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During the Mid-Pleistocene transition (MPT) the dominant periodicity of glacial cycles increased from 41 thousand years (kyr) to an average of 100 kyr, without any appreciable change in the orbital pacing. As the MPT is not a linear response to orbital forcing, it must have resulted from feedback processes in the Earth system. However, the precise mechanisms underlying the transition are still under debate. In this study, we investigate the MPT by simulating the Northern Hemisphere ice sheet evolution over the past 1.5 million years. The transient climate forcing of the ice-sheet model was obtained using a matrix method, by interpolating between two snapshots of global climate model simulations. Changes in climate forcing are caused by variations in CO2, insolation, as well as implicit climate–ice sheet feedbacks. Using this method, we were able to capture glacial-interglacial variability during the past 1.5 million years and reproduce the shift from 41 kyr to 100 kyr cycles without any additional drivers. Instead, the modelled frequency change results from the prescribed CO2 combined with orbital forcing, and ice sheet feedbacks. Early Pleistocene terminations are initiated by insolation maxima. After the MPT, low CO2 levels can compensate insolation maxima which favour deglaciation, leading to an increasing glacial cycle periodicity. These deglaciations are also prevented by a relatively small North American ice sheet, which, through its location and feedback processes, can generate a relatively stable climate. Larger North American ice sheets become more sensitive to small temperature increases. Therefore, Late Pleistocene terminations are facilitated by the large ice-sheet volume, were small changes in temperature lead to self-sustained melt instead. This concept is confirmed by experiments using constant insolation or CO2. The constant CO2 experiments generally capture only the Early Pleistocene cycles, while those with constant insolation only capture Late Pleistocene cycles. Additionally, we find that a lowering of CO2concentrations leads to an increasing number of insolation maxima that fail to initiate terminations. These results therefore suggest a regime shift, where during the Early Pleistocene, glacial cycles are dominated by orbital oscillations, while Late Pleistocene cycles tend to be more dominated by CO2. This implies that the MPT can be explained by a decrease in glacial CO2 concentration superimposed on orbital forcing. [ABSTRACT FROM AUTHOR]

During the glacial cycles of the past 800 000 years, Eurasia and North America were periodically covered by large ice sheets, causing up to 100 m of sea-level change. While Late Pleistocene glacial cycles typically lasted 80 000–120 000 years, the termination phases were completed in only 10 000 years. During these glacial terminations, the North American and Eurasian ice sheets retreated, which created large proglacial lakes in front of the ice-sheet margin. Proglacial lakes accelerate deglaciation as they facilitate the formation of ice shelves at the southern margins of the North American and Eurasian ice sheets. These ice shelves are characterized by basal melting, low surface elevations, and negligible friction at the base. Here, we use an ice-sheet model to quantify the (combined) effects of proglacial lakes on Late Pleistocene glacial terminations by examining their interplay with glacial isostatic adjustment (GIA) and basal sliding. We find that proglacial lakes accelerate the deglaciation of ice sheets mainly because there is an absence of basal friction underneath ice shelves. If friction underneath grounded ice is applied to floating ice, full deglaciation is postponed by a few millennia, resulting in more ice remaining during interglacial periods and no extensive ice shelves forming. Additionally, the large uncertainty in melt rates underneath lacustrine ice shelves translates to an uncertainty in the timing of the termination of up to a millennium. Proglacial lakes are created by depressions in the landscape that remain after an ice sheet has retreated. The depth, size, and timing of proglacial lakes depend on the rate of bedrock rebound. We find that if bedrock rebounds within a few centuries (rather than a few millennia), the mass loss rate of the ice sheet is substantially reduced. This is because fast bedrock rebound prevents the formation of extensive proglacial lakes. Additionally, a decrease in ice thickness is partly compensated for by faster bedrock rebound, resulting in a higher surface elevation; lower temperatures; and a higher surface mass balance, which delays deglaciation. We find that a very long bedrock relaxation time does not substantially affect terminations, but it may lead to a delayed onset of the next glacial period. This is because inception regions, such as northwestern Canada, remain below sea level throughout the preceding interglacial period. [ABSTRACT FROM AUTHOR]

Deltas and coastal plains worldwide developed under the influence of relative sea level rise (RSLR) during the Holocene. In the Netherlands, Holocene RSLR results from both regional sea-level rise and regional subsidence patterns, mainly caused by glacial isostatic adjustment (GIA: Scandinavian forebulge collapse) and longer-term North Sea Basin tectono-sedimentary subsidence. Past coastal and inland water levels are preserved in geological indicators marking the gradual drowning of an area, for example basal peats. Such geological water-level indicators have been used in the Netherlands for varying types of research. However, uniform overviews of these data exist only for smaller local subsets and not for the entire Netherlands. In this paper we present a data set of 712 Holocene water-level indicators from the Dutch coastal plain that are relevant for studying RSLR and regional subsidence, compiled in HOLSEA workbook format. This format was expanded to allow for registering basal-peat type geological indicators, documenting Dutch-setting specific parameters and accompanying uncertainties, to assess indicative meaning, and to appropriately correct the raw vertical positions of the indicators. Overall, our new, internally consistent, expanded documentation provided for the water-level indicators encourages users to choose the information relevant for their research and report RSLR uncertainties transparently. From the indicators, 59 % was collected in 1950–2000, mainly in academic studies and survey mapping campaigns; 37 % was collected in 2000–2020 in academic studies and archaeological surveying projects, 4 % was newly collected (this study), the latter mainly in previously under sampled central and northern Netherlands regions. Prominent regional differences exist in the vertical position and abundance of the indicators. Older indicators in our data set are mostly located in the deeper seaward area of the Netherlands. These indicators correspond well with previous transgression reconstructions, that are partly based on the same data. The younger, landwards set of indicators in the Rhine-Meuse central and Flevoland regions corresponds with the transgression phase reaching further inland, from 8000 cal. BP onwards. Northern indicators of Middle Holocene age (8–5 ka cal. BP), in general lie 2–3 meters lower compared to those in the south. For younger data this difference is less, showing spatial and temporal variation in RSLR throughout the Netherlands. [ABSTRACT FROM AUTHOR]

The strongly varying benthic δ18 O levels of the early and mid-Miocene (23 to 14 Myr ago) are primarily caused by a combination of changes in Antarctic Ice Sheet (AIS) volume and deep-ocean temperatures. These factors are coupled since AIS changes affect deep-ocean temperatures. It has recently been argued that this is due to changes in ice sheet area rather than volume because area changes affect the surface albedo. This finding would be important when the transient AIS grows relatively faster in extent than in thickness, which we test here. We analyse simulations of Miocene AIS variability carried out using the three-dimensional ice sheet model IMAU-ICE forced by warm (high CO 2 , no ice) and cold (low CO 2 , large East AIS) climate snapshots. These simulations comprise equilibrium and idealized quasi-orbital transient runs with strongly varying CO 2 levels (280 to 840 ppm). Our simulations show a limited direct effect of East AIS changes on Miocene orbital-timescale benthic δ18 O variability because of the slow build-up of volume. However, we find that relative to the equilibrium ice sheet size, the AIS area adapts significantly faster and more strongly than volume to the applied forcing variability. Consequently, during certain intervals the ice sheet is receding at the margins, while ice is still building up in the interior. That means the AIS does not adapt to a changing equilibrium size at the same rate or with the same sign everywhere. Our results indicate that the Miocene Antarctic Ice Sheet affects deep-ocean temperatures more than its volume suggests. [ABSTRACT FROM AUTHOR]

During the glacial cycles of the past 800 thousand years, Eurasia and North America were periodically covered by large ice sheets. While the Late Pleistocene glacial cycles typically lasted 80 - 120 thousand years, the termination phases only took 10 thousand years to complete. During these glacial terminations, the North American and Eurasian ice sheets retreated which created large proglacial lakes in front of the ice sheet margin. Proglacial lakes accelerate the deglaciation as they can facilitate ice shelves in the southern margins of the North American and the Eurasian ice sheets. Ice shelves are characterized by basal melting, low surface elevations and negligible friction at the base. Here we quantify the effect of proglacial lakes, and the combined effect with glacial isostatic adjustment (GIA) on Late Pleistocene glacial terminations. We find that proglacial lakes accelerate the deglaciation of the ice sheets mainly because of the absence of basal friction underneath ice shelves. If the friction underneath grounded ice is applied to floating ice, we find that full deglaciation is postponed by a few millennia, the Barents-Kara Sea region does not fully deglaciate, and there are no extensive ice shelves. Additionally, the large uncertainty in melt rates underneath lacustrine ice shelves translates to an uncertainty in the timing of the termination of only a few centuries at most. Proglacial lakes are created by the depression in the landscape that linger after the ice sheet has retreated. The depth, size and timing of proglacial lakes depend on the bedrock rebound. We find that if the bedrock rebounds within a few centuries, instead of a few millennia, the mass loss rate of the ice sheet is substantially reduced. This is because fast bedrock rebound prevents the formation of extensive proglacial lakes. Additionally, a decrease in thickness is partly compensated by the faster bedrock rebound, resulting in a higher surface elevation with lower temperatures and higher surface mass balance delaying deglaciation. We find that a very long bedrock relaxation time does not affect terminations substantially, but will lead to a later inception of the next glacial period. This is because initial inception regions, such as North-Western Canada, remain below sea level throughout the preceding interglacial period. [ABSTRACT FROM AUTHOR]

The strongly varying benthic δ18O levels of the early and mid-Miocene (23 to 14 Myr ago) are primarily caused by a combination of changes in Antarctic ice sheet (AIS) volume and deep ocean temperatures. These factors are coupled since AIS changes affect deep ocean temperatures. It has recently been argued that this is due to changes in ice sheet area rather than volume, because area changes affect the surface albedo. This would be important when the transient AIS grows relatively faster in extent than in thickness, which we test here. We analyse simulations of Miocene AIS variability carried out using the three-dimensional ice-sheet model IMAU-ICE forced by warm (high CO2, no ice) and cold (low CO2, large East-AIS) climate snapshots. These simulations comprise equilibrium and idealised quasi-orbital transient runs with strongly varying CO2 levels (280 to 840 ppm). Our simulations show limited direct effect of East-AIS changes on Miocene orbital timescale benthic δ18O variability, because of the slow build-up of volume. However, we find that AIS area responds significantly faster and more strongly than volume to the applied forcing variability. Consequently, during certain intervals the ice sheet is receding at the margins, while ice is still building up in the interior. That means the AIS does not adapt to a changing equilibrium size at the same rate or with the same sign everywhere. Our results indicate that the Miocene Antarctic ice sheet affects deep ocean temperatures more than its volume suggests. [ABSTRACT FROM AUTHOR]

During the last glacial cycle (LGC), ice sheets covered large parts of Eurasia and North America, which resulted in ∼120 m of sea level change. Ice sheet–climate interactions have considerable influence on temperature and precipitation patterns and therefore need to be included when simulating this time period. Ideally, ice sheet–climate interactions are simulated by a high-resolution Earth system model. While these models are capable of simulating climates at a certain point in time, such as the pre-industrial (PI) or the Last Glacial Maximum (LGM; 21 000 years ago), a full transient glacial cycle is currently computationally unfeasible as it requires a too-large amount of computation time. Nevertheless, ice sheet models require forcing that captures the gradual change in climate over time to calculate the accumulation and melt of ice and its effect on ice sheet extent and volume changes. Here we simulate the LGC using an ice sheet model forced by LGM and PI climates. The gradual change in climate is modelled by transiently interpolating between pre-calculated results from a climate model for the LGM and the PI. To assess the influence of ice sheet–climate interactions, we use two different interpolation methods: the climate matrix method, which includes a temperature–albedo and precipitation–topography feedback, and the glacial index method, which does not. To investigate the sensitivity of the results to the prescribed climate forcing, we use the output of several models that are part of the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3). In these simulations, ice volume is prescribed, and the climate is reconstructed with a general circulation model (GCM). Here we test those models by using their climate to drive an ice sheet model over the LGC. We find that the ice volume differences caused by the climate forcing exceed the differences caused by the interpolation method. Some GCMs produced unrealistic LGM volumes, and only four resulted in reasonable ice sheets, with LGM Northern Hemisphere sea level contribution ranging between 74–113 m with respect to the present day. The glacial index and climate matrix methods result in similar ice volumes at the LGM but yield a different ice evolution with different ice domes during the inception phase of the glacial cycle and different sea level rates during the deglaciation phase. The temperature–albedo feedback is the main cause of differences between the glacial index and climate matrix methods. [ABSTRACT FROM AUTHOR]

Sea-level science has seen many recent developments in observations and modelling of the different contributions and the total mean sea-level change. In this overview, we discuss (1) the evolution of the Intergovernmental Panel on Climate Change (IPCC) projections, (2) how the projections compare to observations and (3) the outlook for further improving projections. We start by discussing how the model projections of 21st century sea-level change have changed from the IPCC AR5 report (2013) to SROCC (2019) and AR6 (2021), highlighting similarities and differences in the methodologies and comparing the global mean and regional projections. This shows that there is good agreement in the median values, but also highlights some differences. In addition, we discuss how the different reports included high-end projections. We then show how the AR5 projections (from 2007 onwards) compare against the observations and find that they are highly consistent with each other. Finally, we discuss how to further improve sea-level projections using high-resolution ocean modelling and recent vertical land motion estimates. [ABSTRACT FROM AUTHOR]

A major source of uncertainty in future sea-level projections is the ocean-driven basal melt of Antarctic ice shelves. Whereas ice sheet models require a kilometer-scale resolution to realistically resolve ice shelf stability and grounding line migration, global or regional 3D ocean models are computationally too expensive to produce basal melt forcing fields at this resolution. To bridge this resolution gap, we introduce the 2D numerical model LADDIE (one-Layer Antarctic model for Dynamical Downscaling of Ice-ocean Exchanges) which allows for the computationally efficient modeling of basal melt rates. The model is flexible, and can be forced with output from coarse 3D ocean models or with vertical profiles of offshore temperature and salinity. In this study, we describe the model equations and numerics. To illustrate and validate the model performance, we apply the model to two test cases: the small Crosson-Dotson Ice Shelf in the warm Amundsen Sea region, and the large Filchner-Ronne Ice Shelf in the cold Weddell Sea. At ice-shelf wide scales, LADDIE reproduces observed patterns of basal melt and freezing that are also well reproduced by 3D ocean models. At scales of 0.5-5 km, which are unresolved by 3D ocean models and poorly constrained by observations, LADDIE produces plausible basal melt patterns. Most significantly, the simulated basal melt patterns are physically consistent with the applied ice shelf topography. These patterns are governed by the topographic steering and Coriolis deflection of meltwater flows, two processes that are poorly represented in basal melt parameterisations. The kilometer-scale melt patterns simulated by LADDIE include enhanced melt rates in basal channels, in some shear margins, and nearby grounding lines. As these regions are critical for ice shelf stability, we conclude that LADDIE can provide detailed basal melt patterns at the essential resolution that ice sheet models require. The physical consistency between the applied geometry and the simulated basal melt fields indicates that LADDIE can play a valuable role in the development of coupled ice-ocean modeling. [ABSTRACT FROM AUTHOR]

Glacial Isostatic Adjustment (GIA) has a stabilizing effect on the evolution of the Antarctic Ice Sheet by reducing the grounding line migration that follows ice melt. The timescale and strength of this feedback depend on the spatially varying viscosity of the Earth's mantle. Most studies assume a relatively high laterally homogenous response time of the bedrock. However, viscosity is spatially variable with a high viscosity beneath East Antarctica, and a low viscosity beneath West Antarctica. For this study, we have developed a new method to couple a 3D GIA model and an ice-sheet model to study the interaction between the Solid Earth and the Antarctic Ice Sheet during the last glacial cycle. The feedback effect into account on a high temporal resolution by using coupling time steps of 500 years. We applied the method using the ice-sheet model ANICE, a 3D GIA FE model, and results from a seismic model to determine the patterns in the viscosity. The results of simulations over the Last Glacial Cycle show that differences in viscosity of an order of magnitude can lead to differences in grounding line position up to 500 km, to differences in ice thickness in the order of 1.5 km. These results underline and quantify the importance of including local GIA feedback effects in ice-sheet models when simulating the Antarctic Ice Sheet evolution over the Last Glacial Cycle. [ABSTRACT FROM AUTHOR]

During the Last Glacial Cycle (LGC), ice sheets covered large parts of Eurasia and North America which resulted in ~120 meters of sea level change. Ice sheet - climate interactions have considerable influence on temperature and precipitation patterns, and therefore need to be included when simulating this time period. Ideally, ice sheet - climate interactions are simulated by a high-resolution earth system model. While these models are capable of simulating climates at a certain point in time, such as the Pre-Industrial (PI) or the Last Glacial Maximum (LGM; 21,000 years ago), a full glacial cycle is currently unfeasible as it requires a too large amount of computation time. Nevertheless, ice-sheet models require forcing that captures the gradual change in climate over time to calculate the accumulation and melt of ice and its effect on ice sheet extent and volume changes. Here we simulate the LGC using an ice sheet model forced by LGM and PI climates. The gradual change in climate is modelled by transiently interpolating between pre-calculated results from a climate model for the LGM and the PI. To assess the influence of ice sheet - climate interactions, we use two different interpolation methods: The climate matrix method, which includes these interactions, and the glacial index method, which does not. To investigate the sensitivity of the results to the prescribed climate forcing, we use the output of several models that are part of the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3). In these simulations, ice volume is prescribed and the climate is reconstructed. Here we test those models by using their climate to drive an ice sheet model over the LGC. We find that the differences caused by the climate forcing exceeds the differences caused by the interpolation method. Some General Circulation Models (GCMs) produced unrealistic LGM volumes and only four resulted in reasonable ice sheets with LGM Northern Hemisphere sea level contribution ranging between 74 - 113 meters with respect to the present day. The glacial index and climate matrix methods result in similar ice volumes at LGM but yield a different ice evolution with different ice domes during the inception phase of the glacial cycle, and different sea-level rates during the deglaciation phase. The temperature-albedo feedback is the main cause of differences between the glacial index and climate matrix methods. [ABSTRACT FROM AUTHOR]