Polar Region Bathymetry: Critical Knowledge for the Prediction of Global Sea Level Rise

The ocean and the marine parts of the cryosphere interact directly with, and are affected by, the seafloor and its primary properties of depth (bathymetry) and shape (morphology) in many ways. Bottom currents are largely constrained by undersea terrain with consequences for both regional and global heat transport. Deep ocean mixing is controlled by seafloor roughness, and the bathymetry directly influences where marine outlet glaciers are susceptible to the inflow relatively warm subsurface waters - an issue of great importance for ice-sheet discharge, i.e., the loss of mass from calving and undersea melting. Mass loss from glaciers and the Greenland and Antarctic ice sheets, is among the primary drivers of global sea-level rise, together now contributing more to sea-level rise than the thermal expansion of the ocean. Recent research suggests that the upper bounds of predicted sea-level rise by the year 2100 under the scenarios presented in IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCCC) likely are conservative because of the many unknowns regarding ice dynamics. In this paper we highlight the poorly mapped seafloor in the Polar regions as a critical knowledge gap that needs to be filled to move marine cryosphere science forward and produce improved understanding of the factors impacting ice-discharge and, with that, improved predictions of, among other things, global sea-level. We analyze the bathymetric data coverage in the Arctic Ocean specifically and use the results to discuss challenges that must be overcome to map the most remotely located areas in the Polar regions in general.

Sedimentation strategies provide effective but limited mitigation of relative sea-level rise in the Mekong delta

The Mekong delta is experiencing rapid environmental change due to anthropogenic activities causing accelerated subsidence, sea-level rise and sediment starvation. Consequentially, the delta is rapidly losing elevation relative to sea level. Designating specific areas for sedimentation is a suggested strategy to encourage elevation-building with nature in deltas. We combined projections of extraction-induced subsidence, natural compaction and global sea-level rise with new projections of fluvial sediment delivery to evaluate the potential effectiveness of sedimentation strategies in the Mekong delta to 2050. Our results reveal that with current rates of subsidence and sediment starvation, fluvial sediments alone can only preserve elevation locally, even under optimistic assumptions, and organic sedimentation could potentially assume a larger role. While sedimentation strategies alone have limited effectiveness in the present context, combined with enhanced organic matter retention and interventions reducing anthropogenic-accelerated subsidence, they can considerably delay future relative sea-level rise, buying the delta crucial time to adapt.

Climate change is raising sea levels in the oceans

Climate change is a global challenge, directly affecting ecosystems, environmental resources, and human life. One of its consequences is the problem of sea and ocean surface area, sea level is increasing day by day. In the long term, global mean sea level will continue to change continuously. The birth of the industrial revolution has made the Earth warmer and warmer, followed by many different causes leading to the rapid increase of global sea level: melting ice, expansion of the sea. water and changes in the Earth's climate system, costing the global economy trillions of dollars with many development consequences.

Path Analysis of Sea-Level Rise and Its Impact

Global sea-level rise has been drawing increasingly greater attention in recent years, as it directly impacts the livelihood and sustainable development of humankind. Our research focuses on identifying causal factors and pathways on sea level changes (both global and regional) and subsequently predicting the magnitude of such changes. To this end, we have designed a novel analysis pipeline including three sequential steps: (1) a dynamic structural equation model (dSEM) to identify pathways between the global mean sea level (GMSL) and various predictors, (2) a vector autoregression model (VAR) to quantify the GMSL changes due to the significant relations identified in the first step, and (3) a generalized additive model (GAM) to model the relationship between regional sea level and GMSL. Historical records of GMSL and other variables from 1992 to 2020 were used to calibrate the analysis pipeline. Our results indicate that greenhouse gases, water, and air temperatures, change in Antarctic and Greenland Ice Sheet mass, sea ice, and historical sea level all play a significant role in future sea-level rise. The resulting 95% upper bound of the sea-level projections was combined with a threshold for extreme flooding to map out the extent of sea-level rise in coastal communities using a digital coastal tracker.

Distribution and Evolution of Supraglacial Lakes in Greenland during the 2016–2018 Melt Seasons

In recent decades, the melting of the Greenland Ice Sheet (GrIS) has become one of the major causes of global sea-level rise. Supraglacial lakes (SGLs) are typical hydrological features produced on the surface of the GrIS during the melt seasons. The existence and evolution of SGLs play an important role in the melting process of the ice sheet surface. To understand the distribution and recent changes of SGLs in Greenland, this study developed a random forest (RF) algorithm incorporating the texture and morphological features to automatically identify SGLs based on the Google Earth Engine (GEE) platform. Sentinel-2 imagery was used to map the SGLs inventory in Greenland during the 2016–2018 melt seasons and to explore the spatial and temporal variability characteristics of SGLs. Our results show changes in SGLs from 2016 to 2018, with the total area decreasing by ~1152.22 km2 and the number increasing by 1134; SGLs are mainly distributed in western Greenland (SW, CW, NW) and northeastern Greenland (NE), where the NE region has the largest number of observed SGLs and the largest SGL was with the surface area of 16.60 km2 (2016). SGLs were found to be most active in the area with the elevation of 800–1600 m and the slope of 0–5°, and showed a phenomenon of retreating to lower elevation areas and developing to steeper slope areas. Our work provided a method for rapid inventory of SGLs. This study will help monitor the mass balance of the GrIS and predict future rapid ice loss from Greenland.

The Challenge of Non-Stationary Feedbacks in Modeling the Response of Debris-Covered Glaciers to Climate Forcing

Ongoing changes in mountain glaciers affect local water resources, hazard potential and global sea level. An increasing proportion of remaining mountain glaciers are affected by the presence of a surface cover of rock debris, and the response of these debris-covered glaciers to climate forcing is different to that of glaciers without a debris cover. Here we take a back-to-basics look at the fundamental terms that control the processes of debris evolution at the glacier surface, to illustrate how the trajectory of debris cover development is partially decoupled from prevailing climate conditions, and that the development of a debris cover over time should prevent the glacier from achieving steady state. We discuss the approaches and limitations of how this has been treated in existing modeling efforts and propose that “surrogate world” numerical representations of debris-covered glaciers would facilitate the development of well-validated parameterizations of surface debris cover that can be used in regional and global glacier models. Finally, we highlight some key research targets that would need to be addressed in order to enable a full representation of debris-covered glacier system response to climate forcing.

GNSS-IR Measurements of Inter Annual Sea Level Variations in Thule, Greenland from 2008–2019

Studies of global sea level often exclude Tide Gauges (TGs) in glaciated regions due to vertical land movement. Recent studies show that geodetic GNSS stations can be used to estimate sea level by taking advantage of the reflections from the ocean surface using GNSS Interferometric Reflectometry (GNSS-IR). This method has the immediate benefit that one can directly correct for bedrock movements as measured by the GNSS station. Here we test whether GNSS-IR can be used for measurements of inter annual sea level variations in Thule, Greenland, which is affected by sea ice and icebergs during much of the year. We do this by comparing annual average sea level variations using the two methods from 2008–2019. Comparing the individual sea level measurements over short timescales we find a root mean square deviation (RMSD) of 13 cm, which is similar to other studies using spectral methods. The RMSD for the annual average sea level variations between TG and GNSS-IR is large (18 mm) compared to the estimated uncertainties concerning the measurements. We expect that this is in part due to the TG not being datum controlled. We find sea level trends from GNSS-IR and TG of −4 and −7 mm/year, respectively. The negative trend can be partly explained by a gravimetric decrease in sea level as a result of ice mass changes. We model the gravimetric sea level from 2008–2017 and find a trend of −3 mm/year.

Progress or Regress? A Systematic Review on Two Decades of Monitoring and Addressing Land Subsidence Hazards in Semarang City

Land subsidence is a major cause of environmental degradation. It increases the exposure of global sea level rise-related disasters in coastal cities lying on young sediment. Ample monitoring, adaptation, and mitigation measures have been taken to tackle the impact of such coastal hazards for decades in Semarang City. However, to date, land subsidence still has a negative impact on people’s quality of life. This brings us to the question of whether the measures are progressing towards better management or going to the opposite side. This paper is aimed to answer that question through an extensive literature review using PRISMA Guidelines to 125 scholarly articles and quantitative supporting analysis. We found that land subsidence is overlooked. Although the monitoring measures are progressing towards better technology utilization, it was not properly integrated into mitigation and adaptation measures. Instead of investing more on developing better urban water management, groundwater extraction still became the preferred water source. Thus, there is a major shift needed with regard to urban activities that need to pay more heed to the environment.

Pliocene glacial-interglacial sea-level change

<p>The mid- to late Pliocene (3.3-2.6 Ma) spans one of the most significant climatic transitions of the Cenozoic. It is characterised by global cooling from a climate with an atmospheric CO2 concentration of ~400 ppm and temperatures of 2-3°C warmer-than-present, to one marked by the progressive expansion of ice sheets on northern hemisphere. Consequently, the mid-Pliocene warm period (MPWP; 3.3-3.0 Ma) provides the most accessible and recent geological analogue for global sea-level variability relevant to future warming. Global mean sea level has been estimated at 22 ± 10 m above present-day for MPWP. However, recent re-evaluations of this estimate suggest that spatially-varying visco-elastic responses of the crust, local gravitational changes and dynamic topography from mantle processes may preclude ever being able to reconstruct peak Pliocene mean sea level. The Whanganui Basin, New Zealand, contains a ~5 km thick stratigraphic succession of Pliocene-Pleistocene (last 5 Ma), shallow-marine, cyclical sedimentary sequences demonstrated to record orbitally-paced, glacial-interglacial global sea-level fluctuations. A limitation of the Whanganui sea level record, to date, has been an inability to resolve the full amplitude of glacial-interglacial water depth change due to the occurrence of cycle bounding unconformities representing sub-aerial erosion during glacial lowstands. This thesis analyses a new ~900 m-thick, mid- (3.3-3.0 Ma) to late Pliocene (3.0-2.6 Ma), shallow-marine, cyclical sedimentary succession from a remote and relatively understudied part of Whanganui Basin. Unlike previous studies, these shelf sediments were continuously deposited, and were not eroded during sea-level lowstands, and thus provide the potential to reconstruct the full amplitude of glacial-interglacial sea-level change. On orbital timescales the influence of mantle dynamic processes is minimal. The approach taken applies lithofacies, sequence stratigraphy, and benthic foraminiferal analyses and a novel depth-dependent sediment grain size method to reconstruct the paleowater depths for, two continuously-cored drill holes, which are integrated with studies of outcropping sections. The thesis presents a new record of the amplitude and frequency of orbitally-paced, global sea-level changes from a wave-graded continental shelf, that is independent of the benthic δ¹⁸O proxy record of global ice-volume change. Paleobathymetric interpretations are underpinned by analysis of extant benthic foraminiferal census data and a statistical correlation with the distribution of modern taxa. In general, water depths derived from foraminiferal modern analogue technique are consistent with variability recorded by lithofacies. The inferred sea-level cycles co-vary with a qualitative climate record reconstructed from a census of extant pollen and spores, and a modern temperature relationship. A high-resolution age model is established using magnetostratigraphy constrained by biostratigraphy, and the dating and correlation of tephra. This integrated chronostratigraphy allows the recognition of 23 individual sedimentary cycles, that are correlated “one-to-one” across the paleo-shelf and are compared to the deep-ocean benthic oxygen isotope (δ ¹⁸O) record. A grain size-water depth technique was developed to quantify the paleobathymetry with more precision than the relatively insensitive benthic foraminifera approach. The method utilises a water depth threshold relationship between wave-induced near bed velocity and the velocity required to transport sand. The resulting paleobathymetric records of the most sensitive sites, the mid-Pliocene Siberia-1 drill core and the late Pliocene Rangitikei River section, were selected to compile a composite paleobathymetry. A one-dimensional backstripping method was then applied to remove the effects of tectonic subsidence, sediment and water loading on the record, to derive a relative sea level (RSL) curve. The contribution of glacio-hydro-isostatic (GIA) processes to the RSL record was evaluated using a process-based forward numerical solid Earth model for a range of plausible meltwater scenarios. The Whanganui Basin RSL record approximates eustatic sea level (ESL) in all scenarios when variability is dominated by Antarctic Ice Sheet meltwater source during the mid-Pliocene, but overestimates ESL once Northern Hemisphere ice sheet variability dominates in the late Pliocene. The RSL record displays 20 kyr precession-paced sea level variability during the MPWP with an average amplitude of ~15 ± 8 m, in-phase with southern high-latitude summer insolation. These are interpreted as ~20 m Antarctic Ice Sheet contributions, offset by ~ 5 m anti-phased Greenland Ice Sheet contribution, in the absence of a significant Northern Hemisphere ice sheets. This interpretation is supported by a previously published ice-proximal precession-paced, ice-berg-rafted debris record recovered off the coast of Wilkes Land. The Whanganui RSL record is not consistent with a dominant 40 kyr pacing observed the benthic oxygen isotope stack at this time. While the deep ocean benthic δ¹⁸O stack is of varying temporal and spatial resolution, during this time interval, the Whanganui RSL record implies a more complex relationship between ice-volume and oxygen isotope composition of sea water (δ¹⁸Oseawater). The relative influences of varying composition of the polar ice sheets, marine versus land based ice, the out-of-phase behaviour of polar ice sheet growth and retreat, and a potential decoupling of ocean bottom water temperature and δ¹⁸Oseawater are explored. The late Pliocene relative sea level record exhibits increasing ~40 kyr obliquity-paced amplitudes of ~20 ± 8 m. This is interpreted as a response to the expansion of Northern Hemisphere ice sheets after ~2.9 Ma. During this time the Antarctic proximal ice-berg rafted debris records display continuing precession-paced ice-volume fluctuations, but with decreasing amplitude suggesting cooling and stabilisation of the East Antarctic Ice Sheet. With the bipolar glaciation, the ocean δ¹⁸O signal became increasingly dominated by northern hemisphere ice-volume. However, the RSL record implies relatively limited ice-volume contributions (up to ~25 m sea level equivalent) prior to ~2.6 Ma. The large amplitude contribution of Antarctic Ice Sheets to global sea level during the MPWP has significant implications for the sensitivity of the Antarctica Ice Sheet to global temperatures 2-3°C above preindustrial levels, and atmospheric CO₂ forecast for the coming decades.</p>