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-In: Transformations and evolution of the Mediterranean coastline -
Monitoring sea-level rise in the
Mediterranean
by
Susanna ZERBINI 1, Hans-Peter PLAG 2, Bernd RICHTER 3 ,
Claudia ROMAGNOLI 4
1
2
Dipartimento di Fisica, Universita di Bologna, Italy.
Institut fiir Geophysik, Christian Albrechts Universitiit zu Kiel, Germany.
3
Institut fiir Angewandte Geodiisie, Frankfurt, Germany.
4
Dipartimento di Scienze della Terra e Geologico-Ambientali,
Universita di Bologna, Italy.
ABSTRACT
Sea-level variations are produced by global, regional and local phenomena. Regional and local phenomena such as subsidence and subsurface
water withdrawal may, in some cases, induce rates of change in sea level at
least one to two orders of magnitude greater than the present estimated rate
of sea-level rise, which is about 2 mm/yr. It is therefore of major importance
to reliably determine, over short time intervals, absolute as well as relative
sea-level changes. Through the use of space geodetic and absolute gravity
techniques, the SELF (SEa Level Fluctuations) Project has already succeeded in defining the height of selected tide gauge stations in the Mediterranean to the subcentimeter accuracy level. The determination of vertical
crustal rates at tide gauge stations and of seasonal variations in mean sealevel with a mm accuracy, through the combined use of satellite and laser
altimetry is now an important objective of researchers.
RESUME
Les variations du niveau de la mer sont produites par des phenomenes
globaux, regionaux et locaux. Les phenomenes regionaux et locaux, comme
la subsidence et le pompage des nappes phreatiques, peuvent provoquer
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187
dans certains cas des variations du niveau de la mer qui sont au moins de un
a deux ordres de grandeur plus elevees que la valeur moyenne de la hausse
globale du niveau de la mer, actuellement estimee a 2 mm/an environ. 11 est
par consequent fondamental de determiner de fac;on fiable, sur de breves
periodes, les variations a la fois absolues et relatives du niveau de la mer. Le
projet SELF a deja reussi, en utilisant les techniques de geodesie spatiale et
de gravite absolue, a definir avec un niveau de precision inferieur au centimetre la hauteur de maregraphes selectionnes en Mediterranee. L'objectif
suivant des chercheurs est de determiner les variations verticales de la
crofite terrestre aux stations maregraphiques et les variations saisonnieres du
niveau de la mer avec une precision de l'ordre du millimetre, grace a !'utilisation combinee de l'altimetrie par satellite et par laser.
INTRODUCTION
The analysis and interpretation of the global set of tide gauge data
collected over the last century provide an estimate for a global sea-level
change in the order of 1 to 3 mm/yr though there are also extreme predictions which indicate an average of about 9 to 10 mm/yr. However, the confidence interval for such predictions has not been quantified (Climate
Change, 1995). This phenomenon is, possibly, one of the primary and most
obvious consequences, though difficult to assess, of global climate change.
As a matter of fact, an increase in the global temperature should induce
melting of the glaciers and of the polar ice caps and ice sheets as well as
thermal expansion of the ocean thus producing a rise in sea level. There is,
however, a considerable level of uncertainty in these predictions, and
concern also arises from the fact that, during this century, the average rise of
sea level is noticeably higher than that infered from geological records relevant to the past few thousand years. The initial time of this higher rate of
rise is uncertain though there are indications that it probably began before
the 1850's (WOODWORTH, 1990; GORNITZ and SOLOW, 1991; DOUGLAS,
1992). These are certainly good reasons for establishing projects both at
global and regional levels to monitor sea-level variations.
The study of sea-level variations at regional scale is equally important as
the global investigations, since local levels depend upon both the global sealevel change and the tectonic behavior of the coastline. It is well known that
rates of subsidence in many coastal areas may, quite significantly, exceed
the global rate in sea level; also they can be influenced both directly and
indirectly by human activities. It is, therefore, of major importance to determine absolute as well as relative changes in sea level. This is being recommended within major international programs such as LOICZ (Land-Ocean
Interactions in the Coastal Zone), a core project of the IGBP Global Change
Programme (IGBP, 1993). LOICZ aims to study issues related to the role
played by coastal areas, which are among those most heavily populated on
Earth, in the global climate system and in the light of ongoing changes.
In Figure 1 four examples are presented showing the importance of
properly monitoring the local tectonic behavior or, more generally, the
local conditions at the tide gauge benchmarks (TGBM) in order to interpret correctly relative sea level. Going from top to bottom in Figure 1, the
first time series of annual means computed from tide gauge data collected at
Stockholm, Sweden, illustrates the influence of post-glacial rebound; this
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causes a land rise corresponding to a decrease in sea level relative to the
ground benchmark of about 4 mm/yr. The second example highlights
human influence on local tectonic conditions. In fact, this time series
collected near Bangkok, in Thailand, shows an average relative sea-level
rise of 13.19 ± 0.73 mm/yr, if the complete record from 1940 to 1994 is
taken into consideration. A closer look at the data reveals two different
trends, the first lasting until 1960 in the order of 3 mm/yr and a second,
from 1960 through 1994 of 20 mm/yr. This latter trend is the result of the
increased extraction of ground-water. The following example refers to the
STOCKHOLM
glacial rebound -3.85 ± 0.18 mm/yr
I
-~\
increased
groundwater
extraction
(,..,.,1960)
§
,:
8
FORTPHRACHULA/BANGKOK
ddl~ 13.19 ±0.73mm/y,
1
~
HONOLULU
"""stable, 1.49± 0.15 mm/yr
NEZUGASEKl
subduction, 7.34 ± 0.77 mm/yr
"\.\
T
earthquake (1964)
1900
1950
2000
Year
Figure 1- Examples of sea-level variability (from EHLER et al., 1996).
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189
Honolulu tide gauge in the Hawaiian islands and is the only one among
those presented here which seems representative of the global sea level rise
(1.49 ± 0.15 mm/yr). This station, in fact, is supposed to be stable. The last
example refers to the Nezugaseki (Japan) tide gauge record exhibiting the
effect of the 1964 earthquake which caused a 15 cm land submergence
along a coast generally dominated by emergence. It is worth mentioning
that regional differences in heating and circulation may also occur, thus
contributing to the sea-level behavior of a specific region.
In the Mediterranean region a project has been developed with the aim
of studying sea-level fluctuations by means of space techniques, gravimetry
and the analysis and interpretation of tide gauge data from a selected
ensemble of tide gauges available in the area of interest. This project, called
SELF (SEa Level Fluctuations: geophysical interpretation and environmental impact), funded by the Commission of the European Union (EU),
has served as a pilot study in the Mediterranean and Black Sea regions. It
has demonstrated, among other findings, the present capability to establish
tide gauge station heights in a global well-defined reference system to the
one centimeter level of accuracy or even better, through campaign-type of
observations of the satellites of the Global Positioning System (GPS)
(ZERBINI et al., 1996). A follow-on project, SELF II (Sea Level Fluctuations in the Mediterranean: interactions with climate processes and vertical crustal movements) (ZERBINI coordinator, 1995), also funded by the EU,
is presently underway. SELF II will rely on a broadly based and highly
interdisciplinary research work to use the determination of absolute sea
level and of its variations in a comprehensive way for the study of interactions, in the present as well as in the recent past, between the ocean, the
atmosphere and the earth's crust and to develop appropriate models to
assess future aspects.
GEOLOGIC TRENDS AND METEOROLOGIC EXTREMES
IN THE MEDITERRANEAN
The total length of the Mediterranean coastline is about 46,000 km, of
which 19,000 km represent island coastlines. About 54% of the Mediterranean coastline is rocky, the rest consists of low sedimentary shores.
Moreover, a significant percentage of the population lives in the coastal
zone, which is of major environmental and socio-economic importance in
the Mediterranean.
From geomorphological and archaeological investigations, and from
long-term tide-gauge records, it seems that the fluctuations of sea level in
the Mediterranean during the Holocene and in historical times are largely
dominated by the effects of local tectonics (HEY, 1978; PIRAZZOLI, 1986;
EMERY et al., 1988; FLEMMING, 1992); a marked variability between the
rates and direction of vertical movements of coastal regions exists,
according to the distribution of closely-spaced structural discontinuities,
which are connected to main tectonic plate boundaries intersecting the
Mediterranean.
The incidences of most rapid upward movements are from tectonically
active convergent margins. The Calabrian Arc shows values for regional
uplifting ranging from 0.3 to 1.4 mm/yr for the last 125,000 years
(COSENTINO and Guozzr, 1988); the Hellenic Arc is associated with consis190
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tent evidence for rapid uplift on the outermost islands, with the most rapid
rates of movement (of the order of 5 mm/yr) observed anywhere in the
Mediterranean associated with dramatically high rates of seismicity
(FLEMMING, 1992; STIROS et al., 1994). In volcanic areas (not necessarily
subduction-related) such as nearby Vesuvio, Etna, Aeolian Islands, shortterm non-linear or alternating vertical movements may occur (STEWART et
al., 1993; ROMAGNOLI et al., 199~).
From available tide-gauge records, most of the Mediterranean coastal
tract appears as submergent, although mostly at a low rate (1 to 2 mm/yr,
which is consistent with the generally accepted range of sea-level rise;
EMERY et al., 1988). The most extensive «quiet» zones in historical times,
as revealed by archeological data, are the northwestern Mediterranean coast
and southern Turkey, which are relatively far from active plate boundaries,
and thus correlate with a relative vertical tectonic stability (FLEMMING,
1992). The areas of larger river deltas or coastal plains, where there is accumulation and compaction of alluvial deposits (e.g. the deltas of the rivers
Nile, Po, Rh6ne, Ebro, etc.) appear on the other hand to have undergone
local subsidence, as shown by the relative sea-level rise greater than the global rise (4 to 7.3 mm/yr from tide gauges; EMERY et al., 1988). So, the
major problem connected with sea-level rise (at least for the near future) is
related to presently low-lying coastal areas, and to those sectors where local
subsidence (due to sedimentary loading and compaction of soft sediments,
tectonics or volcano-tectonics or anthropic causes) may enhance the eustatic
effect. Nearly all the Mediterranean coastal lowlands and their shorelines
are, at present, experiencing damage from erosion and inundation during
storms; a further substantial rise of the sea-level would invade part of them,
accelerate coastal erosion, aggravate coastal flooding, increase the salinity
of aquifers and substantially alter coastal dynamics processes (JELGERSMJ\
and SESTINI, 1992). Damage or destruction of particular wildlife habitats
and coastal ecosystems such as wetlands would be felt as well.
The most important natural wetlands and coastal lowlands around the
Mediterranean (Figure 2) lie in the Ebro delta area (Spain), the Rhone delta
and the Camargue region, in part of the northern Tyrrhenian (Arno, Ombrone,
15° E
30" E
45' N
45" N
3ff' N
JO " N
15" E
30" E
Figure 2 - Map of the Mediterranean, indicating in black the most important coastal lowlands
(adapted from JELGERSMA and SESTINI, 1992).
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191
Tevere river mouths) and north-eastern Adriatic coasts (including the Po
River basin) in Italy, in limited parts of the Albanian, Greek and Turkish
coasts, at the deltaic plain of the Nile River (whose coastal lagoons represent
1/4 of total Mediterranean coastal wetlands; SESTINI, 1992c) and along
stretches of the northern Tunisian coast. The study of such "high-risk" areas
as well as of other Mediterranean coastal tracts shows indeed that humaninduced effects, connected to economic and social activities, greatly
increase the problems connected to sea-level rise, mainly through:
a) a reduction of river sediment supply,
which should have a fundamental role in maintaining the rate of vertical
accretion in deltas and coastal wetlands. If the sediment input is lowered,
either naturally or because of human activities, the vertical accretion may not
match the rate of relative sea-level rise, resulting in increased erosion along
coastal tracts and submergence. The Po and Ebro deltas, for example, have
grown mainly with the high amount of eroded sediment which reached the
coast after the deforestation of their drainage basins since the Middle Ages
(JELGERSMA and SESTINI, 1992). During the last decades, however, both
river basins have been subject to damming and sand mining; sediment supply
to the beaches and deltas has been greatly reduced (down 96% from the last
century for the Ebro River according to MARINO, 1992), with consequences
on the erosion along the delta and nearby shorelines. Similar estimates
(sedimentary discharge to the sea from 40 million tons per year to 4 million
tons in the last century) resulted for the Rhone River mainly as a consequence of damming, causing increased erosion of the coast (JELGERSMA and
SESTINI, 1992). To the east, the retreat of the delta coastline, due to the
strong reduction of Nile River sand discharge since the building of the
Aswan High Dam and to the expanding urbanization, have effects extending
to the coast of Israel, whose beaches are fed with sand derived from the erosion of the Nile delta coast and carried by littoral and drift currents (see
GOLIK, this volume).
b) the destruction of natural shoreline defences,
such as sand dunes and coastal ridges, for coastal urban development
connected to commercial or touristic activities on most of the Mediterranean
coasts (as, for example, in the Gulf of Lyons and northern Adriatic Sea).
The sand dunes belt has to act as a final line of defence and storage for the
shoreline, providing sediment for morphological adjustment and landwards
extension of the active shore zone in response to high energy storm events
(FREESTONE and PETHICK, 1991). Coastal response to rising sea level
involve, in fact, change in coastal morphology (due to increased flood frequency) and the readjustment of the present equilibrium profile. Interference
with coastal processes may also be caused by the construction of defence
structures such as stone jetties or offshore breakwaters, which may have
negative effects on the littoral drift system and on the stability of coastal
dynamics. These structures are very common, for example, over long
stretches of the Veneto and Romagna regions coastlines, where they have
been considerably altering the hydrodynamic regimes of the north Adriatic
lowlands and lagoons (SESTINI, 1992b).
c) the overpumping of ground water or other fluids of economic importance,
which may cause enhanced subsidence due to lowering of the piezometric
surfaces of confined aquifers and due to compaction phenomena. A marked
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increase in the natural subsidence was observed between the 1950s and
1970s in the areas near the Po delta and Venice due to methane extraction
and the overdrawing of underground water for industrial use, causing land
lowering of the order of 40-60 cm (interval 1958-62) and 20 cm (1962-67)
for the Po delta, 10 cm and 14 cm respectively at Venezia and Marghera (for
the interval 1952 to 1969; CARBOGNIN et al., 1981; BONDESAN et al., 1995).
Average subsidence rates of 15 mm/yr occurred between the Po delta and
Ravenna and 5-10 mm/yr further to the south, along the Romagna coast, between 1968 and 1978 (BONDESAN et al., 1995). Altimetric data, collected for
the years 1950s to 1980s in the industrial area located between Ravenna and
Porto Corsini showed a land lowering of more than 125 cm, with average
subsidence rates of 3-4 cm/yr until 1972 and even higher rates in the interval
1972-77, while natural (geological) subsidence in the area is of the order of
1-2.3 mm/yr (BERTONI et al.' 1986; RONCUZZI, 1992). The effects of the
maximum rate of subsidence have been clearly identified in the littoral and
industrial areas as well as in the historical centre of the town (Figure 3).
Figure 3 - Example of the effects induced by subsidence in Ravenna. Permanent inundation of
the crypt in the San Francesco Basilica (IX'h century).
In Venice, where the average rate of geological subsidence is only 0.3 to
0.4 mm/yr since the early Quaternary and 0.6 mm/yr since the last interglacial, tide-gauge records indicate a sea-level rise of 25 cm in the present
century. This represents, however, a ground sinking of about 15 cm from
1890 to 1950 (local subsidence rate of approximately 1.8 mm/yr) and of
8-10 cm from 1950 to 1970 (corresponding to a rate of more than 4 mm/yr,
PIRAZZOLI, 1987; 1991).
Almost all the Adriatic coast of northern Italy represents today a main
high-risk area in respect to future sea-level rise: many parts of its shorelines
arc subject to regression and the wetlands are experiencing increasingly
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193
higher tide and storm surge levels (SESTINI, 1992b). Its present state of instability partly derives from a natural tendency to subsidence, but much more
from the extensive coastal development activities which have been carried
out in the last fifty years with scarce consideration of the natural equilibrium
of the area. These include a widespread enlargement of the urban and industrial settlement at the expense of the beach and dune ridge; the building of
harbours. and coastal defence structures; the land reclamation of wide areas,
causing reduction of the soil level, compaction and oxidation of peat levels;
the lowering of the sediment load to the coast for gravel and sand quarrying
from river beds (see SIMEON! and BONDESAN, this volume).
As a result, tracts below sea level can be found at more than 40 km
inland from the present shorelines and cover a total 2,375. km 2 , being especially widespread around the Po Delta, where a relative sea-level rise of
about 250 cm occurred during the present century (BONDESAN et al., 1995).
The environmental hazard connected to sea-level rise threatens also coastal
cities of major cultural and touristic importance: in over 90% of the city of
Venice the street level is situated at elevations below + l.2 m above the
present Mean Sea Level (MSL) and, for a relative MSL rise of + 20 cm or
+ 30 cm, the lowest point of the city, in St. Mark's Square (which is now
only 0.4 m above the present sea-level) would be flooded respectively by
55 % or 75 % of high tides, instead of by 15 % as today (PIRAZZOLI, 1991).
Storm surges in association with spring tides may generate water levels
1 to 2 m above normal MSL, representing the main threat of flooding and
shore erosion. In the Gulf of Lyons sea level can rise by 1.8 m with southeastern winds, or be depressed to -0.50 m by the onshore northwestern
winds. Storm surges in the Mediterranean cause winter waves, which can be
1 to 5 m high, to raise by 1.5 to 2.5 m (SESTINI, 1992a). Major flooding
events due to river overflow or sea surges, as the one which occurred in
November 1966 on the Veneto, Friuli and Romagna coastal and inland areas
at the time of the greatest storm surge of this century in the north Adriatic
Sea, have been frequently reported in the coastal areas around the Po delta.
Major sea surges are generally related to exceptionally negative
meteorologic configurations; the wind effect from southeast directions is
particularly significant in the northern Adriatic for physiographic and
hydrodynamic reasons and may induce a water level rising windward (as
often observed in the Gulf of Venice and Trieste; see, for example,
BONDESAN et al., 1995). Moreover, a greater recurrence of "extreme" storm
events may represent a northward shift of climatic zones and changes in air
circulation and precipitation patterns (SESTINI, 1992a). An estimation of the
frequency of extreme storm surges in the Venice lagoon, deducted on the
base of the MSL changes in the city of Venice from the highest annual tide
records (PIRAZZOLI, 1991), leads to the observation of an increase in flood
frequency from the time interval 1924-1936 to 1968-1987. With a 20 cm
relative sea-level rise in the future, the most dangerous storm surge levels
would become three times more frequent with respect to the latter interval;
for a MSL rise of 34 cm, exceptional high tidal flooding (such as the one of
November 1966) could recur in Venice every 15 years instead of every 165
years. At Trieste, a 20 cm relative sea-level rise would reduce the repeat
time of the November 1969 exceptional surge from 80 to 40 years
(MAZARELLA and PALUMBO, 1989).
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SEA-LEVEL TRENDS IN THE MEDITERRANEAN
Most of the globally available sea-level data stem from coastal tide
gauges. At many coastal sites, tide gauges have been operated for several
decades and at some places even for two centuries. However, most tide
gauges were (and continuously are) operated by national authorities, and a
more or less homogeneous data base only exists for monthly mean sea
levels derived from the tide gauge recordings. For a major fraction of the
global tide gauges, these monthly means have been and are continuously
collected at the Permanent Service for Mean Sea Level (PSMSL). In general, monthly means are submitted to the PSMSL by national authorities who
are responsible for the installation, maintenance and operation of the tide
gauge, the high-frequency sampling of sea level (mostly hourly values) and
the computation of monthly means from these measurements.
The PSMSL thoroughly checks the quality of the individual time series
(WOODWORTH et al., 1990). Tide gauges measure sea level relative to a
benchmark on land, and the specific knowledge of the relation of the tide
gauge zero to the benchmark is of particular importance in order to create
individual records homogeneously referred to the same height reference.
However, in many cases the required information is not available to the
PSMSL. Consequently, there exist two data sets at the PSMSL, namely a
Metric set, which comprises all data transfered to the PSMSL, and a Revised
Local Reference (RLR) set, which consists of high-quality records. RLR
series are each reduced to a common local reference datum by making use
of the benchmark history as supplied with the data. According to PSMSL
suggestions, only RLR data should be used as a basis investigating the low
frequency spectrum of the records (SPENCER and WOODWORTH, 1993). Of
course, the RLR data set would provide even more geodynamically and
oceanographically valuable information, if the tide-gauge benchmarks were
tied to a common reference system (e.g. geocentric coordinates).
Already since the end of the last century, sea-level variations in the Mediterranean have been monitored at an increasing number of coastal stations
and some of the data have been submitted to the PSMSL. For the
Mediterranean and the Black Sea, the PSMSL currently holds records of
131 tide gauge stations. The locations of the tide gauges are given in
Figure 4 together with the lengths of the records. Clearly, a preference for
the northern coast of the Mediterranean is emphasised as well as a more or
less equidistant coverage of the whole northern coasts by the longer records
with gaps existing on the northern Mediterranean coast of Spain and the
Albanian coast. With the exception of the Egyptian coasts and the strait of
Gibraltar, there are no records available from the northern coast of Africa.
In the Black Sea the distribution of available tide gauges is sparce. The temporal structure of the records is summarised in Table I. It should be noted
that only four RLR records, namely Genova, Marseille, Trieste and Port
Tuapse, have more than 50 years of data. Moreover, the individual records
often do not overlap each other, as illustrated in Figure 5.
In general, most stations cover the second part of the present century.
Nevertheless there is a group of Italian stations that was operational at the
end of the previous century but stopped around 1920. Of course, additional
data exist that are not available to PSMSL. Both from publications (e.g.
PALUMBO and MAZZARELLA, 1985) and the availability of tidal constants it
Bulletin de l'Institut oceanographique, Monaco, n° special 18 (1997)
CIESM Science Series n °3
195
48
Q = 1440
Tide gauges
m.
46
44
42
Q)
"O 40
::s
.µ 38
·r-1
.µ 36
cO
H 34
32
30
28
-12
-8
-4
0
4
8
12
16
20
24
28
32
36
40
44
Longitude
Figure 4 - Location and lengths of the available monthly mean sea-level records. Each cross marks the position of a tide gauge. The size of the circle around
cross is proportional to the monthly values available at the respective station.
gauges
,,
50
1-1
' I\
Q)
...Q
8
1111 '1,
11
1. 1r1
40
\l/lj
30
,1 '
I~
/'
I
1
',
\\
::l 2 0
z
10
0
1850
1880
1910
1940
1970
2000
Year
Figure 5 - Number of Mediterranean and Black Sea tide gauges contributing to the data base as
function of time.
can be inferred that a considerable number of additional tide gauges and
records exist in the Mediterranean including the north coast of Africa and
particularly in the Black Sea.
Despite the consistency checks being applied before a record is accepted
as RLR-record additional error checks are required especially in respect to
any interpretation of the low frequency part of the series. The long-period
variability of sea level may be expected to be regionally coherent.
Therefore, a lack of coherence between a station and its neighbouring ones
can be used to screen out erroneous records. However, a comparison between only two neighbouring stations cannot clarify which of the stations
carries falsified data. Screening all the available RLR record with a multistation comparison revealed that nearly all records contain some suspicious
data points (see Table I). Ambiguous parts thus identified in some of the
series given in Table I were excluded in the subsequent analyses.
In summary, the monthly mean sea-level data base of the Mediterranean
can be characterised as a spatially and temporally non-homogeneous set of
records with a high probability of erroneous data in the individual series.
This assessment is slightly more gentle than that of MOSETTI and PURGA
(1991), who described the condition of mean sea level data in the Mediterranean as "catastrophic". As mentioned above, tide gauges measure local
sea level relative to a benchmark at land. Thus, vertical crustal movements
enter as one factor affecting the local relative sea-level signal. From the
oceanic side, sea level itself is affected by a large number of factors operating at different temporal and spatial scales (see e.g. EMERY and AUBREY,
1991). Among these factors, the periodic movements of the semidiurnal and
diurnal tides belong to the more prominent ones, though being of minor
importance in the Mediterranean. On time scales of days to weeks, the
atmospheric forcing due to air pressure and wind variations causes sea-level
variations easily discernible in the tide gauge records. On longer time scales
of months to decades, changes in the sea-surface topography due to variations in the ocean currents or the temperature and salinity of the water also
contribute to sea level on a regional scale.
Bulletin de l'lnstitut oceanographique, Monaco, n° special 18 (1997)
CIESM Science Series n°3
197
TABLE!
Temporal structure of the PSML sea-level records and local
--·---------- ----------------·--
---
~
"'
~-
R
~'
;:
~
{;
;:-
~,">
~
n iS
51 ~8
s: "
o
"'
~·
en0
'"d
~ B~
~ :
3. ~
D; :;;
::':; §
Station
Marseille
Genova
Trieste
Lagos
Tuapse
Bakar
Split Rt Mar.
Split Harbour
Cagliari
Rovinj
Dubrovnik
Alicante II
Koper
Bar
P. Maurizio
Civitavecchia
Alicante I
Napoli (Man.)
Palermo
Venezia (S.St.)
Port Said
Venezia (Ars.)
Gibraltar
Napoli _{_Arsl_
Lot!_g,
5.35
8.90
13.75
-8.67
39.07
14.53
16.38
16.43
9.17
13.63
18.07
-0.48
13.73
19.08
8.02
11.82
-0.48
14.27
13.33
12.33
32.30
12.35
-5.35
14.27
Lat_
Begin
43.30
1885
44.40
1884
45.65
1905
37.10
1908
44.10
1917
45.30
1930
43.50
1952
43.50
1954
39.20
1896
45.08
1955
42.67
1956
38.33
1960
45.55
1962
42.08
1964
43.87
1896
42.05
1896
38.33
1952
40.87
1896
38.13
1896
45.47
1896
31.25
1923
45.47
1889
36.17
1961
40.87
1899
----------------·-·---
End
N
NG
1989 1157
9
1988
994
IO
1990
960
3
1989
863
13
1990
844
3
1990
600
3
1990
450
3
1990
442
l
1934
433
13
1990
424
2
1990
419
1
1987
4
332
1990
332
8
1990
318
I
1922
309
4
304
1922
6
1987
303
5
1922
301
11
1922
294
13
1920
288
3
1946
287
1
1913
287
8
1989
272
8
1922
263
14
NM
103
266
72
121
44
132
18
2
35
8
1
4
16
6
15
20
129
23
30
12
1
13
76
25
c
*s
*s
*s
x
?
?
s
s
s
s
*
?
x
?
*s
t
l. l
1.3
Ll
1.4
2.2
0.9
0.1
-0.8
1.3
-0.2
-0.1
-1.4
-0.5
1.4
LI
1.2
-2.2
2.1
1.0
4.4
4.7
1.8
-0.7
2.5
ot
0_1
0.1
0.2
0.2
0.3
0 .3
0.5
0.5
0.4
0.5
0.5
0.4
0.6
0.7
0.6
0.7
0.4
0.7
0.7
1.2
1.0
1.4
0.8
1.5
tn
-0.2
-0.2
-0.3
-0.4
-0.1
-0.3
-0.3
-0.3
0.2
-0.3
-0.3
-0.3
-0.3
...()_3
-0.1
0.0
-0.3
0.0
0.3
-0.3
0.3
-0.3
-0.6
0.0
tu.
tMa
1.03
0.98
1.10
1.39
1.99
1.04
-1.29
-0.05
1.15
0.57
0.58
-0.18
1.07
2.57
1.52
0.59
-2.71
2.18
-1.39
2.11
5.20
3.58
0.24
l.94
1.10
1.21
1.17
1.42
2.17
1.69
0.73
1.46
1.74
2.21
2.10
l.15
2.21
3.25
2.71
1.43
-0.24
3.25
l.15
4.72
4.20
4.60
0.15
3.00
1.:c..
0.
0.
0.
0.1
-0.
-0.
0.
0.1
0.
-0.
-0..
0.
-0.
-1.
#-L'
# 0.
L
# ·1
# 0.
# -3.
-2.
#-3.
L'
# -1.
The table lists all stations in the RLR data set located in or within the vicinity of the Mediterranean having at least 240 monthly values. The stations are sorted
according to the available data. N: number of available monthly sea-level values; NG: number of gaps; NM: number of missing monthly values;
C: comments on the data quality, with *denoting records with definite data errors, s indicating spikes in the records (i.e. at least one erroneous monthly value i
present), ? indicating strong doubts concerning the data quality, and x denoting stations with no nearby station to compare; t: sea-level trend in mm/yr;
at: standard error in mm/yr; tp: trend due to postglacial rebound as computed with the ICE-3G model (PELTIER and TUSHINGHAM, 1989); tr,./tMa: trends in
mm/yr determined using Trieste and Marseille, respectively, as base record. For records marked with#, the overlap with Trieste is small, explaining the large
differences compared to the trends derived with Marseille as base station; re: crustal movement rates (positive for uplift) decontaminated for post-glacial
rebound effects; the rates given are calculated from re= -(tMa - tp - e), where e is the eustatic sea-level change. We assumed e = 1.8 mm/yr (DOUGLAS, 1992).
Geological evidence of past sea-level changes and estimates of the
mass added to the ocean as a consequence of the possible present warming
suggest oceanic trends in relative sea level of the order of 1 mm/yr. On the
other hand, interannual coastal sea-level variations may reach up to 10 cm
over a decade (STURGES, 1987; PIRAZZOLI, 1989; GROGER and PLAG,
1993). These variations may result in apparent «trends» of the order of 10
mm/yr prevailing for more than a decade. Therefore, trends estimated from
records spanning only one or two decades are most likely reflecting the
decadal fluctuations and not the long-term trend. The record length required
to separate the oceanic long-term trends from the interannual to multidecadal "noise" depends both on the local magnitude of the decadal sealevel variability and the tolerable uncertainty in the trend estimates. For
Mediterranean stations, BAKER et al. (1995) showed that in order to have
errors of less than 0.5 mm/yr, records spanning at least 40 years are required.
Thus, the number of stations, which can be used for a direct determination of
reliable local trends is drastically reduced. However, local trend estimates
can be improved by modelling the decadal variations.
ZERBINI et al. (1996) base their discussion of the Mediterranean Sea
level spectrum on power spectra of the few records having more than 30
years of data without any gaps (see Table I). The most powerful signals in
the spectrum are those at the annual and semi-annual frequencies, which
correspond to the annual (Sa) and semi-annual (Ssa) tidal constituents,
respectively. The amplitudes of these constituents are at their maxima in
Tuapse, where they are nearly twice as large as at most of the Mediterranean
stations. Marseille and Genova at the western Mediterranean, and Trieste
and Bakar in the Adriatic display only slight inter-station differences in the
amplitudes of these signals. It should be mentioned, however, that both Sa
and Ssa together typically account for nearly 20 % to a maximum of 45 % of
the total variance of the monthly means. The rest of the variance in the sealevel spectrum is distributed more or less uniformly over all other frequencies creating a noise level of typically 5 mm. Only in the low frequency
part of the spectrum, some regionally coherent peaks appear, that each
account for about 2 % of the total variance. It is interesting to note that the
spatial coherency of the intra-annual spectra is generally less pronounced
than for the interannual parts, indicating a positive relation between wavelength and period for the forcing on sea level.
From an oceanographic point of view, vertical crustal movement is a
perturbation in the sea-level measurements. Tectonic measurements, sedimentation, groundwater or oil extraction, all may result in vertical crustal
movements of regional and down to local scales. Post-glacial rebound
contributes to regional scale vertical movement. Furthermore, changes in
the surface mass distribution in both hydrosphere and cryosphere induce a
viscoelastic deformation of the Earth affecting the global geoid and consequently the sea level (FARELL and CLARK, 1976). In many locations on the
globe the crustal component is of the same order or even in excess of the
long-term sea-level variations. Generally, vertical crustal movements
together with associated changes of the geoid are considered as a major
factor masking the sea-level changes due to changes in the volume of the
ocean water.
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199
The trends determined from the longer RLR records available in the
Mediterranean are compiled in Table 1 which also lists the relative sealevel rise expected from isostatic compensation due to post-glacial rebound.
In the Mediterranean this effect is of the order of ±0.3 mm/yr. Thus at tectonically stable sites we should expect a relative sea-level rise close to the
global one. However, the local trends particularly of the shorter records
deviate significantly from the expected 1 to 2 mm/yr.
To improve local trend estimations from shorter records a better understanding of the interannual to multidecadal response of sea level to the
various forcing parameters is needed. Of course, hydrodynamical models
to simulate sea-level variability due to meteorological and oceanographic
forcing would be the appropriate tool to separate these effects from the
long-term sea-level changes.
However, a useful simple method can be based on the assumption that
the interannual to multidecadal sea-level variability is spatially coherent
within properly defined regions. Thus a long and qualitatively good record
may be used as "base record" which contains the information about the
decadal to inter-decadal variability. For shorter records, trends are estimated from the differences of monthly means to the base record, thus eliminating the synchronous part of the sea-level variations (SJOBERG, 1987).
As is obvious from Table I, Marseille, Trieste and Genova are potential
base stations, of which we use the first two. The trends determined with
Marseille as a base station tend to be slightly larger than those using
Trieste. Especially for records spanning the end of the last and the beginning of the present century (marked with an asterisk in Table I), Trieste
introduces a negative bias, which may be due to the small overlap of these
records.
For records spanning the second half of the century, both Marseille and
Trieste as base records improve the trends in the sense that the interstation
scatter is reduced. At one of the two records at Split (Rt. Mar.), the effects
of using Trieste and Marseille are opposite, possibly indicating some data
problems. The overall scatter of the local trends is reduced for both base
stations compared to the original data, suggesting that a large fraction of
the scatter in the trend estimates is not due to crustal movements but rather
to an effect of the variable record spans in conjunction with the interannual
to multidecadal variation in relative sea level.
This is even more obvious from the apparent crustal movement rates
given in Table I, too. These rates are corrected for the isostatic adjustment
and sea-level changes due to the ongoing post-glacial rebound. In general,
the rates of crustal movements are less than ± 1 mm/yr with very few
exceptions, as for example in Venice, where known local effects contribute
to subsidence. At least at the tide gauges included in the present study crustal movements are small compared to the decadal to multidecadal sea-level
variability discussed above, but of the same order as the long-term trend in
sea level. Thus a careful monitoring of crustal movement with the highest
possible accuracy is required if crustal movement is to be separated from
the oceanographic contribution to relative sea-level changes.
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SPACE AND GRAVIMETRIC METHODS
FOR MONITORING SEA LEVEL
Nowadays space geodesy has matured both as regards the technological
capabilities and the relevant methodologies to such a level that it is possible
to monitor, in a global reference system, horizontal as well as vertical velocities of stations on the Earth's surface to the subcentimeter level of accuracy. Satellite Laser Ranging (SLR), Very Long Baseline Interferometry
(VLBI) and GPS measurements are used to fix Tide Gauge Bench Marks,
thus unifying the tide gauge network. Crustal motions at the TGBMs are
determined using space techniques such as GPS, in particular the vertical
component is of relevance in sea-level studies. This makes it possible to
discriminate the true sea-level variations from the component of tectonic
origin in relative sea-level observations. Absolute gravity measurements
provide an estimate of vertical surface elevation changes with accuracy
comparable to that obtainable by means of space methods. This is, on the
one hand, a quite valuable external check, by an independent methodology,
of the height and of its temporal variations of the TGBMs. On the other
hand, the possibility to correlate height variations and temporal variations in
the gravity field, induced by mass movements or density variations within
the Earth's crust, constitutes an important mean to put constraints on the
mechanisms producing the observed deformation.
The recent realization and present development of global and regional
projects such as SELF I and II (ZERBINI et al., 1996; ZERBINI coordinator,
1995) to connect well-established tide gauges on a global well-defined reference system such as, for example, the one established by the global network of SLR and VLBI fiducial stations or the ITRS (International
Terrestrial Reference System) (BOUCHER and ALTAMIMI, 1993a, 1993b),
made it possible to determine the TGBM heights at the one centimeter level
of accuracy or even better. The observational approach adopted in the SELF
I and II projects is schematically described in Figure 6. This figure shows
RADIO SOURCE
GPS SATELLITE
~
SEA SURFACE
TOPOGRAPHY
y~,: j
TIDE
GAUGE
ROD
"f m
GPS
FIDUCIAL
REFERENCE
STATION
SLR
A
6~
V~I
GPS
.A,.
ABSOLUTE
GRAVIMETER
WATER VAPOR
RADIOMETER
...
ABSOLUTE
GRAVIMETER
Figure 6 - Schematic diagram showing tide gauge connections to the SLRNLBI reference system.
Bulletin de /'Jnstitut oceanographique, Monaco,
CJESM Science Series n°3
n° special 18 (1997)
201
that the TGBM heights are measured in the global reference system, realized through the SLR/VLBI fiducial stations (or ITRS), by means of simultaneous GPS observations at both the tide gauge and the nearest fiducial
site. GPS observations are performed simultaneously with Water Vapor
Radiometer (WVR) measurements in order to improve the determination of
the vertical component by the accurate determination of the path delay
along the GPS signal propagation path. Figure 7 illustrates the network of
stations involved in the course of both the SELF projects.
MAS PALOMAS
(Canary Islands)
•
./
•FIDUCIAL REFERENCE STATIONS
-'.TIDE GAUGES
•SLR AND TIDE GAUGE SITES
Figure 7 - The SELF network.
In order to be able to understand true sea-level variations it is necessary
to determine vertical crustal movements of the TGBMs to an accuracy of
1 mm/yr or better (CARTER, 1994). This can be achieved through permanent
occupations of tide gauge stations by means of GPS receivers. However,
continuous GPS observations can only be foreseen at selected tide gauges.
Within the SELF II project, the most appropriate strategy, which should
comply to both scientific and economic requirements, is being investigated
and implemented in order to assess the capability to achieve a reliable
knowledge of the vertical rates of the TGBMs, within limited time intervals,
without the need for continuous collection of GPS observations.
Absolute gravity observations with an accuracy of 2 µgal (2 10-9 g) are
feasible and routinely achievable as ZERBINI et al. (1996) have demonstrated. These are used to detect vertical crustal movements of the order of
1 cm, as mentioned earlier. The measurements are independent of any
satellite-based reference frame and they provide completely independent
measurements of vertical crustal movements. The gravity acceleration is a
physical quantity which is location- and time-dependent. Several sources
are responsible for the time-dependent gravity variations at a given site :
the attraction of sun and moon (earth tides) and the related elastic response
of the Earth, attraction and crustal deformation due to ocean and atmospheric loading, change in the position of the rotation axis of the Earth (polar
motion) and variation of the water table level, of the water content of the
soil, etc. (RICHTER, 1995).
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CIESM Science Series n °3
The gravity variations associated with these events may occur within
hours, days and years and may have local as well as global characteristics.
While some of the above mentioned phenomena are the obvious objectives
of geophysical studies, others belong to the noise component of the observed
signal. A successful interpretation of the gravity time variations strongly
depends upon the possibility to measure, model or eliminate this latter component. Reliable physical models have already been proposed to take into
proper consideration some of these effects on the observed gravity signal.
There is, however, a need to improve the models, mainly those concerning
fluid tides, ocean and atmospheric loading, by means of a more detailed
monitoring and modelling of deformation, gravity potential changes and
environmental parameters at selected sites. This is the aim of an experiment
which is now taking place at the Medicina fiducial station, near Bologna, in
Italy.
This specific experiment at Medicina will assess the accuracy with
which vertical crustal movements can be determined both from a new type
of superconducting gravimeter for continuous gravity registrations in combination with a new generation of absolute gravimeters for episodic gravity
observations and from continuous and episodic GPS measurements.
Medicina is an ideal location for this study, since it is near to the northern
Adriatic and the Po River delta, where the effects of crustal subsidence and
sea-level change are particularly important.
Monitoring long-term changes of the mean sea level by satellite altimetry
has been recognized for several years as a fundamental challenge, but it is
only recently that time variations in the mean sea level began to be detected,
in particular with the very accurate altimetry data of the Topex-Poseidon
(TIP) satellite. The TIP satellite, launched in August 1992, was designed to
study the ocean circulation and its large-scale variations. Sea surface heights
computed from the T/P altimeter data are the most precise ever obtained by
satellite altimetry. Several studies have been performed to determine, at a
global scale, mean sea level trends with TIP data. These studies show that
TIP recovers a clear seasonal signal in sea level. Moreover, based on the onboard estimates of the instrumental drift (and after a correction for a recently
discovered software error), the T/P results indicate a mean sea-level rise of
0.5 mm/yr and 2.8 mm/yr if a tide-gauge based calibration estimate is
applied (NEREM et al., 1996). However, the uncertainties are estimated to be
above 1 mm/yr. Other studies have and are being conducted in the Mediterranean (BONNEFOND, 1994), also in the framework of the SELF II project.
At the scale of this basin, these show that the coverage of the T/P altimeter
profiles is good enough to measure, when orbit information will be further
improved, with a few mm accuracy, temporal sea-level changes. Important,
additional data are available or will become available through the European
ERS-1 and ERS-2 satellite altimetry missions.
The use of laser altimetry for airborne determination of the sea-surface
topography is a very promising technique which can be used to complement
the satellite altimetry information in the proximity of the coasts where, as it
is well known, satellite altimetry cannot provide reliable information. Also
validation of hydrological and sea-surface topography models can be performed in marine areas of particular interest where the theoretical modelling
is presumably biased.
Bulletin de l'Institut oceanographique, Monaco, n° special 18 (1997)
CIESM Science Series n°3
203
CONCLUSIONS
The results obtained in the course of the SELF I project have demonstrated that the approach adopted to study sea-level fluctuations in the
Mediterranean is feasible and reliable. The work developed has made it
possible to define selected tide gauge station heights in a global highlyaccurate reference system to the one centimeter level of accuracy or even
better. This has been accomplished through campaign-type of GPS observations to link the TGBMs to the fiducial reference stations (SLR/VLEI) of
the global network. The realization of a vertical reference system of high
accuracy through the use of space geodetic and absolute gravity techniques
was a necessary requisite in order to lay the basis to properly interpret the
tide-gauge data series and to derive, from the measurement of the tide gauge
station vertical rates, the true oceanic contribution to sea-level rise. The
capability demonstrated by the project to determine reliably to subcentimeter accuracy station heights, through the combined use of GPS and WVRs,
is most important because vertical land movements at coastal stations may
be originated by different processes, such as those of tectonic origin or those
resulting by human activities, which are characterized by different time
scales. Monitoring of sea level, being nowadays feasible, is a major endeavour that should be pursued both at global as well as at regional scale,
since local perturbations of the global trend may be quite significant.
The possible impacts of sea-level rise, or even worse of accelerated sealevel rise, have been investigated in several studies, and they extend over a
wide range of environmental damages which include coastal inundation,
increased erosion, changes in circulation and salinity of estuaries and
lagoons, increased storminess, loss of wetlands, changes in habitat, increased salinity intrusion into groundwater, etc. For each one of the possible
damages there is an associated socio-economic impact, therefore a cost to
be paid, if proper remedies are not adopted in a timely manner. To assess the
danger for each area, the long-term variation of absolute sea level has to be
added to land movements to produce the local sea-level trend. This local
relative trend is useful for long-term coastal management and is directly
observed through tide-gauge measurements. Extensive collection of tide
gauge data therefore is essential to rationally assess the influence of sealevel rise in coastal areas. The long-term trends can easily be derived.
Nevertheless, the short-term danger for sinking areas (for example, Venice)
comes from extreme surges driven by the meteorology. When these coincide with high tides they produce even more destructive results. Therefore
studying the sea-level variability regionally can reveal connections with
meteorological or oceanographic mechanisms that may help in understanding changes in the regional climate.
In the Mediterranean region there are several coastal areas subject to
"high risk" both as regards the damages induced by sea-level rise and those
due to potential catastrophic extreme events. As the outcome of the geologic
study performed by ZERBINI et al. (1996) has highlighted, in the Mediterranean area there is a substantial variability of crustal movements both in
space and time. While the present-day horizontal crustal movements have
been measured for more than 10 years now by means of different space
techniques and start to provide a comprehensive image of the main trends
(KAHLE et al., 1993; NOOMEN et al., 1993; NOOMEN et al., 1996), little
204
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CIESM Science Series n°3
information is available on the vertical rates. It is, therefore, important that
an effort be made in this sense to provide, at the basin scale, reliable vertical
crustal movements. Since it is not feasible, both from the economical and
the data analysis "load" point of view, that all the tide gauges be permanently co-located with continuously observing receivers, an alternative and
effective strategy shall be developed in order to derive GPS reliable vertical
rates over short periods of time (in the order of 5 years).
ACKNOWLEDGMENTS
This work was supported by contracts EV5V-CT91-0049 and ENV4CT95-0087 from the Commission of the European Union.
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