Sea-Level Changes

Abstract

Tide gauge records and satellite altimetry have demonstrated that the sea level is rising on global and relative (regional/local) scales. Globally, the rate of sea-level rise (SLR) in the past two decades is faster than at any time. During the most recent era, 2006–2018, the global SLR rate was 3.7 mm/year, i.e. nearly three times faster than during 1901–1971 (1.3 mm/year). This is mainly attributed to two main reasons: (1) seawater thermal expansion due to climate change and global warming, and (2) ice melting of the Arctic and Antarctic regions. Additionally, the vertical land movement (subsidence/rise) can impact the calculated relative SLR rates. SLR is projected to continue if global warming will continue. SLR has a destructive impact on coastal cities, especially coastal low-lying areas. Factually, it is not only human infrastructures that are at risk from the SLR and coastal flooding, but also coastal environments such as coastal wetlands, seagrass beds, rocky shores, and sandy beaches are vulnerable to such a rise and flooding. This chapter aims at highlighting the SLR issue on global and relative scales, by using both tide gauges and altimeter tools.

1. Introduction

The importance of the sea-level rise (SLR) issue stems from its direct impact on human lives, coastal infrastructures and constructions, and the coastal environment not only on local and regional scales but also on a global one. Throughout this chapter, the problem will be discussed depending on previous research results from both global and relative (regional and local) points of view to adequately evaluate the problem. The chapter consists of five sections, including this introductory one, which introduces the terminologies used to study the sea-level variations, in addition to the impact of climate change on the observed sea levels. Section two focuses on the tools for measuring that can be used to measure sea level. Section three discusses the global SLR causes and impacts. Section four introduces the relative SLR problem and its impact from regional and local perspectives. The chapter ends with conclusions followed by the set of references listed in the main manuscript, to which those who are interested may refer to.

1.1 Sea-level variations

The observed sea-level variations rely on a variety of variables, including storm surges, astronomical (tidal) harmonic components, the seasonal cycle, interannual to secular variability, and, finally, variations at geological and interglacial scales [1, 2, 3]. At any location and at any time (t), the observed sea level (η) is the sum of three elements: the mean sea level (MSL), the tidal component (X), and the residual component (R). This is mathematically expressed by (Eq. (1)):

$$\eta \left(t\right)=MSL+X\left(t\right)+R\left(t\right)$$E1

The mean sea level (MSL) is the average relative sea level over a long enough time to average out transients like waves and tides [4]. According to [5], the MSL is usually defined as the average value of the observed hourly level over at least 1 year, ideally more than 19 years, to average over the nodal cycle of 18.61 years in tidal amplitudes and phases and to average out the weather. The MSL series can be identified by specific titles, such as monthly MSL and annual MSL [6].

The tidal component (X(t)) is the coherent component of sea level that reacts to astronomical forcing directly or indirectly. Tides are thus described as the periodic rise and fall of a body of water caused by gravitational interactions between the Sun, Moon, and Earth [6]. In reality, the relative positions of the three celestial bodies cause the most visible variations in the magnitude of tides [7]. Tides can be represented analytically as the finite sum of harmonic constants [8] as shown in (Eq. (2)):

$$X\left(t\right)={\displaystyle \sum _n{A}_n}cos\left(\frac{2\partial }{T_n}t+{\varphi }_n\right)$$E2

where A_n is the amplitude of a harmonic component (m), T_n is the period of the specified harmonic component (s), and 𝜙_n is the phase of the harmonic component.

The amplitudes and phases of the astronomical harmonic constituents are heavily influenced by the local geography [8, 9].

Approximately 390 tidal constituents were early identified [10], the most important of which are formed by the gravitational attraction between the three celestial bodies: the Sun, Moon, and Earth. The main lunar semidiurnal tide component M₂ is often the largest recognized tidal constituent. This tidal component’s tidal-producing force is twice as strong as that of the K₁ tide, the main diurnal constituent [1]. Five constituents are particularly important in modeling applications: K₁, O₁, M₂, S₂, and N₂ [7, 11, 12, 13]. This is because these constituents are important for any tidal signal and are adequate to compute variations in tidal levels and currents [9]. Two extreme tidal occurrences are associated with the regular astronomical tides: spring and neap tides. These tides are caused primarily by the combined gravitational influences of the Sun and Moon in relation to their relative positions to each other. The extreme gravitational force between the two celestial entities is extracted when the Moon’s course aligns with that of the Sun (new and full Moon phases), resulting in the spring tide. The neap tide, on the other hand, occurs when the Moon’s course is normal to the Sun’s (1st and 3rd quarter phases). Spring tides have the greatest high tides and lowest low tides, while neap tides have the lowest high tides and highest low tides [7].

The residual (R(t)), also known as surge, is the local shift in ocean elevation along a shore caused by a storm. It is calculated by subtracting the astronomic tidal elevation from the overall elevation and usually lasts a few hours [6]. When wind-generated waves ride on top of the surge, the total instantaneous elevation may be much higher than the predicted surge plus the astronomic tide. This is referred to as a storm surge. Storm surges can be disastrous, particularly on low-lying coastlines. Flooding produced by storm surges has the potential to harm not only coastal structures and human infrastructures but also human lives and the ecosystem along the coast, such as wetlands, seagrass beds, and shorelines. The deltas are one of the most vulnerable areas to storm surges and flooding.

Those who are interested to get more knowledge on the fundamentals and theories of sea-level variations may refer to [1, 5].

1.2 Impact of climate change on observed sea level

Extensive and precise climate monitoring showed unequivocal evidence of recent and accelerating global warming. Climate Change is defined as a change in the condition of the climate that can be identified by changes in the mean and/or variability of its attributes and that lasts for an extended period (decades or longer) [14]. Climate change may be caused by natural processes or by ongoing human-induced modifications in the composition of the atmosphere/land use [14]. The effects of climate change will vary considerably by region on a global scale. For example, warming is expected to be greater over continents than over oceans and to be greatest in the world’s polar areas. According to [15], oceans will become increasingly acidic as carbon dioxide is absorbed by marine creatures and combined with water to form carbonic acid. This acidification can harm coral reefs and alter the ecosystems of a variety of fish, shellfish, and other resources on which people rely. The impact of climate change on the observed sea level can be declared in two main terms: the sea-level rise (SLR) and the associated flooding phenomena, e.g. storm surges. Using a combination of satellite altimeter data and conventional measurements of tide gauges, scientists have determined that the sea level is rising worldwide and that the rate of rise is likely to accelerate [16]. The SLR is a significant consequence of climate change, both for societies and the ecosystem. The 20th century warming is very likely to have added considerably to the observed SLR, through the thermal expansion of seawater and widespread loss of land ice [17]. Climate change is expected to decrease the amount of water frozen in glaciers and ice caps due to increased melting and evaporation. Greater melting and evaporation on the Greenland and Antarctic ice sheets are also expected, but this may be offset by higher precipitation [18]. Extreme high water levels, storm surges and coastal flooding will occur with increasing frequency (i.e. with reduced return period) as a result of mean SLR. Their frequency may be further increased if storms become more frequent or severe as a result of climate change [4].

Those who are interested in more details on climate change and the different theories proposed to explain this global phenomenon may refer to [19, 20, 21] in addition to the reports published by the Intergovernmental Panel on Climate Change (IPCC), e.g [17, 18].

2. Measurement of sea level

2.1 Tide gauge equipment

A tidal gauge, also known as a sea-level recorder, is a device that measures the change in sea level relative to a specified reference known as a “datum.” Sensors continually record the water level’s height in relation to a height reference surface near the geoid. Tide gauges are split into two main categories:

1. Non-recording tide gauges, also known as staff or pole gauges (Figure 1), need an observer to record the level data regularly. A staff gauge is a simple style of tidal gauge used to monitor the sea level. It has a graded vertical board with a width of 150–250 mm and a thickness of 100 mm. Staff gauges of various heights are available, and the appropriate height is determined based on the conditions. The height markers are graduated to a minimum count of 5–10 cm. The staff gauge is mounted vertically at a known elevation. If it is unknown, it should be measured by leveling. The differences in water level are manually monitored by viewing the staff directly from a distance.

2. Recording tide gauges, in which the sea level is recorded by the instrument itself. There are basically four types of these gauges: the floating system gauge, the pressure system gauge, the acoustic system gauge and the radar system gauge.

Figure 1.

Tide staff gauge in Alexandria Western Harbor [22].

The floating system gauge, also known as stilling well tide gauge, is likely the most popular of all sea-level measuring techniques on a global scale (Figure 2). These gauges were once used at every port and were the principal technique used to collect sea-level data. A well’s function is to filter out the wave activity so that tides and longer-period processes can be correctly observed. It is most usually linked with having a float gauge in the well driving a pen and chart recorder or, more recently, a shaft encoder so that sea level height readings can be automatically digitized [23].

Figure 2.

Alexandria Western Harbor tide gauge: the still-well (right) and the recording drum sheets (left).

Pressure sensor gauges are fixed directly in the sea beneath the sea surface to monitor subsurface pressure. In the old pressure gauges such as the Water Level Recorder 7 (WLR7) of Aanderaa, the recorded data used to be stored in an internal data storage unit (DSU) (Figure 3), while in the modern gauges, the sensor is often connected through a cable that carries power and signal lines to an onshore control and logging unit (Figure 4).

Figure 3.

WLR7 and its DSU (https://www.comm-tec.com/prods/mfgs/Aanderaa/).

Figure 4.

Modern pressure gauge system [24].

The acoustic system gauge depends on an acoustic transducer, which can be positioned vertically above the water surface to perform this type of measurement. However, the sensor of the acoustic gauge is preferably housed inside a tube that offers some surface stilling and safeguards the apparatus so that it can operate continuously and reliably under any circumstances where the reflected signals may be lost.

Lastly, radar tide gauges comprise the technology and software required to transform radar measurements into sea-level height. An example of this radar system is the Inexpensive Device of Sea Level (IDSL) system installed in many Mediterranean harbors, e.g. Alexandria Eastern Harbor in Egypt, as shown in Figure 5. The output signals are frequently compatible with current data recorders or can be linked to a communication network. They, like many modern systems, can be configured using a portable computer.

Figure 5.

Radar tide gauge in Alexandria Eastern Harbor [25].

2.2 Altimeter approach

Since the early 1990s, altimeter measurement, the measurement of sea surface height from space, has produced an accurate estimate of changes in sea level every 10 days over the open ocean, attributed to the satellite’s frequent sampling capabilities and global coverage [26, 27]. The European Space Agency launched its radar altimeter onboard ERS-1 in 1991. It performed well, but it did not meet the standards for regional or global sea-level change research [28]. Due to land interference within the radar echo in the coastal area, this early altimeter approach built for the open ocean did not produce valid sea-level data within 20 km of the coast [29]. The TOPEX/POSEIDON, which launched in 1992, heralded a new era in satellite altimetry, with the altimeter and orbit errors being only a few centimeters apart, resulting in sea-level observations that were accurate to 3–4 cm [28]. Factually, coastal altimetry has been developed to increase data quality closer to the shore with higher spatial resolution, to extend the satellite-based sea-level record toward the coast with quality comparable to that of the open ocean [30].

Nowadays, coastal flooding, erosion, coastline movement, maritime security, marine pollution, water quality, marine ecology shifts, several marine biophysical features, and atmospheric and oceanic drivers of change have all been effectively monitored using satellite altimetry [31]. The system of sea height altimeter measurement is depicted in Figure 6. The main measurement delivered by a satellite altimeter system is the Range (R) [31], which can be calculated using (Eq. (3)):

Figure 6.

Altimeter system (https://sealevel.jpl.nasa.gov/missions/technology/).

$$R=\frac{c\times t}{2}$$E3

where c is the speed of light and t is the travel time of the radar pulse down and up.

In practice, the satellite altitude converts the predicted range R to the instantaneous sea surface level (height) (H_isl). Satellite altitude, denoted as (H), is defined as the distance in the normal direction between the satellite center of mass and the reference ellipsoid, as indicated in (Eq. (4)) [22]:

$$H_{isl}=H\text{-}R$$E4

3. Global sea-level rise (SLR) issue

Measuring sea-level change and understanding its causes has greatly improved in recent years, owing mostly to the availability of new in situ and remote sensing measurements [32]. While the sea level has stayed nearly constant over the last two to three millennia, fluctuations have been recorded since the beginning of the industrial epoch [33]. Global sea level is rising, according to direct data from long-term tide gauges and global satellite altimetry. Tides gauge data showed a global mean SLR of 1.6 to 1.8 mm/year over the 20th century [34]. Between 1993 and 2009, high-precision satellite altimetry implies a recent worldwide acceleration with rates as high as 3.4 mm/year [35]. The IPCC treated the issue of SLR as a universal concern caused by global climate change and greenhouse gas emissions. The IPCC included a chapter in each of its released reports that outlines the updated problem of SLR and sheds insight on the many predictions and scenarios to explore this worldwide issue. In its recent report [18], it is revealed that between 1901 and 2018, the global mean sea level rose by 0.20 m. Between 1901 and 1971, the average rate of sea level rise was 1.3 mm/year, increasing to 1.9 mm/year between 1971 and 2006, and then to 3.7 mm/year between 2006 and 2018. Throughout the period 1993–2019, the altimeter records revealed a global mean SLR rate of 3.3 mm/year [36].

Several natural phenomena contribute to global SLR [33, 37, 38, 39, 40]: thermal expansion of seawater due to ocean warming, as well as water mass input from glaciers and ice sheets melting. Over the last century, the global ocean has warmed faster than it has since the end of the previous deglacial transition (about 11,000 years ago) [18]. Between 1992–1999 and 2010–2019, the rate of ice sheet loss accelerated by a factor of 4. Between 2006 and 2018, ice sheets and glacier mass loss were the primary contributors to global mean sea-level rise [18]. Since the early 1990s, various remote sensing tools (airborne and satellite radar and laser altimetry; synthetic aperture radar interferometry (InSAR), and, since 2002, space gravimetry from the Gravity Recovery and Climate Experiment (GRACE) mission) have provided reliable data on the polar ice sheets’ mass balance. These findings revealed that mass loss in Greenland and West Antarctica is accelerated [41]. Indeed, ice sheets accounted for less than 15% of global SLR between 1993 and 2003 [4]. However, their contribution has roughly doubled since 2003 [42, 43].

In addition to these natural phenomena, the anthropogenic (man-made) element plays a key influence on the observed SLR. Since at least 1971, human involvement has most certainly been the primary driver of the recent increased global SLR [18].

There is an increasing consensus that an accelerating SLR scenario due to climate warming will have significant impacts on the coastal zone [44]. Changes in the MSL can gradually alter morphological characteristics, pollute subsurface water with salt intrusion, and render coastal areas inhospitable or unsuitable for agriculture [45]. The SLR can have a harmful influence on coastal areas, causing flooding, property damage, and, in some cases, loss of life [45, 46]. Storminess variations may cause additional changes in extremes [45, 47]. Historically, sea-level extremes have increased in lockstep with increases in the MSL in coastal sites. Using this as a foundation, one may relate sea-level extremes to the MSL, allowing one to predict future extremes and return periods [48]. Another way to assess the impact of the SLR is to determine the probabilistic properties of the nontidal residuals (component of storm surge and waves above the tidal variations). Recent studies have focused on the combined consequences of storm intensification, storm surge, and gradual increase in the SLR. On the Atlantic coasts of Europe and Canada, physical factors have been examined using tide/storm surge models [49, 50, 51, 52].

Future global SLR projections are complicated due to uncertainty in modeling the many contributory processes, which rely on the understanding of the processes that drive sea level increases as well as trustworthy data to check and calibrate models [40]. Although historical sea level trends are useful for planning for future changes, they are insufficient for estimating risk in the face of future uncertainties [53]. The main components of climate-driven sea-level rise—thermal expansion, glaciers and ice caps, the Greenland ice sheet, and the Antarctic ice sheet—are now projected, though solid ice discharge (SID) from the ice sheets remains difficult to constrain [18]. These projections are frequently created utilizing sets of climate models ranging from simple climate models to intermediate complexity models, comprehensive climate models, and Earth System Models. These models simulate changes depending on a collection of anthropogenic forcing scenarios. According to the IPCC’s Fourth Assessment Report (AR4), the global mean sea level would rise by up to 60 cm by 2100 as a result of ocean warming and glacier melting [4]. This forecast increased in the AR5 and AR6 to range between 52 and 98 cm by 2100 for the highest emissions scenario and 28–61 cm for the lowest emissions scenario [18, 54]. To plan for changes due to future sea levels at the local level, local forecasts of SLR that allow varying risk tolerances and cover a variety of periods useful for planning purposes are required [55].

4. Relative (Regional/Local) SLR issue

The global impact of SLR is not uniform. This is attributed to the vertical motion (rise or fall) of the sea surface itself and the vertical motion (rise or subsidence) of the land surface near the sea [38, 56]. The supply of sediments, the wave and current climatology, erosion, and gravitational collapse are also elements that cause variations in the observed coastal sea level. This change in coastal sea level is known as the relative sea-level change, and it can be monitored using a tide gauge at specified coastal locations. Recently, a satellite that detects the motion of the sea surface relative to the center of the Earth (known as a geocentric measurement) has been developed as a sophisticated instrument for assessing the relative SLR issue. A proper assessment of the risks associated with the SLR requires distinguishing between global sea level and relative sea level.

Relative sea-level change is, in fact, the most essential metric for measuring the effects of SLR on infrastructure, property, and ecosystems. Local subsidence of the land causes relative SLR to be greater than global SLR in many regions of the world, especially around several large towns built on deltas. Relative SLR increases the frequency and severity of coastal flooding in low-lying locations, as well as coastal erosion along most sandy coasts [18]. Also, deltas, estuaries, barrier islands, and coral reef communities are among the most vulnerable environments to SLR.

Local changes in the ocean temperature and salinity fields can cause local sea-level changes via variations in the density and volume of the water columns (thermosteric and halosteric effects, respectively) [37, 57, 58, 59, 60]. As a result, they cause geographical variation in the rates of sea-level rise. Land discharge fluxes may also affect the observed relative sea level. The input of freshwater from land into the ocean alters the density structure and thus the ocean circulation [61, 62]. This causes regional dynamical alterations in sea level on time intervals ranging from interannual to multi-decadal [37, 61, 62, 63]. Furthermore, the movement of water mass from land to ocean induces an elastic response of the solid Earth, which deforms ocean basins and coastal morphometry, affecting the observed local sea level [37].

Tide gauge records were used at different coastal regions to calculate the relative SLR rates. The relative sea-level rise rate in the Mediterranean basin was 1.1–1.3 mm/year for the 20th century [64], accelerated to 3.4 mm/yr. in its northwest region for the period 1990 to 2009 [65] and 2.4 ± 0.5 mm/yr. for the period 1993 to 2012 [66]. Shirman [67] observed a SLR of 10 mm/yr. off the Israeli coast in the Levantine Basin between 1958 and 2001. The same rate was found between 1992 and 2002 [68]. Relative SLR rates of 10.6 mm/yr. and 9 mm/yr., respectively, in the Ionian Sea and the Adriatic Sea throughout the decade of 1990–2000 were calculated [69]. The eastern Mediterranean basin had a SLR rate of 0.11 m/110 years, i.e. 1.1 mm/yr [70]. Using the Argo data from 2004 to 2008 [71], investigated the steric sea-level variations in the eastern Mediterranean basin. The results revealed that while in the Ionian basin the total steric sea level change is characterized by strong annual variations (amplitude: 5.9 cm) and a positive trend of 17.9 ± 2.6 mm/year (2004–2008); with a dominant thermosteric impact, the steric sea-level change in the Levantine Sea does not show a clear trend over the same period. However, the thermosteric contribution is also dominant in the Levantine Sea. Ref [72] estimated a SLR of 1.5 mm/yr. in the analysis of tide gauge records from around the North Sea, with modest but not significant differences along its different places. A SLR rate of 3.6 ± 1.6 mm/yr. was calculated in Malacca Strait throughout the period 1986–2013 [73]. The coastlines of the UK exhibited a relative SLR rate of 1–2 mm/yr. as mentioned in [74]. Off the Egyptian Mediterranean coast, the SLR rate has ranged between 2.0 and 3.0 mm/yr. over different periods [75, 76, 77, 78, 79, 80, 81].

Altimeter and satellite measurements have proven to be good tools to calculate the relative SLR rate. For the period 1992–2000, altimetry measurements suggested a rapid rising of sea level (20 mm/yr) in the Eastern Mediterranean which has been associated with increases in the sea surface temperature [82]. In [83] a 16-year altimetry data set (1992–2008) was used to investigate the sea-level variations in the Mediterranean Sea. The results revealed that the amplitudes of the annual cycle vary from 4 to 11 cm, except for a small area of value around 16 cm at the southeast of Crete corresponding to the Ierapetra gyre activity. With a combined analysis of altimetry and tide gauge data in the interval 1993–2008, the absolute sea-level rise and crustal motion in the Adriatic Sea were investigated [84]. In the North-Eastern Adriatic, most of the measurements indicated land subsidence with a rate ranging between −0.51 mm/yr. and −0.29 mm/yr. The absolute SLR was 1.9 ± 0.3 mm/yr. in the interval 1993–2008. From 1993 to 2013, the Strait of Malacca exhibited a SLR rate of 4.1 ± 1.9 mm/yr. as revealed by altimeter analysis [85]. Malaysian sea levels have been rising at a spatially variable rate ranging from 1.4 to 4.1 mm/year throughout the period 1993–2008 [86]. Satellite altimetry data were used to assess trends in sea-level rise in the Dumai Sea of Malaysia over 21 years (from 1993 to 2014). The results of the analysis revealed that the SLR rates ranged from 4.80 mm/year to 5.61 mm/year [83]. According to a preliminary examination of SLR rate near Venice, the detected trend by altimetry (4.25 mm/year) is less pronounced than the trends reported by measurements made offshore (5.65 mm/year) and in the lagoon (5.29 mm/year) [26]. Along the western African region, the SLR rate was 2.15 mm/yr. throughout the period 2002–2018 [36]. The regional trends of SLR throughout the period 1993–2019 (Figure 7) were depicted in [87].

Figure 7.

Regional trends in sea level over 1993 to 2019 from satellite altimetry [87].

Impacts from the relative SLR can be assessed and observed in many phenomena, such as storm surges, inundation of seawater, and flooding. Alexandria of Egypt is subsiding at 2 mm/yr. and even without climate change is highly vulnerable to flooding and erosion, as 35% (700 km²) of the land area is below mean sea level [88]. A 50-cm rise in sea level could result in a loss of 13% (0.05 km²) industrial, 8% (0.46 km²) urban area, and 1.6% (21 km²) beach area, and other physical and socioeconomic losses in Port Said governorate (Egypt), costing more than US$2.2 billion [89]. The Nile Delta, Alexandria, Port Said, and Gamassa may suffer not only direct inundation from the SLR but also saltwater intrusion [90]. This will have a direct impact on groundwater resources, soil salinity, agricultural productivity, and quality in the coastal zone. The relative impact of mean SLR in Australia and wind speed in Ireland were examined [91, 92], concluding that SLR has a larger potential than meteorological changes to increase extreme sea levels and flooding probabilities. The higher sea level resulted in increased flooding frequency in several coastal communities, e.g., Boston, Norfolk, and Miami Beach [93, 94]. These frequent flood events, often termed “nuisance flooding,” do not cause major damage but do cause material harm, inconvenience, and economic drag. Recently, research [94] used tide gauge data to calculate accumulated flooding time in 12 locations along the Atlantic coast and showed a significant increase in flooding duration. It is suggested that flood duration is a reliable indicator for the accelerating rate of sea-level rise, which is often difficult to estimate on a regional scale. The trends of sea-level extremes due to atmospheric conditions for a period of 150 years (1951–2100), in the Greek seas, under a future climate scenario with highly increasing concentrations of atmospheric greenhouse gases were explored [95]. The results confirmed that the majority of extreme events may appear primarily in winter and secondarily in spring. However, results showed that there is an increase in summer extremes, especially over southern areas due to the increase in cyclogenesis. The damage in the socioeconomical sector in coastal cities affected by the SLR issue typically increases faster than the sea-level rise itself [96]. A vulnerability map of the Egyptian Mediterranean coast to SLR (Figure 8) was produced in research [97]. The varying vulnerability classification to SLR along the Egyptian coast is mainly attributed to the different composition of strata over this coast and the different behavior and rates of land subsidence.

Figure 8.

Vulnerability map of the Egyptian Mediterranean coast to SLR [97].

5. Conclusion

To conclude, the sea level has been rising globally. The sea-level rise (SLR) is mainly attributed to two reasons: (1) thermal expansion of seawater and (2) ice melting in the large ice masses on Earth. Both are direct consequences of climate change, which is mainly caused by anthropogenic activities since the industrial epoch. Research and studies have proven that the rate of increase is not uniform all over the globe and that the relative (regional/local) SLR is more important, to discuss the impact of the SLR on coastal structures, the environment, the economy, and human activities. Relative sea-level variations are studied using data from tide gauges and satellites, which are complementary tools to assess these variations. Any shortfall in the sea-level data recorded by gauges can be compensated by that collected from altimetry.

Analyses of tide gauge records indicate that a global mean SLR was between 1.6 and 1.8 mm/yr. over the 20th century. This increased to 3.7 mm/yr. throughout the period 2006–2018. High-precision satellite altimetry suggests a recent global acceleration with a rate of 3.4 mm/yr. Though, this rate varies according to geographical location and land vertical movement around coastlines. Extreme surges, flooding, and seawater intrusion are expected phenomena to associate with the SLR, especially in low-lying coastlines.

The SLR issue has been a main topic of interest in all reports of the Intergovernmental Panel on Climate Change (IPCC) since its First Assessment Report in 1990. According to these reports, there are no solutions to control the SLR but to control climate change through effective adaptation and mitigation plans.

Given that nature has changed over the years and decades and is unlikely to return to its prehuman state anytime soon, the necessity of international cooperation, public awareness campaigns, better monitoring tools, numerical models for simulation and predictions, and the expansion of satellite technology development for marine sciences are all emphasized as key future perspectives.