vii].
A significant recent step forward in projecting future sea levels is an improved understanding of the contributions to recent sea-level rise. A recent study found good agreement over the four last decades between observed total global sea-level rise and the sum of known contributions [viii]. Figure/Table 4 summarises the main contributions based on that study. According to this table, thermal expansion was likely to have been the most important contributor to sea-level rise throughout the whole period (1972–2008). Sea-level rise has accelerated in the latter part of that period (1993–2008) when the melting of glaciers and ice caps became the most important source of sea-level rise.
Currently there are two main approaches to projecting future sea level: physically-based models that represent the most important known processes, and statistical models that apply the observed relationship between temperature or radiative forcing on the one hand and sea level on the other hand in the past and extrapolate it to the future. Both approaches produce a spread of results, which results in large uncertainties around future sea-level rise.
The IPCC AR4 contained several statements on future sea level. Most often quoted is the range of sea-level rise projected by physically-based models for thermal expansion, glaciers and small ice caps, the mass balance of the Greenland and West Antarctic ice sheet, and a term to represent the observed dynamic acceleration of the melting of the major ice sheets. The result is a global average increase of between 0.18 m and 0.59 m from the 1980–1999 mean to the 2090–2099 mean. The range depends on both the spread in future GHG emissions and uncertainty from computer models. The largest sea-level rise contribution was projected to come from the thermal expansion (0.10 to 0.41 m), followed by melting of glaciers and ice caps (0.07 to 0.17 m) and Greenland ice sheet (0.01 to 0.12 m). The IPCC AR4 went further by including a simple sensitivity study, which allowed for future linear increases in the dynamic ice sheet component with temperature. Whilst it is not clear that such a relationship would be linear the calculations suggest an additional 17 cm of rise could occur during the 21st century. The report acknowledged that limitations in understanding and models meant that it was not possible to provide with any degree of confidence either a highest plausible 21st century rise or central estimate of rise for all of the component sea-level terms.
Since publication of the IPCC AR4, further progress has been made in understanding and simulating sea-level changes [ix]. However, global physical models are still particularly limited in their representation of ice sheet processes [x]. Since current understanding suggests that the potential for 21st century sea-level rise significantly above the AR4 range would largely result from potential increases in the ice sheet dynamical contributions, the lack of suitable physically-based models is still a significant hindrance to making reliable projections.
Statistical models of sea-level rise are also available. These models use observed relationships between changes in sea level and either surface air temperature or radiative forcing [xi]. The statistical models are then combined with 21st century projections of radiative forcing or temperature and used for projection purposes. Typically, they produce larger sea-level rise projections than current physically-based models. Future projections based on this approach have limitations because the balance of contributions to sea-level rise during the future may not be the same as the balance during the tuning period of these statistical relationships [xii]. However, the differences between the two modelling approaches may also be interpreted as indicating the scale of processes not well represented in physically-based models.
In view of these limitations to future projections purely from models, some studies have combined understanding from current physical models with other strands of evidence to provide information on possible high-end sea-level rise amounts. Evidence stands include maximum rates of sea-level rise at the last interglacial and plausible kinematic constraints on future ice flows. A synthesis of high-end sea-level rise estimates based on all sources of information available is provided in Figure 5.
The major conclusion from recent studies is that it is still not possible to rule out GMSL increases during the next century of up to approximately 2 m. However, the balance of evidence suggests increases significantly in excess of 1 m are still considered much less likely than lower rates of sea-level rise. This is consistent with the results of the Thames Estuary 2100 study in the UK [xiii] and a recent study in the Netherlands [xiv]. The latter, for example, combined modelling and expert judgement to derive a plausible high-end global scenario of 21st century sea-level rise of 0.55 to 1.15 m. However, they again concluded that although the probability of larger increases is small, it was still not possible to rule out increases approaching around 2 m based on palaeo-climatic evidence [xv]. In summary, the highest projections available in the scientific literature should not be treated as likely increases in 21st century sea level, but they are useful for vulnerability tests against flooding in regions where there is a large risk aversion to flooding, or the consequences of flooding are particularly catastrophic.
Specific projections for regional seas
Future projections of the spatial pattern of sea-level rise also remain highly uncertain. There was little improvement in reducing this uncertainty between the IPCC Third and Fourth Assessment Report. Recent model improvements, however, may reduce this uncertainty in the future. One study produced estimates of sea-level rise around the UK based on results from the IPCC AR4 [xvi]. This study estimates absolute sea-level rise (which exclude changes in land level) around the UK for the 21st century in the range of 12 cm (the lower bound of the Low emission scenario) to about 76 cm (the upper bound of the High emission scenario). Larger rises could result from an additional ice sheet term, but this is more uncertain. Another study estimated the plausible high-end scenario for 21st century sea-level rise on the North Sea coast of the Netherlands in the range 40 to 105 cm [xvii]. Making multi-decadal regional projections for relatively small isolated and semi-isolated basins, such as the Mediterranean, is even more difficult than for the global ocean. One study made projections for the Mediterranean Sea based on the output of 12 global climate models for 3 emission scenarios [xviii]. The results project an ocean temperature-driven sea-level rise during the 21st century between 3 and 61 cm over the basin, which needs to be combined with a salinity-driven sea-level change between - 22 and +31 cm.
[i] John A. Church and Neil J. White, „Sea-level rise from the late 19th to the early 21st century“, Surveys in Geophysics 32, Nr. 4–5 (März 30, 2011): 585–602.
[ii] I. Velicogna, „Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE“, Geophysical Research Letters 36, Nr. 19 (Oktober 13, 2009), http://www.agu.org/pubs/crossref/2009/2009GL040222.shtml.
[iii] NASA, „NASA Satellites Detect Pothole on Road to Higher Seas - NASA Jet Propulsion Laboratory“, 2012, http://www.jpl.nasa.gov/news/news.php?release=2011-262.
[iv] Ian Joughin et al., „Greenland flow variability from ice-sheet-wide velocity mapping“, Journal of Glaciology 56, Nr. 197 (2010): 415–430.
[v] Faezeh M. Nick et al., „Large-scale changes in Greenland outlet glacier dynamics triggered at the terminus“, Nature Geoscience 2, Nr. 2 (Januar 11, 2009): 110–114.
[vi] IPCC, „IPCC Fourth Assessment Report: Climate Change 2007 (AR4)“, 2007, http://www.ipcc.ch/publications_and_data/publications_and_data_reports.shtml.
[vii] J. M. Johansson et al., „Continuous GPS measurements of postglacial adjustment in Fennoscandia 1. Geodetic results“, Journal of Geophysical Research 107, Nr. B8 (August 10, 2002): 2157.
[viii] Church and White, „Sea-level rise from the late 19th to the early 21st century“.
[x] R. J. Nicholls et al., „Sea-level rise and its possible impacts given a ‚beyond 4 C world‘ in the twenty-first century“, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 369, Nr. 1934 (November 29, 2010): 161–181.
[xi] S. Rahmstorf, „A Semi-Empirical Approach to Projecting Future Sea-Level Rise“, Science 315, Nr. 5810 (Januar 19, 2007): 368–370; Martin Vermeer and Stefan Rahmstorf, „Global Sea Level Linked to Global Temperature“, Proceedings of the National Academy of Sciences 106, Nr. 51 (Dezember 22, 2009): 21527–21532.
[xii] Jason A. Lowe and Jonathan M. Gregory, „A Sea of Uncertainty“, Nature Reports Climate Change (Januar 4, 2010): 42–43.
[xiii] J. A. Lowe et al., UK Climate Projections science report: Marine and coastal projections (Exeter, UK: Met Office Hadley Centre, 2009), http://ukclimateprojections.defra.gov.uk/media.jsp?mediaid=87905&filetype=pdf.
[xiv] Caroline A. Katsman et al., „Exploring high-end scenarios for local sea level rise to develop flood protection strategies for a low-lying delta—the Netherlands as an example“, Climatic Change 109, Nr. 3–4 (Februar 24, 2011): 617–645.
[xv] E. J. Rohling et al., „High rates of sea-level rise during the last interglacial period“, Nature Geoscience 1, Nr. 1 (Dezember 16, 2007): 38–42.
[xvi] Lowe et al., UK Climate Projections science report: Marine and coastal projections.
[xvii] Katsman et al., „Exploring high-end scenarios for local sea level rise to develop flood protection strategies for a low-lying delta—the Netherlands as an example“.
[xviii] Marta Marcos and Michael N. Tsimplis, „Comparison of results of AOGCMs in the Mediterranean Sea during the 21st century“, Journal of Geophysical Research 113, Nr. C12 (Dezember 31, 2008), http://www.agu.org/pubs/crossref/2008/2008JC004820.shtml.
Mean Sea Level Trend from satellite altimetry (T/P-Jason-1-Jason-2 merged datasets)
provided by Ocean Monitoring and Forecasting (MyOcean)
Permanent Service for Mean Sea Level (PSMSL)
provided by UK Natural Environment Research Council (NERC)
Sea-Level Rise from the Late 19th to the Early 21st Century
provided by Commonwealth Scientific and Industrial Research Organisation (CSIRO)
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