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date: 10 December 2022

Regional Sea Levelfree

Regional Sea Levelfree

  • Thomas WahlThomas WahlUniversity of Central Florida
  •  and Soenke DangendorfSoenke DangendorfOld Dominion University

Summary

Sea level rise leads to an increase in coastal flooding risk for coastal communities throughout the world. Changes in mean sea level are caused by a combination of human-induced global warming and natural variability and are not uniform throughout the world. The key processes leading to mean sea level rise and its variability in space and time are the melting of land-based ice and changes in the hydrological cycle; thermal expansion due to warming oceans; changes in winds, ocean currents, and atmospheric pressure; and, when focusing on the relative changes between the land and the ocean, any vertical motion of the land itself (subsidence or uplift). In addition to the change in mean sea level, which is the main climatic driver for changes in coastal flooding risk in most regions, additional changes in tides, storm surges, or waves can further exacerbate, or offset, the negative effects of mean sea level rise. Hence, it is important to analyze, understand, and ultimately project the changes in all of these sea level components individually and combined, including the complex interactions between them. Advances in sea level science in the 21st century along with new and extended observational records including in situ and remote sensing measurements have paved the path to being able to provide better and more localized information to stakeholders, particularly in the context of making decisions about coastal adaptation to protect the prosperity of coastal communities and ecosystems.

Subjects

  • Climate Impact: Extreme Events
  • Climate Impact: Sea Level Rise
  • Climate and Coasts

Introduction

Sea level rise (SLR) is one of the most obvious consequences of a warmer climate. As such, global mean sea level (GMSL) rise has been widely discussed in the scientific literature and is used as one of the key indicators to track climate change impacts (U.S. Environmental Protection Agency, 2022). Global and regional mean sea level are also designated as Essential Climate Variables by the Global Climate Observing System. In 1990, the Intergovernmental Panel on Climate Change first provided future projections of GMSL rise until 2100 (Houghton et al., 1990). Since then, subsequent assessment reports (ARs) included revised projections for the 21st century and, since AR5, also for longer time horizons, up to 2300, also including, for the first time, projections of regional SLR. For a long time, tide gauge records were the only reliable source of information to track changes in the sea level. Unfortunately, the uneven spatial distribution of tide gauges that provide long-term records of several decades to centuries, with a strong bias toward the Northern Hemisphere, complicated the derivation of GMSL and its changes through time. With the advent of satellite altimetry, which started orbiting the Earth in the early 1990s, much more reliable estimates of GMSL rise (at least over the period covered by the satellite data) became possible. At the same time, a more complete picture of the strong spatial variability in SLR was obtained, which could only be partially revealed by the point measurements taken by the tide gauges. This is of particular importance for coastal adaptation planning, which takes place at the local to regional scale, and hence associated changes in sea level also need to be assessed at the local to regional scale. In this context, regional sea level changes can be the result of changes in any of the components that comprise the instantaneous water levels that can be observed at the coast, including changes in the regional relative mean sea level (MSL; including the important effects of vertical land motion), astronomical tides (with frequencies ranging from minutes to years), storm surges or non-tidal residuals (caused by tropical or extra-tropical storms), and waves. The latter can lead to wave setup, which is the increase in MSL due to the presence of breaking waves, and to wave run-up, which is the maximum vertical extent of wave uprush on a beach or built structure. The different sea level components and the terminology used for water levels that are composed of different combinations of these components are outlined in Figure 1. In this article, the sum of the MSL, tides, and storm surges is referred to as still water level (SWL); when the dynamic wave contribution is included as the wave setup, it is referred to as dynamic still water level, and when the wave run-up is included, it is referred to as total water level (TWL).

Figure 1. Sea level components contributing to total water levels with erosion and flooding potential.

This article discusses the current knowledge on the spatiotemporal changes in the individual components in the past and projections (and associated uncertainties) for the future. It also discusses the nonlinear interactions between some of the components and how these may compound to lead to an increase or decrease in the overall change in TWL, which impacts coastlines worldwide.

A Brief History of Sea Level Science

The first in situ sea level measurements began in the mid-17th century in Amsterdam, Netherlands and Brest, France (Wöppelmann & Pirazzoli, 2005; Wöppelmann et al., 2006), but it took approximately 150 years until more countries throughout the world started their own tide gauge measurement campaigns in the early 19th century. Although originally installed to measure (often only high and low) water levels in support of ship navigation, long records from tide gauges now provide the most valuable information for scientists to track SLR. Since 1933, the Permanent Service of Mean Sea Level in Liverpool has collected and analyzed monthly and annual MSL time series for scientific purposes and holds today more than 60,000 station years from more than 2,000 locations throughout the world (Pugh & Woodworth, 2014). The spatial distribution of tide gauges throughout the world is not homogeneous, with a strong spatial bias toward the Northern Hemisphere and along the coastal mainland, and only few open-ocean sites located on islands (Dangendorf et al., 2017; Holgate et al., 2013). Tide gauges also measure sea level relative to the land on which they are grounded and thus track both changes in sea and land level (also widely known as relative sea level) (Wöppelmann & Marcos, 2016). Attempts to collect and interpret these records in terms of GMSL—that is, the average over the entire surface of the ocean—date back as far as the early 1940s (Gutenberg, 1941). These GMSL reconstructions have been refined during the past several decades (e.g., Church & White, 2011; Dangendorf et al., 2019; Douglas, 1991; Emery, 1980; Frederikse et al., 2020; Hay et al., 2015; Jevrejeva et al., 2006) and consistently show an increase of GMSL on the order of 1.5 mm/yr over the 20th century (Oppenheimer et al., 2019). Since 1992, data from the tide gauge network can be supplemented by information collected through various satellite missions that track sea level changes with near-global coverage and high precision from space (Nerem et al., 2018). Satellite measurements track sea level relative to the Earth’s center of mass and are therefore not affected by vertical movement of the land (this is widely known as absolute or geocentric sea level). Satellites have revolutionized the understanding of sea level and ocean circulation changes and provide the most accurate estimate of GMSL change showing, in agreement with tide gauges over the same period, an average rise of 3.3 ± 0.4 mm/yr since 1993 (Ablain et al., 2019). Although altimetry data are very accurate over the open ocean, the quality declines along the coast, where the measurements suffer from the nearby presence of land. To address this issue, coordinated efforts began in the early 2000s and a growing coastal altimetry community (the International Coastal Altimetry Community) is working toward re-processing and extending the capabilities of altimeters to provide more accurate information along the coast (Cazenave et al., 2022; Vignudelli et al., 2011). There are no direct measurements of sea level available before the Industrial Revolution, but sediment cores from salt marshes or corals provide useful indicators (or proxies) of sea level change that often extend over large parts of the Holocene (Gehrels & Woodworth, 2013). Compilations of those proxy records into GMSL change indicate that the 20th-century increase recorded by tide gauges is indeed unprecedented over at least the past 3,000 years (Kopp et al., 2016). Formal detection and attribution studies using a combination of observations and historical climate model simulations with natural and anthropogenic forcing agents indicate that at least 37% of the observed GMSL rise since 1900 has been produced by human contributions (i.e., increasing greenhouse gases and aerosols) (Marcos et al., 2017; Slangen et al., 2016). This contribution has significantly increased during the second half of the 20th century, thus explaining approximately 70% of the observed increase in GMSL since 1970 (Slangen et al., 2016), as well as the major fraction of the observed GMSL acceleration seen in tide gauges since the 1960s (Dangendorf et al., 2019) and from satellites since the early 1990s (Nerem et al., 2018).

It is important to note that there is strong spatial variability in the rates of SLR throughout the world, leading to much higher or lower rates of rise in many locations compared to the global mean. As outlined previously, these local and regional sea level changes along the coastline are most important to understand how SLR modulates flood risk and how communities need to adapt in the future to limit the negative impacts of SLR. The main physical processes that cause SLR and the spatial variability are discussed in the section on “Current Knowledge on Sea Level Change.”

The other sea level components outlined previously (and discussed in more detail later)—that is, tides, storm surges, and waves—received much less attention in the past. This is because their changes are relatively less important than SLR in most places, but also because observational data are even more sparse (because high-frequency measurements are required) and using numerical models to simulate data over long time periods is computationally very expensive. Hence, more comprehensive assessments of these high-frequency sea level components did not start until the early 2000s (e.g., Cox & Swail, 2001; Menéndez & Woodworth, 2010; Zhang et al., 2000). Currently, national and global databases of tide gauge records with at least hourly resolution (Caldwell et al., 2015; Haigh et al., 2021), including historic information from data archeology (Talke & Jay, 2013, 2017), as well as measurements of relevant wave parameters (e.g., National Oceanic and Atmospheric Administration, 2022), such as wave height and periods, are available and contain records long enough to assess changes in the relevant variables over time. Such studies can be complemented by regional (e.g., Arns et al., 2015; Lin et al., 2019; Marsooli et al., 2019; Vousdoukas et al., 2017) and global (Muis et al., 2016; Reguero et al., 2019; Vousdoukas et al., 2018) re-analysis data sets derived with numerical models.

Current Knowledge on Sea Level Change

Changes in MSL

The physical processes of MSL change are usually separated into global (i.e., GMSL) and regional (for a detailed introduction to the terminology of sea level change, see Gregory et al., 2019). Changes in GMSL are mainly determined by two processes. The first process is related to the exchange of water (mass) between land (e.g., water residing as ice forming glaciers or ice sheets, or in other terrestrial storages such as lakes, dams, or aquifers), the atmosphere, and the ocean, and it is termed barystatic sea level change. The second process is related to changes in the ocean’s density that lead to either expansion (SLR) or contraction (sea level fall) of the water column—the so-called steric sea level change. Steric sea level changes are driven by variations in seawater temperature or salinity. Global mean steric sea level rise is dominated by temperature-driven steric sea level changes (i.e., thermosteric), whereas variations in salinity (halosteric) only have a very minor impact on GMSL, albeit being important regionally (e.g., Jordà & Gomis, 2013). Since 1900, GMSL reconstructions from tide gauge records indicate an increase of approximately 20 cm (Church & White, 2011; Dangendorf et al., 2017, 2019; Frederikse et al., 2020; Hay et al., 2015, 2017; Jevrejeva et al., 2014; Palmer et al., 2021; Ray & Douglas, 2011), with a rate that has persistently been accelerating since the late 1960s (Dangendorf et al., 2019; Nerem et al., 2018) (Figure 2). Approximately two-thirds of the increase since 1900 has been attributed to barystatic sea level changes (predominantly by glacier melting) and only one-third to thermal expansion (Frederikse et al., 2020). The acceleration since the 1960s has likely been initiated by thermal expansion (Dangendorf et al., 2019; Frederikse et al., 2020), but a significant increase in mass loss of the ice sheets in Greenland and Antarctica has been dominating the increase in rates since the beginning of the 2000s (Chen et al., 2017; Frederikse et al., 2020; Nerem et al., 2018).

Changes in regional MSL are generally more complex and lead to strong deviations compared to GMSL and across individual locations (see Figure 2). This is related to a potpourri of additional redistribution processes that cancel out in the global mean (for a detailed review of regional sea level change processes, see Gregory et al. [2019] and Hamlington et al. [2020]) and that are illustrated in Figure 3. First, any barystatic mass change (e.g., due to ice melt in Greenland, Antarctica, or in mountain glacier regions) does not only lead to an increase in GMSL but also is accompanied by changes in Earth’s gravity, rotation, and deformation (Farrell & Clark, 1976; Mitrovica et al., 2001). Large bodies of mass such as ice sheets exert a gravitational pull on the surrounding ocean and compress the solid earth beneath (Figure 3a). When ice melts, the mass of the ice sheet and therefore its gravity onto the ocean is reduced, leading to a migration of the water masses away from the melt source. This leads to an effective sea level fall near the melt source and an above average increase in the far field (Mitrovica et al., 2001). At the same time, the weight of the melt source decreases and the solid earth beneath starts to relax. Together, these processes are also widely known as fingerprints (Bamber & Riva, 2010), and they leave a unique geometry in the ocean’s relative MSL that may last thousands of years. For instance, today’s most significant imprints of those ice-melt fingerprints are still related to melting initiated by the warming after the last ice age, the so-called Glacial Isostatic Adjustment (GIA) (Caron et al., 2018; Peltier, 2004), and they explain why tide gauges from regions that were formerly located beneath large bodies of ice show a relative MSL decrease (e.g., Stockholm in Figure 3; see also Caron et al., 2018). Second, spatially varying rates in ocean heat uptake, changes in ocean circulation, and weather patterns (affecting the ocean via wind forcing) lead to variations in regional MSL (Dangendorf et al., 2021; Wang et al., 2021). Large ocean circulation systems such as the Gulf Stream along the American Atlantic coast carry large amounts of heat, salt, and water mass, and changes in their intensity and position therefore initiate significant changes in regional MSL. For instance, larger than global (and partially wind-forced) rates of SLR persist in the Southern Ocean surrounding the Antarctic Circumpolar Current (Cheng et al., 2022; Dangendorf et al., 2019; Frederikse et al., 2020) (see Figure 2). In general, patterns of steric expansion explain a significant fraction of the observed spatial patterns in sea surface height changes over the Altimetry Era since 1992 (Fasullo & Nerem, 2018; Moreira et al., 2021). Third, in addition to the previously mentioned natural GIA processes, the land surface may be subject to other vertical land motion (VLM), leading to changes in relative MSL as measured by tide gauges (Shirzaei et al., 2021). VLM can generally be separated into natural processes, such as GIA, tectonics, and sediment compaction, or human-induced processes comprising fluid withdrawals (Figure 3c). For instance, many megacities (e.g., Jakarta, Bangkok, and Manila) experienced large population growth with increased drinking water demand during the 20th century, which led to the withdrawal of groundwater and ultimately to subsidence (Nicholls et al., 2021). In the Gulf of Mexico along the coasts of Texas and Louisiana, fluid withdrawals (e.g., through the extraction of oil, gas, or groundwater) have contributed to significant subsidence (Kolker et al., 2011) that has resulted in an almost sixfold increase in relative MSL compared to the global mean (see also the example of Grand Isle, Louisiana, in Figure 2). Finally, changes in the sea level pressure force an inverse barometer response of the ocean beneath (Figure 3d) (Ponte et al., 1991). The higher the atmospheric pressure, the more the ocean gets compressed, whereas the opposite is true for low atmospheric pressure anomalies. The inverse barometer response explains significant portions of the interannual to decadal sea level variability but has less influence on long-term trends (Piecuch et al., 2016).

Figure 2. Global and regional MSL changes over the instrumental era. (Top) Time series of local relative MSL at tide gauges from Stockholm, Sweden, and Grand Isle, Louisiana, together with reconstructed GMSL (extension of Dangendorf et al., 2019). Given trends cover the period since 1950 (dotted line). (Bottom) Reconstructed linear trends over the entire ocean since 1950. Also shown are the locations of the two tide gauges.

Figure 3. Processes of regional sea level change. (a) Gravitation, rotation, and deformation effects accompanying global barystatic sea level changes. (b) Vertical land motion, here illustrated because of groundwater withdrawal. (c) Dynamic sea level changes resulting from a combination of wind forcing and ocean circulation changes. (d) Hydrostatic sea level changes due to local variations in sea level pressure, also known as the inverse barometer effect.

Source: Artwork by Lengxi Dangendorf.

Figure 2 illustrates the regional MSL trends since 1950 as inferred from a state-of-the-art relative MSL reconstruction (based on Dangendorf et al., 2019). Overall, the figure shows that even on timescales of decades to centuries, regional MSL trends may be several times larger or smaller than the trend in GMSL. This demonstrates that for adaption purposes, local sea level, rather than the global mean, is the correct metric for consideration.

Changes in Extreme Sea Levels

Although relative MSL rise is the main oceanographic driver for long-term changes in coastal flooding along coastlines (Menéndez & Woodworth, 2010), changes in other sea level processes (or components) causing instantaneous water levels and superimposed onto MSL can further exacerbate or reduce the effects of SLR. This section summarizes the current knowledge on how tides, storm surges, and waves have changed in the past and how they may change in the future.

Tides

Tides are a result of the gravitational attraction exerted by the Sun and the Moon and the rotation of the Earth. Different tidal regimes exist across the global ocean and along the coastline. Tides can be classified into three regimes based on the periods of the most dominant tidal constituents (of which there are many that superimpose to create the overall tidal signal): semi-diurnal, where two high tides and two low tides occur each day; diurnal, where one high tide and one low tide occur each day; or mixed, where two high tides and low tides occur but with different heights. In addition, tides can be classified based on the tidal range, i.e., the difference between high and low tide. The latter varies strongly along the coast, ranging from only a few centimeters in almost fully enclosed basins such as the Baltic Sea and Mediterranean Sea to several meters in places such as the North Sea or the U.S. northeast coast. When tides enter estuaries or bays, the tidal range can further increase; for example, it reaches more than 15 m in the Severn Estuary in the United Kingdom and in the Bay of Fundy in Canada. Given that tides are caused by the Sun–Moon–Earth system, it was assumed for a long time that they are stationary, and as such they were widely used to define vertical datums, among others. However, research during the past few decades has shown that tides have changed over the period covered by long tide gauge records in many places throughout the world (e.g., Haigh et al., 2020; Müller, 2011; Müller et al., 2011; Talke & Jay, 2020; Woodworth, 2010). There are various reasons why tides can change locally (Haigh et al., 2020; Talke & Jay, 2020); the most important one is dredging, which takes place in many urbanized estuaries where ports and marinas are located and certain channel depths need to be maintained in support of ship traffic. The change in channel topography and associated change in bottom friction can lead to increases or decreases in the tidal range. Land reclamation can have a similar effect. Newly built jetties or piers, however, can lead to turbulence changes and more rapid energy dissipation, which in turn can lead to a reduction in tidal range. Along the open coast, tides can change due to variations in ocean stratification related to varying density changes throughout the water column (Jänicke et al., 2021; Ray & Talke, 2019) or due to sea level rise, which also leads to changes in bottom friction (see the section on “Nonlinear Interactions Between Processes” for more details on this particular aspect).

There are many individual locations and wider regions, such as the North Sea (Jänicke et al., 2021) or Gulf of Maine (Ray & Talke, 2019), where changes in tides have occurred in the past, often leading to more rapid changes in tidal high water levels compared to mean sea level (Dangendorf et al., 2013; Mudersbach et al., 2013). For the U.S. coastline, Li et al. (2021) showed that past changes in tides contributed significantly to the observed increase in nuisance flooding events throughout the country. Of 40 tide gauges analyzed, tidal range increased at 18 (most of which are located in estuaries) in the past, and 27% of the nuisance flooding events that occurred across these locations since 1950 would not have occurred if tides had not changed. This highlights the importance of tidal changes locally because they can exacerbate or alleviate (when tidal range decreases) the negative impacts of SLR.

Only a few studies have assessed potential future changes in tides, and the focus of these studies was exclusively on the effect of SLR on changes in tides. However, the effect of this will likely be relatively small compared to the changes that have occurred in the past due to direct anthropogenic impacts (i.e., system alterations) and will likely continue in the future; but such changes are hardly predictable.

In addition to the dominant diurnal and semidiurnal constituents, tides are also modulated on longer timescales, through the 4.4-year perigean cycle and the 18.6-year nodal cycle. The relative importance of these multiyear tidal cycles depends on the location and varies throughout the world (Haigh et al., 2011), ranging from a few millimeters to several decimeters. This can have effects on the occurrence probabilities of extreme still water levels during certain years (Baranes et al., 2020; Talke et al., 2018; Enríquez et al., 2022) and also affects high-tide flooding, where the frequency of these events can change much more rapidly during certain years/decades due to the combination of SLR and increasing tidal ranges associated with the 4.4.- and 18.6-year cycles (Thompson et al., 2021).

Storm Surges

Storm surge refers here to the meteorologically induced deviation between the observed still water levels and the predicted tide water levels (see Figure 1). Many studies have been conducted to assess past changes in the storm surge climate, ranging from local (e.g., Dangendorf et al., 2014; Talke et al., 2014) to continental (e.g., Marcos & Woodworth, 2017; Wahl & Chambers, 2015) and global (e.g., Mawdsley et al., 2015; Menéndez & Woodworth, 2010). The key conclusion of all of these assessments was that there is no evidence for long-term trends in the magnitude or frequency of storm surges over the 19th and 20th centuries and SLR is considered the main driver for changes in extreme SWLs during the past 150–200 years. However, there are some exceptions in which significant trends have been consistently found in previous studies even though different methods were employed; for example, positive trends in the magnitude of storm surges were found in the Gulf of Mexico, whereas negative trends persist along the north Australian coast, as well as along multiple locations along the European North Atlantic coast. Calafat et al. (2022) identified a dipole in the storm surge trends over Europe for the period from 1960 to 2018, with positive trends over the northern United Kingdom and negative trends in the southern United Kingdom and along the European mainland from Denmark to the Iberian Peninsula. They were the first to perform an attribution study in the context of changes in storm surges, and although they found a detectable anthropogenic signal, the majority of the trends are the result of natural variability. These multi-decadal fluctuations are closely linked to different modes of climate variability and can modulate, for example, 50- or 100-year storm surge levels by up to several decimeters over timescales from years to a few decades (Marcos et al., 2015; Rashid et al., 2019, 2021; Rashid & Wahl, 2020; Wahl & Chambers, 2015, 2016). Therefore it is crucial to interpret trends in storm surges derived from relatively short records carefully because they may merely stem from natural variability as opposed to indicating longer term trends that could possibly be caused by global warming.

Local or regional assessments of future changes in the storm surge climate have been conducted for different areas of the world’s coastlines, using hydrodynamic models driven by atmospheric fields from global or regional climate models (e.g., Lin et al., 2019; Woth et al., 2006) or using synthetic tropical cyclone information for the end of the century (e.g., Marsooli et al., 2019). Global assessments of future changes in the storm surge climate have only become computationally feasible during the past few years, and the number of studies and ability to include larger ensembles from various climate models for a range of emission scenarios are still limited (Muis et al., 2020; Vousdoukas et al., 2018). These studies identify locations where future changes in the storm surge climate (e.g., expressed as changes in return periods) could be significant, but the uncertainties surrounding these projections are still large and the expected changes are typically much smaller compared to the expected due to SLR, which will be the dominant driver of future changes in extreme SWLs.

Waves

Surface ocean waves can strongly affect TWL changes at multiple timescales via wave setup and swash (Hoeke et al., 2013; Melet et al., 2018, 2020). However, their high- and low-frequency contributions to TWLs have been rarely considered in larger scale flood risk assessments until recently (Melet et al., 2018, 2020). The three wave parameters that typically drive such processes are the wave height (most often the significant wave height representing the average of the 33% highest ocean waves measured over a given time span), the wave period, and the wave direction. Most studies of wave climate have been concentrated on changes in the wave height. In situ measurements from wave buoys rarely cover more than 20 or 30 years, making the data less suitable to analyze and interpret past trends in the wave climate, but local assessments have been conducted for areas where sufficient buoy records exist, pointing to significant changes in wave heights, particularly during extreme events (e.g., Ruggiero et al., 2010; Wahl & Plant, 2015). Global-scale observational studies have become possible through the availability of remote sensing data from satellites (e.g., Young & Ribal, 2019; Young et al., 2011). Results from these assessments point to slight increases in significant wave heights in many places, with stronger changes found in extreme events (e.g., events above the 90th percentile), particularly over the Southern Ocean. In the absence of long in situ or remote sensing measurements, model studies have been extensively used to assess past and future changes in the global wave climate (Erikson et al., forthcoming; Morim et al., 2019). These efforts were consolidated through the Coordinated Ocean Wave Climate Project (Hemer et al., 2010, 2012). Meucci et al. (2020), for instance, used three wave reanalysis products to assess trends in significant wave height for the entire 20th century and also to compare these model trends with much shorter satellite records. When considering the entire 20th century, they find statistically significant trends in wave height for large areas of the global coastline. However, these results have to be interpreted with caution because it is known that long atmospheric reanalysis products suffer from inconsistencies that could lead to spurious trends, particularly when including data from the first half of the 20th century (Krueger et al., 2013). More recently, Erikson et al., forthcoming used all the contemporary reanalysis and hindcasts (Morim et al., 2022) available to date to assess trends in the seasonal mean significant wave height, mean wave period, and mean wave direction during the past 35 years (1980–2014) and also to compare these model trends with the satellite records (Young & Ribal, 2019. These results indicate strong signals of upward trending wave heights over up to one-third of the global ocean and along many coastlines. Erikson et al., forthcoming also indicated that relying on single products to quantify changing global scale wave fields could potentially be misleading and that assessments of future long-term change can benefit from using ensemble-based trends.

Although large uncertainties still persist in future global and regional projections (Morim et al., 2018), robust changes throughout the century, including wave height increases and decreases depending on the location, have been established across wide regions of the global ocean (with the magnitude depending on the underlying greenhouse gas emission scenarios, wave modeling methodologies, and global climate model forcing) (Morim et al., 2019). Similar changes have been projected for mean wave period and directions, also depending on location (Morim et al., 2019), with approximately half of the world’s coasts showing robust changes in at least one of these wave parameters (significant wave height, mean wave period, and/or mean wave directions). In terms of extreme events, existing studies indicate that robust changes in the storm wave events (e.g., events above the 90th percentile and 100-year return periods) are limited to the Southern Ocean (Meucci et al., 2020; Morim et al., 2018, 2019). More recently, Morim et al. (2021) reported significant changes in a core set of extreme wave indices (representing high-frequency extreme wave events) across wide regions of the global ocean, and they indicated that the intensity of these storm events can nearly double in some regions.

Nonlinear Interactions Between Processes

The previous sections provided a brief overview of the current knowledge on past and future changes of the key processes that combine to create instantaneous coastal water levels: mean sea level, tides, storm surges, and waves. Importantly, although changes have been observed and are expected in the future in all of these processes individually, complex nonlinear interactions between them can further exacerbate or offset the associated changes in coastal flooding risk (Idier et al., 2019). Some of these interactions have been more extensively studied than others, and the focus here is on the most important ones for which previous assessments provide indications of how the interactions could increase/decrease coastal flooding risk.

For example, MSL and tides interact. As discussed in the section on “Tides” changes in water depths due to channel dredging are among the main reasons why tides can change. MSL rise has a similar effect; as the water depth increases, bottom friction reduces and tidal propagation along the coast is altered. This can lead to an increase or decrease in the tidal range as a response (Green, 2010; Idier et al., 2017; Pelling et al., 2013; Pickering et al., 2017; Schindelegger et al., 2018). By focusing, for example, on the 136 largest port cities, and considering uniform SLR of 2 m and a fixed shoreline (i.e., no flooding takes place), Pickering et al. (2017) found changes in the mean tidal range between –0.25 m (Surabaya, Indonesia) and +0.33 m (Rangoon, Myanmar). An increase in tidal range increases the risk of flooding, hence exacerbating the effect of SLR, whereas a decrease in tidal range offsets some of the effects of SLR in terms of flooding risk. The results can vary when overland flooding is allowed and the shoreline recesses (Pelling et al., 2013), but overall, these previous assessments indicate that changes in the tidal range in response to SLR can be on the order of –15% to +15% of the assumed SLR scenario, with strong spatial variability.

Interactions between tide and surge (referred to commonly as tide–surge interactions) are more pronounced in shallow water where the highest meteorologically induced surges do typically occur around low tide or during rising tide as opposed to high tide (Horsburgh & Wilson, 2007). This means that tide–surge interaction leads to a reduction in still water levels in many places, and hence flood risk is likely overestimated in studies in which these nonlinear interactions are ignored. In the only near-global assessment of tide–surge interaction, Arns et al. (2020) showed that extreme sea levels could be up to 30% (or 70 cm) higher without tide–surge interaction. They also highlighted that the dependence between tide and surge has changed over time in many places spread across the global coast, leading to a strengthening in the tide–surge interaction (i.e., counteracting the negative effects of SLR) at the majority of tide gauge sites that were studied and a weakening in some locations.

Finally, SLR relaxes the depth limitation of waves, resulting in waves with larger periods and greater amplitudes reaching the shoreline and leading to higher wave run-up (Arns et al., 2017), especially along protected coastlines such as in northwestern Europe, where the shoreline is hardened and cannot dynamically evolve in response to the changing conditions.

Arns et al. (2017) showed for the German Bight that combined, the nonlinear interactions between SLR, tides, storm surges, and waves could lead to required changes in the design heights of coastal protection of up to twice the amount of SLR; for example, assuming 1 m of SLR could lead to changes in the design water levels of up to 2 m due to the nonlinear interactions discussed here.

Conclusion

This article provides a brief overview of current knowledge on regional sea level changes, a scientific field that has evolved during approximately the past 80 years. In the context of coastal flooding risk and adaptation planning, regional and local SLR projections are required, which may deviate substantially from global mean SLR projections, due mainly to the processes discussed in the section on “Changes in MSL.” Because coastal flood defenses need to protect from instantaneous extreme water levels, other processes superimposed onto MSL are also important. Tides, storm surges, and waves can also change over different timescales for various reasons. Although the effects of these changes are smaller than the effects of SLR and often occur more localized, they can exacerbate or alleviate the negative impacts of SLR. Other processes, such as freshwater input from rivers, can also contribute to sea level variability at interannual to decadal timescales (Piecuch et al., 2018), but this article focuses only on the oceanographic drivers contributing to extreme coastal water levels and their changes over time.

Significant progress has been made in the broad field of sea level science, and much of the scientific discovery that took place during the past few decades is now informing stakeholders in their quest to make important adaptation decisions in the face of an uncertain future. Reducing these uncertainties through improved understanding of individual processes and the nonlinear interactions between them will be key to support the successful development and implementation of adaptation pathways. Improved data availability—for example, through continuing and new satellite missions (Hamlington et al., 2020), maintaining and expanding in situ measurement networks, data archeology, and database development—should go hand in hand with process-based and data-driven model developments and coordinated efforts to synthesize the results.

Further Reading

  • Fox-Kemper, B., Hewitt, H. T., Xiao, C., Aðalgeirsdóttir, G., Drijfhout, S. S., Edwards, T. L., Golledge, N. R., Hemer, M., Kopp, R. E., Krinner, G., Mix, A., Notz, D., Nowicki, S., Nurhati, I. S., Ruiz, L., Sallée, J.‑B., Slangen, A. B. A., & Yu, Y. (2021). Ocean, cryosphere and sea level change. In V. Masson-Delmotte, P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J. B. R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, & B. Zhou (Eds.), Climate change 2021: The physical science basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 1211–1362). Cambridge University Press.
  • Gregory, J. M., Griffies, S. M., Hughes, C. W., Lowe, J. A., Church, J. A., Fukimori, I., Gomez, N., Kopp, R. E., Landerer, F., Le Cozannet, G., Ponte, R. M., Stammer, D., Tamisiea, M. E., & Van de Wal, R. S. (2019). Concepts and terminology for sea level: Mean, variability and change, both local and global. Surveys in Geophysics, 40(6), 1251–1289.
  • Haigh, I. D., Pickering, M. D., Green, J. M., Arbic, B. K., Arns, A., Dangendorf, S., Hill, D. F., Horsburgh, K., Howard, T., Idier, D., & Jay, D. A. (2020). The tides they are a-changin’: A comprehensive review of past and future nonastronomical changes in tides, their driving mechanisms, and future implications. Reviews of Geophysics, 58(1), e2018RG000636.
  • Hamlington, B. D., Gardner, A. S., Ivins, E., Lenaerts, J. T., Reager, J. T., Trossman, D. S., Zaron, E. D., Adhikari, S., Arendt, A., Aschwanden, A., & Beckley, B. D. (2020). Understanding of contemporary regional sea‐level change and the implications for the future. Reviews of Geophysics, 58(3), e2019RG000672.
  • Jordà, G., & Gomis, D. (2013). On the interpretation of the steric and mass components of sea level variability: The case of the Mediterranean basin. Journal of Geophysical Research: Oceans, 118(2), 953–963.
  • Morim, J., Hemer, M., Wang, X. L., Cartwright, N., Trenham, C., Semedo, A., Young, I., Bricheno, L., Camus, P., Casas-Prat, M., Erikson, L., Mentaschi, L., Mori, N., Shimura, T., Timmermans, B., Aarnes, O., Breivik, Ø., Behrens, A., Dobrynin, M., . . . Andutta, F. (2019). Robustness and uncertainties in global multivariate wind–wave climate projections. Nature Climate Change, 9, 711–718.
  • NASA. (2022). NASA sea level change.
  • Oppenheimer, M., Glavovic, B. C., Hinkel, J., van de Wal, R., Magnan, A. K., Abd-Elgawad, A., Cai, R., Cifuentes-Jara, M., DeConto, R. M., Ghosh, T., Hay, J., Isla, F., Marzeion, B., Meyssignac, B., & Sebesvari, Z. (2019). Sea level rise and implications for low-lying islands, coasts and communities. In 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.), Special report on the ocean and cryosphere in a changing climate. Intergovernmental Panel on Climate Change (pp. 321–445). Cambridge University Press.
  • Pugh, D., & Woodworth, P. (2014). Sea-level science: Understanding tides, surges, tsunamis and mean sea-level changes. Cambridge University Press.

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