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In less than a decade, analyses of ancient genomes have transformed our understanding of the Indigenous peopling and population history of the Americas. These studies have shown that this history, which began in the late Pleistocene epoch and continued episodically into the Holocene epoch, was far more complex than previously thought. It is now evident that the initial dispersal involved the movement from northeast Asia of distinct and previously unknown populations, including some for whom there are no currently known descendants. The first peoples, once south of the continental ice sheets, spread widely, expanded rapidly and branched into multiple populations. Their descendants--over the next fifteen millennia--experienced varying degrees of isolation, admixture, continuity and replacement, and their genomes help to illuminate the relationships among major subgroups of Native American populations. Notably, all ancient individuals in the Americas, save for later-arriving Arctic peoples, are more closely related to contemporary Indigenous American individuals than to any other population elsewhere, which challenges the claim--which is based on anatomical evidence--that there was an early, non-Native American population in the Americas. Here we review the patterns revealed by ancient genomics that help to shed light on the past peoples who created the archaeological landscape, and together lead to deeper insights into the population and cultural history of the Americas. A review of the ancient human genomic record sheds light on the peopling processes of the Americas., Author(s): Eske Willerslev [sup.1] [sup.2] [sup.3] , David J. Meltzer [sup.2] [sup.4] Author Affiliations: (1) GeoGenetics Group, Department of Zoology, University of Cambridge, Cambridge, UK (2) Lundbeck Foundation GeoGenetics Centre, [...]

Author(s): Natalya Gomez [sup.1] , Michael E. Weber [sup.2] , Peter U. Clark [sup.3] [sup.4] , Jerry X. Mitrovica [sup.5] , Holly K. Han [sup.1] Author Affiliations: (1) Department of [...], Sea-level rise due to ice loss in the Northern Hemisphere in response to insolation and greenhouse gas forcing is thought to have caused grounding-line retreat of marine-based sectors of the Antarctic Ice Sheet (AIS).sup.1-3. Such interhemispheric sea-level forcing may explain the synchronous evolution of global ice sheets over ice-age cycles. Recent studies that indicate that the AIS experienced substantial millennial-scale variability during and after the last deglaciation.sup.4-7 (roughly 20,000 to 9,000 years ago) provide further evidence of this sea-level forcing. However, global sea-level change as a result of mass loss from ice sheets is strongly nonuniform, owing to gravitational, deformational and Earth rotational effects.sup.8, suggesting that the response of AIS grounding lines to Northern Hemisphere sea-level forcing is more complicated than previously modelled.sup.1,2,6. Here, using an ice-sheet model coupled to a global sea-level model, we show that AIS dynamics are amplified by Northern Hemisphere sea-level forcing. As a result of this interhemispheric interaction, a large or rapid Northern Hemisphere sea-level forcing enhances grounding-line advance and associated mass gain of the AIS during glaciation, and grounding-line retreat and mass loss during deglaciation. Relative to models without these interactions, the inclusion of Northern Hemisphere sea-level forcing in our model increases the volume of the AIS during the Last Glacial Maximum (about 26,000 to 20,000 years ago), triggers an earlier retreat of the grounding line and leads to millennial-scale variability throughout the last deglaciation. These findings are consistent with geologic reconstructions of the extent of the AIS during the Last Glacial Maximum and subsequent ice-sheet retreat, and with relative sea-level change in Antarctica.sup.3-7,9,10. Changes in Northern Hemisphere ice-sheet size during ice-age cycles enhance the advance and retreat of the grounding line of the Antarctic Ice Sheet, owing to interhemispheric sea-level forcing.

Author(s): T. Blackburn [sup.1] , G. H. Edwards [sup.1] , S. Tulaczyk [sup.1] , M. Scudder [sup.1] , G. Piccione [sup.1] , B. Hallet [sup.2] , N. McLean [sup.3] , [...], Efforts to improve sea level forecasting on a warming planet have focused on determining the temperature, sea level and extent of polar ice sheets during Earth's past interglacial warm periods.sup.1-3. About 400,000 years ago, during the interglacial period known as Marine Isotopic Stage 11 (MIS11), the global temperature was 1 to 2 degrees Celsius greater.sup.2 and sea level was 6 to 13 metres higher.sup.1,3. Sea level estimates in excess of about 10 metres, however, have been discounted because these require a contribution from the East Antarctic Ice Sheet.sup.3, which has been argued to have remained stable for millions of years before and includes MIS11.sup.4,5. Here we show how the evolution of .sup.234U enrichment within the subglacial waters of East Antarctica recorded the ice sheet's response to MIS11 warming. Within the Wilkes Basin, subglacial chemical precipitates of opal and calcite record accumulation of .sup.234U (the product of rock-water contact within an isolated subglacial reservoir) up to 20 times higher than that found in marine waters. The timescales of .sup.234U enrichment place the inception of this reservoir at MIS11. Informed by the .sup.234U cycling observed in the Laurentide Ice Sheet, where .sup.234U accumulated during periods of ice stability.sup.6 and was flushed to global oceans in response to deglaciation.sup.7, we interpret our East Antarctic dataset to represent ice loss within the Wilkes Basin at MIS11. The .sup.234U accumulation within the Wilkes Basin is also observed in the McMurdo Dry Valleys brines.sup.8-10, indicating.sup.11 that the brine originated beneath the adjacent East Antarctic Ice Sheet. The marine origin of brine salts.sup.10 and bacteria.sup.12 implies that MIS11 ice loss was coupled with marine flooding. Collectively, these data indicate that during one of the warmest Pleistocene interglacials, the ice sheet margin at the Wilkes Basin retreated to near the precipitate location, about 700 kilometres inland from the current position of the ice margin, which--assuming current ice volumes--would have contributed about 3 to 4 metres.sup.13 to global sea levels. Uranium isotopes in subglacial precipitates from the Wilkes Basin of the East Antarctic Ice Sheet reveal ice retreat during a warm Pleistocene interglacial period about 400,000 years ago.

Author(s): J. Labidi [sup.1] [sup.11] , P. H. Barry [sup.2] , D. V. Bekaert [sup.3] , M. W. Broadley [sup.3] , B. Marty [sup.3] , T. Giunta [sup.4] , O. [...], Nitrogen is the main constituent of the Earth's atmosphere, but its provenance in the Earth's mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth's accretion versus that subducted from the Earth's surface is unclear.sup.1-6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare .sup.15N.sup.15N isotopologue of N.sub.2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle [delta].sup.15N (the fractional difference in .sup.15N/.sup.14N from air), N.sub.2/.sup.36Ar and N.sub.2/.sup.3He. Our results show that negative [delta].sup.15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen.sup.7-10, in fact represent dominantly air-derived N.sub.2 that experienced .sup.15N/.sup.14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the .sup.15N.sup.15N data allow extrapolations that characterize mantle endmember [delta].sup.15N, N.sub.2/.sup.36Ar and N.sub.2/.sup.3He values. We show that the Eifel region has slightly increased [delta].sup.15N and N.sub.2/.sup.36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts.sup.11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has [delta].sup.15N values substantially greater than that of the convective mantle, resembling surface components.sup.12-15, its N.sub.2/.sup.36Ar and N.sub.2/.sup.3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume [delta].sup.15N values may both be dominantly primordial features. A rare nitrogen isotopologue is used to detect contamination by air in volcanic gas effusions, and thereby derive the isotopic compositions of mantle endmembers.

Author(s): Peter U. Clark [sup.1] [sup.2] , Feng He [sup.3] , Nicholas R. Golledge [sup.4] [sup.5] , Jerry X. Mitrovica [sup.6] , Andrea Dutton [sup.7] [sup.10] , Jeremy S. Hoffman [...], Sea-level histories during the two most recent deglacial-interglacial intervals show substantial differences.sup.1-3 despite both periods undergoing similar changes in global mean temperature.sup.4,5 and forcing from greenhouse gases.sup.6. Although the last interglaciation (LIG) experienced stronger boreal summer insolation forcing than the present interglaciation.sup.7, understanding why LIG global mean sea level may have been six to nine metres higher than today has proven particularly challenging.sup.2. Extensive areas of polar ice sheets were grounded below sea level during both glacial and interglacial periods, with grounding lines and fringing ice shelves extending onto continental shelves.sup.8. This suggests that oceanic forcing by subsurface warming may also have contributed to ice-sheet loss.sup.9-12 analogous to ongoing changes in the Antarctic.sup.13,14 and Greenland.sup.15 ice sheets. Such forcing would have been especially effective during glacial periods, when the Atlantic Meridional Overturning Circulation (AMOC) experienced large variations on millennial timescales.sup.16, with a reduction of the AMOC causing subsurface warming throughout much of the Atlantic basin.sup.9,12,17. Here we show that greater subsurface warming induced by the longer period of reduced AMOC during the penultimate deglaciation can explain the more-rapid sea-level rise compared with the last deglaciation. This greater forcing also contributed to excess loss from the Greenland and Antarctic ice sheets during the LIG, causing global mean sea level to rise at least four metres above modern levels. When accounting for the combined influences of penultimate and LIG deglaciation on glacial isostatic adjustment, this excess loss of polar ice during the LIG can explain much of the relative sea level recorded by fossil coral reefs and speleothems at intermediate- and far-field sites. A reduction in the strength of the Atlantic Meridional Overturning Circulation initiated during the penultimate deglaciation led to excess polar ice losses, contributing to higher sea levels during the last interglacial period.

Author(s): G. R. Grant [sup.1] [sup.2] , T. R. Naish [sup.1] , G. B. Dunbar [sup.1] , P. Stocchi [sup.3] , M. A. Kominz [sup.4] , P. J. J. Kamp [...], Earth is heading towards a climate that last existed more than three million years ago (Ma) during the 'mid-Pliocene warm period'.sup.1, when atmospheric carbon dioxide concentrations were about 400 parts per million, global sea level oscillated in response to orbital forcing.sup.2,3 and peak global-mean sea level (GMSL) may have reached about 20 metres above the present-day value.sup.4,5. For sea-level rise of this magnitude, extensive retreat or collapse of the Greenland, West Antarctic and marine-based sectors of the East Antarctic ice sheets is required. Yet the relative amplitude of sea-level variations within glacial-interglacial cycles remains poorly constrained. To address this, we calibrate a theoretical relationship between modern sediment transport by waves and water depth, and then apply the technique to grain size in a continuous 800-metre-thick Pliocene sequence of shallow-marine sediments from Whanganui Basin, New Zealand. Water-depth variations obtained in this way, after corrections for tectonic subsidence, yield cyclic relative sea-level (RSL) variations. Here we show that sea level varied on average by 13 [plus or minus] 5 metres over glacial-interglacial cycles during the middle-to-late Pliocene (about 3.3-2.5 Ma). The resulting record is independent of the global ice volume proxy.sup.3 (as derived from the deep-ocean oxygen isotope record) and sea-level cycles are in phase with 20-thousand-year (kyr) periodic changes in insolation over Antarctica, paced by eccentricity-modulated orbital precession.sup.6 between 3.3 and 2.7 Ma. Thereafter, sea-level fluctuations are paced by the 41-kyr period of cycles in Earth's axial tilt as ice sheets stabilize on Antarctica and intensify in the Northern Hemisphere.sup.3,6. Strictly, we provide the amplitude of RSL change, rather than absolute GMSL change. However, simulations of RSL change based on glacio-isostatic adjustment show that our record approximates eustatic sea level, defined here as GMSL unregistered to the centre of the Earth. Nonetheless, under conservative assumptions, our estimates limit maximum Pliocene sea-level rise to less than 25 metres and provide new constraints on polar ice-volume variability under the climate conditions predicted for this century. Sea level varied by 13 [plus or minus] 5 metres on average, but up to 25 metres, over glacial-interglacial cycles during the Pliocene epoch, due to partial collapses of Antarctic Ice Sheets.

Author(s): Oana A. Dumitru [sup.1] , Jacqueline Austermann [sup.2] , Victor J. Polyak [sup.3] , Joan J. Fornós [sup.4] , Yemane Asmerom [sup.3] , Joaquín Ginés [sup.4] , Angel Ginés [...], Reconstructing the evolution of sea level during past warmer epochs such as the Pliocene provides insight into the response of sea level and ice sheets to prolonged warming.sup.1. Although estimates of the global mean sea level (GMSL) during this time do exist, they vary by several tens of metres.sup.2-4, hindering the assessment of past and future ice-sheet stability. Here we show that during the mid-Piacenzian Warm Period, which was on average two to three degrees Celsius warmer than the pre-industrial period.sup.5, the GMSL was about 16.2 metres higher than today owing to global ice-volume changes, and around 17.4 metres when thermal expansion of the oceans is included. During the even warmer Pliocene Climatic Optimum (about four degrees Celsius warmer than pre-industrial levels).sup.6, our results show that the GMSL was 23.5 metres above the present level, with an additional 1.6 metres from thermal expansion. We provide six GMSL data points, ranging from 4.39 to 3.27 million years ago, that are based on phreatic overgrowths on speleothems from the western Mediterranean (Mallorca, Spain). This record is unique owing to its clear relationship to sea level, its reliable U-Pb ages and its long timespan, which allows us to quantify uncertainties on potential uplift. Our data indicate that ice sheets are very sensitive to warming and provide important calibration targets for future ice-sheet models.sup.7. Using phreatic overgrowths on speleothems, sea level during the mid-Piacenzian Warm Period, which was about two to three degrees Celsius warmer than the pre-industrial period, is shown to have been about 16 metres higher than today.

Author(s): Henri Samuel [sup.1] , Philippe Lognonné [sup.1] , Mark Panning [sup.2] , Valéry Lainey [sup.2] Author Affiliations: (1) Institut de Physique du Globe de Paris, CNRS, Université Sorbonne Paris [...], The evolution and internal structure of Mars are, by comparison to its present-day surface, poorly known--although evidence of recent volcanic activity.sup.1 suggests that its deep interior remains hot and convectively cooling. The cooling rate of Mars is related to its early thermal state and to its rheology, which determines its ability to deform and to dynamically evolve.sup.2. Attempts to reconstruct the dynamic history of Mars and reveal its present-day structure, by combining the study of thermal evolution with surface observations, are limited by the interplay between several key quantities--including temperature, composition and rheology. Here we show that by considering Phobos (the closest satellite of Mars)--the orbital evolution of which is governed by the thermochemical history of Mars, through tidal interactions--we can gain insight into the thermal history and rheology of the planet. We investigated the long-term evolution of the main envelopes of Mars; these comprise a liquid metallic core that is overlain by a homogeneous silicate convecting mantle underneath an evolving heterogeneous lithospheric lid that includes a crust enriched in radiogenic elements. By exploiting the relationship between Mars and Phobos within an established in situ scenario for the early origin of the moons of Mars.sup.3, we find that--initially--Mars was moderately hotter (100 to 200 kelvin) than it is today, and that its mantle sluggishly deforms in the dislocation creep regime. This corresponds to a reference viscosity of 10.sup.22.2 [plus or minus] 0.5 pascal seconds and to a moderate to relatively weak intrinsic sensitivity of viscosity to temperature and pressure. Our approach predicts a present-day average crustal thickness of 40 [plus or minus] 25 kilometres and a surface heat flow of 20 [plus or minus] 1 milliwatts per square metre. We show that combining these predictions with data from future and ongoing space missions--such as InSight--could reduce uncertainties in Martian thermal and rheological histories, and help to uncover the origin of Phobos. Insights into the thermal and rheological history of Mars are generated by considering the interplay between the thermochemistry of the planet and the orbital evolution of its closest satellite, Phobos.

Author(s): Christopher G. Piecuch [sup.1] , Peter Huybers [sup.2] , Carling C. Hay [sup.3] , Andrew C. Kemp [sup.4] , Christopher M. Little [sup.5] , Jerry X. Mitrovica [sup.2] , [...], Identifying the causes of historical trends in relative sea level--the height of the sea surface relative to Earth's crust--is a prerequisite for predicting future changes. Rates of change along the eastern coast of the USA (the US East Coast) during the past century were spatially variable, and relative sea level rose faster along the Mid-Atlantic Bight than along the South Atlantic Bight and the Gulf of Maine. Past studies suggest that Earth's ongoing response to the last deglaciation.sup.1-5, surface redistribution of ice and water.sup.5-9 and changes in ocean circulation.sup.9-13 contributed considerably to this large-scale spatial pattern. Here we analyse instrumental data.sup.14,15 and proxy reconstructions.sup.4,12 using probabilistic methods.sup.16-18 to show that vertical motions of Earth's crust exerted the dominant control on regional spatial differences in relative sea-level trends along the US East Coast during 1900-2017, explaining most of the large-scale spatial variance. Rates of coastal subsidence caused by ongoing relaxation of the peripheral forebulge associated with the last deglaciation are strongest near North Carolina, Maryland and Virginia. Such structure indicates that Earth's elastic lithosphere is thicker than has been assumed in other models.sup.19-22. We also find a substantial coastal gradient in relative sea-level trends over this period that is unrelated to deglaciation and suggests contributions from twentieth-century redistribution of ice and water. Our results indicate that the majority of large-scale spatial variation in long-term rates of relative sea-level rise on the US East Coast is due to geological processes that will persist at similar rates for centuries.Vertical motions of Earth's crust had the greatest effect on regional spatial differences in relative sea-level trends along the eastern coast of the USA during 1900-2017, explaining most of the large-scale spatial variance in regional rates of sea-level rise.

Author(s): David Parkes [sup.1] [sup.2] , Ben Marzeion [sup.2] Author Affiliations: (1) Institute of Atmospheric and Cryospheric Sciences, University of Innsbruck, Innsbruck, Austria (2) Institute of Geography, University of Bremen, [...], Global-mean sea-level rise (GMSLR) during the twentieth century was primarily caused by glacier and ice-sheet mass loss, thermal expansion of ocean water and changes in terrestrial water storage.sup.1. Whether based on observations.sup.2 or results of climate models.sup.3,4, however, the sum of estimates of each of these contributions tends to fall short of the observed GMSLR. Current estimates of the glacier contribution to GMSLR rely on the analysis of glacier inventory data, which are known to undersample the smallest glacier size classes.sup.5,6. Here we show that from 1901 to 2015, missing and disappeared glaciers produced a sea-level equivalent (SLE) of approximately 16.7 to 48.0 millimetres. Missing glaciers are those small glaciers that we expect to exist today, owing to regional analyses and theoretical scaling relationships, but that are not represented in the inventories. These glaciers contributed approximately 12.3 to 42.7 millimetres to the historical SLE. Additionally, disappeared glaciers (those that existed in 1901 but had melted away by 2015, and that therefore cannot be included in modern global glacier inventories) made an estimated contribution of between 4.4 and 5.3 millimetres. Failure to consider these uncharted glaciers may be an important cause of difficulties in closing the GMSLR budget during the twentieth century: their contribution is on average between 0.17 and 0.53 millimetres of SLE per year, compared to a budget discrepancy of about 0.5 millimetres of GMSLR per year between 1901 and 1990. Although the uncharted glaciers will have a minimal role in sea-level rise in the future, and are less important after 1990, these findings imply that undiscovered physical processes are not required to close the historical sea-level budget. From 1901 to 2015, missing and disappeared glaciers produced a rise in sea level that may enable the historical budget for global-mean sea-level rise to be closed without recourse to an undiscovered physical process.

Author(s): Yusuke Yokoyama [sup.1] [sup.2] [sup.3] , Tezer M. Esat [sup.4] [sup.5] , William G. Thompson [sup.6] , Alexander L. Thomas [sup.7] , Jody M. Webster [sup.8] , Yosuke Miyairi [...], The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth's recent climate history.sup.1. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower.sup.2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago.sup.4 and was followed by about 10,000 years of rapid deglaciation into the Holocene.sup.1. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to -118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago.sup.1,5, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth's response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about -125 to -130 metres.Details of the drops in sea level associated with glaciation during the Last Glacial Maximum are revealed using a coral proxy from the Great Barrier Reef.

Author(s): J. Kingslake [sup.1] , R. P. Scherer [sup.2] , T. Albrecht [sup.3] , J. Coenen [sup.2] , R. D. Powell [sup.2] , R. Reese [sup.3] , N. D. Stansell [...], To predict the future contributions of the Antarctic ice sheets to sea-level rise, numerical models use reconstructions of past ice-sheet retreat after the Last Glacial Maximum to tune model parameters.sup.1. Reconstructions of the West Antarctic Ice Sheet have assumed that it retreated progressively throughout the Holocene epoch (the past 11,500 years or so).sup.2-4. Here we show, however, that over this period the grounding line of the West Antarctic Ice Sheet (which marks the point at which it is no longer in contact with the ground and becomes a floating ice shelf) retreated several hundred kilometres inland of today's grounding line, before isostatic rebound caused it to re-advance to its present position. Our evidence includes, first, radiocarbon dating of sediment cores recovered from beneath the ice streams of the Ross Sea sector, indicating widespread Holocene marine exposure; and second, ice-penetrating radar observations of englacial structure in the Weddell Sea sector, indicating ice-shelf grounding. We explore the implications of these findings with an ice-sheet model. Modelled re-advance of the grounding line in the Holocene requires ice-shelf grounding caused by isostatic rebound. Our findings overturn the assumption of progressive retreat of the grounding line during the Holocene in West Antarctica, and corroborate previous suggestions of ice-sheet re-advance.sup.5. Rebound-driven stabilizing processes were apparently able to halt and reverse climate-initiated ice loss. Whether these processes can reverse present-day ice loss.sup.6 on millennial timescales will depend on bedrock topography and mantle viscosity--parameters that are difficult to measure and to incorporate into ice-sheet models.Radiocarbon dating of sediment cores and ice-penetrating radar observations are used to demonstrate that the West Antarctic Ice Sheet has not retreated progressively during the Holocene epoch, but has instead showed periods of retreat and re-advance.

Author(s): Jeremy D. Shakun [sup.1] , Lee B. Corbett [sup.2] , Paul R. Bierman [sup.2] , Kristen Underwood [sup.3] , Donna M. Rizzo [sup.3] , Susan R. Zimmerman [sup.4] , [...], The East Antarctic Ice Sheet (EAIS) is the largest potential contributor to sea-level rise. However, efforts to predict the future evolution of the EAIS are hindered by uncertainty in how it responded to past warm periods, for example, during the Pliocene epoch (5.3 to 2.6 million years ago), when atmospheric carbon dioxide concentrations were last higher than 400 parts per million. Geological evidence indicates that some marine-based portions of the EAIS and the West Antarctic Ice Sheet retreated during parts of the Pliocene.sup.1,2, but it remains unclear whether ice grounded above sea level also experienced retreat. This uncertainty persists because global sea-level estimates for the Pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss.sup.3, and also because direct geological evidence bearing on past ice retreat on land is lacking. Here we show that land-based sectors of the EAIS that drain into the Ross Sea have been stable throughout the past eight million years. We base this conclusion on the extremely low concentrations of cosmogenic .sup.10Be and .sup.26Al isotopes found in quartz sand extracted from a land-proximal marine sediment core. This sediment had been eroded from the continent, and its low levels of cosmogenic nuclides indicate that it experienced only minimal exposure to cosmic radiation, suggesting that the sediment source regions were covered in ice. These findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread or long-lasting meltback of the EAIS margin onto land. We suggest that variations in Antarctic ice volume in response to the range of global temperatures experienced over this period--up to 2-3 degrees Celsius above preindustrial temperatures.sup.4, corresponding to future scenarios involving carbon dioxide concentrations of between 400 and 500 parts per million--were instead driven mostly by the retreat of marine ice margins, in agreement with the latest models.sup.5,6.Analysis of cosmogenic isotopes from a marine sediment core shows that much of the land-based East Antarctic Ice Sheet has remained stable for the past eight million years, including during the warm Pliocene epoch.

Author(s): M. Rodell [sup.1] , J. S. Famiglietti [sup.2] [sup.5] , D. N. Wiese [sup.2] , J. T. Reager [sup.2] , H. K. Beaudoing [sup.1] [sup.3] , F. W. Landerer [...], Freshwater availability is changing worldwide. Here we quantify 34 trends in terrestrial water storage observed by the Gravity Recovery and Climate Experiment (GRACE) satellites during 2002-2016 and categorize their drivers as natural interannual variability, unsustainable groundwater consumption, climate change or combinations thereof. Several of these trends had been lacking thorough investigation and attribution, including massive changes in northwestern China and the Okavango Delta. Others are consistent with climate model predictions. This observation-based assessment of how the world's water landscape is responding to human impacts and climate variations provides a blueprint for evaluating and predicting emerging threats to water and food security.Analysis of 2002-2016 GRACE satellite observations of terrestrial water storage reveals substantial changes in freshwater resources globally, which are driven by natural and anthropogenic climate variability and human activities.

Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics--including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs--that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years. Climate and ice-sheet modelling that includes ice fracture dynamics reveals that Antarctica could contribute more than a metre of sea-level rise by 2100 and more than 13 metres by 2500, if greenhouse gas emissions continue unabated. A 500-year model of Antarctica's contribution to future sea-level rise Robert DeConto and David Pollard use a newly improved numerical ice-sheet model calibrated to Pliocene and Last Interglacial sea-level estimates to develop projections of Antarctica's evolution over the next five centuries, driven by a range of greenhouse gas scenarios. The modelling shows that the Antarctic ice sheet has the potential to contribute between almost nothing, to contributing more than a metre of sea-level rise by 2100 and more than 15 metres by 2500. The startling high-end estimate arises from unabated emissions and previously underappreciated mechanisms: ice-fracturing by surface meltwater and collapse of large ice cliffs. The low end shows that a scenario of strong climate mitigation can radically reduce societal exposure to higher sea levels., Author(s): Robert M. DeConto [sup.1] , David Pollard [sup.2] Author Affiliations: (1) Department of Geosciences, University of Massachusetts, Amherst, USA (2) Earth and Environmental Systems Institute, Pennsylvania State University, University [...]

Author(s): M. A. Siegler [sup.1] [sup.2] , R. S. Miller [sup.3] , J. T. Keane [sup.4] , M. Laneuville [sup.5] , D. A. Paige [sup.6] , I. Matsuyama [sup.4] , [...], Polar hydrogen deposits on the Moon provide evidence that its spin axis has shifted; analysis of the locations of these deposits and of the lunar figure suggests that the shift occurred as a result of changes in the Moon's moments of inertia caused by a low-density thermal anomaly beneath the Procellarum region. Polar wandering on the Moon Matthew Siegler and co-authors show that the polar hydrogen deposits detected on the Moon by orbital neutron spectroscopy pose something of a mystery: the spatial distribution of hydrogen, which is thought to arise from the presence of water ice, does not match that expected from present-day lunar temperatures. The explanation may lie in the phenomenon known as 'true polar wander', in which a reference point on a solid body rotates with respect to its spin axis. Based on an analysis of polar deposit locations and of the lunar figure, the authors suggest that the shift occurred as a result of changes in the Moon's moment of inertia caused by a low-density thermal anomaly beneath the Procellarum region. Procellarum was most geologically active early in lunar history, which would imply that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. The earliest dynamic and thermal history of the Moon is not well understood. The hydrogen content of deposits near the lunar poles may yield insight into this history, because these deposits (which are probably composed of water ice) survive only if they remain in permanent shadow. If the orientation of the Moon has changed, then the locations of the shadowed regions will also have changed. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers.sup.1,2,3, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures.sup.4,5. This finding is in contrast to the distribution of volatiles observed in similar thermal environments at Mercury's poles.sup.6. Here we show that polar hydrogen preserves evidence that the spin axis of the Moon has shifted: the hydrogen deposits are antipodal and displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and from analysis of the moments of inertia of the Moon, we hypothesize that this change in the spin axis, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this region resulted in the bulk of lunar mare volcanism.sup.7,8,9,10,11 and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This thermal anomaly still exists and, in part, controls the current orientation of the Moon. The Procellarum region was most geologically active early in lunar history.sup.7,8,9, which implies that polar wander initiated billions of years ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner Solar System. Our hypothesis provides an explanation for the antipodal distribution of lunar polar hydrogen, and connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early Solar System.

A synthesis of new and existing data allows Heinrich Stadial 11 (HS11), a prominent Northern Hemisphere cold event, to be linked to the timing of peak sea-level rise during glacial termination T-II, whereas rapid sea-level rise in T-I is shown to clearly post-date Heinrich Stadial 1, so fundamentally different mechanisms seem to be at work during glacial terminations. Anatomy of a glacial termination A central goal of palaeoclimate research is that of deciphering the mechanisms responsible for major state shifts in the Earth system, such as between glacial and interglacial conditions. This has proven difficult enough even for the last glacial termination (T-I), much less termination II (T-II), which ended glacial conditions about 130,000 years ago. Gianluca Marino et al. use new and existing data to demonstrate a link, within uncertainties, between Heinrich Stadial 11 (HS11) -- a prominent Northern Hemisphere cold event -- and the timing of peak sea level rise during T-II. A strong Southern Hemisphere warming also occurred during HS11, consistent with the idea of a bipolar seesaw that would probably have promoted Antarctic ice sheet melting. In contrast, rapid sea level rise in T-1 clearly postdated Heinrich Stadial 1. Possibly in response to differing CO.sub.2 and insolation conditions during T-I and T-II, fundamentally different mechanisms seem to be at work in triggering glacial terminations. Our current understanding of ocean-atmosphere-cryosphere interactions at ice-age terminations relies largely on assessments of the most recent (last) glacial-interglacial transition.sup.1,2,3, Termination I (T-I). But the extent to which T-I is representative of previous terminations remains unclear. Testing the consistency of termination processes requires comparison of time series of critical climate parameters with detailed absolute and relative age control. However, such age control has been lacking for even the penultimate glacial termination (T-II), which culminated in a sea-level highstand during the last interglacial period that was several metres above present.sup.4. Here we show that Heinrich Stadial 11 (HS11), a prominent North Atlantic cold episode.sup.5,6, occurred between 135 [plus or minus] 1 and 130 [plus or minus] 2 thousand years ago and was linked with rapid sea-level rise during T-II. Our conclusions are based on new and existing.sup.6,7,8,9 data for T-II and the last interglacial that we collate onto a single, radiometrically constrained chronology. The HS11 cold episode.sup.5,6 punctuated T-II and coincided directly with a major deglacial meltwater pulse, which predominantly entered the North Atlantic Ocean and accounted for about 70 per cent of the glacial-interglacial sea-level rise.sup.8,9. We conclude that, possibly in response to stronger insolation and CO.sub.2 forcing earlier in T-II, the relationship between climate and ice-volume changes differed fundamentally from that of T-I. In T-I, the major sea-level rise clearly post-dates.sup.3,10,11 Heinrich Stadial 1. We also find that HS11 coincided with sustained Antarctic warming, probably through a bipolar seesaw temperature response.sup.12, and propose that this heat gain at high southern latitudes promoted Antarctic ice-sheet melting that fuelled the last interglacial sea-level peak., Author(s): G. Marino [sup.1] , E. J. Rohling [sup.1] [sup.2] , L. Rodríguez-Sanz [sup.1] , K. M. Grant [sup.1] , D. Heslop [sup.1] , A. P. Roberts [sup.1] , J. [...]

Author(s): Tamsin L. Edwards [sup.1] , Mark A. Brandon [sup.2] , Gael Durand [sup.3] , Neil R. Edwards [sup.2] , Nicholas R. Golledge [sup.4] [sup.5] , Philip B. Holden [sup.2] [...], Predictions for sea-level rise this century due to melt from Antarctica range from zero to more than one metre. The highest predictions are driven by the controversial marine ice-cliff instability (MICI) hypothesis, which assumes that coastal ice cliffs can rapidly collapse after ice shelves disintegrate, as a result of surface and sub-shelf melting caused by global warming. But MICI has not been observed in the modern era and it remains unclear whether it is required to reproduce sea-level variations in the geological past. Here we quantify ice-sheet modelling uncertainties for the original MICI study and show that the probability distributions are skewed towards lower values (under very high greenhouse gas concentrations, the most likely value is 45 centimetres). However, MICI is not required to reproduce sea-level changes due to Antarctic ice loss in the mid-Pliocene epoch, the last interglacial period or 1992-2017; without it we find that the projections agree with previous studies (all 95th percentiles are less than 43 centimetres). We conclude that previous interpretations of these MICI projections over-estimate sea-level rise this century; because the MICI hypothesis is not well constrained, confidence in projections with MICI would require a greater range of observationally constrained models of ice-shelf vulnerability and ice-cliff collapse.By better quantifying uncertainties for marine ice-cliff instability, future Antarctic ice loss is predicted to be much lower than previously estimated.

Author(s): Nicholas R. Golledge [sup.1] [sup.2] , Elizabeth D. Keller [sup.2] , Natalya Gomez [sup.3] , Kaitlin A. Naughten [sup.4] , Jorge Bernales [sup.5] , Luke D. Trusel [sup.6] , [...], Government policies currently commit us to surface warming of three to four degrees Celsius above pre-industrial levels by 2100, which will lead to enhanced ice-sheet melt. Ice-sheet discharge was not explicitly included in Coupled Model Intercomparison Project phase 5, so effects on climate from this melt are not currently captured in the simulations most commonly used to inform governmental policy. Here we show, using simulations of the Greenland and Antarctic ice sheets constrained by satellite-based measurements of recent changes in ice mass, that increasing meltwater from Greenland will lead to substantial slowing of the Atlantic overturning circulation, and that meltwater from Antarctica will trap warm water below the sea surface, creating a positive feedback that increases Antarctic ice loss. In our simulations, future ice-sheet melt enhances global temperature variability and contributes up to 25 centimetres to sea level by 2100. However, uncertainties in the way in which future changes in ice dynamics are modelled remain, underlining the need for continued observations and comprehensive multi-model assessments.Increased meltwater from the Greenland and Antarctic ice sheets will slow the Atlantic overturning circulation and warm the subsurface ocean around Antarctica, further increasing Antarctic ice loss.

Author(s): Sean D. Willett [sup.1] , Scott W. McCoy [sup.2] , Helen W. Beeson [sup.2] Author Affiliations:(1) Department of Earth Sciences, ETH, Zurich, Switzerland(2) Department of Geological Sciences and Engineering, [...], Ecosystem diversity and human activity in dry climates depend not just on the magnitude of rainfall, but also on the landscape's ability to retain water. This is illustrated dramatically in the High Plains of North America, where despite the semi-arid modern and past climate, the hydrologic conditions are diverse. Large rivers sourced in the Rocky Mountains cut through elevated plains that exhibit limited river drainage but widespread surface water in the form of ephemeral (seasonal) playa lakes.sup.1, as well as extensive groundwater hosted in the High Plains aquifer of the Ogallala formations.sup.2. Here we present a model, with supporting evidence, which shows that the High Plains landscape is currently in a transient state, in which the landscape has bifurcated into an older region with an inefficient river network and a younger, more efficient, river channel network that is progressively cannibalizing the older region. The older landscape represents the remnants of the Ogallala sediments that once covered the entirety of the High Plains, forming depositional fans that buried the pre-existing river network and effectively 'repaved' the High Plains.sup.3-6. Today we are witnessing the establishment of a new river network that is dissecting the landscape, capturing channels and eroding these sediment fans. Through quantitative analysis of the geometry of the river network, we show how network reorganization has resulted in a distinctive pattern of erosion, whereby the largest rivers have incised the older surface, removed fan heads near the Rocky Mountains and eroded the fan toes, but left portions of the central fan surface and the Ogallala sediments largely intact. These preserved fan surfaces have poor surface water drainage, and thus retain ephemeral water for wetlands and groundwater recharge. Our findings suggest that the surface hydrology and associated ecosystems are transient features on million-year timescales, and reflect the mode of landscape evolution.The High Plains region of North America is in a transient state, with a younger, efficient network of river channels progressively cannibalizing an older, less efficient region, aiding water retention for wetlands and groundwater recharge.

Abstract Results from a new mid Holocene site in the central-western Torres Strait, north-eastern Australia are presented. AMS determinations from Dabangai on Mabuyag provide evidence for two settlement periods. Phase [...]

Way down south The recent discovery of active water-venting from the surface of Saturn's icy moon Enceladus has caused a ripple of excitement as it suggests the possibility of subsurface liquid water. Geologically, Enceladus poses problems. One, the reason why the hotspot of geological activity is centred precisely on the south pole, has been tackled by Francis Nimmo and Robert T. Pappalardo. They identify reorientation of Enceladus's rotation axis as the likely cause, and an upwelling of low-density material to the surface as the cause of that reorientation. Comparison of the distribution of impact craters on the 'leading' and 'trailing' hemispheres will be a good test of the reorientation part of the theory. Enceladus is a small icy satellite of Saturn. Its south polar region consists of young, tectonically deformed terrain and has an anomalously high heat flux.sup.1,2. This heat flux is probably due to localized tidal dissipation within either the ice shell.sup.3 or the underlying silicate core.sup.4. The surface deformation is plausibly due to upwelling of low-density material (diapirism.sup.5) as a result of this tidal heating. Here we show that the current polar location of the hotspot can be explained by reorientation of the satellite's rotation axis because of the presence of a low-density diapir. If the diapir is in the ice shell, then the shell must be relatively thick and maintain significant rigidity (elastic thickness greater than ~0.5 km); if the diapir is in the silicate core, then Enceladus cannot possess a global subsurface ocean, because the core must be coupled to the overlying ice for reorientation to occur. The reorientation generates large (~10 MPa) tectonic stress patterns.sup.6 that are compatible with the observed deformation of the south polar region.sup.2. We predict that the distribution of impact craters on the surface will not show the usual leading hemisphere-trailing hemisphere asymmetry. A low-density diapir also yields a potentially observable negative gravity anomaly., Author(s): Francis Nimmo [sup.1] , Robert T. Pappalardo [sup.2] [sup.3] Author Affiliations: (1) Department of Earth Sciences, University of California Santa Cruz, Santa Cruz, USA (2) Laboratory for Atmospheric and [...]

It is difficult to obtain a dynamic picture of the Earth's mantle. A study involving geophysical observations with geochemical implications shows that compositionally distinct megablobs contribute to the ebb and flow., Author(s): Michael Manga [sup.1] Author Affiliations: (1) Department of Earth and Planetary Sciences, University of California, Berkeley, USA The slow and continual deformation of the Earth's mantle is seen in [...]

Researchers struggle to project how fast, how high and how far the oceans will rise., Author(s): Nicola Jones [sup.1] Author Affiliations: (1) Nicola Jones is a freelance writer near Vancouver, Canada., PETE RYAN/NATL GEOGRAPHIC/GETTY The world's leading climate scientists kicked up a storm in 2007, [...]