Sinkender Meeresspiegel auf den Malediven während der Kleinen Eiszeit und der Kältephase der Völkerwanderungszeit

SchildkrötenFoto: dronepicr / CC BY (https://creativecommons.org/licenses/by/2.0)

Thema heute: Der Meeresspiegel in Asien. Wir beginnen im tektonisch aktiven Sumatra. Dort war der Meeresspiegel in den letzten 3800 Jahren offenbar stabil, wie Kelsey et al. 2015 berichteten:

Accommodation space, relative sea level, and the archiving of paleo-earthquakes along subduction zones

The spatial variability of Holocene relative sea-level (RSL) change influences the capacities of coastal environments to accommodate a sedimentary record of paleoenvironmental change. In this study we couch a specific investigation in more general terms in order to demonstrate the applicability of the relative sea-level history approach to paleoseismic investigations. Using subsidence stratigraphy, we trace the different modes of coastal sedimentation over the course of time in the eastern Indian Ocean where RSL change evolved from rapidly rising to static from 8000 yr ago to present. Initially, the coastal sites from the Aceh, Sumatra, coastal plain, which are subject to repeated great earthquakes and tsunamis, built up a sedimentary sequence in response to a RSL rise of 1.4 mm/yr. The sequence found at 2 sites 8 km apart contained 3 soils of a mangrove origin (Rhizophora,Bruguiera/Ceriops, Avicennia pollen, and/or intertidal foraminifera) buried by sudden submergence related to coseismic subsidence and 6 tsunami sands that contain pristine subtidal and planktic foraminifera. After 3800 cal yr B.P. (years before A.D. 1950), sea level stabilized and remained such to the present. The stable relative sea level reduced accommodation space in the late Holocene, suggesting that the continued aggradation of the coastal plain was a consequence of periodic coastal inundation by tsunamis.

Rutgers University 2017 mit einer Pressemitteilung zu einer mysteriösen Meeresspiegelschwankung in Südostasien in ferner vorindustrieller Zeit (via Science Daily):

Sea-level change in Southeast Asia 6,000 years ago has implications for today

Fluctuations of nearly 2 feet took place without assistance from human-influenced climate change

For the 100 million people who live within 3 feet of sea level in East and Southeast Asia, the news that sea level in their region fluctuated wildly more than 6,000 years ago is important, according to research published by a team of ocean scientists and statisticians, including Rutgers professors Benjamin Horton and Robert Kopp and Rutgers Ph.D. student Erica Ashe. That’s because those fluctuations occurred without the assistance of human-influenced climate change.

In a paper published in Nature Communications, Horton, Kopp, Ashe, lead author Aron Meltzner and others report that the relative sea level around Belitung Island in Indonesia rose twice just under 2 feet in the period from 6,850 years ago to 6,500 years ago. That this oscillation took place without any human-assisted climate change suggests to Kopp, Horton and their co-authors that such a change in sea level could happen again now, on top of the rise in sea level that is already projected to result from climate change. This could be catastrophic for people living so close to the sea.

„This research is a very important piece of work that illustrates the potential rates of sea-level rise that can happen from natural variability alone,“ says Horton, professor of marine and coastal sciences in the School of Environmental and Biological Sciences. „If a similar oscillation were to occur in East and Southeast Asia in the next two centuries, it could impact tens of millions of people and associated ecosystems.“

Meltzner, a senior research fellow at Earth Observatory of Singapore at Nanyang Technological University, along with Horton, Kopp and their co-authors, used coral microatolls to understand when, and by how much, the sea level had risen and fallen near the Indonesian island of Belitung, which lies between Sumatra and Borneo. A microatoll is a circular coral colony, typically no more than about 20 feet across, in which the topmost coral is dead and the bottom part living and growing. By taking samples from microatolls in different places, scientists can date rises and falls of sea level.

The microatolls are what scientists call a „proxy“ — a natural process that provides a reliable record of past events. „In any region, you try to find the proxy controlled by sea level,“ Horton says. „In New Jersey, we have no corals, so we use salt marshes. In the tropics, corals are the go-to proxy.“

The scientists studied microatolls at two sites on opposite sides of the island. Meltzner says they didn’t expect the fluctuations they found because those changes in sea level contradicted what they knew about sea level in Southeast Asia. „Our conventional understanding of ocean circulation and ice-melting history told us that such fluctuations should not occur, so we were a bit mystified at the results from our first site,“ Meltzner says. „But after finding a similar pattern at a second site 80 kilometers to the southeast, and ruling out other plausible explanations, it was clear that the coral growth patterns must reflect regional changes in sea level. There would be way too many coincidences otherwise.“

The paper comes out of a long-running research project aimed at understanding the physical processes involved in sea-level rise. Such understanding, Kopp says, is necessary to help scientists understand the present and likely future state of the ocean. „This is a basic science problem,“ Kopp says. „It’s about understanding past changes. Understanding what drove those changes is what allows us to test the climate models we use to predict future changes.“

Paper: Aron J. Meltzner, Adam D. Switzer, Benjamin P. Horton, Erica Ashe, Qiang Qiu, David F. Hill, Sarah L. Bradley, Robert E. Kopp, Emma M. Hill, Jędrzej M. Majewski, Danny H. Natawidjaja, Bambang W. Suwargadi. Half-metre sea-level fluctuations on centennial timescales from mid-Holocene corals of Southeast Asia. Nature Communications, 2017; 8: 14387 DOI: 10.1038/ncomms14387

Kench et al. 2020 stellten eine Meeresspiegelrekonstruktion für die vergangenen 2000 Jahre für die Malediven im Indischen Ozean vor. Die Forscher fanden zwei Phasen mit fallendem Meeresspiegel, die mit den Kälteperioden der Völkerwanderungszeit (500 n. Chr.) und der Kleinen Eiszeit (1600 n. Chr.) zusammenfielen. Zudem konnten Kench und Kollegen zeigen, dass die heutige Meeresspiegelanstiegsrate sowie das Niveau bereits in der vorindustriellen Vergangenheit der letzten 2000 Jahre erreicht wurden. Abstract:

Climate-forced sea-level lowstands in the Indian Ocean during the last two millennia

Sea-level reconstructions over the past two millennia provide a pre-industrial context to assess whether the magnitude and rate of modern sea-level change is unprecedented. Sea-level records from the Indian Ocean over the past 2,000 years are sparse, while records from the Atlantic and Pacific Oceans show variations less than 0.25 m and no significant negative excursions. Here, we present evidence of two low sea-level phases in the Maldives, Indian Ocean, based on fossil coral microatolls. Microatoll growth is constrained by low water levels and, consequently, they are robust recorders of past sea level. U–Th dating of the Maldivian corals identified lowstands at ad 234–605 and ad 1481–1807 when sea level fell to maximum depths of −0.88 m and −0.89 m respectively. These lowstands are synchronous with reductions in radiative forcing and sea surface temperature associated with the Late Antiquity Little Ice Age and the Little Ice Age. Our results provide high-fidelity observations of lower sea levels during these cool periods and show rates of change of up to 4.24 mm yr−1. Our data also confirm the acceleration of relative sea-level rise over the past two centuries and suggest that the current magnitude and rate of sea-level rise is not unprecedented.

Die Malediven hätten gerne Klimagelder, weil sie sich als Klimawandelopfer sehen. Da kommt ihnen die obige Studie sicher nicht so recht. Tony Heller hat einige hochinteressante Fakten zu den Malediven und ihrem Flirt mit dem Klimawandel:

Wetlands International warnte 2015, dass Küstensumpfgebiete in Südostasien vom Meer überflutet werden könnten, wenn die Trockenlegung zwecks Anpflanzung von Ölpalmen nicht stoppt. PM via EAAFP:

Peatlands of South East Asia are heading towards a socio-economic disaster

Agricultural production in vast regions of South East Asia will be lost in the coming decades as a result of flooding of extensive lowland landscapes due to unsustainable development and management of peat soils. About 82% of the Rajang Delta in Sarawak (East Malaysia) will be irreversibly flooded within 100 years and substantial areas are already experiencing drainage problems. This will increasingly impact local communities, the economy and biodiversity and will develop over time into disastrous proportions unless land-use on the region’s peatlands is radically changed. Therefore Wetlands International calls for conservation and sustainable management of peatlands in South East Asia.

A study commissioned by Wetlands International and executed by Deltares suggests that extensive drainage of peatlands for oil palm cultivation in the Rajang river delta results in such massive land subsidence. This will lead to extensive and devastating flooding incidents in the coming decades.

The study analysed an area of 850,000 hectares of coastal peatlands in Sarawak, and produced a model that demonstrates that in 25 years 42% of the area will experience flooding problems. In 50 years the percentage affected will increase to 56% while the nature of flooding becomes more serious and permanent, and in 100 years 82% of the peatlands will be affected.

Such extensive flooding is due to massive conversion of peat swamp forests to agriculture, mainly oil palm plantations: currently only 16% of Sarawak’s natural peat forests remain. These valuable crops require drainage in order to be profitable.

“The results of the model clearly shows the need for a radical change in peatland landuse not only in Sarawak but in all peat landscapes in the region”, said Lee Shin Shin, Senior Technical Officer of Wetlands International-Malaysia. “Current trends whereby vast areas of peatlands are opened up for drainage-based activities will render these areas unproductive and useless and this will adversely impact communities, industries and biodiversity that rely on such areas for their very survival and existence.”

Peat soils are made up of 10% accumulated organic material (carbon) and 90% water. When water is drained from the peat soil, the carbon in the peat soil is turned into CO2 and emitted into the atmosphere causing climate change. This carbon loss reduces the peat volume and thus causes the peat soil to subside. This process continues as long as drainage is continued and until the soil surface reaches sea or river levels constraining the outflow of water and thus leading to flooding. In tropical conditions, peat drainage causes the soil to subsidence at a rate of 1 to 2 metres in the first years of drainage, and 3 to 5 centimetres per year in subsequent years. This results in the subsidence of the soil by up to 1.5 metres within 5 years and 4 to 5 metres within 100 years.

“The study results are very relevant to Indonesia as well, where we observe the same patterns of peat swamp forest conversion, drainage and expansion of oil palm and Acacia for pulp wood plantations”, said Nyoman Suryadiputra, Director of Wetlands International – Indonesia. “Thousands of square kilometres in Sumatra and Kalimantan may become flooded in the same way as the Rajang delta, affecting millions of people who depend on these areas for their livelihoods”.

“Highly developed countries or regions in temperate areas, such as the Netherlands, cope with soil subsidence by building dykes and pump-operated drainage systems, but this is impossible in Malaysia or Indonesia”, explained Marcel Silvius, Programme Head for Climate Smart Land Use at Wetlands International. “The predominantly rural economy along thousands of kilometres of coastline and rivers, combined with the intense tropical rainfall makes it economically and practically impossible to implement such costly water management measures in the Southeast Asian region”.

Governments and industry should therefore immediately stop the conversion of remaining peat forests to agricultural or other use, and actively promote peatland conservation and restoration. Industry will need to phase out drainage-based plantations on peatlands, as these areas will be increasingly subject to flooding and eventually become unsuitable for any form of productive land-use. Effective policies should be drawn up, implemented and enforced to conserve and ensure the wise use of peatlands. There are many crops that can be cultivated on peatlands without drainage. Over 200 commercial local peat forest tree species have been identified, such as Tengkawang (Shorea spp.) which produces an edible oil and Jelutung, a latex producing tree. These plants can provide alternative and sustainable livelihood opportunities for local communities but require piloting, improvement of varieties and up-scaling for industrial plantations.

Der Meeresspiegel hat keinen monotonen Trend, sondern wird von Ozeanzyklen beeinflusst. Nidheesh et al. 2017 beschreiben einen ENSO-Effekt aus dem Indo-Pazifik. Schwacher Monsun, steigender Meeresspiegel:

Robustness of observation‐based decadal sea level variability in the Indo‐Pacific Ocean

We examine the consistency of Indo‐Pacific decadal sea level variability in 10 gridded, observation‐based sea level products for the 1960–2010 period. Decadal sea level variations are robust in the Pacific, with more than 50% of variance explained by decadal modulation of two flavors of El Niño–Southern Oscillation (classical ENSO and Modoki). Amplitude of decadal sea level variability is weaker in the Indian Ocean than in the Pacific. All data sets indicate a transmission of decadal sea level signals from the western Pacific to the northwest Australian coast through the Indonesian throughflow. The southern tropical Indian Ocean sea level variability is associated with decadal modulations of ENSO in reconstructions but not in reanalyses or in situ data set. The Pacific‐independent Indian Ocean decadal sea level variability is not robust but tends to be maximum in the southwestern tropical Indian Ocean. The inconsistency of Indian Ocean decadal variability across the sea level products calls for caution in making definitive conclusions on decadal sea level variability in this basin.

Auch im nördlichen Indik gibt es multidekadische Effekte beim Meeresspiegel. Das fanden Swapna et al. 2017:

Multidecadal Weakening of Indian Summer Monsoon Circulation Induces an Increasing Northern Indian Ocean Sea Level

North Indian Ocean sea level has shown significant increase during last three to four decades. Analyses of long‐term climate data sets and ocean model sensitivity experiments identify a mechanism for multidecadal sea level variability relative to global mean. Our results indicate that North Indian Ocean sea level rise is accompanied by a weakening summer monsoon circulation. Given that Indian Ocean meridional heat transport is primarily regulated by the annual cycle of monsoon winds, weakening of summer monsoon circulation has resulted in reduced upwelling off Arabia and Somalia and decreased southward heat transport, and corresponding increase of heat storage in the North Indian Ocean. These changes in turn lead to increased retention of heat and increased thermosteric sea level rise in the North Indian Ocean, especially in the Arabian Sea. These findings imply that rising North Indian Ocean sea level due to weakening of monsoon circulation demands adaptive strategies to enable a resilient South Asian population.

Überschwemmungen im Ganges‐Brahmaputra‐Meghna Delta? Kein Wunder, denn das Dekta sinkt stark ab und zwar genauso schnell wie der globale Meeresspiegel steigt. Krien et al. 2019:

Present‐Day Subsidence in the Ganges‐Brahmaputra‐Meghna Delta: Eastern Amplification of the Holocene Sediment Loading Contribution

The subsidence of the Ganges‐Brahmaputra‐Meghna Delta (GBMD) drastically increases the adverse impacts of coastal flooding and exacerbates the vulnerability of populations from ongoing rapid sea level rise. We focus here on estimating the present‐day subsidence rates induced by the loading of sediments continuously deposited within the GBMD over the past 11,000 years. By constructing a realistic GBMD 3‐D numerical model with laterally variable mantle and lithospheric structure, we demonstrate for the first time that the presence of the strong Indian Craton and the weakened Indo‐Burma margin results in significant amplification of subsidence driven by sediment loading in the eastern part of the delta, where the population density is the highest (>1,000 habitants per km2). Although uncertainties remain regarding the amplitude of subsidence, the rate estimates (2–3 mm/year) are found to be comparable to the present‐day global mean sea level rise.

In Taiwan ist der Meeresspiegel in den letzten 25 Jahren laut Küstenpegelmessungen um 2,2 mm pro Jahr angestiegen wie Lan et al. 2017 berichteten. Zudem stoppte der Meeresspiegelanstieg kürzlich für eine Weile, was mit der PDO und ENSO zu tun hatte.

Impact of Geophysical and Datum Corrections on Absolute Sea-Level Trends from Tide Gauges around Taiwan, 1993–2015

The Taiwanese government has established a complete tide gauge network along the coastline for accurate sea-level monitoring. In this study, we analyze several factors impacting the determination of absolute or geocentric sea-level trends—including ocean tides, inverted barometer effect, datum shift, and vertical land motion—using tide gauge records near Taiwan, from 1993–2015. The results show that datum shifts and vertical land motion have a significant impact on sea-level trends with a respective average contribution of 7.3 and 8.0 mm/yr, whereas ocean tides and inverted barometer effects have a relatively minor impact, representing 9% and 14% of the observed trend, respectively. These results indicate that datum shifts and vertical land motion effects have to be removed in the tide gauge records for accurate sea-level estimates. Meanwhile, the estimated land motions show that the southwestern plain has larger subsidence rates, for example, the Boziliao, Dongshi, and Wengang tide gauge stations exhibit a rate of 24–31 mm/yr as a result of groundwater pumping. We find that the absolute sea-level trends around Taiwan derived from tide gauges or satellite altimetry agree well with each other, and are estimated to be 2.2 mm/yr for 1993–2015, which is significantly slower than the global average sea-level rise trend of 3.2 mm/yr from satellite altimeters. Finally, a recent hiatus in sea-level rise in this region exhibits good agreement with the interannual and decadal variabilities associated with the El Niño-Southern Oscillation and Pacific Decadal Oscillation.

Zum Abschluss noch die holozäne Meeresspiegelgeschichte an der Nordküste der South China Sea. Zwischen 10.500-7000 Jahre vor heute stieg der Meeresspiegel rasant mit bis zu 33 mm pro Jahr. Während der letzten 7000 Jahre blieb der Meeresspiegl aber weitgehend konstant. Abstract von Xiong et al. 2018:

Holocene sea-level history of the northern coast of South China Sea

This study has collected and analyzed seven sediment cores from the Pearl River delta, from which 16 new and high-quality sea-level index points are generated using a new approach. This study has also re-checked and re-calibrated the previously published sea-level data from China’s southeast coast with corrections made for tectonic subsidence and sediment compaction factors. These sea-level data indicate a rise of relative sea level from −49.3 ± 0.8 m to the present height between 10,500 and 7000 cal. a BP. This sea-level history is similar to those recorded from other far-field locations and ice-volume equivalent sea-level models. The early to early-middle Holocene sea-level history in the study area shows a phase of accelerated rise at a rate increasing rapidly from 16.4 ± 6.1 mm/a at 10,500 cal. a BP to 33.0 ± 7.1 mm/a at 9500 cal. a BP. This phase was followed by a period of rapid decrease in the rate of sea-level rise to 8.8 ± 1.9 mm/a at 8500 cal. a BP and 1.7 ± 1.3 mm/a at 7500 cal. a BP. During the past 7000 years, the relative sea level in the study area changed very little. This new and complete history of Holocene sea-level change supports the following findings: (1) no obvious higher-than-present sea-level highstand in the Holocene is found from the northern South China Sea; (2) certain proportion of the effects of the predicted glacial isostatic adjustment were cancelled out by the effects of the weak upper mantle viscosity in the study area; (3) meltwater pulse 1b likely exists spanning into the early Holocene; (4) there are significant misfit between sea-level data and glacial isostatic adjustment models, and a revision to the existing ice melting history for the early Holocene is possibly needed.

Soviel zu den neuen Erkenntnissen zum Meeresspiegel in Asien