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Antarctic sea-ice forms a complex and dynamic system that drives many ecological processes in the Southern Ocean. Sea-ice microalgae and their associated microbial communities are understood to influence nutrient flow and allocation in marine polar environments. Sea-ice microalgae and their microbiota can have high seasonal and regional (>1000 km2) compositional and abundance variation, driven by factors modulating their growth, symbiotic interactions and function. In contrast, our knowledge of small-scale variation in these communities is limited. Understanding variation across multiple scales and its potential drivers is critical for informing on how multiple stressors impact sea-ice communities and the functions they provide. Here, we characterized bacterial communities associated with sea-ice microalgae and the potential drivers that influence their variation across a range of spatial scales (metres to >10 kms) in a previously understudied area in Commonwealth Bay, East Antarctica where anomalous events have substantially and rapidly expanded local sea-ice coverage. We found a higher abundance and different composition of bacterial communities living in sea-ice microalgae closer to the shore compared to those further from the coast. Variation in community structure increased linearly with distance between samples. Ice thickness and depth to the seabed were found to be poor predictors of these communities. Further research on the small-scale environmental drivers influencing these communities is needed to fully understand how large-scale regional events can affect local function and ecosystem processes.

Our study on the exact timing and the potential climatic, environmental, and evolutionary consequences of the Laschamps Geomagnetic Excursion has generated the hypothesis that geomagnetism represents an unrecognized driver in environmental and evolutionary change. It is important for this hypothesis to be tested with new data, and encouragingly, none of the studies presented by Picin et al . undermine our model.

Beneficiaries prefer renewable energy-based systems over grid-connected electricity. The cost of energy is viewed as the most influential factor while choosing renewable energy systems. Beneficiaries chose to stay linked with renewa­ble energy systems even when they received grid-connected electricity at a lower tariff.Net-metering and feed-in tariff mechanisms, as well as tax cuts and subsidies for renewable energy projects, could be the catalyst for fostering greater uptake of renewables in the electricity generation mix.

The sub-Antarctic islands of New Zealand are biodiversity hotspots in the Southern Ocean, containing numerous endemic species and providing breeding grounds for seabirds and marine mammals. However, due to their remoteness and harsh environments, many of their marine ecosystems are relatively unexplored and potentially at risk from alien invasive species. To better understand nearshore marine ecosystems at three New Zealand sub-Antarctic islands (Snares Islands, Auckland Islands and Campbell Island), we sampled nektobenthic fish and mobile macroinvertebrates at 40 sites (15–20 m depth) using baited remote underwater videos (BRUVs). MaxN of each species in the videos was recorded in 5-min intervals for 45 min, allowing analyses of MaxN over the whole deployment, as well as change through time during the deployment. Species distributions appeared to reflect both the geomorphological and biogeographic traits of the islands. The Auckland Islands and Campbell Island contain large inlets dominated by mobile crustaceans, and biological trends followed gradients in marine exposure along inlets. In contrast, the Snares Islands are mostly exposed coast and contained a higher diversity of fish species common with mainland New Zealand. We suggest that differences in nearshore marine communities between these islands are likely due to the combined effects of habitat availability, biogeography, and ocean temperature.

The Antarctic Ice Sheet (AIS) response to past warming consistent with the 1.5–2°C “safe limit” of the United Nations Paris Agreement is currently not well known. Empirical evidence from the most recent comparable period, the Last Interglaciation, is sparse, and transient ice-sheet experiments are few and inconsistent. Here, we present new, transient, GCM-forced ice-sheet simulations validated against proxy reconstructions. This is the first time such an evaluation has been attempted. Our empirically constrained simulations indicate that the AIS contributed 4 m to global mean sea level by 126 ka BP, with ice lost primarily from the Amundsen, but not Ross or Weddell Sea, sectors. We resolve the conflict between previous work and show that the AIS thinned in the Wilkes Subglacial Basin but did not retreat. We also find that the West AIS may be predisposed to future collapse even in the absence of further environmental change, consistent with previous studies.

Emerging ice-sheet modeling suggests once initiated, retreat of the Antarctic Ice Sheet (AIS) can continue for centuries. Unfortunately, the short observational record cannot resolve the tipping points, rate of change, and timescale of responses. Iceberg-rafted debris data from Iceberg Alley identify eight retreat phases after the Last Glacial Maximum that each destabilized the AIS within a decade, contributing to global sea-level rise for centuries to a millennium, which subsequently re-stabilized equally rapidly. This dynamic response of the AIS is supported by (i) a West Antarctic blue ice record of ice-elevation drawdown >600 m during three such retreat events related to globally recognized deglacial meltwater pulses, (ii) step-wise retreat up to 400 km across the Ross Sea shelf, (iii) independent ice sheet modeling, and (iv) tipping point analysis. Our findings are consistent with a growing body of evidence suggesting the recent acceleration of AIS mass loss may mark the beginning of a prolonged period of ice sheet retreat and substantial global sea level rise.

Satellite observations offering detailed records of global environmental change are only available from 1979. Emerging studies combining high-quality instrumental and natural observations highlight that the Earth system experienced a substantial shift across the mid-20th century, one that appears to have taken place before the Great Acceleration of human activities from the 1950s. These new results have far-reaching implications for understanding ice-ocean-atmospheric interactions in the Anthropocene and highlight the urgent need for drastic cuts in carbon emissions to limit the impact of future warming.

Reversing the field Do terrestrial geomagnetic field reversals have an effect on Earth's climate? Cooper et al. created a precisely dated radiocarbon record around the time of the Laschamps geomagnetic reversal about 41,000 years ago from the rings of New Zealand swamp kauri trees. This record reveals a substantial increase in the carbon-14 content of the atmosphere culminating during the period of weakening magnetic field strength preceding the polarity switch. The authors modeled the consequences of this event and concluded that the geomagnetic field minimum caused substantial changes in atmospheric ozone concentration that drove synchronous global climate and environmental shifts. Science , this issue p. 811

The Antarctic Ice Sheet (AIS) is out of equilibrium with the current anthropogenic‐enhanced climate forcing. Paleoenvironmental records and ice sheet models reveal that the AIS has been tightly coupled to the climate system during the past and indicate the potential for accelerated and sustained Antarctic ice mass loss into the future. Modern observations by contrast suggest that the AIS has only just started to respond to climate change in recent decades. The maximum projected sea level contribution from Antarctica to 2100 has increased significantly since the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report, although estimates continue to evolve with new observational and theoretical advances. This review brings together recent literature highlighting the progress made on the known processes and feedbacks that influence the stability of the AIS. Reducing the uncertainty in the magnitude and timing of the future sea level response to AIS change requires a multidisciplinary approach that integrates knowledge of the interactions between the ice sheet, solid Earth, atmosphere, and ocean systems and across time scales of days to millennia. We start by reviewing the processes affecting AIS mass change, from atmospheric and oceanic processes acting on short time scales (days to decades), through to ice processes acting on intermediate time scales (decades to centuries) and the response to solid Earth interactions over longer time scales (decades to millennia). We then review the evidence of AIS changes from the Pliocene to the present and consider the projections of global sea level rise and their consequences. We highlight priority research areas required to improve our understanding of the processes and feedbacks governing AIS change.

The Southern Ocean occupies 14% of the Earth’s surface and plays a fundamental role in the global carbon cycle and climate. It provides a direct connection to the deep ocean carbon reservoir through biogeochemical processes that include surface primary productivity, remineralization at depth and the upwelling of carbon-rich water masses. However, the role of these different processes in modulating past and future air–sea carbon flux remains poorly understood. A key period in this regard is the Antarctic Cold Reversal (ACR, 14.6–12.7 kyr bp), when mid- to high-latitude Southern Hemisphere cooling coincided with a sustained plateau in the global deglacial increase in atmospheric CO2. Here we reconstruct high-latitude Southern Ocean surface productivity from marine-derived aerosols captured in a highly resolved horizontal ice core. Our multiproxy reconstruction reveals a sustained signal of enhanced marine productivity across the ACR. Transient climate modelling indicates this period coincided with maximum seasonal variability in sea-ice extent, implying that sea-ice biological feedbacks enhanced CO2 sequestration and created a substantial regional marine carbon sink, which contributed to the plateau in CO2 during the ACR. Our results highlight the role Antarctic sea ice plays in controlling global CO2, and demonstrate the need to incorporate such feedbacks into climate–carbon models.

Subglacial glaciohydraulic supercooling can form basal ice and affect glacier dynamics, sediment transfer and geomorphology. Whilst isotopic analysis (δ18O-δD) of basal ice has demonstrated the significance of supercooling, questions remain as to what extent the identification of supercooling depends on sampling resolution. We conducted laboratory experiments in which ice was frozen from supercooled water and sampled at a micro-scale (1.5 millilitre) to identify highly localized variations in isotopic compositions that might be lost in bulk-scale sampling. Three distinctive ice facies produced by the freezing process demonstrated diagnostic isotopic signatures that were distinguished when the facies were sampled independently. However, their respective isotopic signatures were lost when bulk-scale sampling combined the two facies, demonstrating the requirement of micro-scale sampling when identifying supercooling in basal ice facies. These findings indicate that sampling for isotopic compositions of ice facies frozen from supercooled water should be conducted at a scale that prevents the amalgamation of different facies to highlight a detailed isotopic signature. We conclude that micro-scale sampling is imperative to understanding and quantifying this subglacial process.

Irreversible shifts of large-scale components of the Earth system (so-called ‘tipping elements’) on policy-relevant timescales are a major source of uncertainty for projecting the impacts of future climate change. The high latitudes are particularly vulnerable to positive feedbacks that amplify change through atmosphere-ocean-ice interactions. Unfortunately, the short instrumental record does not capture the full range of past or projected climate scenarios (a situation particularly acute in the high latitudes). Natural archives from past periods warmer than present day, however, can be used to explore drivers and responses to forcing, and provide data against which to test models, thereby offering insights into the future. The Last Interglacial (129–116,000 years before present) — the warmest interglacial of the last 800,000 years — was the most recent period during which global temperatures were comparable with low-end 21st Century projections (up to 2 °C warmer, with temperature increase amplified over polar latitudes), providing a potentially useful analogue for future change. Substantial environmental changes happened during this time. Here we synthesise the nature and timing of potential high-latitude tipping elements during the Last Interglacial, including sea ice, extent of the boreal forest, permafrost, ocean circulation, and ice sheets/sea level. We also review the thresholds and feedbacks that likely operated through this period. Notably, substantial ice mass loss from Greenland, the West Antarctic, and possibly sectors of the East Antarctic drove a 6–9 m rise in global sea level. This was accompanied by reduced summer sea-ice extent, poleward-extended boreal forest, and reduced areas of permafrost. Despite current chronological uncertainties, we find that tipping elements in the high latitudes all experienced rapid and abrupt change (within 1–2 millennia of each other) across both hemispheres, while recovery to prior conditions took place over multi-millennia. Our synthesis demonstrates important feedback loops between tipping elements, amplifying polar and global change during the Last Interglacial. The high sensitivity and tight interconnections between polar tipping elements suggests that they could exhibit similar thresholds of vulnerability in the future, particularly if the aspirations of the Paris Agreement are not met.

The UN Sustainable Development Goals provide a framework towards a more sustainable future. Although each goal can be targeted separately, the greatest benefit is to be had in ensuring that projects exploit synergies between different goals, are developed with an interdisciplinary perspective, and integrate different stakeholders across academia, business, government, NGOs, and communities.In combination, Renewable Energy Systems (RES) and distributed ‘smart’ energy networks (SEN) provide opportunities to drive down CO2 emissions, clearly addressing SDG13, ‘climate action’. However, significant potential exists to positively contribute to a wider suite of goals as well as the potential to negatively impact other aspects. Addressing these tensions and opportunities requires development of a detailed understanding of the full societal, economic and environmental impacts of such developments.Such integrated renewable energy systems and smart energy networks are in the early stages of development. Taking a ‘living laboratory approach’ enables the development and live-testing of new energy systems, including the opportunity to consider full life cycle assessment impacts and benefits, as well as investigate and co-develop interactions with end-users. Here we outline the potential of one of Europe’s largest ‘at scale’ multi-vector smart energy systems, developed as a ‘living laboratory’ at Keele University in the UK, to demonstrate an integrated approach to addressing the UN’s SDGs through integrated RES-SEN systems. The scale and scope of the project provides the opportunity for the detailed analysis required to provide a model of a scalable, integrated RES-SEN approach as part of an evolving energy landscape, where multi-vector renewables, and distributed energy and storage provide new models for decarbonisation, whilst also contributing more widely to the UN’s SDGs.This project represents an ambitious and innovative demonstrator programme that brings together multiple stakeholders to explore the potential for addressing the core SDGs of ‘climate action’, ‘affordable and clean energy’, ‘sustainable cities and communities’, ‘decent work and economic growth’, and ‘industry, innovation and infrastructure’, while exploring the additional potential impacts and benefits to ‘quality education’, ‘life on land’ and ‘partnerships for the goal’. The programme of work focusses on technical developments, societal adoption and full economic life-cycle assessment, which combined are developing a blue print for the integration of RES-SEN technologies across the evolving energy landscape by working in partnership with key industrial and commercial partners to contribute to a wide array of the UN’s SDGs.

Current efforts to examine and quantify so-called ‘biomarkers’ present in polar ice samples offer exciting potential as biological and biochemical proxies for past climate and ocean dynamics. Here we present a new rapid and easily replicable method to provide measurements of the microscopic particulate content of ice samples from polar environments. Using an Amnis® Imagestream® Imaging Flow Cytometer, melted snow and ice samples from Patriot Hills in the Ellsworth Mountains, Antarctica were analysed for their particulate (biological and non-biological) content. Selective use of a nucleic acid stain pre-treatment allows for a straightforward gating strategy that resolves both autofluorescent and non-autofluorescent biological material in sample replicates. In the Patriot Hills samples this method clearly identifies marine picoplankton, along with non-biological particulates such as tephra and minerogenic material. Crucially, the 60x Brightfield images provided by the Imagestream offer a significant additional capability above standard flow cytometry systems; each object identified by the machine can be visually differentiated (automatically or manually) from particulates with similar fluorescence properties. Back-trajectory analysis with the NOAA Hybrid Single-Particle Lagrangian Integrated Trajectory (HySPLIT) model indicates that these ice-bound marine organisms originate from the Weddell and Amundsen-Bellingshausen Seas. This technique, when paired with established chemical and biochemical methods, shows considerable potential in providing valuable information about the nature and origin of aerosols and biomarker signals trapped in past ice layers.

The future response of the Antarctic ice sheet to rising temperatures remains highly uncertain. A useful period for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG; 129-116 kyr), which experienced warmer polar temperatures and higher global mean sea level (GMSL +6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (~2 m), ocean thermal expansion and melting mountain glaciers (~1 m), suggesting substantial Antarctic mass loss was initiated by warming of Southern Ocean waters, resulting from a weakening Atlantic Meridional Overturning Circulation in response to North Atlantic surface freshening. Here we report a blue-ice record of ice-sheet and environmental change from the Weddell Sea Embayment at the periphery of the marine-based West Antarctic Ice Sheet (WAIS) which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide the first evidence for substantial mass loss across the Weddell Sea Embayment during the Last Interglacial, most likely driven by ocean warming and associated with destabilization of sub-glacial hydrates. Ice-sheet modelling supports this interpretation and suggests that millennial-scale warming of the Southern Ocean could have triggered a multi-meter rise in global sea levels. Our data indicate that Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice-climate feedbacks that further amplify warming.

Significance Fifty years ago, it was speculated that the marine-based West Antarctic Ice Sheet is vulnerable to warming and may have melted in the past. Testing this hypothesis has proved challenging due to the difficulty of developing in situ records of ice sheet and environmental change spanning warm periods. We present a multiproxy record that implies loss of the West Antarctic Ice Sheet during the Last Interglacial (129,000 to 116,000 y ago), associated with ocean warming and the release of greenhouse gas methane from marine sediments. Our ice sheet modeling predicts that Antarctica may have contributed several meters to global sea level at this time, suggesting that this ice sheet lies close to a “tipping point” under projected warming., The future response of the Antarctic ice sheet to rising temperatures remains highly uncertain. A useful period for assessing the sensitivity of Antarctica to warming is the Last Interglacial (LIG) (129 to 116 ky), which experienced warmer polar temperatures and higher global mean sea level (GMSL) (+6 to 9 m) relative to present day. LIG sea level cannot be fully explained by Greenland Ice Sheet melt (∼2 m), ocean thermal expansion, and melting mountain glaciers (∼1 m), suggesting substantial Antarctic mass loss was initiated by warming of Southern Ocean waters, resulting from a weakening Atlantic meridional overturning circulation in response to North Atlantic surface freshening. Here, we report a blue-ice record of ice sheet and environmental change from the Weddell Sea Embayment at the periphery of the marine-based West Antarctic Ice Sheet (WAIS), which is underlain by major methane hydrate reserves. Constrained by a widespread volcanic horizon and supported by ancient microbial DNA analyses, we provide evidence for substantial mass loss across the Weddell Sea Embayment during the LIG, most likely driven by ocean warming and associated with destabilization of subglacial hydrates. Ice sheet modeling supports this interpretation and suggests that millennial-scale warming of the Southern Ocean could have triggered a multimeter rise in global sea levels. Our data indicate that Antarctica is highly vulnerable to projected increases in ocean temperatures and may drive ice–climate feedbacks that further amplify warming.

A valuable analogue for assessing Earth’s sensitivity to warming is the Last Interglacial (LIG; 129–116 kyr), when global temperatures (0−+2 °C) and mean sea level (+6–11 m) were higher than today. The direct contribution of warmer conditions to global sea level (thermosteric) are uncertain. We report here a global network of LIG sea surface temperatures (SST) obtained from various published temperature proxies (e.g. faunal/floral assemblages, Mg/Ca ratios of calcareous plankton, alkenone UK’37). Each reconstruction is averaged across the LIG (anomalies relative to 1981–2010), corrected for ocean drift and with varying seasonality (189 annual, 99 December-February, and 92 June–August records). We summarise the current limitations of SST reconstructions for the LIG and the spatial temperature features of a naturally warmer world. Because of local δ18O seawater changes, uncertainty in the age models of marine cores, and differences in sampling resolution and/or sedimentation rates, the reconstructions are restricted to mean conditions. To avoid bias towards individual LIG SSTs based on only a single (and potentially erroneous) measurement or a single interpolated data point, here we average across the entire LIG. To investigate the sensitivity of the reconstruction to high temperatures, we also report maximum values during the first 5 ka of the LIG (129–124 kyr). The global dataset provides a remarkably coherent pattern of higher SST increases at polar latitudes than in the tropics, with comparable estimates between different SST proxies. We report mean global annual SST anomalies of 0.2 ± 0.1 °C and a maximum of 0.9 ± 0.2 °C respectively. Using the reconstructed SSTs suggests a mean thermosteric sea level rise of 0.01 ± 0.1 m and a maximum of 0.13 ± 0.1 m respectively. The data provide an important natural baseline for a warmer world, constraining the contributions of Greenland and Antarctic ice sheets to global sea level during a geographically widespread expression of high sea level, and can be used to test the next inter-comparison of models for projecting future climate change. The dataset described in this paper, including summary temperature and thermosteric sea-level reconstructions, are available at https://doi.org/10.1594/PANGAEA.904381 (Turney et al., 2019).

Precise radiocarbon (14C) dating of sedimentary sequences is important for developing robust chronologies of environmental change, but sampling of suitable components can be challenging in highly dynamic landscapes. Here we investigate radiocarbon determinations of different peat size fractions from six peat sites, representing a range of geomorphological contexts on the South Atlantic subantarctic islands of the Falklands and South Georgia. To investigate the most suitable fraction for dating, 112 measurements were obtained from three components within selected horizons: a fine fraction 0.2 mm, and bulk material. We find site selection is critical, with locations surrounded by high-ground and/or relatively slowly accumulating sites more susceptible to the translocation of older carbon. Importantly, in locations with reduced potential for redeposition of material, our results show that there is no significant or systematic difference between ages derived from bulk material, fine or coarse (plant macrofossil) material, providing confidence in the resulting age model. Crucially, in areas comprising complex terrain with extreme relief, we recommend dating macrofossils or bulk carbon rather than a fine fraction, or employing comprehensive dating of multiple sedimentary fractions to determine the most reliable fraction(s) for developing a robust chronological framework.

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.

The Amundsen Sea Low (ASL) plays a major role in the climate and environment of Antarctica and the Southern Ocean, including surface air temperature and sea ice concentration changes. Unfortunately, a relative dearth of observational data across the Amundsen and Bellingshausen seas prior to the satellite era (post-1979) limits our understanding of the past behaviour and impact of the ASL. The limited proxy evidence for changes in the ASL are primarily restricted to the Antarctic where ice core evidence suggests a deepening of the atmospheric pressure system during the late Holocene. However, no data have previously been reported from the northern side of the ASL. Here we report a high-resolution, multi-proxy study of a 5000-year-long peat record from the Falkland Islands, a location sensitive to contemporary ASL dynamics which modulates northerly and westerly airflow across the southwestern South Atlantic sector of the Southern Ocean. In combination with climate reanalysis, we find a marked period of wetter, colder conditions most likely the result of enhanced southerly airflow between 5000 and 2500 years ago, suggesting limited ASL influence over the region. After 2500 years ago, drier and warmer conditions were established, implying more westerly airflow and the increased projection of the ASL onto the South Atlantic. The possible role of the equatorial Pacific via atmospheric teleconnections in driving this change is discussed. Our results are in agreement with Antarctic ice core records and fjord sediments from the southern South American coast, and suggest that the Falkland Islands provide a valuable location for reconstructing high southern latitude atmospheric circulation changes on multi-decadal to millennial timescales.

Understanding feedbacks between the Greenland Ice Sheet (GrIS) and the Atlantic Meridional Overturning Circulation (AMOC) is crucial for reducing uncertainties over future sea level and ocean circulation change. Reconstructing past GrIS dynamics can extend the observational record and elucidate mechanisms that operate on multi-decadal timescales. We report a highly-constrained last glacial vertical profile of cosmogenic isotope exposure ages from Sermilik Fjord, a marine-terminating ice stream in the southeast sector of the GrIS. Our reconstruction reveals substantial ice-mass loss throughout the Younger Dryas (12.9-11.7 ka), a period of marked atmospheric and sea-surface cooling. Earth-system modelling reveals that southern GrIS marginal melt was likely driven by strengthening of the Irminger Current at depth due to a weakening of the AMOC during the Younger Dryas. This change in North Atlantic circulation appears to have drawn warm subsurface waters to southeast Greenland despite markedly cooler sea surface temperatures, enhancing thermal erosion at the grounding lines of palaeo ice-streams, supporting interpretation of regional marine-sediment cores. Given current rates of GrIS meltwater input into the North Atlantic and the vulnerability of major ice streams to water temperature changes at the grounding line, this mechanism has important implications for future AMOC changes and northern hemisphere heat transport.

The influence of wildfire on surface soil and hydrology has been widely investigated, while its impact on the karst vadose zone is still poorly understood. A moderate to severe experimental fire was conducted on a plot (10 m × 10 m) above the shallow Wildman's Cave at Wombeyan Caves, New South Wales, Australia in May 2016. Continuous sampling of water stable isotopes, inorganic geochemistry and drip rates were conducted from Dec 2014 to May 2017. After the fire, drip discharge patterns were significantly altered, which is interpreted as the result of increased preferential flows and decreased diffuse flows in the soil. Post-fire drip water δ18O decreased by 6.3‰ in the first month relative to the average pre-fire isotopic composition. Post-fire monitoring showed an increase in drip water δ18O in the following six months. Bedrock related solutes (calcium, magnesium, strontium) decreased rapidly after the fire due to reduced limestone dissolution time and potentially reduced soil CO2. Soil- and ash-derived solutes (boron, lead, potassium, sodium, silicon, iodine and iron) all decreased after the fire due to volatilisation at high temperatures, except for SO42−. This is the first study to understand the hydrological impact from severe fires conducted on a karst system. It provides new insights on the cave recharge process, with a potential explanation for the decreased d18O in speleothem-based fire study, and also utilise the decreased bedrock solutes to assess the wildfire impacts both in short and long time scales.

The arrival of iceberg B09B in Commonwealth Bay, East Antarctica, and subsequent fast ice expansion has dramatically increased the distance Adélie penguins (Pygoscelis adeliae) breeding at Cape Denison must travel in search of food. This has provided a natural experiment to investigate the impact of iceberg stranding events and sea ice expansion along the East Antarctic coast. As part of the Australasian Antarctic Expedition 2013–14, the Adélie penguin colony at Cape Denison was censused to compare to historic counts. Whilst some 5520 pairs still bred at Cape Denison there has been an order of magnitude decline in Adélie numbers in the area in comparison to the first counts a century ago and, critically, recent estimates based on satellite images and a census in 1997. In contrast, an Adélie population on the eastern fringe of Commonwealth Bay just 8 km from the fast ice edge was thriving, indicating the arrival of B09B and fast ice expansion was probably responsible for the observed recent population decline. In conclusion, the Cape Denison population could be extirpated within 20 years unless B09B relocates or the now perennial fast ice within the bay breaks out. Our results have important implications for wider East Antarctic if the current increasing sea ice trend continues.

The stratigraphic relationships of palaeoecological proxies and use of changepoint analyses to determine the cause and effect relationships between past events has allowed a better understanding of the relative contributions of humans and environmental drivers to Late Quaternary extinctions and of their effects on terrestrial ecosystems. Few studies, however, have validated these approaches at localities where past interactions between vegetation communities and large herbivores are well-documented. Here, we use a peat core from subantarctic Enderby Island to present the first study tracing the spores of dung fungi alongside pollen at a site where the history of mammalian herbivore introductions (and subsequent eradication), and their effects on the vegetation, are precisely known. We find a strong connection between spore influx rates of the dung-fungus Sporormiella and PCA axis 1 of the pollen assemblages, suggesting that past vegetation change caused by herbivore introductions and eradications at the core site can be readily deduced from the palaeoecological record. The response of the vegetation community to the removal of herbivores was so rapid, however, that a difference in timing between changepoints relating to specific pollen taxa, the overall pollen community, and the decline of Sporormiella spores, could not be resolved in our record, despite a sampling resolution of

The Southern Ocean plays a significant role in driving global climate–ocean–carbon dynamics. Unfortunately, a relative dearth of datasets across the region limits our ability to understand past and future mechanisms of change. Here we report a new dataset from the south-west Pacific: radiocarbon-dated subfossil tree stumps (Dracophyllum) eroding out from peat exposures on Campbell and Auckland Islands (52–50°S). Dracophyllum are the southernmost growing trees in the south-west Pacific and their growth on exposed sites and at altitude is strongly controlled by the prevailing westerly airflow, providing a unique measure of past changes in zonal wind strength. Here we demonstrate a significant collapse in the altitudinal limit of growth between approximately 2000 and 1000 years ago (hereafter 2–1 ka), consistent with other records across the Southern Hemisphere that indicate westerly airflow was significantly enhanced. Importantly, this period in the late Holocene was one of marked change across the broader region, suggesting westerly airflow played a key role in driving Southern Ocean variability at this time.

Future changes in atmospheric circulation and associated modes of variability are a major source of uncertainty in climate projections. Nowhere is this issue more acute than across the mid-latitudes to high latitudes of the Southern Hemisphere (SH), which over the last few decades have experienced extreme and regionally variable trends in precipitation, ocean circulation and temperature, with major implications for Antarctic ice melt and surface mass balance. Unfortunately there is a relative dearth of observational data, limiting our understanding of the driving mechanism(s). Here we report a new 130-year annually resolved record of δD – a proxy for temperature – from the geographic South Pole where we find a significant influence from extratropical pressure anomalies which act as "gatekeepers" to the meridional exchange of air masses. Reanalysis of global atmospheric circulation suggests these pressure anomalies play a significant influence on mid- to high-latitude SH climate, modulated by the tropical Pacific Ocean. This work adds to a growing body of literature confirming the important roles of tropical and mid-latitude atmospheric circulation variability on Antarctic temperatures. Our findings suggest that future increasing tropical warmth will strengthen meridional circulation, exaggerating current trends, with potentially significant impacts on Antarctic surface mass balance.

The Southern Ocean plays a critical role in global climate, yet the mixing properties of the circulation in this part of the ocean remain poorly understood. Here dispersion in the vicinity of the Southern Antarctic Circumpolar Current Front, one of the branches of the Antarctic Circumpolar Current, is studied using 10 pairs of surface drifters deployed systematically across the frontal jet and its flanks. Drifter pairs were deployed with an initial separation of 13 m and report their position every hour. The separation of the pairs over 7 months, in terms of their Finite-Scale Lyapunov Exponents (FSLE), dispersion, and diffusivity, is characterized and related to expected behavior from Quasi-Geostrophic (QG) and Surface Quasi-Geostrophic (SQG) theories. The FSLE analysis reveals two submesoscale regimes, with SQG-like behavior at scales below 3.2 km and mixed QG/SQG behavior at scales between 3.2 and 73 km. The dispersion analysis, however, suggests QG-like behavior for the smallest scales. Both dispersion and diffusivity appear isotropic for scales up to 500 km. Finally, there is no clear indication of a cross-jet variation of drifter dispersion.

Atmospheric warming is projected to increase global mean surface temperatures by 0.3 to 4.8 degrees Celsius above pre-industrial values by the end of this century. If anthropogenic emissions continue unchecked, the warming increase may reach 8-10 degrees Celsius by 2300 (ref. 2). The contribution that large ice sheets will make to sea-level rise under such warming scenarios is difficult to quantify because the equilibrium-response timescale of ice sheets is longer than those of the atmosphere or ocean. Here we use a coupled ice-sheet/ice-shelf model to show that if atmospheric warming exceeds 1.5 to 2 degrees Celsius above present, collapse of the major Antarctic ice shelves triggers a centennial- to millennial-scale response of the Antarctic ice sheet in which enhanced viscous flow produces a long-term commitment (an unstoppable contribution) to sea-level rise. Our simulations represent the response of the present-day Antarctic ice-sheet system to the oceanic and climatic changes of four representative concentration pathways (RCPs) from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that substantial Antarctic ice loss can be prevented only by limiting greenhouse gas emissions to RCP 2.6 levels. Higher-emissions scenarios lead to ice loss from Antarctic that will raise sea level by 0.6-3 metres by the year 2300. Our results imply that greenhouse gas emissions in the next few decades will strongly influence the long-term contribution of the Antarctic ice sheet to global sea level.

Sea-ice is a key physical driver of Antarctic marine ecosystems. Understanding ecological effects of sea-ice is particularly important given current and future climate change, but a major obstacle is the impracticality of manipulating sea-ice at a relevant scale. However, large-scale anomalous events, such as those occurring in Commonwealth Bay, East Antarctica, provide opportunities for natural experiments. Historically, katabatic winds have kept Commonwealth Bay ice-free for most of each year, but since 2010, a massive grounded iceberg has resulted in year-round sea-ice cover. We surveyed benthic communities in Commonwealth Bay approximately 3 years after continuous sea-ice cover began and found algal bed communities in severe decline. The majority (~75 %) of large macroalgae were decomposing, and the remainder were discoloured or bleached, while approximately 40 % of encrusting coralline algae were bleached. Accompanying this, the presence of invertebrates such as ophiuroids and polychaetes suggests that communities are in the early stages of transitioning to an invertebrate-dominated state. With a known start date, monitoring benthic communities in Commonwealth Bay will allow quantification of rates of benthic regime shifts in response to sea-ice cover, and improve understanding of the vulnerability of polar ecosystems to climate change.

Satellite observations demonstrate Antarctic sea ice extent increased between late-1978 and 2015, with significant spatial and seasonal variability. Late spring retreat off George V Land is a major component of the observed increase, but the paucity of proxy records makes interpretation of trends (and impacts) challenging. Here Earth-system modelling and reanalysis demonstrate tropical Pacific warming can trigger an atmospheric Rossby wave response during the austral spring, delaying sea-ice retreat off George V Land. Our results provide new insights into the spatial and temporal role low latitudes play in Antarctic sea-ice production, drift and ocean circulation on decadal to centennial timescales.

The stability of Antarctic ice sheets and their potential contribution to sea level under projected future warming remains highly uncertain. The Last Interglacial (135 000-116 000 years ago) provides a potential analogue, with global temperatures 2uC higher and rates of sea-level rise >5.6m ka 1 , leading to sea levels 6.6- 9.4m higher than present. The source(s) of this sea-level rise remain fiercely debated. Here we report a series of independent model simulations exploring the effects of migrating Southern Hemisphere Westerlies (SHWs) on Southern Ocean circulation and Antarctic ice-sheet dynamics. We suggest that southerly shifts in winds may have significantly impacted the sub-polar gyres, inducing pervasive warming (0.2-0.8uC in the upper 1200m) adjacent to sectors of the East Antarctic Ice Sheet (EAIS), which due to their geometries and connectivity to the Southern Ocean are highly sensitive to ocean forcing. We conclude that the EAIS potentially made a substantial, hitherto unsuspected, contribution to interglacial sea levels, and given 21st-century projections in the Southern Annular Mode and associated SHW migration, we highlight how pervasive circum-Antarctic warming may threaten EAIS stability. Copyright # 2013 John Wiley & Sons, Ltd.

The transition from the late Pleistocene to the Holocene (30 000–5000 years ago) was a period of considerable climate variability, which has been associated with changes in deep water formation and the intensity of the Meridional Overturning Circulation. Although numerous records exist across the North Atlantic region, few Antarctic ice core records have been obtained from the south. Here we exploit the potential of upwelling ancient ice – so-called blue ice areas (BIAs) – from the Patriot Hills in the Ellsworth Mountains to derive the first deuterium isotope record (δD) from continental Antarctica south of the Weddell Sea. Gas analysis and glaciological considerations provide a first relative chronology. Inferred temperature trends from the Patriot Hills BIA and snow pit suggest changing climate influences during the transition between the last glacial period and Holocene. Under modern conditions, the interplay between the Antarctic high-pressure system and the Southern Annular Mode appears to play a significant role in controlling katabatic wind flow over the site while the BIA record suggests that greater sea ice extent during the last glacial period was a major control. Our results demonstrate the considerable potential of the Patriot Hills site for reconstructing past climate change in the south Atlantic region.

Dynamical changes in contemporary ice sheets account for significant proportions of their current rates of mass loss, but assessing whether or not these processes are a natural part of ice-sheet evolution requires inference from palaeo-glaciological records. However, a robust mechanism for translating sparse geological data into meaningful interpretations of past glacier dynamics at the continental scale is lacking, since geological archives can be ambiguous, and often their chronology is only poorly constrained. To address this, we combine the interpretation of high-resolution Antarctic ice sheet model results with continent-wide geological evidence pertinent to the dynamical configuration of the ice sheet during the last, and possibly preceding, glacial maxima. We first focus on the thermal regime of the ice sheet, its pattern and velocity of flow, variability in likely subglacial erosion and sediment transport, and how these characteristics evolve during glacial transitions. We show that rapid basal sliding was restricted to discrete outlets that eroded and advected sediment toward and across the continental shelf primarily during the early stages of advance and retreat of the ice sheet, highlighting the need to consider time-transgressive behaviour in the interpretation of geological archives. Secondly, we present new modelling that attempts to improve the fit of our numerical model to geologically-based reconstructions in the Ross Sea. By accounting for locally-enhanced ablation in McMurdo Sound, our new simulation achieves a much closer fit to empirically-derived flow patterns than previously. Growth of the modelled Last Glacial Maximum ice sheet takes place primarily by marine ice accretion in the major embayments, as a consequence of cooler ocean temperatures and reduced sub-ice-shelf melting, and at its maximal extent represents a grounded ice volume excess above present of approximately 8.3 m sea-level equivalent. This figure thus provides an upper bound on the possible Antarctic contribution to deglacial meltwater pulses.

Occupying about 14 % of the world's surface, the Southern Ocean plays a fundamental role in ocean and atmosphere circulation, carbon cycling and Antarctic ice-sheet dynamics. Unfortunately, high interannual variability and a dearth of instrumental observations before the 1950s limits our understanding of how marine–atmosphere–ice domains interact on multi-decadal timescales and the impact of anthropogenic forcing. Here we integrate climate-sensitive tree growth with ocean and atmospheric observations on southwest Pacific subantarctic islands that lie at the boundary of polar and subtropical climates (52–54° S). Our annually resolved temperature reconstruction captures regional change since the 1870s and demonstrates a significant increase in variability from the 1940s, a phenomenon predating the observational record. Climate reanalysis and modelling show a parallel change in tropical Pacific sea surface temperatures that generate an atmospheric Rossby wave train which propagates across a large part of the Southern Hemisphere during the austral spring and summer. Our results suggest that modern observed high interannual variability was established across the mid-twentieth century, and that the influence of contemporary equatorial Pacific temperatures may now be a permanent feature across the mid- to high latitudes.

Reconstructing the dynamic response of the Antarctic ice sheets to warming during the Last Glacial Termination (LGT; 18,000–11,650 yrs ago) allows us to disentangle ice-climate feedbacks that are key to improving future projections. Whilst the sequence of events during this period is reasonably well-known, relatively poor chronological control has precluded precise alignment of ice, atmospheric and marine records, making it difficult to assess relationships between Antarctic ice-sheet (AIS) dynamics, climate change and sea level. Here we present results from a highly-resolved ‘horizontal ice core’ from the Weddell Sea Embayment, which records millennial-scale AIS dynamics across this extensive region. Counterintuitively, we find AIS mass-loss across the full duration of the Antarctic Cold Reversal (ACR; 14,600–12,700 yrs ago), with stabilisation during the subsequent millennia of atmospheric warming. Earth-system and ice-sheet modelling suggests these contrasting trends were likely Antarctic-wide, sustained by feedbacks amplified by the delivery of Circumpolar Deep Water onto the continental shelf. Given the anti-phase relationship between inter-hemispheric climate trends across the LGT our findings demonstrate that Southern Ocean-AIS feedbacks were controlled by global atmospheric teleconnections. With increasing stratification of the Southern Ocean and intensification of mid-latitude westerly winds today, such teleconnections could amplify AIS mass loss and accelerate global sea-level rise.

Here we report a new quantitative mean July temperature reconstruction using non-biting midges (chironomids) from the Danish Last Interglacial (LIG) site Hollerup (spanning 127-116 ka). We find that peak mean July temperatures of 17.5 °C, similar to those of the present day (1961-1990 CE), were reached shortly before the onset of the regional Carpinus pollen zone. Through comparison to terrestrial and marine sequences we demonstrate that peak summer warmth took place some three millennia after the onset of LIG warming in Europe, a marked delay in line with records from the North Atlantic. Crucially, the warmest northern European summer temperatures appear to follow maximum Greenland Ice Sheet mass loss, implying that meltwater substantially reduced Atlantic Meridional Overturning Circulation and depressed European temperatures during the early part of the interglacial. (Less)

The dramatic calving of the Mertz Glacier Tongue in 2010, triggered by the impact of iceberg B09B, reshaped the oceanographic regime across the Mertz Polynya and Commonwealth Bay, regions where high salinity shelf water (HSSW) is formed, the precursor to Antarctic bottom water (AABW). Here we compare post-calving observations with high-resolution ocean modelling which suggest that this reconfiguration has led to the development of a new polynya off Commonwealth Bay, where HSSW production continues by the grounding of B09B. Our findings demonstrate how local changes in icescape can impact formation of AABW, with implications for large-scale ocean circulation and climate.

The dramatic calving of the Mertz Glacier tongue in 2010, precipitated by the movement of iceberg B09B, reshaped the oceanographic regime across the Mertz Polynya and Commonwealth Bay, regions where high-salinity shelf water (HSSW) – the precursor to Antarctic bottom water (AABW) – is formed. Here we present post-calving observations that suggest that this reconfiguration and subsequent grounding of B09B have driven the development of a new polynya and associated HSSW production off Commonwealth Bay. Supported by satellite observations and modelling, our findings demonstrate how local icescape changes may impact the formation of HSSW, with potential implications for large-scale ocean circulation.

Northern Hemisphere-wide cooling during the Little Ice Age (LIA; 1650–1775 Common Era, C.E.) was associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO2, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally resolved radiocarbon (14C) levels in a network of tree-ring series spanning 1700–1950 C.E. located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of 14CO2-depleted abyssal waters. We find the interhemispheric 14C offset approaches zero during increasing global atmospheric CO2 at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO2 increase and reduction in the interhemispheric 14C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed.

Robustly testing hypotheses of geographic synchroneity of abrupt and extreme change during the late Pleistocene (60,000 to 11,650 years ago) requires a level of chronological precision often lacking in ice, marine and terrestrial sequences. Here we report a bidecadally-resolved New Zealand kauri (Agathis australis) tree-ring sequence spanning two millennia that preserves a record of atmospheric radiocarbon (14C) during ice-rafted debris event Heinrich Event 3 (HE3) in the North Atlantic and Antarctic Isotope Maximum 4 (AIM4) in the Southern Hemisphere. Using 14C in the marine Cariaco Basin and 10Be preserved in Greenland ice, the kauri 14C sequence allows us to precisely align sequences across this period. We observe no significant difference between atmospheric and marine 14C records during HE3, suggesting no stratification of surface waters and collapse in Atlantic Meridional Overturning Circulation (AMOC). Instead our results support recent evidence for a weakened AMOC across at least two millennia of the glacial period. Our work adds to a growing body of literature confirming that Heinrich events are not the cause of stadial cooling and suggests changes in the AMOC were not the primary driver of antiphase temperature trends between the hemispheres. Decadally-resolved 14C in ancient kauri offers a powerful new (and complementary) approach to polar ice core CH4 alignment for testing hypotheses of abrupt and extreme climate change.