Sea level changes occur at various time scales. Throughout geologic eras, sea levels have changed drastically many times, primarily following tectonic processes and glacial cycles [1
]. During the Last Glacial Maximum (LGM), sea levels were about 130 m lower than today, because of the large amount of water held by glaciers and ice sheets [2
]. After the major deglaciation (~21,000 years ago), sea levels have remained almost stable over the last 2–3 millennia [3
]. However, with the beginning of the industrial age (late 18th to early 19th century), global sea level rise has accelerated [5
], triggered by abrupt changes in temperature, ice cover, precipitation, etc., rather than being part of a natural cycle. Furthermore, if considering possible greenhouse gas concentration scenarios, by the end of the 21st century, global mean sea levels may rise in the range of 43 cm to 84 cm [10
Regional sea level changes deviate substantially from that of the global mean, and some regions even reveal a condition opposite that of the global trend [11
]. In this case, in addition to the global sea level change and its causes, it is essential to understand the regional variability in rates of this change (i.e., its evolution with time and space and its drivers) in order to assess the potential impacts of sea level rise in coastal areas [12
]. General forcing for regional (or local) sea level patterns can be basically linked to (1) surface warming and cooling of the ocean, (2) exchange of freshwater with the atmosphere and land through evaporation, precipitation, and runoff, and (3) changes in the surface wind stress [11
]. The complex response of the ocean to these forcing mechanisms causes changes in ocean circulation (hence density) and mass transport.
Sea level rise poses a significant threat to areas with low topography such as coasts, islands, and deltas. From the past to present, coastal regions have always attracted high interest in terms of their social and economic impacts [13
]. Increasing human migration to these regions has made the possible consequences of sea level rise even more important. Flooding, inundation, storm, erosion, habitat loss, ecosystem damage, and contamination of underground water are the most damaging/catastrophic effects of sea level rise in coastal areas. The importance of these effects depends on the character of the coastal environment. Nevertheless, it is clear that some of them can threaten human life and coastal installations [10
]. Eventually, in the near-future, rising sea levels and potentially more intense storms will exacerbate possible consequences, and more frequent extreme sea level events will occur. Therefore, effective management and sustainable use of coastal areas need multidisciplinary studies about the reasons and effects of sea level rise.
Tide gauges are one of the oldest instruments for measuring sea level changes [8
]. A tide gauge measures the sea level relative to a fixed point on land, and therefore vertical movement of the point affects sea level measurements. It is necessary to perform geodetic measurements to determine sea level changes, independent of land movements at tide gauge stations. Ideally, Global Navigation Satellite System (GNSS) equipment is attached directly to the tide gauge or located nearby [18
]. In addition to this, a network consisting of tide gauge stations that are referenced to the same datum and have a good distribution is needed to monitor long-term sea level changes. With the development of satellite systems, satellite altimetric techniques have been used for sea level measurements. Since 1993, the modern satellite altimetry record has provided accurate measurements of sea surface height with near-global coverage within latitudes of about 60° N and S. This technique is based on high precision measurement of the distance between the satellite and sea surface. Sea surface height is achieved by combining this information with precise satellite positional data [19
The Black Sea in southeastern Europe (Figure 1
), which is semi-enclosed, has different characteristics from other seas. It is an isolated deep body of water (average depth ~1200 m) with a restricted saltwater exchange with the Mediterranean Sea through the Turkish Straits System (the Bosporus Strait–the Sea of Marmara–the Dardanelles Strait). Unlike the Mediterranean Sea (a concentration basin), the Black Sea is an estuarine basin fed by major European rivers [20
]. Additionally, another feature supported by these conditions is that the Black Sea has a specific density stratification separated by a permanent halocline [21
]. Due to its geographical location, the Black Sea has been of immense strategic importance over the centuries. Its coasts have favorable natural conditions in terms of ecosystem, warm climate, fertile soils, etc., so from antiquity to the present, it has been a desirable region for human habitation [22
]. Consequently, the Black Sea and its coastal zone are very sensitive to climate change and anthropogenic forcing, and thus is an area that has attracted the considerable interest of scientists.
Coastal erosion and saltwater intrusion are major threats for the Black Sea coasts [24
]. It is known that an important part of the most critical coastal erosion areas in Europe is the Black Sea coastline [28
], and in particular, some coastal zones such as those in Bulgaria, Romania, and Turkey have far less protection than in other places.
Both tide gauge and altimetry observations show that sea level trends in the Black Sea vary over time. However, from the beginning of available tide gauge observations in the Black Sea, that is from the 1860s to the first decade of the 21st century, on average, an increase in sea level has been generally observed, with alternating periods of rise and fall. The Black Sea level has increased by 20 cm in the last 100 years [29
]. A rise in the mean sea level of 1.83 ± 0.7 mm/year from the mid-1920s to about 1985 was mentioned in [30
]. Forty-seven tide gauge observations, which were collected along the Black Sea coast except for the Anatolian coast before the year 1985, were evaluated by Boguslavsky et al. [32
]. Considering the effects of continental discharge, atmospheric pressure, and density distribution, they asserted that the Black Sea mean level rate was 1.6 mm per year during the observational period. A rate of 2.2 mm/year from 1960 to the early 1990s was also determined by Tsimplis and Baker [33
]. A rate of increase in the Black Sea level of 27.3 ± 2.5 mm/year for a period of six years (1993–1998), determined from satellite altimetry and tide gauge data, was estimated in [34
]. Sea level change in the Black Sea obtained from along-track altimetry data indicated that sea level rose at a rate of 13.4 ± 0.11 mm/year over 1993–2008; in the western and eastern regions, this rate became 14.2 ± 0.16 and 12.8 ± 0.12 mm/year, respectively [30
]. In the same study, the in situ and satellite results were compared and correlation coefficients ranging 0.4 to 0.7 were calculated between the tide gauge and altimeter measurements. Minimum values were obtained for tide gauges at the western and eastern coasts, whereas maximum ones were at the northern and southern coasts. The altimeter-derived Black Sea levels and corresponding independent in situ measurements have also been compared in other studies. Reasonable correlations between the data from tide gauges and the matching TOPEX/Poseidon along-track passes in the period 1992–1996 were found by Stanev et al. [35
], which were 0.76, 0.68, 0.65, and 0.51 at the Tuapse, Bourgas, Varna, and Nesebar Stations, respectively. From the comparison of data over 1992–1998, the following correlation coefficients were achieved by Goryachkin and Ivanov [31
], as referred to in Ginzburg et al. [30
]: 0.93 for Sevastopol, 0.92 for Yalta, and 0.77 for Tuapse. High correlation coefficients varying from 0.66 to 0.89 at the tide gauge stations along the Black Sea coast (except for Batumi) have also been reported for the changing periods during 1993–2014 using gridded altimetry data [36
Regarding coastal sea level changes, the highest change was recorded at the Poti tide gauge station (8.2 mm/year) along the Black Sea coast, whereas the lowest change was recorded at Kerci tide gauge station (1.3 mm/year) between 1860 and 1990 [37
]. While at the Varna, Constantza, Sulina, Odessa, and Sevastopol Stations, the rates of sea level rise were 3.3, 2.7, 3.7, 7.1, and 3.0 mm/year, respectively; the mean subsidence rates were about 5.2, 1.1, and 6.5 mm/year at Odessa, Sevastopol and Poti, respectively. At Samsun Station, the sea level fell at a rate of −6.9 mm/year from 1963 to 1977. In order for a comparison with tide gauge records, the altimetry time series at the closest grid points to the tide gauge locations along the Black Sea coast were analyzed over the common data periods by Avsar et al. [38
]. For stations with long-term records such as Poti and Tuapse, the rates of sea level changes from the satellite altimetry and tide gauges showed good agreement, and by considering the vertical land motion, the results were greatly improved. Kubryakov and Stanichnyi [39
] asserted that due to the cyclonic rim current intensification for the period of 1992–2005, the sea level was rising 1.5–2 times faster in areas close to the shore than in the offshore (8–9 mm/year versus 4.5–6 mm/year). The spatial distribution of the Black Sea level trends over 1993–2014 showed that rates of sea level change during this period varied from 0.2 to 5.0 mm/year [40
]. The southeastern region indicated a faster rise than in the other parts. Kubryakov et al. [41
] pointed out that the spatial differences observed in the sea level rise were again related to basin dynamics on account of the intensification of cyclonic wind curl (3.2–4 mm/year in the coastal areas versus 1.5–2.5 mm/year in the offshore area).
In order to estimate and model regional sea level change accurately, it is important to detect sea level forcing mechanisms. According to Volkov and Landerer [42
], the forcing of sea level in the Black Sea is dominated by the basin’s freshwater budget (river + precipitation inputs > evaporation output) and water exchange through the Bosporus Strait as well as depth-integrated changes in seawater density. This means that changes in the water balance are the main factors for sea level variability in the Black Sea. First, it requires an investigation of long-term total sea level change in the Black Sea. This study presents an analysis of sea level changes in the Black Sea using satellite and in situ data. It aims to provide a reliable estimate of the present-day sea level rise using the data from tide gauge stations along the Black Sea coast and satellite altimetry. This study including information on absolute sea level change obtained from satellite data in the Black Sea, contributes to the relative sea level estimates by Avsar and Kutoglu [43
]. Sea level observations from satellite altimetry as well as tide gauge stations have been used to infer trends in changes in Black Sea levels and their periodicity. In addition, in order to determine vertical land motion along the Black Sea coast, the data of six continuous GNSS stations, which are nearly co-located with the available tide gauge locations, were used in this study. Thus, the contribution of land motion to the coastal sea level change was also investigated.
In the Black Sea, having a limited interaction with the Atlantic Ocean, there are strong temporal mass variations due to its wide drainage area covering a large part of Europe and Asia, and sea level change is closely related to its hydrological balance. The results of this study confirmed that the Black Sea level has continued to rise over the near satellite altimetry era (1993–2017). In this context, monitoring sea level change in the Black Sea is critical for determining its long-term variability and mitigating its negative impacts.
focuses on the spatial distributions of low-lying areas surrounding the Black Sea. Since coastal slope is the main indicator, these areas are highly vulnerable to sea level rise. In order to estimate the vulnerability of these areas, the general characteristic of the regions should be examined in terms of soil type, land use, population, income, etc.
The level of the Black Sea has been rising at a mean rate of ~2.5 mm/year from January 1993 to May 2017, although a slowdown of this rate was recorded over the last about three years. Nevertheless, in order to confirm this supposition, the dominant cycles in the Black Sea level time series should be examined spectrally. Thus, the recent rate of sea level rise can be estimated more accurately. Note that, the dominant cycles of sea level change indicate that the Black Sea rose at a rate of about 3.2 ± 0.6 mm/year until December 2014. This rate was nearly identical to the global trend, which was reported by Legeais et al. [61
]; this common tendency may be attributed to global warming [30
]. Here, it is appropriate to summarize the available literature on the Black Sea level changes for a rightful evaluation (Table 4
). When considering the rate values in Table 4
as well as the character of sea level fluctuations in the Black Sea, the sea level generally tends to rise in the long-term. The estimated rates for the short periods were higher than the estimates for the long periods (note that a period under five years is not significant for statistics). The rate of sea level rise estimated in this study for a period of approximately 22 years was about 1.8 times greater than the rate in the preceding half a century (1920–1985), as quoted from Goryachkin and Ivanov [31
]. However, the fluctuations in the Black Sea from 1993 to 2014 were not uniform: the sea level rose in general for the period of January 1993–June 1999, fell during July 1999–April 2006 (even though it slightly increased from the beginning of 2001 to about 2005), and rose again until 2014. Then, it started to fall again; throughout the 1993–2017 period mentioned in this study, the mean sea level displayed a positive trend of 2.5 ± 0.5 mm per year. According to Ginzburg et al. [30
], the sea level increase from 1993 to 1999, and then decrease from 1999 to 2001 are in agreement with data on the Danube River discharge. In addition, it was mentioned in Cazenave et al. [34
], Avsar et al. [62
], and Vigo et al. [63
] that sea level rise over 1993–1999 showed good correlation with the increase in sea surface temperature in the Black Sea over this period.
Along the coasts, complex ocean dynamics occur at shorter spatial and temporal scales. For example, tides are much more complex near the shore than in the open sea. Moreover, the high frequency variations due to atmospheric pressure and tides must be taken into account in these areas. Over shorter periods, sea levels rise faster at the coasts than offshore [65
]. The main feature of sea dynamics in the Black Sea is the cyclonic rim current flowing along the continental slope, and it leads to lower sea level in the interior of the basin and higher sea level along the coast [40
]. Table 5
presents an extensive review of the rates of the sea level changes at common tide gauges from different studies. Although the data periods are different, the results obtained in this study showed similar characteristics with Alpar et al. [37
] and Avsar et al. [38
], especially for Poti and Tuapse. At Poti Station, a high sea level trend has been generally estimated. This relative sea level rise may have resulted from subsidence at the Poti coast [30
]. At Tuapse, ground subsidence may be a determinant in the sea level rise at this station [38
]. While our results indicate that the rate of sea level rise during 1945–1994 slowed down at the Sevastopol tide gauge station, the trend estimates in Alpar et al. [37
] and Kubryakov and Stanichnyi [39
] were higher. The trend estimates in this study were calculated after removing the seasonal cycles. Thus, there was a small discrepancy in the trend estimates along the southern coast of the Black Sea compared to Avsar et al. [38
depicts the coastal areas in the Black Sea, which would be under water if the sea level rises 1 m. As previously outlined, coastal erosion is also a remarkable problem along the Black Sea coastline. The observed rise rates (see Table 2
) along the Black Sea coast and the basin-averaged rate (from the gridded altimetry data) of the sea level rise may be significant for the threat of coastal erosion. It has been estimated by Allenbach et al. [24
] that a 50 cm rise in sea level might lead to about a 50% reduction in the Black Sea beach area. According to Goryachkin and Ivanov [31
], the shore might retreat 1–2 m for a rise in the sea level by 1 cm [41
Numerous studies have been carried out to determine the long-term variability of sea level in the Black Sea. These studies, based on altimetry and tide gauge data, have revealed that the level of the Black Sea has risen. Our study dealt with recent sea level changes in the Black Sea over a time period for which data from tide gauges and satellite altimetry are available. The mean rate of the sea level rise has been estimated as 2.5 ± 0.5 mm/year over the entire Black Sea by using the gridded satellite altimetry data covering January 1993–May 2017. During this period, it was seen that inter-annual variability of non-seasonal sea level change was quite strong (with a standard deviation of about 6.7 cm). In addition, coastal sea level changes were analyzed from 12 tide gauge stations along the Black Sea coast. However, most tide gauge data are not up to date and the spatial distribution of the stations is sparse. Nevertheless, using the available data, relative sea level changes along the Black Sea were assessed, and the results generally reflect a rise in the sea level. The highest rate of rise (7.01 ± 0.12 mm/year) was at the Poti tide gauge station. These results, combined with the vertical rates of GNSS stations, showed that at some tide gauge locations, there were significant vertical movements. This study suggests that a regional network of tide gauge stations with a suitable spatial distribution, along with co-located continuous GNSS stations along the Black Sea coastline, should be established. Continuous geodetic measurements can be used to monitor vertical land movements to estimate absolute sea level changes independent of vertical motions of the land.
The results of this study demonstrate that accurate modeling of sea level changes depending on time and location in the Black Sea, which is semi-enclosed, is crucial for risk assessments related to sea level rise, analysis of coastal change, and planning of coastal area use. Local, regional, and national patterns of potential consequences of sea level rise should be assessed, and coastal vulnerabilities should be identified in this region. The implications of sea level rise should be considered for population location, economic, infrastructure, and construction planning. This issue should be regarded as higher priority in coastal management. The related governments and local authorities should design long-term policy for coastal planning. The necessary precautions for reducing the effects of sea level rise should be implemented for all coastal areas.