Florida Institute of Technology: Südseeriffe vermögen mit dem Meeresspiegelanstieg mitzuwachsen

Der Meeresspiegelanstieg war bereits vielfach Thema hier im Blog, auch im Bereich der Pazifikinseln. Die Forschung ist weiterhin aktiv, so dass es sich lohnt, einen Blick in die neuesten Resultate zu werfen. Im Juli 2015 gab das Florida Institute of Technology per Pressemitteilung bekannt, dass die kleinen Koralleninseln duchaus eine Chance haben, mit dem Meeresspiegelanstieg Schritt zu halten:

New Study from Florida Tech Finds Pacific Reef Growth Can Match Rising Sea

For Reef Growth to Keep Up, However, Ocean Temperatures Must Stabilize

The coral reefs that have protected Pacific Islanders from storm waves for thousands of years could grow rapidly enough to keep up with escalating sea levels if ocean temperatures do not rise too quickly, according to a new study from Florida Institute of Technology. The study, published Wednesday in the journal Royal Society Open Science, provides the first evidence that well-managed reefs will be able to keep up with sea-level rise through vertical growth. But that can happen only if carbon dioxide levels in the atmosphere stay below 670 parts-per million (ppm). Carbon dioxide is the principal greenhouse gas responsible for most of global warming, which in turn increases ocean temperatures.

Today, the level of carbon dioxide is 400 ppm. Beyond 670 ppm – which represents a 3.5 degree Fahrenheit ocean temperature increase and could be reached within the next 100 years – even healthy reefs will not be able keep up. “Reefs will continue to keep up with sea-level rise if we reduce our emission of greenhouse gases,” said Florida Tech’s Rob van Woesik, a professor in the Department of Biological Sciences. “If reefs lose their capacity to keep up with sea-level rise they will drown.” Van Woesik was joined by researchers from the University of Queensland and the Palau International Coral Reef Center in the study, which took place in Palau in the western Pacific Ocean.

Coral reefs are an intricate part of island culture, and they are considered a precious resource in the Pacific Ocean. If global temperatures continue to rise and thus retard the growth of these natural storm barriers, the homelands of millions of people on lands throughout the Pacific Ocean will be in jeopardy. The paper can be found at http://rsos.royalsocietypublishing.org/content/2/7/150181.

Hier noch der Abstract der besprochenen Arbeit von van Woesik et al. 2015:

Keep up or drown: adjustment of western Pacific coral reefs to sea-level rise in the 21st century
Since the Mid-Holocene, some 5000 years ago, coral reefs in the Pacific Ocean have been vertically constrained by sea level. Contemporary sea-level rise is releasing these constraints, providing accommodation space for vertical reef expansion. Here, we show that Porites microatolls, from reef-flat environments in Palau (western Pacific Ocean), are ‘keeping up’ with contemporary sea-level rise. Measurements of 570 reef-flat Porites microatolls at 10 locations around Palau revealed recent vertical skeletal extension (78±13 mm) over the last 6–8 years, which is consistent with the timing of the recent increase in sea level. We modelled whether microatoll growth rates will potentially ‘keep up’ with predicted sea-level rise in the near future, based upon average growth, and assuming a decline in growth for every 1°C increase in temperature. We then compared these estimated extension rates with rates of sea-level rise under four Representative Concentration Pathways (RCPs). Our model suggests that under low–mid RCP scenarios, reef-coral growth will keep up with sea-level rise, but if greenhouse gas concentrations exceed 670 ppm atmospheric CO2 levels and with +2.2°C sea-surface temperature by 2100 (RCP 6.0 W m−2), our predictions indicate that Porites microatolls will be unable to keep up with projected rates of sea-level rise in the twenty-first century.

Das sind gute Nachrichten. Gibt es überhaupt zwingende Hinweise auf anthropogenes Zutun zum pazifischen Meeresspiegelgeschehen? Palanisamy et al. 2015 haben die diversen Ozeanzyklen aus dem pazifischen Meeresspiegelsignal herausgerechnet und fanden kaum einen Rest. Sie schlussfolgern, dass es bisher keinen robusten Hinweis auf einen „menschlichen Fingerabdruck“ beim pazifischen Meeresspiegelanstieg gibt. Hier der Abstract aus den Environmental Research Letters:

Is anthropogenic sea level fingerprint already detectable in the Pacific Ocean?
Sea level rates up to three times the global mean rate are being observed in the western tropical Pacific since 1993 by satellite altimetry. From recently published studies, it is not yet clear whether the sea level spatial trend patterns of the Pacific Ocean observed by satellite altimetry are mostly due to internal climate variability or if some anthropogenic fingerprint is already detectable. A number of recent studies have shown that the removal of the signal corresponding to the Pacific Decadal Oscillation (PDO)/Interdecadal Pacific Oscillation (IPO) from the observed altimetry sea level data over 1993–2010/2012 results in some significant residual trend pattern in the western tropical Pacific. It has thus been suggested that the PDO/IPO-related internal climate variability alone cannot account for all of the observed trend patterns in the western tropical Pacific and that the residual signal could be the fingerprint of the anthropogenic forcing. In this study, we investigate if there is any other internal climate variability signal still present in the residual trend pattern after the removal of IPO contribution from the altimetry-based sea level over 1993–2013. We show that subtraction of the IPO contribution to sea level trends through the method of linear regression does not totally remove the internal variability, leaving significant signal related to the non-linear response of sea level to El Niño Southern Oscillation (ENSO). In addition, by making use of 21 CMIP5 coupled climate models, we study the contribution of external forcing to the Pacific Ocean regional sea level variability over 1993–2013, and show that according to climate models, externally forced and thereby the anthropogenic sea level fingerprint on regional sea level trends in the tropical Pacific is still too small to be observable by satellite altimetry.

Wenn man die Ozeanzyklenmuster und ihre Beeinflussung des pazifischen Meeresspiegels erst einmal verstanden hat, eröffnet dies gute Prognosemöglichkeiten, wie Hamlington et al. 2016 darlegen konnten:

An ongoing shift in Pacific Ocean sea level
Based on the satellite altimeter data, sea level off the west coast of the United States has increased over the past 5 years, while sea level in the western tropical Pacific has declined. Understanding whether this is a short-term shift or the beginning of a longer-term change in sea level has important implications for coastal planning efforts in the coming decades. Here, we identify and quantify the recent shift in Pacific Ocean sea level, and also seek to describe the variability in a manner consistent with recent descriptions of El Nino-Southern Oscillation (ENSO) and particularly the Pacific Decadal Oscillation (PDO). More specifically, we extract two dominant modes of sea level variability, one related to the biennial oscillation associated with ENSO and the other representative of lower-frequency variability with a strong signal in the northern Pacific. We rely on cyclostationary empirical orthogonal function (CSEOF) analysis along with sea level reconstructions to describe these modes and provide historical context for the recent sea level changes observed in the Pacific. As a result, we find that a shift in sea level has occurred in the Pacific Ocean over the past few years that will likely persist in the coming years, leading to substantially higher sea level off the west coast of the United States and lower sea level in the western tropical Pacific.

Damit Korallen mitwachsen können und Mangroven gegen Erosion schützen, müssen diese aber auch am Leben gelassen werden. Hier müssen die Pazifikanwohner selber aktiv werden: Korallensand von aktiven Riffen abzubaggern, Abhacken von Mangroven und Einleiten von ungeklärten Abwässern kann es nicht geben, insbesondere wenn sich einige der „Täter“ gleichzeitig als Klimaopfer darstellen. Zum Thema Mangrovenschutz veröffentlichte die University of Southampton im Juli 2015 die folgende Pressemitteilung:

Mangroves help protect against sea level rise

Mangrove forests could play a crucial role in protecting coastal areas from sea level rise caused by climate change, according to new research involving the University of Southampton. 

A joint study between researchers at the University of Southampton along with colleagues from the Universities of Auckland and Waikato in New Zealand used cutting-edge mathematical simulations to study how mangrove forests respond to elevated sea levels. Taking New Zealand mangrove data as the basis of a new modelling system, the team were able to predict what will happen to different types of estuaries and river deltas when sea levels rise. They found areas without mangroves are likely to widen from erosion and more water will encroach inwards, whereas mangrove regions prevent this effect – which is likely due to soil building up around their mesh-like roots and acting to reduce energy from waves and tidal currents.

Coastal estuaries and recesses in coastlines that form bays receive the run-off from erosion on steep catchments, which give them the tendency to fill in over time. As they infill, the movement of the tidal currents over the shallow areas create networks of sandbanks and channels. The sand banks grow upward to keep pace with water level changes, while the channels get deeper to efficiently drain the excess water out to sea. The researchers’ latest work shows that mangroves can facilitate this process, by adding leaf and root structures into the accumulating sediment, which increase the elevation while enhancing the trapping of new sediment arriving from the catchment. Dr Barend van Maanen from the University of Southampton explains: “As a mangrove forest begins to develop, the creation of a network of channels is relatively fast. Tidal currents, sediment transport and mangroves significantly modify the estuarine environment, creating a dense channel network.

“Within the mangrove forest, these channels become shallower through organic matter from the trees, reduced sediment resuspensions (caused by the mangroves) and sediment trapping (also caused by the mangroves) and the sea bed begins to rise, with bed elevation increasing a few millimetres per year until the area is no longer inundated by the tide.” In modelling of sea level rise in the study, the ability of mangrove forest to gradually create a buffer between sea and land occurs even when the area is subjected to potential sea level rises of up to 0.5mm per year. Even after sea level rise, the mangroves showed an enhanced ability to maintain an elevation in the upper intertidal zone.

Associate Professor Karin Bryan, from the University of Waikato, says the spread of mangroves is changing the New Zealand coastal landscape. “In New Zealand, mangroves have been traditionally viewed as undesirable as they take over areas where there were once sandy beaches. In other countries, this is not the case as they are seen as a buffer for climate change in low level areas.” “Now we know that they also could play a critical role in buffering our coastal land from the effects of sea level rise. Although the study is on Avicennia marina (the only species of mangrove that occurs in New Zealand), Avicennia occurs in every major mangrove habitat in the world.”

Overseas studies have shown mangroves have the ability to remove carbon from the atmosphere and protect people from hazards such as tsunami. The research team hopes that this work will enhance the case for protecting global fringing wetlands from the threats of drainage and clearance caused by development and aquaculture pressures. “These findings show that mangrove forests play a central role in estuarine and salt marsh environments,” Associate Professor Giovanni Coco from the University of Auckland says. “As we anticipate changes caused by climate change, it’s important to know the effect sea level rise might have, particularly around our coasts. “Mangroves appear to be resilient to sea level rise and are likely to be able to sustain such climatic change. The implications for the New Zealand coastline are considerable and will require new thinking in terms of sediment budgets and response to climatic changes.”

Schließlich sei noch auf eine wahrhaftige Katastrophe aus der Südsee hingewiesen. Diese ereignete sich allerdings bereits vor 700 Jahren. Damals gegen 1300 n.Chr. sackten Temperaturen und Meeresspiegel im Pazifikraum drastisch ab, wie Nunn 2000 berichtete. Der Meeresspiegelabfall führte an vielen Stellen zum Abbruch des Korallenriffwachstums und löste sogar kriegerische Konflikte aus.

Environmental catastrophe in the Pacific Islands around A.D. 1300
Paleosea-level data for the Pacific Islands suggest that sea level in the region fell, possibly in two stages, between 680 and 475 cal yr B.P. (A.D. 1270–1475). This was associated with a ∼1.5°C fall in temperature (determined from oxygen-isotope analysis) and an observed increase in El Niño frequency. For a long time, it has been clear that these changes—characterized as the “A.D. 1300 event”—brought about environmental and cultural changes on Pacific Islands. These are documented here systematically for the first time. Temperature fall, sea-level fall, and possibly short-lived precipitation increase are the principal effects of the A.D. 1300 event. Temperature fall stressed ecosystems, but its effects are difficult to separate from those of the others. Sea-level fall saw dramatic falls of nearshore coral-reef productivity and the formation of (habitable) reef islands (motu). Precipitation rise increased upland erosion and lowland sedimentation. The human outcomes of these environmental changes are organized in three groups: conflict, settlement-pattern changes, and the end of long-distance voyaging. Conflict increased during/after the A.D. 1300 event because of an abrupt fall in the food resource base. This also caused large coastal settlements on many islands to be abandoned in favor of caves and/or smaller fortified hilltop settlements. Successful long-distance voyages ceased during/after the A.D. 1300 event, as did interisland exchange within many archipelagoes. The regional (rather than local) extent of the A.D. 1300 event is demonstrated. Questions remain as to its synchronicity and duration.