Zyklen modulieren den pazifischen Meeresspiegel

Unser heutiges Thema: Meeresspiegel im Pazifik. Am 18.8.2016 brachte die University of Arizona (UA) eine Pressemitteilung (PM), die eine Vorhersagemöglichkeit der globalen Temperatur über den pazifischen Meeresspiegel beschreibt. Die Autoren skizzieren hochspannende Zusammenhänge von Meeresspiegelungleichgewichten im Pazifik, und wie sie sich auf die globale Temperaturen auswirken. Dahinter stecken die PDO- und ENSO-Ozeanzyklik. Auch der Hiatus wird besprochen. Lessenswert!

Pacific Sea Level Predicts Global Temperature Changes

A UA-led research team figured out that sea level changes in the Pacific Ocean can be used to estimate future global average surface temperatures.

The amount of sea level rise in the Pacific Ocean can be used to estimate future global surface temperatures, according to a new report led by University of Arizona geoscientists. Based on the Pacific Ocean’s sea level in 2015, the team estimates by the end of 2016 the world’s average surface temperature will increase up to 0.5 F (0.28 C) more than in 2014. In 2015 alone, the average global surface temperature increased by 0.32 F (0.18 C). „Our prediction is through the end of 2016,“ said first author Cheryl Peyser. „The prediction is looking on target so far.“

Scientists knew that both the rate at which global surface temperature is rising and sea level in the western Pacific varied, but had not connected the two phenomena, said Peyser, a UA doctoral candidate in geosciences. „We’re using sea level in a different way, by using the pattern of sea level changes in the Pacific to look at global surface temperatures–and this hasn’t been done before,“ she said. Peyser and her colleagues used measurements of sea level changes taken by NASA/NOAA/European satellites starting in 1993.

Using sea surface height rather than sea surface temperatures provides a more accurate reflection of the heat stored in the entire water column, said co-author Jianjun Yin, a UA associate professor of geosciences. „We are the first to use sea level observations to quantify the global surface temperature variability,“ Yin said. The team found when sea level in the western Pacific rises more than average — as it did from 1998 to 2012 — the rise in global surface temperatures slows.

In contrast, when sea level drops in the western Pacific but increases in the eastern Pacific as it did in 2015, global surface temperatures bump up because the heat stored in the ocean is released, Yin said. The paper by Peyser, Yin, Felix Landerer of NASA’s Jet Propulsion Laboratory, Pasadena, California, and Julia Cole, a UA professor of geosciences, titled, „Pacific Sea Level Rise Patterns and Global Surface Temperature Variability,“ is being published online in Geophysical Research Letters. 

People already knew the tropical Pacific Ocean was relatively higher in the west –the trade winds blow from east to west, piling up water on the western side of the Pacific. However, the degree of the tilt from west to east changes over time, much like a seesaw. Sometimes the western Pacific near Asia is much higher than the ocean’s eastern coast with the Americas. At other times, Pacific sea level in the west is not much greater than sea level in the east. Others had documented that two different climate cycles, the Pacific Decadal Oscillation and the El Niño/La Niña cycle, affected how much the surface of the Pacific Ocean tilted from west to east.

From 1998 to 2012, the rate at which the global surface temperature increased slowed down–a phenomenon dubbed „the global warming hiatus.“ During the same time period, sea level in the western tropical Pacific Ocean increased four times faster than the average global sea level rise. Yin wondered if the two phenomena–sea level and global surface temperature–were related and asked Peyser, his graduate student, to investigate.

To figure out whether there was a connection, Peyser used state-of-the-art climate models that show what the climate system would do in the absence of global warming. The models showed that changes in sea level in the western Pacific were correlated with changes in global surface temperature. Verifying the correlation allowed the researchers to calculate the numerical relationship between amount of tilt and global surface temperature. Once the researchers had the correlation, they used actual Pacific sea level data from satellites to calculate the Pacific Ocean’s contribution to global surface temperature. „What I found was that during years when the tilt was steep in the western Pacific, global average temperature was cooler,“ she said. „And when the seesaw is tilted more toward the eastern Pacific, it’s warmer.“

„We could say that for a certain amount of change in the tilt, you could expect a certain change in the temperature,“ she said. „Natural variability is a really important part of the climate cycle.“ Understanding the variability is crucial for understanding the mechanisms underlying the warming hiatus, Yin said. During the global warming hiatus, more heat was being stored in the deeper layers of the western Pacific Ocean, muting warming at the surface, the researchers said. Because warmer water expands, that stored heat contributed to the extreme sea level rise in the western Pacific during that time.

Starting in 2014 the ocean’s tilt started to flatten out as the climate cycle changed to an El Niño pattern. The heat previously stored in the ocean was being released, warming the Earth’s surface and reducing sea level in the western Pacific. Yin was surprised to find the Pacific Ocean plays such an important role in the global surface temperature. He said, „Our research shows that the internal variability of the global climate system can conceal anthropogenic global warming, and at other times the internal variability of the system can enhance anthropogenic warming.“ The next step, he said, is figuring out the mechanisms that allow the Pacific to change the global surface temperature so quickly.

NASA funded the research, including through the Strategic University Research Partnership Program of NASA’s Jet Propulsion Laboratory.

Hier das dazugehörige Paper von Peyser et al. 2016:

Pacific sea level rise patterns and global surface temperature variability

During 1998–2012, climate change and sea level rise (SLR) exhibit two notable features: a slowdown of global surface warming (hiatus) and a rapid SLR in the tropical western Pacific. To quantify their relationship, we analyze the long‐term control simulations of 38 climate models. We find a significant and robust correlation between the east‐west contrast of dynamic sea level (DSL) in the Pacific and global mean surface temperature (GST) variability on both interannual and decadal time scales. Based on linear regression of the multimodel ensemble mean, the anomalously fast SLR in the western tropical Pacific observed during 1998–2012 indicates suppression of a potential global surface warming of 0.16° ± 0.06°C. In contrast, the Pacific contributed 0.29° ± 0.10°C to the significant interannual GST increase in 1997/1998. The Pacific DSL anomalies observed in 2015 suggest that the strong El Niño in 2015/2016 could lead to a 0.21° ± 0.07°C GST jump.

PDO und ENSO spielen auch bei Royston et al. 2018 eine Rolle:

Sea‐Level Trend Uncertainty With Pacific Climatic Variability and Temporally‐Correlated Noise

Recent studies have identified climatic drivers of the east‐west see‐saw of Pacific Ocean satellite altimetry era sea level trends and a number of sea‐level trend and acceleration assessments attempt to account for this. We investigate the effect of Pacific climate variability, together with temporally‐correlated noise, on linear trend error estimates and determine new time‐of‐emergence (ToE) estimates across the Indian and Pacific Oceans. Sea‐level trend studies often advocate the use of auto‐regressive (AR) noise models to adequately assess formal uncertainties, yet sea level often exhibits colored but non‐AR(1) noise. Standard error estimates are over‐ or under‐estimated by an AR(1) model for much of the Indo‐Pacific sea level. Allowing for PDO and ENSO variability in the trend estimate only reduces standard errors across the tropics and we find noise characteristics are largely unaffected. Of importance for trend and acceleration detection studies, formal error estimates remain on average up to 1.6 times those from an AR(1) model for long‐duration tide gauge data. There is an even chance that the observed trend from the satellite altimetry era exceeds the noise in patches of the tropical Pacific and Indian Oceans and the south‐west and north‐east Pacific gyres. By including climate indices in the trend analysis, the time it takes for the observed linear sea‐level trend to emerge from the noise reduces by up to 2 decades.

PDO und ENSO auch bei Moon et al. 2015:

PDO and ENSO modulations intensified decadal sea level variability in the tropical Pacific

According to long‐term sea level reconstruction and steric sea level data, regional sea levels in the tropical Pacific have oscillated between east and west on a decadal time scale over the past 60 years, but the oscillation has been intensified significantly in the last three decades. Using conditional composite analysis, we show that the recent intensification in sea level variability is caused by modulation between the Pacific Decadal Oscillation (PDO) and El Niño‐Southern Oscillation (ENSO), i.e., an El Niño in a positive PDO or a La Niña in a negative PDO phase. Our analysis of meteorological fields indicates that atmospheric circulation associated with the changes in ENSO‐PDO phase relationship plays a positive role in enhancing the decadal sea level oscillation. The intensified sea level oscillation, when superimposed on the global trend of sea level rise, will have profound implications for coastal communities, therefore, the combined effect of PDO and ENSO should be taken into account in the decadal sea level prediction in the tropical Pacific.

Und nochmal ENSO und PDO. Hamlington et al. 2016:

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.

Die Meeresspiegelentwicklung im Südpazifik unterliegt starken natürlichen Schwankungen, die von den Klimamodellen noch nicht nachvollzogen werden können, schreiben Albrecht et al. 2019:

Understanding Sea Level Change in the South Pacific During the Late 20th Century and Early 21st Century

A spatially nonuniform sea level rise was observed in the South Pacific with high trends of up to 8 mm/year in the southwest subtropical Pacific and much lower values in the eastern ocean during the 1993 to 2015 altimetry data period. Negative trends were observed further south. In recent decades, these trends have been interpreted as a spin‐up of subtropical gyre circulation. Here an analysis of altimetry data and data from the Coupled Model Intercomparison Project Phase 5 is presented, which attempts to separate natural climate variability from sea level changes. In order to quantify the impact of natural variability, a multiple linear regression was performed between sea level and dominant climate indices in the region, with the latter representing interannual to interdecadal climate variability. Our results indicate that the observed pattern cannot completely be explained by the climate modes but that a large residual trend remains.

Und noch ein Paper zum selben Thema, diesmal von Piecuch et al. 2019. Änderungen im Wind führen zu Änderungen im ozeanischen Wärmeinhalt, was sich in Meeresspiegeländerungen auswirkt:

What Caused Recent Shifts in Tropical Pacific Decadal Sea‐Level Trends?

Satellite altimetry reveals substantial decadal variability in sea level ζ across the tropical Pacific during 1993–2015. An ocean state estimate that faithfully reproduces the observations is used to elucidate the origin of these low‐frequency tropical Pacific ζ variations. Analysis of the hydrostatic equation reveals that recent decadal ζ changes in the tropical Pacific are mainly thermosteric in nature, related to changes in upper‐ocean heat content. A forcing experiment performed with the numerical model suggests that anomalous wind stress was an important driver of the relevant heat storage and thermosteric variation. Closed budget diagnostics further clarify that the wind‐stress‐related thermosteric ζ variation resulted from the joint actions of large‐scale ocean advection and local surface heat flux, such that advection controlled the budget over shorter, intraseasonal to interannual time scales, and local surface heat flux became increasingly influential at longer decadal periods. In particular, local surface heat flux was important in contributing to a recent reversal of decadal ζ trends in the tropical Pacific. Contributions from local surface heat flux partly reflect damping latent heat flux tied to wind‐stress‐driven sea‐surface‐temperature variations.

Weiter gehts mit den Koralleninseln. Wussten Sie, dass Stürme Inseln schaffen können? PM der Geological Society of America vom 17.9.2018:

Catastrophic construction: Storms can build reef islands in atoll regions

Boulder, Colo., USA: Tropical storms, with waves reaching up to 10-meters-high, can wallop coral reef islands. As global temperatures increase, some scientists suggest that such storms will become more frequent and intense over the next few decades. Additionally, potential sea level rise is perceived as a threat to the continued existence of these remote, low-lying communities.

Many coral reef islands, or atolls, are created by water moving sand and gravel, piling it up into consecutive ridged layers, „a bit like onions,“ says Paul Kench, earth scientist and dean of science at Simon Fraser University, Burnaby, Canada. However, new research by Kench and colleagues recently published online ahead of print for Geology uncovered a different type of island construction: storm-deposited boulders.

On Tutago island in Tuvalu’s Funafuti atoll, giant coral blocks sit on top of a flat reef. Researchers noted the jumble of upended corals didn’t grow there, but instead were tumbled onto the site. As storm waves hit the reefs, it’s not just fist or head-sized gravel chunks that are moving—Kench says coral boulders, which can be meters in diameter, are getting plucked and hurled into piles, forming the island.

To better understand how storms shaped Tutago, the team collected radiocarbon dates on the piled coral blocks. Within these boulder deposits, they uncovered distinct clusters of ages, revealing that Tutago island formed about 750 years ago, and at least two major storm events occurred about 600 and 350 years ago.

Kench says the 300-year hiatus between these storm events may have given nearby reefs a chance to recover after being plucked by the storm. „For coral to grow to two meters diameter, it may take it a hundred and fifty years,“ he says. „Island genesis is not only dependent on the storm, it’s also dependent on the rate of coral production.”

Understanding how Tutago formed can be helpful to other researchers interested in atoll behavior, coral reef regeneration, and ancient storm events. „I think there’s certainly purchase in this notion that the islands are archives of storm sequences,“ says Kench, adding that storm records in the remote ocean can be tough to find.

Storm events have built Tutago up vertically, but that isn’t always the case for islands. Kench says that understanding how an increase in storm frequency and intensity might modify atolls—especially whether they add or destroy land—is extremely important to island communities and governments. „It’s quite real for some of these communities,“ he says. „They’re dealing with environmental change on a daily basis.“

FEATURED ARTICLE

Kench, P., McLean, R.F., Owen, S.D., Tuck, M., and Ford, M.R., Storm-deposited coral blocks: A mechanism of island genesis, Tutaga island, Funafuti atoll, Tuvalu: Geology, https://doi.org/10.1130/G45045.1. Contact: Paul Kench, p.kench@auckland.ac.nz.

Der Spiegel am 23.2.2018:

Warum Tuvalu nun doch nicht untergeht

Seit Jahrzehnten warnt die Uno, dass der steigende Meeresspiegel flache Inseln zerstört. Doch stattdessen wachsen einige sogar. Wie ist das möglich?

[…] Ein neuseeländischer Forscher hat nun untersucht, wie der Klimawandel das Inselreich verändert – und ist zu verblüffenden Ergebnissen gekommen: Paul Kench von der University of Auckland hat alle 101 Tuvalu-Inseln auf Luftaufnahmen und Satellitenbildern aus den Jahren 1971 bis 2014 studiert. In dieser Zeit ist der Meeresspiegel in Tuvalu jedes Jahr um vier Millimeter gestiegen, etwa doppelt so schnell wie durchschnittlich in den Weltmeeren. Wie viel Land mag das bedrängte Inselreich in diesen Jahrzehnten an den Ozean verloren haben? Kenchs Resultat, jetzt veröffentlicht im Fachblatt „Nature Communications“: Nur ein unbewohntes Eiland ist erwartungsgetreu abgesoffen. 27 Inseln sind geschrumpft, einzelne sogar um mehr als die Hälfte. Der größte Teil von Tuvalu aber ist auf wundersame Weise gewachsen – netto um fast drei Prozent seit 1971. Obwohl das Wasser höher steht, hat das niedrige Inselreich sein Terrain um 73,5 Hektar erweitert, eine Fläche, die mehr als hundert Fußballfeldern entspricht. […]

Ganzen Artikel im Spiegel lesen. Siehe auch Bericht auf phys.org.

Hier das dazugehörige Paper von Kench et al. 2018:

Patterns of island change and persistence offer alternate adaptation pathways for atoll nations

Sea-level rise and climatic change threaten the existence of atoll nations. Inundation and erosion are expected to render islands uninhabitable over the next century, forcing human migration. Here we present analysis of shoreline change in all 101 islands in the Pacific atoll nation of Tuvalu. Using remotely sensed data, change is analysed over the past four decades, a period when local sea level has risen at twice the global average (~3.90 ± 0.4 mm.yr−1). Results highlight a net increase in land area in Tuvalu of 73.5 ha (2.9%), despite sea-level rise, and land area increase in eight of nine atolls. Island change has lacked uniformity with 74% increasing and 27% decreasing in size. Results challenge perceptions of island loss, showing islands are dynamic features that will persist as sites for habitation over the next century, presenting alternate opportunities for adaptation that embrace the heterogeneity of island types and their dynamics.

Atoll-Inseln haben in den letzten Jahrzehnten trotz Meeresspiegelanstieg nicht an Fläche verloren, fand Duvat 2018:

A global assessment of atoll island planform changes over the past decades

Over the past decades, atoll islands exhibited no widespread sign of physical destabilization in the face of sea‐level rise. A reanalysis of available data, which cover 30 Pacific and Indian Ocean atolls including 709 islands, reveals that no atoll lost land area and that 88.6% of islands were either stable or increased in area, while only 11.4% contracted. Atoll islands affected by rapid sea‐level rise did not show a distinct behavior compared to islands on other atolls. Island behavior correlated with island size, and no island larger than 10 ha decreased in size. This threshold could be used to define the minimum island size required for human occupancy and to assess atoll countries and territories‘ vulnerability to climate change. Beyond emphasizing the major role of climate drivers in causing substantial changes in the configuration of islands, this reanalysis of available data indicates that these drivers explain subregional variations in atoll behavior and within‐atoll variations in island and shoreline (lagoon vs. ocean) behavior, following atoll‐specific patterns. Increasing human disturbances, especially land reclamation and human structure construction, operated on atoll‐to‐shoreline spatial scales, explaining marked within‐atoll variations in island and shoreline behavior. Collectively, these findings highlight the heterogeneity of atoll situations. Further research needs include addressing geographical gaps (Indian Ocean, Caribbean, north‐western Pacific atolls), using standardized protocols to allow comparative analyses of island and shoreline behavior across ocean regions, investigating the role of ecological drivers, and promoting interdisciplinary approaches. Such efforts would assist in anticipating potential future changes in the contributions and interactions of key drivers.

Perry et al. 2018 machten sich düstere Gedanken, welche Meeresspiegelanstiegsraten Korallenriffe wohl mit Wachstum ausgleichen könnten. Das wundert schon ein wenig, denn die Riffe haben auch die schnellen Anstiege vor 10.000 Jahren gemeistert…

Ähnliche Resultate in Französisch Polynesien von Duvat et al. 2017:

Drivers of shoreline change in atoll reef islands of the Tuamotu Archipelago, French Polynesia

This paper increases by around 30% the sample of atoll reef islands studied from a shoreline change perspective, and covers an under-studied geographical area, i.e. the French Tuamotu Archipelago. It brings new irrefutable evidences on the persistence of reef islands over the last decades, as 77% of the 111 study islands exhibited areal stability while 15% and 8% showed expansion and contraction, respectively. This paper also addresses a key research gap by interpreting the major local drivers controlling recent shoreline and island change, i.e. tropical cyclones and seasonal swells, sediment supply by coral reefs and human activities. The 1983 tropical cyclones had contrasting impacts, depending on the shoreline indicator considered. While they generally caused a marked retreat of the stability line, the base of the beach advanced at some locations, as a result of either sediment reworking or fresh sediment inputs. The post-cyclone fair weather period was characterised by reversed trends indicating island morphological readjustment. Cyclonic waves contributed to island upwards growth, which reached up to 1 m in places, through the transfer of sediments up onto the island surface. However, the steep outer slopes of atolls limited sediment transfers to the reef flat and island system. We found that 57% of the study islands are disturbed by human activities, including ‘rural’ and uninhabited islands. Twenty-six percent of these islands have lost the capacity to respond to ocean-climate related pressures, including the ‘capital’ islands concentrating atolls‘ population, infrastructures and economic activities, which is preoccupying under climate change.

Im Funafuti Atoll hat Wirbelsturm Pam die Inselfläche um 0,13% (!) verkleinert. Hisabayashi et al. 2017:

Quantifying shoreline change in Funafuti Atoll, Tuvalu using a time series of Quickbird, Worldview and Landsat data

Funafuti Atoll, Tuvalu is located in the southwestern Pacific Ocean, which has experienced some of the highest rates of global sea-level rise over the past 60 years. Atoll islands are low-lying accumulations of reef-derived sediment that provide the only habitable land in Tuvalu, and are considered vulnerable to the myriad possible impacts of climate change, especially sea-level rise. This study examines the shoreline change of twenty-eight islands in Funafuti Atoll between 2005 and 2015 using 0.65 m QuickBird, 0.46 m WorldView-2, and 0.31 m WorldView-3 imagery using an image segmentation and decision tree classification. Shoreline change estimates are compared to previous study that used a visual interpretation approach. The feasibility of estimating island area with Landsat-8 Operational Land Imager (OLI) data is explored using CLASlite software. Results indicate a 0.13% (0.35 ha) decrease in net island area over the study time period, with 13 islands decreasing in area and 15 islands increasing in area. Substantial decreases in island area occurred on the islands of Fuagea, Tefala and Vasafua, which coincides with the timing of Cyclone Pam in March, 2015. Comparison between the WorldView-2 shoreline maps and those created from Landstat-8 indicate that the estimates tend to be in higher agreement for islands that have an area > 0.5 ha, a compact shape, and no built structures. Ten islands had > 90% agreement, with percent disagreements ranging from 2.78 to 100%. The methods and results of this study speak to the potential of automated EoV shoreline monitoring through segmentation and classification tree approach, which would reduce down data processing and analysis time. With the growing constellation of high and medium spatial resolution satellite-based sensors and the development of semi or fully automated image processing technology, it is now possible to remotely assess the short and medium-term shoreline dynamics on dynamic atolls. Landsat estimates were reasonably matched to those derived from fine resolution imagery, with some caveats about island size and shape.

Gute Nachrichten der University of Auckland am 16.7.2019:

Sea level rise may not drown low-lying Pacific atolls

Sea-level rise simulation suggests that low-lying Pacific islands such as those in Tuvalu, Tokelau and Kiribati are likely to adapt to the effects of climate change rather than simply sink beneath the waves, according to Kiwi researchers. The researchers created 1:50 scale replica of the uninhabited island of Fatato in Tuvalu and submitted the model to rising water levels and mock storm-generated waves. They found that the highest part of the island actually got higher as rising sea levels and strong wave action washed sand and gravel toward it. They say this shows the islands may be more resilient than we thought as they may be able to change shape to adapt to the climate.

Pacific atolls can adapt to rising seas and extreme storms – new study

Low-lying Pacific islands in atoll archipelagos such as Tuvalu, Tokelau and Kiribati are likely to adapt to the effects of climate change rather than simply sink beneath the waves, a new study shows.

Tuvalu, Tokelau and Kiribati are widely considered under threat from rising seas and severe storms due to climate change with their residents becoming ‘climate refugees’. Researchers from the University of Auckland’s School of Environment recreated a scale model of tiny Fatato Island on the southeast rim of Funafuti Atoll in Tuvalu to test the ability of the real island to withstand predicted climate affects. The study simulated higher sea levels and storm-generated waves up to 4m in a 20m-long water chute or ‘flume’ to replicate real-world sea levels of 0.5m and 1m in a purpose built laboratory at the University of Plymouth in the United Kingdom.

The team chose uninhabited Fatato as a model because they were able to create an accurate 1:50 scale replica using data collected from previous field surveys and research. In the real world, the tiny island is just 90m at its widest point and 860m long. The scale replica created for the laboratory tests was 0.6m wide, 2.6m long with a highest point of 10cm. Using lasers to closely monitor changes in the model, and translating those to a real-world scenario, the researchers found the crest of the island – its highest ground – actually increased 1.13m height as higher sea levels and strong wave action washed sand and gravel toward it.

While that elevation was achieved at the expense of lower-lying areas, simultaneously reducing the amount of low-lying land as the crest got higher. But that might not happen in the real world where islands are continually replenished by sediment from the surrounding reef. Importantly, the island also moved laterally, migrating across the coral reef as sand and gravel shifted position with the action of waves and higher water levels. Lead author and University of Auckland doctoral candidate Megan Tuck says the findings, along with previous research, has profound implications for understanding the physical vulnerability of reef islands and challenges the assumption that they will simply drown.

“Atoll islands do not sit inert on the reef, instead the gravel and sand they are made up of shifts on the reef itself so that the land changes in response to environmental conditions. Interestingly, the elevation of the atoll crest – the highest ground – mirrored the rise in sea levels which suggests sea level may be an important controlling factor on island elevation.” Co-researcher Dr Murray Ford, also from the University of Auckland, says the study shows islands are more resilient than previously thought, able to change shape or physically adjust to higher sea levels and more severe storms. “While the effect on particular islands of climate-induced changes will vary, there is plenty of evidence to suggest these islands are more resilient than commonly thought,” he says.

“The effects on individual islands will vary so that while some areas may become uninhabitable, while areas will keep pace with rising seas. It will be up to governments and communities to decide how to respond over time but we think this study highlights the fact that nature provides a template for adaptation and island communities may need to adapt too.” Previous research by the team, which used aerial photos going back as far as 1943 to track changes to the 101 islands that make up the Tuvalu archipelago, found that overall there was a net gain in land area of 2.9 percent or 73.5ha over the past 40 years. Professor Paul Kench, formerly of the University of Auckland, now at Simon Fraser University in Canada, and Professor Gerd Masselink from the University of Plymouth were co-researchers in the study, published by the Geological Society of America in Geology.

Hier das dazugehörige Paper von Tuck et al. 2019:

Physical modelling of the response of reef islands to sea-level rise

Sea-level rise and increased storminess are expected to destabilize low-lying reef islands formed on coral reef platforms, and increased flooding is expected to render them uninhabitable within the coming decades. Such projections are founded on the assumption that islands are geologically static landforms that will simply drown as sea-level rises. Here, we present evidence from physical model experiments of a reef island that demonstrates islands have the capability to morphodynamically respond to rising sea level through island accretion. Challenging outputs from existing models based on the assumption that islands are geomorphologically inert, results demonstrate that islands not only move laterally on reef platforms, but overwash processes provide a mechanism to build and maintain the freeboard of islands above sea level. Implications of island building are profound, as it will offset existing scenarios of dramatic increases in island flooding. Future predictive models must include the morphodynamic behavior of islands to better resolve flood impacts and future island vulnerability.

Der ehemalige Präsident von Kiribati, Anote Tong, hat genug von der ganzen Meeresspiegelhysterie und erklärt im Interview 2018, dass er keine negativen Auswirkungen des Meeresspiegelanstiegs feststellen kann. Auch sein Nachfolger Taneti Maamau ermuntert seine Landsleute zum Verbleib auf der Inselgruppe und fordert mehr Tourismus.

Montaggioni et al. 2019 untersuchten eine Koralleninsel in Französisch Polynesien. Überraschenderweise lag der Meeresspiegel dort vor 4500 Jahren 80 cm über dem heutigen Niveau. Vor 2300 Jahren lag der Meeresspiegel 60 cm über dem heutigen Wert und fiel dann langsam bis heute:

New insights into the Holocene development history of a Pacific, low-lying coral reef island: Takapoto Atoll, French Polynesia

Low-lying atoll islands are known to be made up of unconsolidated, coral-rich detritus, together with high amounts of foraminifera, derived from both outer-reef and lagoonal environments. While regarded as highly vulnerable to ongoing global changes, these islands are poorly constrained in term of development history. Herein is presented a detailed chronostratigraphic study of Tuamotu atoll islands (French Polynesia, south central Pacific) based on analyses of sedimentary sequences through seven excavations along two transects across both windward and leeward rim areas around Takapoto Atoll. The island accretionary chronology is supported by radiometric dating of 62 coral and molluscan clasts. The sequences range between 3.80 m and 1.20 m in thickness, from the oceanic shoreline lagoonwards. Four lithofacies were identified from sediment composition and texture: a coral (pocilloporid)-rich, gravel-supported, preferentially located in the outermost rim areas; a coral-rich, gravelly sand-supported, locally interbedded with gravel-supported units; a foraminifera (amphisteginid)-rich, sand-dominated, mainly located at the central and innermost rim settings; and an organic-rich, sand facies atop of some sequences. A model of atoll islet formation is drawn up in relation to mid to late Holocene sea-level changes. The foundations of islets (motus), namely conglomerate platforms, started to form with deposition of patchy, rubble spreads over the upper reef-rim surfaces from ca 4,500 yr BP as sea level was about 0.80 m above its present mean level. On these platforms, islets started to accrete not before ca 2,300 yr BP, from isolated depocentres located midway between outer-reef and lagoon margins. At that time, sea level at about +0.60 m above present mean sea level was starting to slowly decrease to its present position. The major growth phases occurred in a context of continued sea-level fall. Islets continued to accrete through concentric ridges mainly until the last 300 years. Accretion was dominantly driven by low-frequency, high-energy wave-surge events. From dating of coral clasts, a number of one to two events by century were identified as having apparently contributed to island formation at Takapoto. Regionally predicted increase in the rates of sea-level rise may have negative impacts on such islands since these have evolved under conditions of falling sea level.