Meeresspiegel-Jo-Jo in Kroatien während der vergangenen 1500 Jahre

Der globale Meeresspiegel ist laut SROCC-Bericht des IPCC von 2019 in den letzten 115 Jahren um 16 cm gestiegen, was einer durchschnittlichen Rate von 1,4 mm pro Jahr entspricht. Kurz nach dem Ende der Kleinen Eiszeit musst der Anstieg erstmal in Fahrt kommen, denn während der davor liegenden Kleinen Eiszeit hatte sich sehr viel Wasser in Form von Eis auf den Kontinenten gestapelt, so dass der Meeresspiegel in der letzten Kältephase abgesackt war. Im Laufe des 20. Jahrhunderts nahm der Meeresspiegelanstieg dann aber Fahrt auf. Der langfristige Anstieg ist in den meisten Ozeanen der Erde durch 60-jährige Zyklen geprägt, die den Anstieg mal beschleunigen und mal abbremsen. Die Beschleunigung der letzten Jahrzehnte könnte Teil dieser Dynamik sein, aber ganz so genau weiß man es noch nicht.

Soweit der allgemeine Kenntnisstand im Telegrammstil. Heute schauen wir, was die Wissenschaft in den letzten Jahren Neues zum Thema Meeresspiegel gefunden hat.

Karabil et al. 2017 haben sich die Meeresspiegeltrends in der Ostsee angeschaut. Der Meeresspiegel ist von Schwankungen im Jahrzehnt-Maßstab geprägt. Die Korrelation mit dem AMO-Zyklus ist eher schlecht. Teile der Ostsee korrelieren aber ganz gut mit der NAO. Die NAO wiederum treibt einen gehörigen Teil der Niederschläge in Nord- und Mitteleuropa an. Die beste Korrelation mit dem Meeresspiegel zeigen die langmaßstäblichen Veränderungen in den Regenfällen. Ein spannendes Puzzle. Abstract:

Mechanisms of variability in decadal sea-level trends in the Baltic Sea over the 20th century

Coastal sea-level trends in the Baltic Sea display decadal-scale variations around a long-term centennial trend. In this study, we analyse the spatial and temporal characteristics of the decadal trend variations and investigate the links between coastal sea-level trends and atmospheric forcing on a decadal timescale. For this analysis, we use monthly means of sea-level and climatic data sets. The sea-level data set is composed of long tide gauge records and gridded sea surface height (SSH) reconstructions. Climatic data sets are composed of sea-level pressure, air temperature, precipitation, evaporation, and climatic variability indices. The analysis indicates that atmospheric forcing is a driving factor of decadal sea-level trends. However, its effect is geographically heterogeneous. This impact is large in the northern and eastern regions of the Baltic Sea. In the southern Baltic Sea area, the impacts of atmospheric circulation on decadal sea-level trends are smaller.

To identify the influence of the large-scale factors other than the effect of atmospheric circulation in the same season on Baltic Sea sea-level trends, we filter out the direct signature of atmospheric circulation for each season separately on the Baltic Sea level through a multivariate linear regression model and analyse the residuals of this regression model. These residuals hint at a common underlying factor that coherently drives the decadal sea-level trends in the whole Baltic Sea. We found that this underlying effect is partly a consequence of decadal precipitation trends in the Baltic Sea basin in the previous season.

The investigation of the relation between the AMO index and sea-level trends implies that this detected underlying factor is not connected to oceanic forcing driven from the North Atlantic region.

Weiter gehts im nördlichen Mittelmeer. Zerbini et al. 2017 haben Pegeldaten ausgewertet und kamen für die letzten 140 Jahre auf einen durchschnittlichen Anstieg von 1,2-1,3 mm pro Jahr. An einigen Pegeln mussten Korrekturen angebracht werden, weil Grundwasserentnahme zu Subsidenz geführt hat. Abstract:

Sea-level change in the Northern Mediterranean Sea from long-period tide gauge time series

The oldest tide gauge observations date back to the 18th century. Although, globally, they are available in limited number, these centuries-old sea level time series are the only data records providing information on the long-period rates of change of the mean ocean surface. Knowledge of the past sea level behavior can contribute key insights to the understanding of climate change impacts. We highlight the greatest importance of monitoring sea-level changes at all spatial scales, from global to local, using terrestrial and space techniques and outline the physical processes, natural and man-induced, responsible for such changes. In general, tide gauge data are made available through different archiving facilities serving both international and national developments. Tide gauges measure local sea-level relative to a benchmark on land, hence, correctly interpreting these observations is challenging since it demands, among other requirements, a proper knowledge of vertical land motions at the stations. In general, it is not easy to find well documented historical data; moreover, benchmarks were not frequently leveled. For more than two decades, space geodetic techniques, such as GNSS (Global Navigation Satellite System) and InSAR (Interferometric Synthetic Aperture Radar), have provided the opportunity to accurately position points in the surroundings of tide gauge sites, potentially giving rise to a large amount of information. However, despite the availability of these techniques, the evolution of the international efforts aiming at realizing consistent observational infrastructures for sea level networks is undergoing only a slow development. In the Mediterranean area, there are a few centennial tide gauge records. Our study focuses on the time series of Alicante, in Spain, Marseille, in France, Genoa, Marina di Ravenna (formerly Porto Corsini), Venice and Trieste, in Italy. After briefly reviewing the gauge types presently in use for sea level measurements, a comprehensive historical description is given for each time series, which may assist understanding an assessment of the problems these stations have experienced over more than one century of operations. Two Italian stations, Marina di Ravenna and Venice, are affected by both natural and anthropogenic subsidence, the latter was particularly intense during a few decades in the 20th century because of ground fluid withdrawal. For these two stations, we have retrieved leveling data of benchmarks close to the tide gauges from the end of the 19th century and, for the last couple of decades, we have evaluated GPS and InSAR heights in close proximity to the stations. The GPS (Global Positioning System) and SAR results were carefully compared. Modeling of the long-period non-linear behavior of subsidence was successfully accomplished by using an ensemble of leveling, GPS and SAR data. After removing the vertical land motions in Venice and Marina di Ravenna, and the inverted barometer effect at all the sites, the linear long period sea-level rates were estimated. The results are in excellent agreement ranging between + 1.2 and + 1.3 mm/year for the overall period from the last decades of the 19th century till 2012. The associated errors, computed by accounting for serial autocorrelation, are of the order of 0.2–0.3 mm/year for all stations, except Alicante, for which the error turns out to be 0.5 mm/year.

Our estimated rates for the northern Mediterranean, a relatively small regional sea, are slightly lower than the global mean rate, + 1.7 ± 0.2 mm/year, recently published in the IPCC AR5 (Intergovernmental Panel on Climate Change 5th Assessment Report) (Church et al., 2013), but close enough, if uncertainties are taken into account. It is known that Mediterranean stations had always had lower trends than the global-average ones. Our regional results, however, are in close agreement with the global mean rate, + 1.2 mm/year, published by Hay et al. (2015) which is currently being discussed by the oceanographic community (see, for example, Hamlington and Thompson, 2015). The six time series were also analyzed by means of the EOF (Empirical Orthogonal Functions) technique over the 1934–2012 common period. As a result, about 50% of the total variance is explained by the first mode, which is characterized by a coherent behavior of the six stations.

Und hier noch ein spannendes Paper aus Kroatien von Faivre et al. 2019. Anhand von fossilen Algenstrukturen rekonstruierten sie die Meeresspiegelgeschichte der letzten 1500 Jahre. Während der Kältephase der Völkerwanderungszeit fiel der Meeresspiegel. Während der nachfolgenden Mittelalterlichen Wärmeperiode stieg das Wasser wieder. Zu Beginn der Kleinen Eiszeit stagnierte der Anstieg, und in der zweiten Hälfte fiel der Meeresspiegel wieder. Im Übergang zur Modernen Wärmeperiode (CWP) stieg der Meeresspiegel dann wieder. Ein tolles Meeresspiegel-Jojo. Abstract:

Relative sea-level change and climate change in the Northeastern Adriatic during the last 1.5 ka (Istria, Croatia)

A new high-resolution relative sea-level (RSL) reconstruction is presented for the past 1500 years based on four bio-constructions formed by alga Lithophyllum byssoides (algal rims). Two algal structures have been studied on the southern (Premantura site) and two on the eastern Istrian coast (Uboka and Brseč sites) in the Northeastern Adriatic. The data from the algal rims (47 radiocarbon data points) enabled the distinction of four major phases of RSL change which corresponds to periods of climate change. RSL between AD 400 and 800 during the Dark Ages Cold Period (DACP), was almost stable. After AD 800, during the Medieval Climate Anomaly (MCA) the RSL increased up to ∼0.8 mm/yr. The following Little Ice Age period, (LIA) interval I (AD 1400 till 1600) is again characterised by RSL stability (RSL slowed down) which allows the rims at the southern coast to reach the width of ∼40–80 cm at their uppermost part and up to 20 cm for those along the eastern coast. Between AD ∼1600 and 1750, during the colder LIA II interval, algal rims do not form, as LIA II is assumed to be a period of RSL fall. Algal rims reveal that from the second part of the 19th century the RSL rose by 13–15 cm at the Premantura location and around 10 cm at the Brseč and Uboka areas, providing rates between 1 and 0.7 mm/yr respectively for the Current Warm Period (CWP).

The sea-level trends were quantitatively defined using an Errors-In-Variables Integrated Gaussian Process (EIV-IGP) model, with full consideration of the available uncertainty. Following correction for the total land-level change (assumed to be around −0.4 mm/yr), four successive trends in sea-level change were confirmed. Sea-level dropped during the DACP at a mean rate of −0.4 mm/yr and increased to 0.5 mm/y as a consequence of Medieval warmth. Thereafter it was relatively stable during LIA I, fell up to −0.1 mm/yr during LIA II interval and has been slowly rising again during the CWP. Moreover, L. byssoides δ18O records show that these periods of sea-level changes are consistent with changes in temperature and thus with periods of rapid climate change.

Weiter auf die Kanarischen Inseln nach Fuerteventura. Meco et al. 2018 rekonstruierten dort die Meeresspiegelgeschichte der letzten 4500 Jahre. Vor 4200 Jahren und zur Zeit der Kälteperiode der Völkerwanderungszeit war der Meeresspiegel 2,5 bzw. 2 m höher als heute. Dazwischen fiel der Meeresspiegel. Die Forscher vermuten, dass isostatische Ausgleichsprozess in Bezug zu Eisveränderungen in der Südlichen Hemisphäre der Auslöser waren. Abstract:

Mid and Late Holocene sea level variations in the Canary Islands

The eastern coast of Fuerteventura (Canary Islands, Spain) hosts the most complete and representative emergent Holocene marine deposits in the middle latitudes (27°N to 30°N) of the eastern Atlantic Ocean. The deposits consist of berms of gravel and foreshore sands which form beach rocks comprising >62 bed sets, with each bed set containing dozens of individual laminations suggesting a cyclical cause as, for example, the orbital movement of the Earth. Calibrated radiocarbon ages place a group of older Holocene highstands of Fuerteventura at around the Mid-Late Holocene boundary (around 4.2 kyr B.P.), and another group of more recent highstands in the Dark Age Period (around 1.4 kyr B.P.) of the Northern Hemisphere. They have been recorded at 4 m and 3.5 m apmsl respectively. The present-day tidal amplitude is around 3 m. Assuming a similar value for the Holocene, the corresponding relative sea level rises were around 2.5 m and 2 m apmsl respectively. Moreover, terrestrial deposits intercalated between the marine deposits indicate a lowering of the sea level at ca. 3 kyr B.P. When the sea levels reached their highstands during these two periods, the sea surface temperatures (SST) at the southern tip of the cold Canary Current were, respectively, 0.5 and 1.5 °C colder than the present-day SST (21.23 °C). The fossil fauna content confirms that these highstands occurred in cold conditions. In contrast, the present SST reaches the range of temperatures of the Holocene Optimum in the area.

Fuerteventura has been vertically stable since the last interglacial period, and the sedimentology of the deposits precludes an origin from tsunamis or storms. This island is in a far-field region in terms of glacial isostatic adjustment (GIA) processes. Here, we suggest that the deposits may be related to GIA and ice mass losses effects in the Southern Hemisphere.