Der Meeresspiegel steigt aktuell mit 2-3 mm pro Jahr. Die Anhänger des Klimakatastrophismus sind sich einig: Schuld an diesem Anstieg ist auf jeden Fall der Mensch, denn derart hohe Werte habe es in der Vergangenheit nicht gegeben. Aber sind die aktuellen Meeresspiegel-Anstiegsraten wirklich so einzigartig, wie sie immer dargestellt werden? Zwei kürzliche Studien zeigen, dass Skepsis angezeigt ist.
Angesichts der Tatsache, dass Küstenpegelmessungen nur gut 100 Jahre in die Vergangenheit zurückreichen, hat eine Forschergruppe um Thomas Cronin nun mit geologischen Methoden versucht, die Meeresspiegelgeschichte der US-Ostküste für die vergangenen 2000 Jahre zurück zu verfolgen. Dabei fanden sie etwas überaus Erstaunliches: Der Meeresspiegel unterlag bereits in vorindustrieller Zeit stets spürbaren Schwankungen im Zeitmaßstab von wenigen Jahrzehnten. Bislang gingen viele Kollegen davon aus, dass der Meeresspiegel der letzten Jahrtausende vor 1850 ziemlich konstant gewesen wäre. Ein Irrtum, wie sich jetzt herausstellte.
Cronin und Kollegen sehen hier einen deutlichen Zusammenhang mit atlantischen Ozeanzyklen, die den Meeresspiegel systematisch beeinflussten. Zur Zeit der Mittelalterlichen Wärmeperiode und Kleinen Eiszeit wurden interessanterweise ähnlich hohe Änderungsraten im Meeresspiegel erreicht wie heute. Der Artikel erschien im August 2014 im Fachblatt Paleoceanography. Anbei die Kurzfassung:
Late Holocene sea level variability and Atlantic Meridional Overturning Circulation
Pre-twentieth century sea level (SL) variability remains poorly understood due to limits of tide gauge records, low temporal resolution of tidal marsh records, and regional anomalies caused by dynamic ocean processes, notably multidecadal changes in Atlantic Meridional Overturning Circulation (AMOC). We examined SL and AMOC variability along the eastern United States over the last 2000 years, using a SL curve constructed from proxy sea surface temperature (SST) records from Chesapeake Bay, and twentieth century SL-sea surface temperature (SST) relations derived from tide gauges and instrumental SST. The SL curve shows multidecadal-scale variability (20–30 years) during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), as well as the twentieth century. During these SL oscillations, short-term rates ranged from 2 to 4 mm yr−1, roughly similar to those of the last few decades. These oscillations likely represent internal modes of climate variability related to AMOC variability and originating at high latitudes, although the exact mechanisms remain unclear. Results imply that dynamic ocean changes, in addition to thermosteric, glacio-eustatic, or glacio-isostatic processes are an inherent part of SL variability in coastal regions, even during millennial-scale climate oscillations such as the MCA and LIA and should be factored into efforts that use tide gauges and tidal marsh sediments to understand global sea level rise.
Nur einen Monat später dann der zweite Paukenschlag. Im September 2014 erinnerte die University of Southampton in einer Pressemitteilung daran, dass der Meeresspiegelanstieg im Anschluss an die letzten 5 Eiszeiten enorme 50 mm pro Jahr betragen hat, also ein Anstieg fast 20 mal schneller als heute:
Study tracks global sea-levels over the last five ice ages
Land-ice decay at the end of the last five ice-ages caused global sea-levels to rise at rates of up to 5.5 metres per century, according to a new study. An international team of researchers developed a 500,000-year record of sea-level variability, to provide the first account of how quickly sea-level changed during the last five ice-age cycles. The results, published in the latest issue of Nature Communications, also found that more than 100 smaller events of sea-level rise took place in between the five major events.Dr Katharine Grant, from the Australian National University (ANU), Canberra, who led the study, says: “The really fast rates of sea-level rise typically seem to have happened at the end of periods with exceptionally large ice sheets, when there was two or more times more ice on the Earth than today. “Time periods with less than twice the modern global ice volume show almost no indications of sea-level rise faster than about 2 metres per century. Those with close to the modern amount of ice on Earth, show rates of up to 1 to 1.5 metres per century.”
Co-author Professor Eelco Rohling, of both the University of Southampton and ANU, explains that the study also sheds light on the timescales of change. He says: “For the first time, we have data from a sufficiently large set of events to systematically study the timescale over which ice-sheet responses developed from initial change to maximum retreat. This happened within 400 years for 68 per cent of all 120 cases considered, and within 1100 years for 95 per cent. In other words, once triggered, ice-sheet reduction (and therefore sea-level rise) kept accelerating relentlessly over periods of many centuries.”
Professor Rohling speculates that there may be an important lesson for our future: “Man-made warming spans 150 years already and studies have documented clear increases in mass-loss from the Antarctic and Greenland ice sheets. Once under way, this response may be irreversible for many centuries to come.”
The team reconstructed sea-levels using data from sediment cores from the Red Sea, an area that is very sensitive to sea-level changes because it’s only natural connection with the open (Indian) ocean is through the very shallow (137 metre) Bab-el-Mandab Strait. These sediment samples record wind-blown dust variations, which the team linked to a well-dated climate record from Chinese stalagmites. Due to a common process, both dust and stalagmite records show a pronounced change at the end of each ice age, which allowed the team to date the sea-level record in detail. The researchers emphasise that their values for sea-level change are 500-year averages, so brief pulses of faster change cannot be excluded. The study was funded primarily by Natural Environment Research Council (NERC) and the Australian Research Council (ARC).
