Meeresspiegelforschung aktuell: Neue Studien aus Europa

Im Januar 2014 hatten wir an dieser Stelle über eine Studie der Universität Siegen mit einem interessanten Fazit berichtet. Eine gründliche Datenauswertung hatte ergeben, dass der Meeresspiegel in der Nordsee seit 100 Jahren mit konstanter Geschwindigkeit steigt, ohne Beschleunigung. Im Oktober 2014 legte die Forschergruppe im Journal of Geophysical Research nach. Den Wissenschaftlern war aufgefallen, dass sich hinter dem gleichmäßigen langfristigen Anstieg des Meeresspiegels Schwankungen versteckten. In ihrer neuen Studie konnten sie zeigen, dass diese Schwankungen nicht nur in der Nordsee auftraten, sondern von der Norwegischen See bis zu den Kanarischen Inseln im Atlantik reichten. Die Forschergruppe um Sönke Dangendorf vermutet, dass Änderungen der küstenparallelen Winde den Meeresspiegelanstieg systematisch beeinflusst haben könnten. Im Folgenden die Kurzfassung des Artikels:

Mean sea level variability in the North Sea: Processes and implications
Mean sea level (MSL) variations across a range of time scales are examined for the North Sea under the consideration of different forcing factors since the late 19th century. We use multiple linear regression models, which are validated for the second half of the 20th century against the output of a tide+surge model, to determine the barotropic response of the ocean to fluctuations in atmospheric forcing. We find that local atmospheric forcing mainly initiates MSL variability on time scales up to a few years, with the inverted barometric effect dominating the variability along the UK and Norwegian coastlines and wind controlling the MSL variability in the south from Belgium up to Denmark. On decadal time scales, MSL variability mainly reflects steric changes, which are largely forced remotely. A spatial correlation analysis of altimetry observations and gridded steric heights suggests evidence for a coherent signal extending from the Norwegian shelf down to the Canary Islands. This fits with the theory of longshore wind forcing along the eastern boundary of the North Atlantic causing coastally trapped waves to propagate over thousands of kilometers along the continental slope. Implications of these findings are assessed with statistical Monte-Carlo experiments. It is demonstrated that the removal of known variability increases the signal to noise ratio with the result that: (i) linear trends can be estimated more accurately; (ii) possible accelerations (as expected, e.g., due to anthropogenic climate change) can be detected much earlier. Such information is of crucial importance for anticipatory coastal management, engineering, and planning.

Den Wind hat auch eine Arbeitsgruppe um Margot Saher im Verdacht. Saher und Kollegen dokumentierten im Januar 2015 in den Quaternary Science Reviews die Meeresspiegelgeschichte Islands für die vergangenen 500 Jahre. Sie fanden, dass sich der Meeresspiegelanstieg auf drei Phasen beschränkte: Um 1600, 1810 und 1980. Den Phasen war gemeinsam, dass sich zu diesen Zeiten die größten Veränderungen in der Nordatlantischen Oszillation (NAO) ereigneten, einem bedeutenden Ozeanzyklus. Die Änderungen in der NAO haben die Windverhältnisse offenbar so umgestellt, dass der Meeresspiegel jeweils nach oben schnellte. Im Folgenden die Kurzfassung der Arbeit:

Sea-level changes in Iceland and the influence of the North Atlantic Oscillation during the last half millennium
We present a new, diatom-based sea-level reconstruction for Iceland spanning the last 500 years, and investigate the possible mechanisms driving the sea-level changes. A sea-level reconstruction from near the Icelandic low pressure system is important as it can improve understanding of ocean–atmosphere forcing on North Atlantic sea-level variability over multi-decadal to centennial timescales. Our reconstruction is from Viðarhólmi salt marsh in Snæfellsnes in western Iceland, a site from where we previously obtained a 2000-yr record based upon less precise sea-level indicators (salt-marsh foraminifera). The 20th century part of our record is corroborated by tide-gauge data from Reykjavik. Overall, the new reconstruction shows ca 0.6 m rise of relative sea level during the last four centuries, of which ca 0.2 m occurred during the 20th century. Low-amplitude and high-frequency sea-level variability is super-imposed on the pre-industrial long-term rising trend of 0.65 m per 1000 years. Most of the relative sea-level rise occurred in three distinct periods: AD 1620–1650, AD 1780–1850 and AD 1950–2000, with maximum rates of 3 ± 2 mm/yr during the latter two of these periods. Maximum rates were achieved at the end of large shifts (from negative to positive) of the winter North Atlantic Oscillation (NAO) Index as reconstructed from proxy data. Instrumental data demonstrate that a strong and sustained positive NAO (a deep Icelandic Low) generates setup on the west coast of Iceland resulting in rising sea levels. There is no strong evidence that the periods of rapid sea-level rise were caused by ocean mass changes, glacial isostatic adjustment or regional steric change. We suggest that wind forcing plays an important role in causing regional-scale coastal sea-level variability in the North Atlantic, not only on (multi-)annual timescales, but also on multi-decadal to centennial timescales.

Im arktischen Norwegen gibt es ein ganz seltsames Phänomen, das nichts mit dem Wind zu tun hat. Dort hat sich nach Ende der letzten Eiszeit und Schmelzen der skandinavischen Eismassen das Land stark gehoben. Ein Forscherteam um Robert Barnett hat im Januar 2015 in den Quaternary Science Reviews eine Meeresspiegelrekonstruktion für eine Lofoteninsel für die vergangenen 3300 Jahre vorgestellt. Sie fanden über den Zeitraum eine Meeresspiegelabsenkung von knapp einem Millimeter pro Jahr. Auf den Lofoten wird es wohl vorerst keine Klimaflüchtlinge geben. Hier die Kurzfassung der Arbeit:

Late Holocene sea-level change in Arctic Norway
Relative sea-level data from the pre-industrial era are required for validating geophysical models of glacio-isostatic adjustment as well as for testing models used to make sea-level predictions based on future climate change scenarios. We present the first late Holocene (past 3300 years) relative sea-level reconstruction for northwestern Norway based on investigations in South Hinnøya in the Vesterålen – Lofoton archipelago. Sea-level changes are reconstructed from analyses of salt-marsh and estuarine sediments and the micro-organisms (foraminifera and testate amoebae) preserved within. The ‘indicative meaning’ of the microfauna is established from their modern distributions. Records are dated by radiocarbon, 201Pb, 137Cs and chemostratigraphical analyses. Our results show a continuous relative sea-level decline of 0.7–0.9 mm yr−1for South Hinnøya during the late Holocene. The reconstruction extends the relative sea-level trend recorded by local tide gauge data which is only available for the past 25 years. Our reconstruction demonstrates that existing models of shoreline elevations and GIA overpredict sea-level positions during the late Holocene. We suggest that models might be adjusted in order to reconcile modelled and reconstructed sea-level changes and ultimately improve understanding of GIA in Fennoscandia.

In Schottland gibt es in abgeschwächter Form einen ähnlichen Trend. Natasha Barlow und Kollegen konnten im September 2014 in den Quaternary Science Reviews zeigen, dass der Meeresspiegel in Nordwest Schottland während der letzten 2000 Jahre allmählich mit 0,4 mm pro Jahr fiel. Insgesamt verzeichneten die Autoren jedoch eine ziemliche Stabilität. Hier die Kurzfassung:

Salt-marsh reconstructions of relative sea-level change in the North Atlantic during the last 2000 years
Sea-level changes record changes in the mass balance of ice sheets and mountain glaciers, as well as dynamic ocean–atmosphere processes. Unravelling the contribution of each of these mechanisms on Late Holocene timescales ideally requires observations from a number of sites on several coasts within one or more oceans. We present the first 2000 year-long continuous salt marsh-based reconstructions of relative sea-level (RSL) change from the eastern North Atlantic and uniquely from a slowly uplifting coastline. We develop three RSL histories from two sites in north west Scotland to test for regional changes in sea-level tendency (a positive tendency indicating an increase in the proximity of marine conditions and a negative tendency the reverse), whilst at the same time highlighting methodological issues, including the problems of dataset noise when applying transfer functions to fossil salt-marsh sequences. The records show that RSL has been stable (±0.4 m) during the last two millennia, and that the regional sea-level tendency has been negative throughout most of the record lengths. A recent switch in the biostratigraphy of all three records, indicating a regional positive tendency, means we cannot reject the hypothesis of a 20th century sea-level acceleration occurring in north west Scotland that must have exceeded the rate of background RSL fall (−0.4 mm yr−1), but this signal appears muted and later than recorded from the western North Atlantic.

Die Hebungsgeschichte der Ostsee ist ebenfalls hochinteressant. Hier haben sich Hebung und globaler Meeresspiegelanstieg immer wieder gegenseitig überholt, so dass es mehrere Wechsel von Verlandung und Überflutung in den letzten 10.000 Jahren gegeben hat. Siehe Zusammenfassungen von Meyer (2002) und Hillmann (2004).

Im Januar 2015 erschien eine Arbeit von Jens Morten Hansen, Troels Aagaard und Antoon Kuijpers online im Journal of Coastal Research. Die Gruppe fahndete nach Zyklen in der Meeresspiegelentwicklung in Nordsee und Ostsee. Dabei stießen sie auf Zyklen mit Perioden von 19, 60 und 75 Jahren, die sie mit Gezeitenzyklen des Mondes korrelierten. Hier der Abstract:

Sea-level forcing by synchronization of 56- and 74-year oscillations with the Moon’s nodal tide on the northwest European Shelf (eastern North Sea to central Baltic Sea)
The North Sea and Baltic Sea long-term records reveal a strong correlation (0.997) between sea-level changes and the sum of identified harmonic oscillations, corresponding to the lunar nodal period and four multiples of it. We developed a transparent method for iterative least residual sine regression that is capable of identifying harmonic sea-level oscillations, e.g., gravitational sea-level effects of the lunar nodal oscillation. Three relatively large harmonic sea-level oscillations with period lengths of 18.6 (18.6), 60.5 (55.8), and 76.1 (74.4) years correspond well to factors 1, 3, and 4 of the 18.6-year lunar nodal period (multiple periods in parentheses). The sum of these oscillations leaves small residuals that can be resolved into two further, statistically less significant oscillations with apparent period lengths of 28.1 (27.9) and 111.1 (111.7) years, corresponding to factors 1½ and 6 of the lunar nodal period. Periods and amplitudes expose strong entrainment, i.e. phase synchronization at rational ratios of the identified oscillations‘ periods as well as amplitude locking at reciprocal rational ratios of 1/2, 1/3, and 2/3 of the three largest oscillations. On top of the region’s general sea-level rise (1.18 mm/y), strong quasi-oscillations occur when the two largest oscillations are in phase. Thus, a large quasi-oscillation commenced in 1971 adding a 40-year sea-level rise of 1.0–1.2 mm/y to the region’s general sea-level rise. If our theory is correct, the ongoing quasi-oscillation should culminate in 2011, and the suggestion may be tested after completion of the ongoing 18.6-year nodal oscillation, i.e. in 2020–21. A purely mathematical extension of the oscillation parameters identified by the applied method suggests that the sum of harmonic oscillations produces 223-year pulses of quasi-oscillations, which can be divided into 158-year periods (e.g., 1747–1905 and after 1970) with large oscillations (60–65 mm), followed by 65-year periods (e.g., 1905–70) with much smaller oscillations (2–16 mm).

Aus Wales berichtete Spiegel Online im Februar 2014 über einen aus den Fluten auftauchenden Wald:

Wilde Stürme vor der walisischen Küste haben eine surreale Landschaft freigelegt. Als der Sand in Cardigan Bay verschwand, tauchten plötzlich Hunderte Stämme abgestorbener Bäume auf: Reste eines prähistorischen Waldes, der über 4500 Jahre alt ist.

In den Niederlanden baut man neuerdings fleissig Strände gegen den Meeresspiegelanstieg, wie Die Welt im November 2014 meldete:

Niederlande bauen Strände gegen den Klimawandel
Der steigende Meeresspiegel und die Zunahme von Stürmen bedrohen die Niederlande. 40 Prozent des Landes liegen unter dem Meeresspiegel. Weil Deiche als Schutz nicht ausreichen, sucht man neue Wege.

Abschließen möchten wir unseren europäischen Streifzug durch die aktuelle Meeresspiegelliteratur auf der Iberischen Halbinsel. Aus Nordwest Spanien beschreibt eine Studie von Sobrino et al. ein interessantes Phänomen: Zu Beginn der Kältephase der Völkerwanderungszeit um 350 n. Chr. stoppte der Meeresspiegelanstieg plötzlich, zumindest verlangsamte er sich enorm. Was könnte der Grund sein? Gletscher und Eiskappen lieferten in der Kältephase offenbar viel weniger Schmelzwasser, die Weltmeere kühlten ab und das kältere Wasser verlor an Volumen. Die Studie erschien im Juni 2014 in den Quaternary Science Reviews. Hier ein Auszug aus der Kurzfassung:

Climate and anthropogenic factors influencing an estuarine ecosystem from NW Iberia: new high resolution multiproxy analyses from San Simón Bay (Ría de Vigo)
Two sedimentary sequences (coastal and subtidal) were studied in San Simón Bay (Ría de Vigo), situated on the Atlantic coast of NW Iberia. The coastal record is a shallowing upward sequence which evidences a locally-developed low marsh, situated below the current beach, and dated at the second half of the 4th century. During the following decades this low marsh was progressively replaced by an alder swamp which formed on it. This suggests an apparent stabilisation or slow-down of the relative sea-level (RSL), in this site, at the beginning of the Dark Ages (DA). […]

Bayern 2 hatte im November 2014 folgende Horrorgeschichte aus Portugal im Programm:

Das Land bricht weg Klimawandel und Küstenerosion in Portugal
Das Ansteigen des Meeresspiegels und schwere Stürme lassen die Westküste Portugals auf breiter Front wegbrechen. Immer größere Gebiete sind inzwischen akut überschwemmungsgefährdet.
Seit Jahren warnen Fachleute vor den Folgen des Klimawandels und der Küstenerosion, doch ohne Erfolg.

Im Beitrag geht es um abbrechende Klippen samt den auf ihnen thronenden Ferienhäusern. Für die Hausbesitzer eine blöde Angelegenheit. Was aber hat der Klimawandel damit zu tun? Der Meeresspiegel steigt seit 10.000 Jahren und Steilküsten verlagern sich immer weiter ins Inland. Dieses Problem auf den anthropogenen Klimawandel zu schieben ist Unsinn. Die Meeresspiegel-Entwicklung Portugals haben wir kürzlich in unserer Länderanalyse „Surprising facts about climate change in Portugal“ beschrieben:

Based on theoretical considerations, IPCC scientists commonly warn of a sea level rising by half a metre, one metre or even two metres by the end of this century. The sea level data that has been measured at tide gauges along the coasts of the globe, however, tell us to be sceptical of such horror scenarios. Based on hard observed data, sea level has been rising by only about 1.0 – 2.0 millimetres per year. Sea level measurements in Cascais over the last 100 years have yielded a total sea level rise of about 150 mm, corresponding to 1.5 mm per year. There are hardly any vertical land movements in the Cascais region so that the measured sea-level value can be considered as characteristic for the Portuguese Atlantic coast. Interestingly, there has been no sea level rise acceleration in Cascais, or even globally, over the past 40 years despite a global warming of half a degree over the same period. If the present sea level trend continues, the sea would only rise only 10-20 cm by 2100, and not 50-200 cm as some scientists predicted.