Late Holocene (0–6 ka) sea-level changes in the Makassar Strait, Indonesia

The Spermonde Archipelago, off the coast of southwest Sulawesi, consists of more than 100 small islands and hundreds of shallow-water reef areas. Most of the islands are bordered by coral reefs that grew in the past in response to paleo relative sea-level changes. Remnants of these reefs are preserved today in the form of fossil microatolls. In this study, we report the elevation, age, and paleo relative sea-level estimates derived from fossil microatolls surveyed in five islands of the Spermonde Archipelago. We describe 24 new sea-level index points, and we compare our dataset with both previously published proxies and with relative sea-level predictions from a set of 54 glacial isostatic adjustment (GIA) models, using different assumptions on both ice melting histories and mantle structure and viscosity. We use our new data and models to discuss Late Holocene (0–6 ka) relative sea-level changes in our study area and their implications in terms of modern relative sea-level estimates in the broader South and Southeast Asia region

Validation of Recent Altimeter Missions at Non-Dedicated Tide Gauge Stations in the Southeastern North Sea

Consistent calibration and monitoring is a basic prerequisite for providing a reliable time series of global and regional sea-level variations from altimetry. The precisions of sea-level measurements and regional biases for six altimeter missions (Jason-1/2/3, Envisat, Saral, Sentinel-3A) are assessed in this study at 11 GNSS-controlled tide gauge stations in the German Bight (SE North Sea) for the period 2002 to 2019. The gauges are partly located at the open water, and partly at the coast close to mudflats. The altimetry is extracted at virtual stations with distances from 2 to 24 km from the gauges. The processing is optimized for the region and adjusted for the comparison with instantaneous tide gauge readings. An empirical correction is developed to account for mean height gradients and slight differences of the tidal dynamics between the gauge and altimetry, which improves the agreement between the two data sets by 15–75%. The precision of the altimeters depends on the location and mission and ranges from 1.8 to 3.7 cm if the precision of the gauges is 2 cm. The accuracy of the regional mission biases is strongly dependent on the mean sea surface heights near the stations. The most consistent biases are obtained based on the CLS2011 model with mission-dependent accuracies from 1.3 to 3.4 cm. Hence, the GNSS-controlled tide gauges operated operationally by the German Waterway and Shipping Administration (WSV) might complement the calibration and monitoring activities at dedicated CalVal stations

The ‘Wickedness’ of Governing Land Subsidence: Policy Perspectives From Urban Southeast Asia

Drawing on Jakarta, Metro Manila and Singapore as case studies, we explore the paradox of slow political action in addressing subsiding land, particularly along high-density urban coastlines with empirical insights from coastal geography, geodesy analysis, geology, and urban planning. In framing land subsidence as a classic ‘wicked’ policy problem, and also as a hybrid geological and anthropogenic phenomenon that is unevenly experienced across urban contexts, the paper uses a three-step analysis. First, satellite-derived InSAR maps are integrated with Sentinel-1A data in order to reveal the socio-temporal variability of subsidence rates which in turn pose challenges in uniformly applying regulatory action. Second, a multi-sectoral mapping of diverse policies and practices spanning urban water supply, groundwater extraction, land use zoning, building codes, tenurial security, and land reclamation reveal the extent to which the broader coastal governance landscape remains fragmented and incongruous, particularly in arresting a multi-dimensional phenomenon such as subsidence. Finally, in reference to distinct coastal identities of each city–the ‘Sinking Capital’ (Jakarta), ‘Fortress Singapore’, and the ‘Disaster Capital’ (Manila) the paper illustrates how land subsidence is portrayed across the three metropolises in markedly similar ways: as a reversible, quasi-natural, and/or a highly individualized problem

Fossil Java Sea corals record Laurentide ice sheet disappearance

The Laurentide ice sheet was the largest late Pleistocene ice mass and the largest contributor to Holocene pre-industrial sea-level rise. While glaciological dates suggest final ice sheet melting between 8 and 6 ka, inversion of sea-level data indicates deglaciation at ca. 7 ka. Here, we present new chronostratigraphic constraints on Laurentide ice sheet disappearance based on Holocene relative sea-level observations from the tectonically stable north coast of Java, Indonesia. Age-elevation data from the flat upper surfaces of 13 fossil intertidal corals (i.e., microatolls) indicate that the Java Sea experienced a relative sea level of 1.3 ± 0.7 m above present between 6.9 and 5.3 ka. To determine uncaptured relative sea-level trends within the observational uncertainties of this apparently constant highstand, we analyzed the internal structure of three sliced microatolls from the same site to produce a high-resolution data set. These data were used to statistically model relative sea-level rates and trends. Employing the data with the model provided evidence for a short-lived rise of relative sea level from 1.0 ± 0.3 m above present at 6.7 ± 0.1 ka to 1.9 ± 0.3 m above present at 6.4 ± 0.1 ka. The end of this rise likely represents the last input of meltwater from the vast Laurentide ice sheet, which, consequently, collapsed at least 400 yr later than assumed by some widely used models of glacial isostatic adjustment. Incorporating these new results into such predictive models will help to better understand the geographical variability of future sea-level rise as a result of global warming

Water in Central Asia - Reservoir Monitoring with Radar Altimetry along the Naryn and Syr Darya River

Water is a scarce resource in many regions of the world. In all Central Asian countries, the society depends on the availability of water either for hydro-power generation or irrigation. Since the collapse of the Soviet Union and the separation into independent countries, water management became a political issue. Public information on water resources is now unavailable for many lakes and reservoirs and the data exchange about in- and outflow or actual storage capacity between the national hydrometeorological services is limited. While the initial purpose of radar altimetry has been to measure heights over the open ocean, it proved to be a suitable tool for inland water body monitoring and to partially provide data about lake and reservoir level and volume changes. By using this technology, the water heights of the Toktogul, Kairakum and Shardara Reservoirs and Lake Aydarkol have been extracted from 1995 onward. Using additional, external information like remote sensing, the derived heights have been converted into water volume changes, the primary desired information. This study demonstrates that since 2011 the available water volume decreased and the water from the Toktogul and Shardara Reservoirs are overused. Merely the Kairakum Reservoir with a relatively small storage capacity is replenished every year. Likewise, a decrease of water level and volume of Lake Aydarkol is clearly visible

Land subsidence in Jakarta and Semarang Bay - The relationship between physical processes, risk perception, and household adaptation

Sea level rise (SLR) is among the most pressing challenges for urban coastal areas. While geocentric (eustatic) SLR receives widespread attention in politics and media, relative SLR at the coast, mainly caused by land subsidence, is still comparatively under-researched despite much higher rates. This paper introduces a combined natural and social science study to bring subsidence more to the forefront of coastal hazard research. We use data from radar altimetry, GNSS controlled tide gauge stations, and InSAR mapping to characterize regional and relative SLR at Jakarta and Semarang Bay, and focus-group discussions and a standardized household survey to analyze risk perceptions and adaptation. Our analysis of InSAR, radar altimetry, and corrected tide gauges clearly identifies subsidence as the major coastal threat in our study areas. The InSAR analysis for Semarang shows stable trends of subsidence up to -100 mm/a. For Jakarta, our analysis reveals more complex spatial and temporal patterns with rates around 60 mm/a; revealing significant changes to previous studies. Our analysis of radar altimetry data since 1993 shows a moderate regional SLR of 2.1 mm/a off Semarang and 3.2 mm/a off Jakarta. The InSAR data are integrated into our statistical analysis of household responses towards subsidence. We found, that in contrast to fast-onset events, constantly proceeding subsidence becomes normalized in peoples' perceptions and responses are integrated into day-to-day habits. Thus, risk perception is a far lesser determinant of responses towards subsidence than it is for fast-onset events. Hence, our results relativize former assumptions that risk perception and not actual exposure lead to action. Moreover, we found that local people are not willing to vacate highly exposed areas. Their views need to be included in municipal disaster risk reduction, the urgency of which clearly lies on mitigating subsidence effects rather than on building protection against regionally rising sea levels