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\"What do we mean when we talk about sea level? How and why did people begin to measure it? With Wilko Graf von Hardenberg as our guide, we follow these questions and more to the muddy littoral spaces of Venice and Amsterdam, the coasts of the Baltic Sea, the Panama and Suez canals, and through the expansion of European colonial empires and the science funding boom of the Cold War. This book is the first history of sea level as a concept and of its theoretical and practical uses. It breaks new ground by offering an innovative outlook on how human societies worldwide have revisited and reinterpreted the relationship between land and sea in modern times. What is more, as a conceptual history of one of the most widely used baselines of environmental change, Sea Level provides a much-needed historical contextualization of anthropogenic sea level rise and its impact on the global coast. By narrating how sea level has morphed from a stable geodetic baseline to a marker of anthropogenic change, von Hardenberg sheds new light on the Anthropocene itself\"-- Provided by publisher.

Reconstructing past sea levels can help constrain uncertainties surrounding the rate of change, magnitude, and impacts of the projected increase through the 21st century. Of significance is the mid-Holocene relative sea-level highstand in tectonically stable and remote (far-field) locations from major ice sheets. The east coast of Australia provides an excellent arena in which to investigate changes in relative sea level during the Holocene. Considerable debate surrounds both the peak level and timing of the east coast highstand. The southeast Australian site of Bulli Beach provides the earliest evidence for the establishment of a highstand in the Southern Hemisphere, although questions have been raised about the pretreatment and type of material that was radiocarbon dated for the development of the regional sea-level curve. Here we undertake a detailed morpho- and chronostratigraphic study at Bulli Beach to better constrain the timing of the Holocene highstand in eastern Australia. In contrast to wood and charcoal samples that may provide anomalously old ages, probably due to inbuilt age, we find that short-lived terrestrial plant macrofossils provide a robust chronological framework. Bayesian modelling of the ages provide improved dating of the earliest evidence for a highstand at 6,880±50 cal BP, approximately a millennium later than previously reported. Our results from Bulli now closely align with other sea-level reconstructions along the east coast of Australia, and provide evidence for a synchronous relative sea-level highstand that extends from the Gulf of Carpentaria to Tasmania. Our refined age appears to be coincident with major ice mass loss from Northern Hemisphere and Antarctic ice sheets, supporting previous studies that suggest these may have played a role in the relative sea-level highstand. Further work is now needed to investigate the environmental impacts of regional sea levels, and refine the timing of the subsequent sea-level fall in the Holocene and its influence on coastal evolution.

A statistical reassessment of the tide gauge record concludes that sea level rose at a rate of about 1.2 millimetres per year from 1901 to 1990, slightly lower than prior estimates and now consistent with estimates based on individual contributions to sea-level change; the estimates reported here from 1990 onwards are consistent with other work, suggesting that the recent acceleration in sea-level rise is greater than previously thought. Twentieth century sea levels revisited Rates of sea-level rise calculated from tide gauge data tend to exceed bottom-up estimates derived from summing loss of ice mass, thermal expansion and changes in land storage. Carling Hay et al . provide a statistical reassessment of the tide gauge record — which is subject to bias due to sparse and non-uniform geographic coverage and other uncertainties — and conclude that sea-level rose by about 1.2 millimetres per year from 1901 to 1990. This is slightly lower than prior estimates and is consistent with the bottom-up estimates. The same analysis applied to the period 1993–2010, however, indicates a sea-level rise of about three millimetres per year, consistent with other work and suggesting that the recent acceleration in sea-level rise has been greater than previously thought. Estimating and accounting for twentieth-century global mean sea-level (GMSL) rise is critical to characterizing current and future human-induced sea-level change. Several previous analyses of tide gauge records 1 , 2 , 3 , 4 , 5 , 6 —employing different methods to accommodate the spatial sparsity and temporal incompleteness of the data and to constrain the geometry of long-term sea-level change—have concluded that GMSL rose over the twentieth century at a mean rate of 1.6 to 1.9 millimetres per year. Efforts to account for this rate by summing estimates of individual contributions from glacier and ice-sheet mass loss, ocean thermal expansion, and changes in land water storage fall significantly short in the period before 1990 7 . The failure to close the budget of GMSL during this period has led to suggestions that several contributions may have been systematically underestimated 8 . However, the extent to which the limitations of tide gauge analyses have affected estimates of the GMSL rate of change is unclear. Here we revisit estimates of twentieth-century GMSL rise using probabilistic techniques 9 , 10 and find a rate of GMSL rise from 1901 to 1990 of 1.2 ± 0.2 millimetres per year (90% confidence interval). Based on individual contributions tabulated in the Fifth Assessment Report 7 of the Intergovernmental Panel on Climate Change, this estimate closes the twentieth-century sea-level budget. Our analysis, which combines tide gauge records with physics-based and model-derived geometries of the various contributing signals, also indicates that GMSL rose at a rate of 3.0 ± 0.7 millimetres per year between 1993 and 2010, consistent with prior estimates from tide gauge records 4 . The increase in rate relative to the 1901–90 trend is accordingly larger than previously thought; this revision may affect some projections 11 of future sea-level rise.

Upper-ocean warming and sea-level rise Changes in the climate system's energy budget are predominantly revealed in ocean temperatures and the associated thermal expansion contribution to sea-level rise. Climate models, however, do not reproduce the large decadal variability in globally averaged ocean heat content inferred from observations, even when volcanic and other variable climate forcings are included. Domingues et al . report improved estimates of near-global ocean heat content and thermal expansion for the upper ocean from 1950 to 2003, applying corrections to reduce systematic biases in the most common ocean temperature observations. Their ocean warming and thermal expansion trends for 1961 to 2003 are about 50 per cent larger than earlier estimates but about 40 per cent smaller for 1993 to 2003, consistent with the recognition that previously estimated rates for the 1990s were biased by instrumental errors. The authors add observational estimates of upper-ocean thermal expansion to other contributions to sea-level rise, and find that the sum of contributions from 1961 to 2003 is in good agreement with their updated estimate of near-global mean sea level. This paper reports improved estimates of near-global ocean heat content and thermal expansion for the upper ocean from 1950–2003, applying corrections to reduce systematic biases in the most common ocean temperature observations. The ocean warming and thermal expansion trends for 1961–2003 are about 50 per cent larger than earlier estimates but about 40 per cent smaller for 1993–2003, consistent with the recognition that previously estimated rates for the 1990s were biased by instrumental errors. Changes in the climate system’s energy budget are predominantly revealed in ocean temperatures 1 , 2 and the associated thermal expansion contribution to sea-level rise 2 . Climate models, however, do not reproduce the large decadal variability in globally averaged ocean heat content inferred from the sparse observational database 3 , 4 , even when volcanic and other variable climate forcings are included. The sum of the observed contributions has also not adequately explained the overall multi-decadal rise 2 . Here we report improved estimates of near-global ocean heat content and thermal expansion for the upper 300 m and 700 m of the ocean for 1950–2003, using statistical techniques that allow for sparse data coverage 5 , 6 , 7 and applying recent corrections 8 to reduce systematic biases in the most common ocean temperature observations 9 . Our ocean warming and thermal expansion trends for 1961–2003 are about 50 per cent larger than earlier estimates but about 40 per cent smaller for 1993–2003, which is consistent with the recognition that previously estimated rates for the 1990s had a positive bias as a result of instrumental errors 8 , 9 , 10 . On average, the decadal variability of the climate models with volcanic forcing now agrees approximately with the observations, but the modelled multi-decadal trends are smaller than observed. We add our observational estimate of upper-ocean thermal expansion to other contributions to sea-level rise and find that the sum of contributions from 1961 to 2003 is about 1.5 ± 0.4 mm yr -1 , in good agreement with our updated estimate of near-global mean sea-level rise (using techniques established in earlier studies 6 , 7 ) of 1.6 ± 0.2 mm yr -1 .

In the sixteenth century, Cornaro wrote a Treatise on Waters with personal observations and conclusions regarding the Lagoon of Venice, e.g., the sea level rise over the centuries, the difference between normal tides for astronomical forces and storm surges driven by meteorological factors, and water exchanges between the Lagoon and the Sea. He witnessed the continuous rise of the sea level since the Middle Ages and listed some public works made to adapt to this challenge, i.e., raising city paving and floors, and rebuilding bridges that had become too low. Cornaro dealt with the mark left by the algae on walls that was kept as an official (zero) reference of sea level. Using this key to measure flooding depths, and knowing the relationship between the algae front and mean sea level, a revision of the historical floods (from 1240 to 1867) is made to assess precise depths. During the deepest floods, it was possible to reach San Marco square by gondola and float on the square. The draught of past gondola types has been another key to interpret flood depths. From 1200 to 1500, the most extreme flooding depths were higher than that of 1966, i.e., the highest in the instrumental record since 1871; from 1500 to 1799, they have been quite homogeneous, close to the value observed in 1966; in the nineteenth century, they returned to be higher than that in 1966. Over eight centuries, the deepest historical floods exceeded 7 times by 40 cm the 1966 extreme depth. The city should be prepared to face this risk.

Ice loss from the Antarctic ice sheet (AIS) is expected to become the major contributor to sea level in the next centuries. Projections of the AIS response to climate change based on numerical ice-sheet models remain challenging due to the complexity of physical processes involved in ice-sheet dynamics, including instability mechanisms that can destabilise marine basins with retrograde slopes. Moreover, uncertainties in ice-sheet models limit the ability to provide accurate sea-level rise projections. Here, we apply probabilistic methods to a hybrid ice-sheet model to investigate the influence of several sources of uncertainty, namely sources of uncertainty in atmospheric forcing, basal sliding, grounding-line flux parameterisation, calving, sub-shelf melting, ice-shelf rheology and bedrock relaxation, on the continental response of the Antarctic ice sheet to climate change over the next millennium. We provide probabilistic projections of sea-level rise and grounding-line retreat, and we carry out stochastic sensitivity analysis to determine the most influential sources of uncertainty. We find that all investigated sources of uncertainty, except bedrock relaxation time, contribute to the uncertainty in the projections. We show that the sensitivity of the projections to uncertainties increases and the contribution of the uncertainty in sub-shelf melting to the uncertainty in the projections becomes more and more dominant as atmospheric and oceanic temperatures rise, with a contribution to the uncertainty in sea-level rise projections that goes from 5 % to 25 % in RCP 2.6 to more than 90 % in RCP 8.5. We show that the significance of the AIS contribution to sea level is controlled by the marine ice-sheet instability (MISI) in marine basins, with the biggest contribution stemming from the more vulnerable West Antarctic ice sheet. We find that, irrespective of parametric uncertainty, the strongly mitigated RCP 2.6 scenario prevents the collapse of the West Antarctic ice sheet, that in both the RCP 4.5 and RCP 6.0 scenarios the occurrence of MISI in marine basins is more sensitive to parametric uncertainty, and that, almost irrespective of parametric uncertainty, RCP 8.5 triggers the collapse of the West Antarctic ice sheet.

Melt supply key to speedy ice sheets Recent observations in Greenland have led to the notion that surface meltwater supply to the ice-sheet bed lubricates ice flow, suggesting that climate warming could lead to runaway glacial acceleration. Christian Schoof uses a physically based model that captures drainage channelization under the ice to challenge this view. The model shows that increased melt supply leads to channelization and a drop in water pressure. This causes ice flow to slow down rather than speed up. However, this effect can be overcome if melt supply is variable over short timescales, when temporary pressure spikes can lead to accelerated flow. The positive melt/dynamic thinning feedback is therefore still viable, but is heavily dependent on daily temperature variations and rain events that are largely ignored in current ice sheet models. Increased melting is often assumed to cause acceleration of ice sheets and glaciers through basal lubrication, possibly leading to increased rates of sea level rise. Now a physically-based model challenges this view, illustrating that above a critical threshold, increased melt will suppress the dynamic thinning process. Short-term spikes in water delivery, as from lake drainage or precipitation, still have the potential to generate spikes in velocity, but overall increases in melt do not appear likely to cause velocity increases. Increased ice velocities in Greenland 1 are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice–ocean interactions in water-terminating outlet glaciers 2 and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration 2 , 3 , 4 , 5 , 6 have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another 7 , suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt 4 but an increase in water input variability 8 that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure 9 , 10 , 11 , with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure 12 , 13 , 14 , 15 . Using a model that captures the dynamic switching 12 between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning 16 , indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles 4 , 14 and an increase in rain and surface lake drainage events 8 , 17 , 18 rather than an increase in mean melt supply 3 , 4 .

Radar satellite altimeters enable the determination of the mean sea surface to centimeter accuracy, which can be degraded in coastal areas because of the lack of valid altimetry observations due to land contamination and the altimeter footprint size. In 2018, the National Aeronautics and Space Administration launched ICESat-2, a laser altimetry mission equipped with the Advanced Topographic Laser Altimeter System, providing measurements every 0.7 m in the along-track direction. Taking into account the complexity of the Norwegian coastline, this study aims to evaluate coastal observations from ICESat-2 in order to use it to update the existing mean sea surface for Norway, NMBU18. We, therefore, determined the mean sea surface using only ICESat-2 observations and compared it with mean sea level observations from 23 permanent tide gauges along the entire coast and 21 temporary tide gauges in Norway’s largest and deepest fjord, Sognefjorden. We also included two global mean sea surface models and NMBU18 for comparison. The results have shown that ICESat-2 is indeed able to provide more valid observations in the coastal zone, which can be used to improve the mean sea surface model, especially along the coast.

Atlantic Water (AW) advection plays an important role in climatic, oceanographic and environmental conditions in the eastern Arctic. Situated along the only deep connection between the Atlantic and the Arctic oceans, the Svalbard Archipelago is an ideal location to reconstruct the past AW advection history and document its linkage with local glacier dynamics, as illustrated in the present study of a 275 cm long sedimentary record from Woodfjorden (northern Spitsbergen; water depth: 171 m) spanning the last  ∼  15 500 years. Sedimentological, micropalaeontological and geochemical analyses were used to reconstruct changes in marine environmental conditions, sea ice cover and glacier activity. Data illustrate a partial break-up of the Svalbard–Barents Sea Ice Sheet from Heinrich Stadial 1 onwards (until  ∼  14.6 ka). During the Bølling–Allerød ( ∼  14.6–12.7 ka), AW penetrated as a bottom water mass into the fjord system and contributed significantly to the destabilization of local glaciers. During the Younger Dryas ( ∼  12.7–11.7 ka), it intruded into intermediate waters while evidence for a glacier advance is lacking. A short-term deepening of the halocline occurred at the very end of this interval. During the early Holocene ( ∼  11.7–7.8 ka), mild conditions led to glacier retreat, a reduced sea ice cover and increasing sea surface temperatures, with a brief interruption during the Preboreal Oscillation ( ∼  11.1–10.8 ka). Due to a  ∼  6000-year gap, the mid-Holocene is not recorded in this sediment core. During the late Holocene ( ∼  1.8–0.4 ka), a slightly reduced AW inflow and lower sea surface temperatures compared to the early Holocene are reconstructed. Glaciers, which previously retreated to the shallower inner parts of the Woodfjorden system, likely advanced during the late Holocene. In particular, topographic control in concert with the reduced summer insolation partly decoupled glacier dynamics from AW advection during this recent interval.

Automatic sea-level measurements in Nouméa, South Pacific, started in 1957 for the International Geophysical year. Data from this location exist in paper record for the 1957–1967 period, and in two distinct electronic records for the 1967–2005 and 2005–2015 period. In this study, we digitize the early record, and established a link between the two electronic records to create a unique, nearly 60 year-long instrumental sea-level record. This work creates one of the longest instrumental sea-level records in the Pacific Islands. These data are critical for the study of regional and interannual variations of sea level. This new data set is then used to infer rates of vertical movements by comparing it to (1) the entire satellite altimetric record (1993–2013) and (2) a global sea-level reconstruction (1957–2010). These inferred rates show an uplift of 1.3–1.4 mm/year, opposite to the currently accepted values of subsidence found in the geological and geodetic literature, and underlie the importance of systematic geodetic measurements at, over very near tide gauges.