Recent studies disagree about the contribution of variations in temperature and salinity of the oceans—steric change—to the observed sea‐level change. This article explores two sources of uncertainty ...to both global mean and regional steric sea‐level trends. First, we analyze the influence of different temperature and salinity data sets on the estimated steric sea‐level change. Next, we investigate the impact of different stochastic noise models on the estimation of trends and their uncertainties. By varying both the data sets and noise models, the global mean steric sea‐level trend and uncertainty can vary from 0.69 to 2.40 and 0.02 to 1.56 mm/year, respectively, for 1993–2017. This range is even larger on regional scales, reaching up to 30 mm/year. Our results show that a first‐order autoregressive model is the most appropriate choice to describe the residual behavior of the ensemble mean of all data sets for the global mean steric sea‐level change over the last 25 years, which consequently leads to the most representative uncertainty. Using the ensemble mean and the first‐order autoregressive noise model, we find a global mean steric sea‐level change of 1.36 ± 0.10 mm/year for 1993–2017 and 1.08 ± 0.07 mm/year for 2005–2015. Regionally, a combination of different noise models is the best descriptor of the steric sea‐level change and its uncertainty. The spatial coherence in the noise model preference indicates clusters that may be best suited to investigate the regional sea‐level budget.
Plain Language Summary
Ocean temperature and salinity variations lead to changes in sea level, known as steric sea‐level change. Steric variations are important contributors to sea‐level change and reflect how the oceans have been responding to global warming. For this reason, several recent studies have quantified the contribution of steric variations to global and regional sea‐level change. However, the reported rates largely differ between studies. In this paper, we look at how the use of different temperature and salinity data sets can be one of the causes of the different estimates of steric sea‐level change published so far. We also investigate how different methods (noise models) used to obtain the rate of change can be another source of different results. We find that the rate of change can vary up to 2 mm/year for the global mean as a result of different data sets and methods used. Regionally, differences can reach up to several tens of millimeters per year. We show that the noise models should always be carefully chosen for each region, so that the rate of change is accurately estimated.
Key Points
Several data sets and noise models are used to compute global and regional steric sea‐level change for 1993–2017 and 2005–2015
Global mean steric sea‐level trend estimates differ up to 2 mm/year depending on the data set and noise model used
Regional sea‐level trends require more complex noise models and exhibit spatial coherency in the noise model preference
The small number of reliable long‐term (i.e., >50 yrs) tide gauges in tropical locations is a major source of uncertainty in modern sea‐level change. Coral microatolls record relative sea‐level (RSL) ...change over their lifetimes and have the potential to extend the instrumental record. Here, we examined a 20th and 21st century RSL record from two living coral microatolls from Mapur Island, Indonesia, which produced 16 sea‐level index points. We validated and combined the living coral microatoll data with tide gauge data to show RSL at Mapur Island was 0.0 ± 1.6 mm/yr (2σ) from 1915 to 1990 and 1.0 ± 2.1 mm/yr (2σ) from 1990 to 2019. Through the addition of microatoll RSL data we extended the record of modern sea‐level change by over 50 years and reduced its uncertainty by ∼50%.
Plain Language Summary
Long instrumental sea‐level records from tide‐gauges are required to understand regional sea‐level variability, but in most tropical regions tide‐gauge records only began in the latter half of the 20th century. Therefore, there is considerable uncertainty in the regional sea‐level projections in tropical regions, which are necessary for effective coastal risk management. Coral microatolls can extend the duration of tide‐gauge records and provide completely new sea‐level records because their growth patterns reveal important information about sea level during the coral's lifetime. In this study, we examined sea‐level records from living coral microatolls from Mapur Island, Indonesia. We validated the coral microatoll technique to reconstruct sea level with data from the nearby Tanjong Pagar tide gauge in Singapore. The combined coral microatoll and tide gauge data showed sea level change of 0.0 ± 1.6 mm/yr (2σ) from 1915 to 1990 and 1.0 ± 2.1 mm/yr (2σ) from 1990 to 2019. Coral microatolls can extend sea level records for other tropical regions where tide gauges have only recently been installed and provide a new data source in locations where tide gauges have not and might not be installed.
Key Points
Validating the coral microatoll technique to reconstruct relative sea level
Producing 16 new sea‐level index points from two microatolls between 1915 and 2012
Extending the record of modern sea‐level change by over 50 years and reducing its uncertainty by ∼50%
Regional variability of global sea-level rise remains an important area of study given the vulnerability of sediment-starved coastlines to coastal inundation, especially those in proximity to large ...population centers. Galveston Bay, Texas, is currently experiencing more than double the global rate of sea-level rise and is particularly vulnerable to storm inundation that will further destabilize the coastline. Limitations in instrumental observations necessitate the use of the geologic record preserved offshore modern Galveston Bay to understand how this particular coastline responds to periods of rapid sea-level rise. We present micropaleontological analysis of sediment cores combined with high-resolution seismic data to reconstruct the Holocene paleoestuary offshore Galveston Bay and its evolution since initial inundation ∼10 ka through marine transgression ∼6 ka. We find that despite rapid sea-level rise, the Galveston paleoestuary maintained relatively stable outer boundaries, and within the bay environmental shifts occurred as a result of probable marine incursions due to tidal inlet migrations. Paleoenvironmental changes in the early Holocene coincide with flooding events within other Texas Gulf Coast bays suggesting global sea-level rise played a prominent role. Middle to late Holocene changes occurred when rates of sea-level rise slowed, suggesting regional hydroclimate change played a more dominant role.
•Micropaleontology as environmental context for seismic and sedimentologic analysis.•Long-term (∼2 kyr) stable estuary amid early Holocene sea-level rise.•Early Holocene paleoestuary changes dominated by rate of sea-level rise.•Middle Holocene transgression influenced by regional climate.
The results from a carefully implemented GPS analysis, using a strategy adapted to determine accurate vertical station velocities, are presented. The stochastic properties of our globally distributed ...GPS position time series were inferred, allowing the computation of reliable velocity uncertainties. Most uncertainties were several times smaller than the 1–3 mm/yr global sea level change, and hence the vertical velocities could be applied to correct the long tide gauge records for land motion. The sea level trends obtained in the ITRF2005 reference frame are more consistent than in the ITRF2000 or corrected for Glacial‐Isostatic Adjustment (GIA) model predictions, both on the global and the regional scale, leading to a reconciled global rate of geocentric sea level rise of 1.61 ± 0.19mm/yr over the past century in good agreement with the most recent estimates.
Nearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the ...autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea‐level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two‐dimensional or unconfined three‐dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea‐level rise and sediment supply: progradation, autoretreat and post‐autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised‐valley geometries, under conditions of constant rates of relative sea‐level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross‐sectional profiles, valleys with triangular cross‐sections tend to contain deltas that experience faster rates of progradation, autoretreat and post‐autobreak transgression under rising sea level, and exhibit a more prominent convex‐seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex‐seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea‐level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence‐stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record.
No consensus currently exists regarding the magnitude of Cretaceous short-term (less than 3Ma in duration) eustatic sea-level change. The lack of a consensus limits the ability to predict sedimentary ...facies and architecture and to assess the potential drivers of eustasy during a period of Earth history considered as significantly warmer than today. Consequently, this review documents, weighs, synthesises, and summarises records of short-term relative sea-level change and evaluates the observed trends in magnitude within the context of potential climatic drivers and their eustatic expression.
Although Cretaceous sea-level change is addressed in many publications, estimates of absolute values are relatively limited, often cover short time intervals, and use different methods. Based upon integrated geological and statistical analyses, four broad episodes of magnitude change have been identified. Three of these episodes reflect trends of increasing magnitudes of sea-level change from the Berriasian to early Hauterivian, late Hauterivian to Aptian, and Santonian to Maastrichtian. The fourth episode reflects a decreasing magnitude trend from the Albian to Coniacian. In addition, the maximum magnitude of sea-level change, at an approximate stage level, has been identified and categorised as slight (less than 10m), modest (10 to 40m), or significant (41 to 65m). Significant magnitudes are inferred for the Valanginian, Aptian, Albian, and Maastrichtian; exclusively slight magnitudes are restricted to the Berriasian. Such an assessment casts doubt on the repeated and stratigraphically widespread episodes of very large magnitudes (more than 75m) advocated by some workers, and instead defines distinct periods and magnitudes of sea-level change that should be globally reflected in sedimentary facies patterns. For example, intervals of sea-level fall of significant magnitude are commonly associated with the increased delivery of sediment into basinal settings, including the marked progradation of shallow-marine sediments, whilst up-systems tract there can be enhanced development of karst and erosional features.
Because climatically driven eustasy is the likely cause of short-term sea-level change, an assessment of the characteristic maximum magnitude limits of the principal climatic drivers (thermo-, aquifer-, and glacio-eustasy) has been made. Such a comparison argues for glacio-eustasy as the driver of significant short-term sea-level change. In addition, climate proxy data demonstrates that the Valanginian, Aptian, Albian, and Maastrichtian are intervals of cooling within the Cretaceous, thereby supporting the link between significant magnitudes and glacio-eustasy.
Glacial isostatic adjustment (GIA) modeling is not only useful for understanding past relative sea‐level change but also for projecting future sea‐level change due to ongoing land deformation. ...However, GIA model predictions are subject to a range of uncertainties, most notably due to uncertainty in the input ice history. An effective way to reduce this uncertainty is to perform data‐model comparisons over a large ensemble of possible ice histories, but this is often impossible due to computational limitations. Here we address this problem by building a deep‐learning‐based GIA emulator that can mimic the behavior of a physics‐based GIA model while being computationally cheap to evaluate. Assuming a single 1‐D Earth rheology, our emulator shows 0.54 m mean absolute error on 150 out‐of‐sample testing data with <0.5 s emulation time. Using this emulator, two illustrative applications related to the calculation of barystatic sea level are provided for use by the sea‐level community.
Plain Language Summary
Piecing together the history of ice sheet change during past glacial cycles is not only important for understanding past sea‐level change but also for predicting how ongoing glacial rebound contributes to future sea‐level change. Traditionally, a physics‐based “sea‐level model” is used to predict the sea‐level change associated with a particular reconstruction of past ice sheet change and compare the results with geological records of past sea level. However, a fundamental limitation of this approach is the need to compute sea‐level change for a large number of plausible ice histories, which is often prohibited by the computational resources required to repeatedly solve the complex physical equations. In this paper, we describe a machine‐learning‐based statistical model that can mimic the behavior of a physics‐based sea‐level model. This statistical model is computationally cheap and we demonstrate that it is able to accurately predict global sea‐level change for a suite of 150 “unseen” ice histories. Our statistical model predicts sea‐level change 100–1,000 times faster than a physics‐based model, making it an ideal tool for investigating and improving our understanding of global ice sheet change.
Key Points
The first attempt to build a deep‐learning based Glacial isostatic adjustment (GIA) emulator that can accurately predict global sea‐level change based on a given ice model
This emulator (GEORGIA) can predict global sea‐level change history within 0.5 s with minor emulation error
This GIA emulator along with two illustrative applications are available for use by the wider sea‐level community
ABSTRACT
Peat layers are well represented in the Holocene coastal deposits of the southern North Sea and provide evidence as to the extent and nature of the fens and bogs that occupied the region in ...the mid and late Holocene. While natural processes contributed to their demise, without human interference extensive areas of peatland would remain. We review the characteristics of the vegetation of these peatlands along with the processes that influenced their development. Spatial and temporal trends are explored through the use of palaeogeographic maps from three areas: the East Anglian Fenland, the Romney Marsh area and the Netherlands. The palaeoecological evidence indicates that eutrophic vegetation promoted by rising relative sea level (RSL) dominated in the mid Holocene, with a trend towards the development of oligotrophic and ombrotrophic vegetation in the late Holocene as the rate of RSL rise declined. Nevertheless, areas of eutrophic vegetation appear capable of long‐term stability with areas of fen woodland and herbaceous fen persisting at some locations for several thousand years in the mid and late Holocene. Areas of active peat growth in the region are now largely confined to small remnants within agricultural settings. To retain their characteristic biodiversity these remnants have been managed using traditional practices, although their small size and fragmented distribution limits their biodiversity value. Biodiversity concerns and the ecosystem services peatlands provide, notably carbon sequestration and flood attenuation, underlie recent restoration projects. These efforts are likely to receive additional impetus as a consequence of rising water levels, given projected rates of RSL rise. Future large‐scale restoration can be informed by a greater understanding of the processes that formed and sustained coastal peatlands in the past. We identify advances in palaeoenvironmental research that could enhance restoration efforts and help maximise the ecosystem services delivered through such projects.
The Earth's cryosphere represents a huge climate-sensitive carbon reservoir capable of releasing carbon dioxide (CO2) and methane (CH4) from permafrost soils or gas reservoirs capped by permafrost ...and ice caps upon rising global temperatures. Carbon release from these reservoirs has the potential to further accelerate global warming. Present day cryosphere demise is a focus of scientific research. The potential role of cryosphere carbon reservoirs in Mesozoic climate perturbations is even lesser known and currently underinvestigated. In contrast to previous views of a constantly warm Early Jurassic period, virtually lacking a cryosphere, recent studies have identified icehouse conditions for this time interval. Following these icehouse conditions, global warming occurred during the early Toarcian (~183 Ma) and was accompanied by a major carbon cycle anomaly as manifested in recurring negative carbon isotope excursions (CIEs). We propose that an initially volcanic-driven gentle rise of atmospheric temperature in the Early Toarcian triggered a melt-down of Earth's cryosphere which during the preceding Pliensbachian had expanded to the mid-latitudes and thus was highly vulnerable to warming. The rapid release of greenhouse gases, mainly as 13C-depleted CH4, or its oxidation product CO2, is recorded in the carbon isotope ratios of sedimentary organic matter and carbonates. Toarcian sediments display a series of orbitally-forced negative CIEs characterized by a frequency shift from eccentricity to obliquity cycles comparable to Pleistocene climate rhythms. This pattern is explained by a self-sustaining destabilization of labile cryosphere carbon reservoirs which started at mid-latitudes where eccentricity is most effective and then rhythmically progressed poleward to latitudes where obliquity dominates. The hitherto underestimated presence of a temperature-sensitive Pliensbachian cryosphere constituted an essential precondition for the early Toarcian climate change and its associated sea-level rise. The Pliensbachian cooling had transferred water into the terrestrial cryosphere causing a severe sea-level fall. Transgressive pulses at the Pliensbachian-Toarcian boundary and in the early Toarcian occurred concomitant to rising global temperatures and resulted from the meltdown of continental ice caps. This ice-volume effect and the massive discharge of freshwater into the oceans is well preserved in the exceptionally low δ18O values of carbonates formed during the cryosphere demise and sea-level increase. Carbon and oxygen isotope ratios, climate and sea-level shifts thus underpin the presence of an Early Jurassic cryosphere and thereby highlight the role of glacio-eustatic mechanisms as main drivers of late Pliensbachian to early Toarcian geodynamics.
Display omitted
•Unifying model for the Early Toarcian climate and carbon cycle perturbations.•Sea level rise, preceding carbon cycle perturbation, indicate ice sheet collapse.•Recurrent C-isotope excursions reflect astronomically-paced methane blowout events.
Long‐term Caspian Sea level change Chen, J. L.; Pekker, T.; Wilson, C. R. ...
Geophysical research letters,
16 July 2017, 2017-07-16, 20170716, Letnik:
44, Številka:
13
Journal Article
Recenzirano
Caspian Sea level (CSL) has undergone substantial fluctuations during the past several hundred years. The causes over the entire historical period are uncertain, but we investigate here large changes ...seen in the past several decades. We use climate model‐predicted precipitation (P), evaporation (E), and observed river runoff (R) to reconstruct long‐term CSL changes for 1979–2015 and show that PER (P‐E + R) flux predictions agree very well with observed CSL changes. The observed rapid CSL increase (about 12.74 cm/yr) and significant drop (~−6.72 cm/yr) during the periods 1979–1995 and 1996–2015 are well accounted for by integrated PER flux predictions of ~+12.38 and ~−6.79 cm/yr, respectively. We show that increased evaporation rates over the Caspian Sea play a dominant role in reversing the increasing trend in CSL during the past 37 years. The current long‐term decline in CSL is expected to continue into the foreseeable future, under global warming scenarios.
Key Points
PER flux‐reconstructed Caspian Sea level change agrees remarkably well with tide gauge and satellite measurements
Increased evaporation rates over the Caspian Sea play a dominant role in reversing the Caspian Sea level trends during the past 37 years
The current Caspian Sea level decline is expected to continue into the foreseeable future, under global warming scenarios
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