# Sustained Antarctic Research: A 21st Century Imperative

Mahlon Kennicutt, David Bromwich, Daniela Liggett, Birgit Njåstad, Lloyd Peck, Stephen Rintoul, Catherine Ritz, Martin Siegert, Alan Aitken, Cassandra Brooks, et al.

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Mahlon Kennicutt, David Bromwich, Daniela Liggett, Birgit Njåstad, Lloyd Peck, et al. Sustained Antarctic Research: A 21st Century Imperative. One Earth, 2019, 1 (1), pp.95-113. 10.1016/j.oneear.2019.08.014. hal-02353429

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# Sustained Antarctic Research: A 21st Century Imperative

Mahlon C. Kennicutt II,¹,*, David Bromwich,² Daniela Liggett,³ Birgit Njåstad,⁴ Lloyd Peck,⁵ Stephen R. Rintoul,⁶

Catherine Ritz,⁷ Martin J. Siegert,⁸ Alan Aitken,⁹ Cassandra M. Brooks,¹⁰ John Cassano,¹¹ Sanjay Chaturvedi,¹²

Dake Chen,¹³ Klaus Dodds,¹⁴ Nicholas R. Golledge,¹⁵ Céline Le Bohec,¹⁶,¹⁷ Marcelo Leppe,¹⁸ Alison Murray,¹⁹

P. Chandrika Nath,²⁰ Marilyn N. Raphael,²¹ Michelle Rogan-Finnemore,²² Dustin M. Schroeder,²³ Lynne Talley,²⁴

Tony Travouillon,²⁵ David G. Vaughan,⁵ Lifan Wang,²⁶ Allan T. Weatherwax,²⁷ Huigen Yang,²⁸ and Steven L. Chown²⁹

¹Department of Oceanography, Texas A&M University, College Station, TX 77843, USA

²Department of Geography, The Ohio State University, Columbus, OH 43210, USA

³Gateway Antarctica, Department of Geography, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand

⁴Norwegian Polar Institute, 9296 Tromso, Norway

⁵British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

⁶CSIRO Oceans & Atmosphere, Centre for Southern Hemisphere Oceans Research, Australian Antarctic Program Partnership, Hobart, 7000 TAS, Australia

⁷Institut des Géosciences de l'Environnement, University of Grenoble Alpes/CNRS, 38 058 Grenoble Cedex 9, France

⁸Grantham Institute and Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK

⁹School of Earth Sciences, University of Western Australia, Perth, WA 6008, Australia

¹⁰Environmental Studies Program, University of Colorado Boulder, Boulder, CO 80309, USA

¹¹Department of Atmospheric and Oceanic Sciences, University of Colorado Boulder, Boulder, CO 80309, USA

¹²Faculty of Social Sciences, Department of International Relations South Asian University, Akbar Bhavan, Chankyapuri, New Delhi 110021, India

¹³State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, MNR, Hangzhou 310012, China

¹⁴Department of Geography, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

¹⁵Antarctic Research Centre, Victoria University of Wellington, 6140 Wellington, New Zealand

¹⁶Université de Strasbourg, CNRS, IPHC UMR 7178, 23 rue Becquerel, 67000 Strasbourg, France

¹⁷Centre Scientifique de Monaco, Département de Biologie Polaire, 8, quai Antoine 1er, MC 98000 Monaco, Principality of Monaco

¹⁸Chilean Antarctic Institute, Punta Arenas, Chile

¹⁹Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA

²⁰Scientific Committee on Antarctic Research, c/o Scott Polar Research Institute, University of Cambridge, Lensfield Road, Cambridge CB2 1ER, UK

²¹University of California Los Angeles, Department of Geography, Los Angeles, CA 90095, USA

²²Council of Managers of National Antarctic Programs Secretariat, Christchurch 8140, New Zealand

²³Department of Geophysics, Stanford University, Palo Alto, CA, USA

²⁴Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92037, USA

²⁵Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australia

²⁶Department of Physics & Astronomy, Texas A&M University, College Station, TX 77843, USA

²⁷Merrimack College, 315 Turnpike Street, North Andover, MA 01845, USA

²⁸Polar Research Institute of China, 451 Jinqiao Road, Pudong Xinqu, Shanghai Shi, China

²⁹School of Biological Sciences, Monash University, Clayton, VIC 3800, Australia

*Correspondence: mckennicutt@gmail.com

https://doi.org/10.1016/j.oneear.2019.08.014

The view from the south is, more than ever, dominated by ominous signs of change. Antarctica and the Southern Ocean are intrinsic to the Earth system, and their evolution is intertwined with and influences the course of the Anthropocene. In turn, changes in the Antarctic affect and presage humanity's future. Growing understanding is countering popular beliefs that Antarctica is pristine, stable, isolated, and reliably frozen. An aspirational roadmap for Antarctic science has facilitated research since 2014. A renewed commitment to gathering further knowledge will quicken the pace of understanding of Earth systems and beyond. Progress is already evident, such as addressing uncertainties in the causes and pace of ice loss and global sea-level rise. However, much remains to be learned. As an iconic global "commons," the rapidity of Antarctic change will provoke further political action. Antarctic research is more vital than ever to a sustainable future for this One Earth.

## Introduction

Antarctica and the Southern Ocean ("the Antarctic") are intrinsic to the Earth system. Although remote, the Antarctic region is interconnected with the northern world by oceanic and atmospheric couplings, geopolitics, and international agreements. Climate variability and change are transmitted from low to high

latitudes. In turn, change in the Antarctic has profound implications for the rest of the planet. The fate of Antarctic ice sheets determines, to a large degree, sea level, and the Southern Ocean plays a dominant role in global heat and greenhouse gas budgets. Therefore, scientific investigations of the Antarctic are critical to understanding the history and future trajectories of
our planet.¹ In the latter case, this raises critical questions about the viability of current socioeconomic arrangements as the planet evolves to states beyond that experienced throughout human history.²⁻⁶

The Antarctic region also sustains some of the planet's most iconic species (e.g., whales, penguins, and albatrosses) and provides a range of important ecosystem services.⁷ Despite past whaling, sealing, fisheries, and krill harvesting, no anthropogenic extinctions have been recorded in the region, but consensus is growing that changing climate and resource exploitation interests pose threats to the region.⁷⁻⁹ Calls for expanding long-term research across the region have become more strident in anticipation of regulatory challenges that will require information on system changes.⁷,¹⁰⁻¹³

The global value of sustained scientific research in the Antarctic is best illustrated by policy responses to observations of ozone depletion over Antarctica. Long-term stratospheric ozone monitoring from the Antarctic continent led to the recognition of a developing ozone hole above Antarctica in the mid-1980s.¹⁴ Realization of the implications for life on Earth was swift and yielded an unprecedented rapid, globally agreed response to phase out the chlorofluorocarbons (CFCs) responsible for depletion. Discerning a causal link between the strengthening and poleward shift of the westerly winds over the Southern Ocean, along with their influence on Antarctic life, transformed the debate.¹⁵⁻¹⁷ Continued long-term assessment of these changes and system-wide effects will be critical if international goals are to be met. Some complexity remains, with indications that, despite the universal ratification of the Montreal Protocol¹⁸ and its instruments, CFC-11 (trichlorofluoromethane) concentrations in the atmosphere are increasing.¹⁹ This is an example of how Antarctic observations and research are critical to identifying global threats and assessing the efficacy of control measures. Today, Antarctic observations play a similar role regarding climate change and sea-level rise.

Five years ago, a community-driven process identified the highest priorities and set an ambitious agenda for Antarctic research (Box 1).²⁰,²¹ Horizon Scanning—a systematic approach

to retrieve, sort, organize, and prioritize information pertinent to the question posed—was used to identify the most important scientific questions from many.²² The first Antarctic Science Horizon Scan (“the Scan”) was followed by an assessment of the technology and infrastructure required to deliver the research. The Antarctic Roadmap Challenges (“the ARC”; Box 1) assessment included estimates of both cost and time to delivery.²³ It was recognized that identifying questions was a first step, but answers were the goal. The ARC provided a path to implementation.

Since then, the imperatives for Antarctic research have grown. Climate change poses an existential threat to society and the future of the planet, with the urgent need “... to bring all nations into a common cause to undertake ambitious efforts to combat climate change and adapt to its effects ....”²⁶ Scientific understanding of the Antarctic is essential for this common cause. This is clearly articulated in the alternatives for the region presented from a 2070 vantage: one presenting an environmentally, as well as a politically, unrecognizable Antarctic region and world; and the other closer to that experienced throughout human history.⁹

Here, we review progress against the priorities set out by the Scan and ARC, recognizing across each theme where progress has been made, where it is lagging, and what new challenges have arisen. In doing so, we recognize that the delivery of evidence does not guarantee a change in policy and that opinions vary on what policies should be adopted among the diverse stakeholders, states, and constituencies that are the 21st century world.

The progress assessment is ordered according to the seven clusters of questions identified by the Scan: (1) Antarctic atmosphere and global connections, (2) Southern Ocean and sea ice in a warming world, (3) ice sheets and sea level, (4) the dynamic Earth: probing beneath Antarctic ice, (5) life on the precipice, (6) near-Earth space and beyond, and (7) human presence in Antarctica.²¹ An eighth topic, regarding effective engagement of diverse audiences, assesses the impact, delivery, and uptake of the Scan and ARC outputs with a goal of discerning lessons

Box 1. The First Antarctic and Southern Ocean Science Horizon Scan and the Antarctic Roadmap Challenges (ARC) Project: The Process

The first Antarctic and Southern Ocean Science Horizon Scan (the Scan)²⁰ was based on wide consultation with the community to develop a collective, international view of the most important future directions in Antarctic research.²⁰,²¹ A final list of 80 highest priority questions, distilled from nearly 1,000 questions submitted by the community, was agreed at a retreat attended by 75 representatives from 22 countries.²⁴ Attendees included researchers, national program directors or managers, and policy makers. Retreat participants were selected to ensure balance among disciplinary expertise, geographical origins, gender, stage of career, and representation of SCAR partner organizations and other stakeholders. The Scan outcomes were articulated as an “Antarctic Science Roadmap” (the Roadmap).²¹ A new team of 60 experts was assembled to conduct the Antarctic Roadmap Challenges (ARC) project.²³,²⁵ Participants included logisticians and operations experts, experienced Antarctic researchers, policy makers, select Scan contributors, and national Antarctic program personnel from 22 countries. A workshop was convened to consider a series of papers submitted by the Antarctic science community, survey results, summaries from the Scan, and other documents addressing future Antarctic research directions, essential technologies, and logistics requirements. The ARC project answered the question, “How will national Antarctic programs meet the challenges of delivery of Antarctic science over the next 20 years?” As entities that fund and support Antarctic science, national Antarctic programs face many practical and technical issues. ARC addressed four of seven challenges: availability of essential technologies, extraordinary logistics requirements (access), supporting infrastructure and international cooperation.²³ Challenges related to human resources, energy demands, and long-term sustainable funding were not considered.
learned for effective communication that influences societal actions. Experts assessed progress by reviewing the scientific literature published in the last 5 years (Tables S1-S15). A transdisciplinary and critical perspective on progress was assured by including stakeholder representatives and others not involved in the Scan or ARC projects. Indications of progress do not infer that the Scan was the cause, as much research was already underway and other non-scientific factors were at play. The notation Q.## refers to specific Scan questions (Tables S1-S14).²⁰,²¹

## Antarctic Atmosphere and Global Connections

Tropical oceans influence Antarctic climate on a variety of time scales via atmospheric teleconnections (Figure 1 and Tables S1 and S2).²⁷ These tropical impacts are most apparent today in West Antarctica and are primarily linked to the tropical and subtropical Pacific Ocean. El Niño-Southern Oscillation (ENSO) variability on interannual timescales is the most prominent influence. These tropical forces modulate the impacts from the ozone hole in the stratosphere above Antarctica that propagate into the weather-active troposphere. These tropical and polar forces govern the behavior of the westerly winds around Antarctica affecting Southern Ocean circulation, sea-ice extent, heat and carbon sequestration, and oceanic biogeochemistry. The north/south pressure gradient over the Antarctic is expressed as the Southern Annular Mode, and understanding of

its variation and change, and the causes and consequences thereof, are improving (Q.1, Q.3, Q.4, and Q.11). There is a growing understanding of the global atmospheric-oceanic coupled system (aka “oceanic-atmospheric bridge”) from model simulations and correlations of observations and how polar modes are relayed through northern and southern mid to low latitudes possibly influencing, and predicting, distant global weather phenomena (e.g., monsoon rainfall patterns).²⁸,²⁹

While descriptions of climate variability and change in Antarctica are improving, direct continent-wide observations of atmospheric variables, such as temperature and pressure, only date to the 1950s. Indirect measures of temperatures from ice core records augment observations as far back as 2000 years before present, and the number of ice cores is growing.³⁰ In these records, broad-scale cooling was apparent until 1900 followed by warming in the Antarctic western hemisphere. Spatial extrapolations of surface air temperature measurements demonstrated that warming extends from the Antarctic Peninsula into central West Antarctica, but there has been little or no recent change in East Antarctica.³¹ Trends in the Southern Annular Mode and tropical influences are suggested as causal factors. Antarctic precipitation for the last 200 years is also derived from reconstructions of ice core records and here too, both record availability and understanding of the underlying variation and its mechanisms is advancing.³² Large but opposing trends are found across West Antarctica, especially for recent

### El Niño condition

<figure><img src="image_2.png" /><figcaption>**Figure 1.** A Schematic Illustration of Key Aspects of the “Antarctic Atmosphere and Global Connections” Theme</figcaption></figure>

Remote atmospheric circulation changes are caused by warm sea-surface temperatures in the tropical Pacific Ocean accompanying the El Niño-Southern Oscillation (ENSO). The mechanisms involved include (1) Rossby wave trains emanating from the tropical Pacific, leading to an anomalous high pressure center in the Amundsen Sea (weakened Amundsen Sea low); (2) meridional circulations exhibiting zonal asymmetry because of contrasting sea surface temperature anomalies in the tropical Pacific and tropical Atlantic: the Hadley cell is strengthened and contracted (weakened) in the South Pacific (South Atlantic); (3) equatorward shifting of the subtropical jet and storm tracks in the South Pacific and poleward shifting of storm tracks in the South Atlantic; and (4) an enhanced (weakened) Ferrel cell in the South Pacific (South Atlantic). This contributes to more poleward heat transport in the lower atmosphere of the South Pacific and less poleward heat transport in the South Atlantic. As a result, storm activity decreases in the Pacific sector of Antarctica but increases in the Atlantic sector.²⁷

© American Meteorological Society. Used with permission. This figure has been modified from the original source.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** A Schematic Illustration of Key Aspects of the "Southern Ocean and Sea Ice" Theme</figcaption></figure>

The ocean circulation is driven by wind forcing and exchange of heat and freshwater at the sea surface. The Antarctic Circumpolar Current circles the continent from west to east. Deep water flows southward and upwells to the sea surface. Part of the upwelled water returns north as dense Antarctic bottom water, and the rest returns as lighter water that supplies the intermediate layers of the ocean, producing an overturning circulation with two counter-rotating cells. Sea ice plays an important role in driving the overturning circulation, contributing to the formation of both dense bottom and lighter intermediate water.

decades, while precipitation changes are muted in East Antarctica. After considering the influence of the Southern Annular Mode, a steady spatially variable increase in precipitation remains, likely caused by global warming (Q.6 and Q.8).³¹

The role of extreme atmospheric events in the surface air mass balance above Antarctica is being explored. The impact of the top 10% of daily precipitation events across Antarctica has been evaluated using a regional atmospheric model simulation.³³ A key attribute of precipitation events is the penetration of warm, moist air masses over the ice sheet. Extreme precipitation events dominated the annual total being primarily responsible for interannual variations in snowfall. These results complicate interpretation of ice core records based on annual samples, pointing to the need for finer-scale records. The importance of surface melting for the future evolution of the Antarctic ice sheet was emphasized by the "ice-cliff instability" hypothesis, discussed below and the realization that widespread melting on Antarctic ice shelves could lead to break-up.³,³⁴ The extended summer melting event on the Ross Ice Shelf and Marie Byrd Land in 2016 originated from the poleward advection of maritime air into the continent linked to a strong ENSO event in the tropical Pacific Ocean.³⁵ Such extreme events may become more frequent as strong ENSOs become more common with consequences for the stability of the Ross and other large ice shelves (Q.2, Q.8, and Q.9). Projected increases in precipitation due to a changing

climate may mitigate sea-level rise by partially offsetting ice melt loss.³⁶

Cloud prediction is the largest uncertainty in atmospheric models over land ice, sea ice, and the ocean, with profound impacts on coupling with the underlying surfaces.³⁷ Predicting the correct proportions of cloud liquid water and cloud ice that govern the downward radiative fluxes from the atmosphere to the Earth's surface is especially challenging. A sensitivity study empirically demonstrated the importance of these surface radiation errors for simulations of large-scale Southern Hemisphere atmospheric and oceanic circulation.³⁸ Projections of future atmospheric change over the Southern Ocean remain uncertain, and persistent biases in climate models (including representation of clouds, winds, sea ice and ocean circulation, and stratification) require improvement (Q.7).

## Southern Ocean and Sea Ice in a Warming World

The Southern Ocean connects the upper and lower limbs of the global overturning circulation that largely sets the capacity of the ocean to store and transport heat and greenhouse gases, especially carbon dioxide (Figure 2 and Tables S3 and S4). Recent insights into the dynamics of the overturning circulation suggest the upwelling and downwelling limbs of the circulation are localized by interactions of water flow with sea-floor topography (Q.12, Q.14, Q.19, and Q.21).³⁹⁻⁴¹ The buoyancy added by northward transport and melt of sea ice is now recognized
as essential to transforming deep water to intermediate water in the upper cell of Southern Ocean overturning.⁴²⁻⁴⁴ The strength of the Southern Ocean overturning circulation varies from decade to decade,⁴⁵ but understanding of sensitivities to changes in forcings remains incomplete.

Compelling evidence from models and observations shows that the Southern Ocean is the dominant contributor to ocean storage of anthropogenic heat and carbon dioxide, which is then swept northward by the overturning circulation, delaying warming near Antarctica and increasing the ocean inventory of anthropogenic heat and carbon dioxide further north (Q.6, Q.12, Q.14, Q.22, and Q.23).⁴⁶⁻⁴⁸ At the time of the Scan, evidence from models and atmospheric observations suggested that the Southern Ocean carbon sink had declined, raising the prospect of a potential positive climate feedback. Recent ocean observations suggest that the decline in the 2000s was due to unanticipated decadal variability in the strength of the carbon sink, which has since returned to values observed in the 1990s.⁴⁹

Since the Scan, Antarctic sea ice has shifted from record high to record low extents (Q.15, Q.17, and Q.23).⁵⁰ This dramatic, and unanticipated, shift underscored incomplete understanding of processes influencing Antarctic sea ice distributions. The decline in sea-ice extent has been linked to several local and remote forcing mechanisms.¹⁷,⁵¹⁻⁵³ Little was known about the impact of ocean surface waves on sea ice and ice shelves in 2014. Several studies have now demonstrated that surface waves can drive the break-up of sea ice and, in the absence of this protective buffer, contribute to destabilizing ice shelves (Q.18).⁵⁴⁻⁵⁶ Basal ice-shelf melt by ocean heat transport beneath

ice shelves, discussed further below, varies with time and is linked by atmospheric teleconnections to low latitude climate variability described above.⁵⁷ It is now known that melt from ice shelves and icebergs influences ocean circulation, sea-ice extent, and the rate of global temperature rise (see Q.14–16 and Q.23).⁵⁸,⁵⁹

Patterns of change in the Southern Ocean have been shaped by ocean circulation, particularly the overturning cells (Q.12–Q.23). Southern Hemisphere oceans are responsible for most of the last 15 years of increase in global ocean heat content.⁶⁰ Antarctic bottom water continues to warm, freshen, and reduce in volume, contributing to changes in ocean heat content and sea level,⁶¹ reflecting multi-decadal trends and responses to episodic events such as iceberg calving.⁶²,⁶³ The ocean and sea ice respond to and drive extreme events, for example, the recurrence of the Weddell Polynya⁶⁴ and recent reductions in sea-ice extent. Future responses will be driven by passive heat advection, freshwater inputs, and changing ocean currents.⁴¹,⁴⁷,⁴⁸

## Antarctic Ice Sheets and Sea Level

Present-day continental Antarctic ice substantially contributes to global sea-level rise and will be increasingly important in the future (Figure 3 and Tables S5 and S6). While Antarctica's contribution remains the major uncertainty in extreme sea-level projections, especially on timescales of centuries to millennia, significant progress in addressing this uncertainty has been made. The loss of Antarctic ice on land is expressed far beyond the southern polar regions as global sea-level rise has widespread socioeconomic consequences.⁶⁵

<figure><img src="image_4.png" /><figcaption>Figure 3. A Schematic Illustration of Key Aspects of the “Antarctic Ice Sheets and Sea Level” Theme</figcaption></figure>

The principal influences on Antarctic glaciers such as snow, winds, and calving fronts are pictured using Thwaites Glacier as an example (CDW, Circumpolar Deep Water; ASL, the Amundsen Sea Low). Changes in marine ice sheets are initiated by changes in atmospheric and oceanic drivers that affect ocean circulation, surface precipitation accumulation, and summer surface-melt rate. Warm, dense mid-depth ocean water surrounding the Antarctic continental shelf is upwelled onto the continental shelf toward the ice fronts and ice-shelf grounding zones along troughs, causing increased melting and retreat at the ice-ocean interfaces. This thins the ice shelves, reducing drag along their sides and at local pinning points on sea-floor highs, which in turn reduces the buttressing. Thinning ice shelves lead to faster grounded-ice flow, leading to further thinning and causing previously grounded ice to float as the grounding zone retreats. Marine ice-sheet retreat is accelerated by surface-meltwater-driven hydrofracturing or other processes that lead to rapid calving of the ice shelf and ice front. Loss of the ice shelf may lead to cliff failure dramatically increasing the rate of grounded marine-terminating glacier calving. Changes in snow accumulation over glacier catchments can affect the timing of collapse and mitigate a portion of the sea-level contribution. This figure has been modified from the original source.⁶⁶
Improved satellite observations indicate that the contribution of Antarctica to sea-level rise has increased in recent years.⁶⁷,⁶⁸ The ice loss is concentrated in West Antarctica where the thinning of floating ice shelves is causing glacier flow to accelerate and grounding lines to retreat.⁶⁹,⁷⁰ Several Scan questions relate to the need for a better understanding of processes underlying ice loss (Q.24–Q.27, and Q.29). As described above, thinning of ice shelves by ocean-driven basal melt is the primary contributor to accelerated mass loss from the Antarctic ice sheet (Q.30 and Q.31).⁶⁷,⁷¹ While most effort has focused on West Antarctica, recent studies reveal that some ice shelves in East Antarctica, once thought to be frozen in time, are also exposed to ocean heat and are experiencing rapid rates of basal melt.⁷² These studies have concluded that the East Antarctic Ice Sheet as a whole is losing mass.⁶⁹ There is emerging agreement that the Marine Ice-Sheet Instability grounding-line retreat is triggered by ice-shelf thinning or destruction. Ice-shelf weakening is intensified, in addition to the basal melting by ocean warming described above, by changes in ocean circulation, hydrofracturing, and the loss of sea ice (Figure 3).³⁴,⁵⁵,⁷³,⁷⁴ Biases in oceanic general circulation models (OGCMs) lead to uncertainties in estimates of the magnitude of future warm-water incursions.⁷⁵ With the advent of model intercomparison projects,⁷⁶ some OGCMs now include sub-ice-shelf cavities in their formulations.

Precise bathymetry is important for coupled ice-ocean models, but more detailed data are needed to improve forecasts (Q.24). Similarly, while understanding of internal ice-sheet processes has improved, feedbacks between them that may underly rapid ice-sheet retreat remain undefined. A feedback with the solid Earth, in which ice retreat leads to bedrock uplift and stabilization, may slow future ice loss (Q.40).⁷⁷,⁷⁸ Other ice-ocean-atmosphere feedbacks have only recently been identified, and some, such as increasing meltwater slowing overturning circulation, have the potential to increase contributions to global sea-level rise.⁵

Recently, numerical ice-sheet models have incorporated more rigorous simulations of grounding line retreat. Several model intercomparisons have established benchmarks for simulations of Marine Ice-Sheet Instability (Q.25).⁷⁹ A second process, Marine Ice-Cliff Instability, has emerged as potentially significant for extreme projections,³ although the necessity of including this process in models remains unclear.⁸⁰ Recent work has shown that the Thwaites Glacier, a major ice stream draining into the Amundsen Sea, may be under threat of collapse due to the nature and rates of bed changes. Some models simulate that a threshold for irreversible grounding line retreat has been, or is about to be, crossed in the next century and the probability of retreat is higher in warmer scenarios (Figure 3).⁸¹,⁸² There have been several attempts to identify tipping points in terms of mean global warming, where parts of Antarctica ice sheets begin irreversible retreat (Q.28). Some suggest that avoidance of serious retreat requires a commitment to representative concentration pathway of 2.6 Watts/m².³,⁴ Others suggest that a long-term tipping point exists at ~2°C of global warming.⁷⁹ However, tipping points are difficult to predict as ice-sheet dynamics are complex, and not all parts of an ice sheet are expected to simultaneously or similarly respond to global warming. Improved, finer-scale models are essential to validating these predictions.

## Dynamic Earth: Probing beneath Antarctic Ice

The Dynamic Earth questions address the geological characteristics and processes beneath the ice (Tables S7 and S8). With only ~2% of the continent's bedrock exposed, these questions are best advanced by geophysical surveys and direct-access drilling (Figure 4). The challenge, given the scale of the continent, is survey coverage and density of sampling. Proxy-based studies of geological records provide further insights. The Scan questions remain essential to comprehending how geology is linked to ice sheet and climate processes in the past, present, and future. Progress has been made on a few of the questions, but most remain largely unanswered due to several factors.

The geophysical exploration of the continent is incomplete, hampering answers to questions in this theme. Despite 50 years of airborne geophysical surveying of Antarctica, continental coverage remains limited due to remoteness and the hostile conditions. The utility of existing data is restricted by spatial extent and resolution, logistical compromises, and platform and equipment limitations. However, there has been progress in measuring the magnetic field anomaly through airborne geophysical campaigns.⁸³,⁸⁴ Similarly, knowledge of subglacial topography remains limited (Q.39), but existing and planned data compilations and analyses are progressing (Q.24, Q.26, and Q.27).⁸⁵,⁸⁶ Magnetic field observations, seismic tomography, and radiometric radar analyses assist in defining geothermal heat flux (Q.36). Contradictory results indicate that more integrated regional and continental surveys will be necessary to advance understanding of the role of geothermal heat flux in geological processes and ice flow.⁸⁷,⁸⁸ Although several nations are conducting systematic surveys, international cooperation and data intercomparability are lacking. A successful program, in this regard, is the Polar Earth Observing Network (POLENET), which has led to advances in knowledge of lithospheric properties through a combination of satellite, airborne, and ground-based measurements (Q.37 and Q.38).⁸⁹,⁹⁰ This network of sensors is investigating systems-scale interactions of the solid earth, the cryosphere, the oceans, and the atmosphere. These measurements evaluate ice-sheet "budgets" to better understand polar ice-sheet contributions to global sea levels.

Progress in accessing climate records in subglacial environments is limited (Q.34). Attempts have been made to advance drilling technologies to access sub-ice targets⁹¹,⁹², but none have achieved routine use. Ice coring is internationally coordinated⁹³ and, while these activities focus on retrieving ice samples for paleoclimate records, access to and sampling of the underlying bed are essential for validating models.⁹⁴ A rock-coring project to extend records of climate to the interior of the continent remains an aspiration.⁹⁵ Progress has been made in collecting proxy data from oceanic coring.⁹⁶ The challenge will be to integrate oceanic geological records with those from the interior of the continent, providing a more complete and varied record of past climates.

Existing data would be more impactful if they were organized within multi-disciplinary frameworks. Promisingly, international collaborations are emerging to use radar to investigate ice-sheet internal structure.⁹⁷ These efforts will advance understanding of geothermal heat flux, gaining knowledge of geological processes and their impact on ice flow.⁹⁸ There is a growing appreciation of multi-technique analyses in geophysics, and some
<figure><img src="image_5.png" /><figcaption>**Figure 4.** A Schematic Illustration of Key Aspects of the “Dynamic Earth: Probing beneath the Ice” Theme</figcaption></figure>

The techniques and locations of past and current Antarctic subsurface sampling and/or measurements are presented. Questions addressing the geological characteristics and processes beneath the ice are best advanced by geophysical surveys and direct-access drilling on land and in the ocean. Surveys; sediment, ice, and rock sampling; and proxy-based studies of geological records provide insights into how geology is linked to ice-sheet and climate processes in the past, present, and future. The geophysical exploration of the continent is incomplete, and continental coverage remains limited due to remoteness and the hostile conditions. Integration and adoption of standard methodologies based on field geological surveys, deep ice coring (including basement sampling), ocean drilling, geophysical surveys, and continental airborne surveys are essential. Note that no deep sedimentary cores have been obtained from beneath the present grounded-ice cover.

projects are making data widely available.⁹⁹ Improvements in numerical modeling of geological processes are essential.¹⁰⁰ While vital to quantitative knowledge, and the only means to forecast future responses, models are currently limited by definition of inputs and the availability of validating datasets, such as those discussed above, beneath Antarctic ice sheets and in the interior of the continent. Other important questions have emerged since the Scan, including detecting and quantifying the presence of groundwater in Antarctica.¹⁰¹ Projects are underway to make the first measurements; however, the geological and glaciological significance of continental groundwater remains unknown.

## Antarctic Life on the Precipice

Antarctic Life on the Precipice is an expansive area of research given the scope of the life sciences, including consideration of the human-environment interface, the state of and trends in life-sustaining processes, questions of adaptational responses to change, and the efficacy of conservation practices given threats to biodiversity (Figure 5 and Tables S9 and S10).

Biodiversity and ecosystem responses to environmental changes are a major focus of Antarctic life sciences research. Progress has been made on understanding the effects of extreme events on biodiversity (Q.63). For example, ice-shelf loss leads to the loss of under ice-shelf communities while
<figure><img src="image_6.png" /><figcaption>**Figure 5.** A Schematic Illustration of Key Aspects of the “Antarctic Life on the Precipice” Theme</figcaption></figure>

Changing living environments in Antarctica provide new challenges for life sciences research. Recent research findings include (i) the discovery of new communities where ice shelves have disintegrated, (ii) high speciation rates among fish and brittle stars in marine systems, (iii) altered plant abundances and distributions and changing bird life histories in response to changing westerly winds, (iv) declining penguin abundance for some species and rapidly shifting distributions for others, (v) significant impacts of infrastructure including facilitation of the introduction of invasive species, and (vi) the discovery of microbial life in subglacial settings.

opening ocean and seabed areas to new colonization.<sup>102-104</sup>

These events alter community structure and diversity, favoring carbon sequestration and resulting in greater CO₂ uptake.<sup>105</sup>

The Brunt Ice Shelf collapse in the Weddell Sea provides a recent example of an extreme event that has eliminated habitat for the world's second largest Emperor penguin colony,¹⁰⁶ reprising a previous extreme event in East Antarctica.¹⁰⁷ In terrestrial systems, a flood event during 2001–2002 in the McMurdo Dry Valleys changed the system dramatically in the years that followed, with asynchronous responses among different components of the living environment.¹⁰⁸ Responses to slower change (e.g., non-extreme events) have also been documented. For example, changes in climate, in part due to an increasingly positive Southern Annular Mode, have led to drying in East Antarctica, re-arranging moss assemblages from those dominated by moisture-preferring endemics to those dominated by more drought

tolerant widely distributed species.¹⁰⁹ On the Antarctic Peninsula, moss communities have shown rapid increases in microbial productivity and moss growth and accumulation rates since the 1950s, across a record spanning of more than 150 years.¹¹⁰ In marine systems along the Antarctic Peninsula, southward shifts in the distribution of krill, leading to declines of abundance in the north and increases in the south, have also been documented, with profound implications for predators, ecosystems, and their interactions with fishing interests in krill.¹¹¹ Questions that remain largely unaddressed include the effects of year-round ice-free intertidal conditions on biodiversity (Q.57 and Q.60), the impact of introductions of alien species (Q.54 and Q.55), the response of Antarctic marine species to changing soundscapes (Q.51), and the synergistic effects of multiple stressors on Antarctic biota (Q.50). Studies of the effects of changing ocean front dynamics are also limited (Q.22 and
Q.65). What future sea-ice loss and habitat loss will mean for winter-breeding seals, effects on pelagic and epontic productivity, and impacts on benthic resource supply remains poorly known (Q.59 and Q.60).⁷ Prominent Antarctic drivers are altered climate and resulting changes in Southern Ocean physical and chemical properties, sea-ice extent and phenology, pollution, invasive species, and direct human impacts, including fisheries activities.¹¹² Approaches have broadened, but large-scale studies assessing multiple, potentially cumulative drivers acting in synergy remain scarce. Likewise, questions in the life sciences theme require long-term monitoring, repeated observations, and/or extended time series. Such questions include those that address the impacts of changing environments, tectonics, volcanism, and ice-sheet mass loss on biodiversity, and the effects of ozone hole recovery on ecosystems (Q.11, Q.23, Q.41, Q.46, and Q.63). Yet such long-term research is uncommon.

Answers to questions about biological adaptations, including the resilience of organisms and changes in ecosystem functioning, have advanced. Understanding of species-level physiological responses to acidification and to temperature change continues to develop, and temperature effects appear to elicit greater responses (Q.49).¹¹³ Advances have been made in identifying the most vulnerable ecosystems and predicting their responses to environmental changes (Q.48). However, there are few studies of the cascading effects from future sea-ice reduction or habitat or ecosystem loss. Genome-enabled studies have improved understanding of adaptations,¹¹⁴,¹¹⁵ mutation rates, and gene flow (Q.43, Q.44, Q.54, and Q.64). Characterization of the Antarctic icefish genome revealed cold-adapted phenotypes (a lack of functional hemoglobin genes and red blood cells), genes encoding antifreeze glycoprotein involved in protection from ice damage, adaptations to the high concentration of oxygen dissolved in cold Antarctic waters, and possibly compromised control of biological rhythms in polar light environments.¹¹⁵ Better comprehension of differential expression patterns of genes and dispersal in Antarctic environments in macrobiota and microbial systems have contributed to knowledge of adaptation.¹¹⁶⁻¹¹⁸ Other areas, such as “-omics” analyses of biodiversity for ecological forecasting, have made less progress (Q.64). Understanding of paleo-ecosystem responses to previous warmer periods and past extreme events has improved with the identification of refugia during glacial maxima, which made the survival of life possible during ice ages, and the contribution of Antarctic biodiversity to global species richness (Q.45 and Q.46), furthering knowledge of the evolution of Antarctic biodiversity.¹¹⁹⁻¹²² Some progress has been made on subglacial ecosystems and biogeochemical processes in isolated systems (Q.45).¹²³

Novel contaminants are arriving in Antarctica, and progress has been made in understanding impacts at the cellular, physiological, and population levels, furthering knowledge of the responses of potentially sensitive biota (Q.52 and Q.53).¹²⁴⁻¹²⁶ The ubiquity of plastics and microplastics in the Southern Ocean is now widely recognized, but impacts on Antarctic biota remain to be fully investigated, although it is known that macro-plastics pose a significant threat to seabirds.¹²⁷,¹²⁸

Understanding of climate interactions with invasive species and diseases is limited. Invasive species research has identified

the sources and types of propagules as well as their pathways to Antarctica (Q.54).⁸,¹²⁹,¹³⁰ Distinguishing range shifts from introductions remains underinvestigated, as do studies of potential impacts (Q.55).¹³¹,¹³² Climate-change impacts on establishment likelihood have been evaluated, although species distribution modeling is underutilized. There are few investigations of how climate-change effects on sea ice and physical access to fishing grounds will affect fisheries, krill stocks, and krill-dependent predators. Little is known about changes in marine biogeochemistry that might be caused by fishery-induced fluctuations in krill stocks (Q.58 and Q.61).¹¹¹,¹³³,¹³⁴

Understanding of direct human impacts on the Antarctic, such as large-scale human modifications associated with the emplacement of infrastructure, will require long time-series monitoring (Q.74, Q.75, Q.76, and Q.80). While relatively few Antarctic terrestrial sites have witnessed large-scale modification, more than half of the accessible coastline is estimated to have been affected.¹³⁵ “Human footprint” assessments of infrastructure and associated activities in ice-free areas are developing using remote sensing techniques. Indications of large-scale environmental modification are emerging. Threats to Antarctic biodiversity from increasing global exploitation of ecosystem services, among other stressors, remains an unrealized research priority (Q.58 and Q.61), although the importance of investigating global stressors has been identified.¹¹²,¹³⁶

Human-associated viruses have infected Antarctic marine mammals and birds, and diseases have been studied using genomic technologies (Q.56 and Q.80).¹³⁷ Climate-change effects on these interactions remains largely unknown and are an important focus for future research. Improving biosecurity measures in Antarctica is fundamental, as it has been shown that alien species and pathogens can be resilient.

The recently adopted Ross Sea Marine Protected Area (MPA), the second international MPA adopted in Antarctica, is an example of progress in implementing conservation measures. However, the efficacy of these MPAs in meeting protection objectives (Q.66) remains unknown, particularly given some of the trade-offs required for approval.¹²,¹³⁸ Research is underway to assess and monitor the MPAs, including how they might protect ecosystem processes despite resource extraction. By contrast, in terrestrial systems, substantial work has covered the effectiveness of the current Antarctic Specially Protected Area system in representing the Antarctic's ecoregions (known as Antarctic conservation biogeographic regions) and biodiversity more generally.¹³⁹,¹⁴⁰ The work has also indicated how the protected area system can be expanded and what might be required to meet either regional or global aspirations.⁸ Evolutionary potential assessments of protected areas have not, but they should be made. A systematic evaluation of *ex situ* conservation has not been undertaken, although the first Antarctic genetic repository has been established (Q.67). This effort needs expansion and procedures to preserve and make samples widely available need to be agreed.

## Near-Earth Space and Beyond: Eyes on the Sky

The Eyes on the Sky questions address two foci, astronomy and near-Earth space (geospace), that use Antarctica as a platform to gaze spaceward. The stable atmospheric conditions, radio quiet areas, and year-round observations have led to an array
<figure><img src="image_7.png" /><figcaption>**Figure 6.** An Image of Important Infrastructure in the Antarctic that Underpins Key Research in the “Near-Earth Space and Beyond: Eyes on the Sky” Theme</figcaption></figure>

Antarctica has several important advantages for studies of space-related phenomena, both solar-terrestrial and astronomical. Extremely stable atmospheric conditions and expanses of radio quiet areas, together with the ability to observe objects continuously throughout the long winter or summer, offer unique conditions for observing the aurora australis (southern lights), distant stars, supernova explosions, and the cosmic microwave background. Pictured above is the 10 m South Pole Telescope (left) and the IceCube Neutrino Detector (right) at South Pole Station, framed by the aurora. The IceCube array searches for elusive particles called neutrinos, believed to emanate from exotic astrophysical objects such as quasars and black holes, while the South Pole Telescope explores the afterglow of the Big Bang. Closer to Earth, optical and radio experiments investigate the southern lights, which are produced by electrons (and protons) that strike the gases in the upper atmosphere. Electrical currents that are produced during such auroral displays can have deleterious effects on space-borne and ground-based technological systems, disrupting satellite electronics, global positioning satellite signals, and power grids and exposing people to radiation.¹⁴¹

of world-class facilities and laboratories in Antarctica (Figure 6 and Tables S11 and S12).

Astronomy questions address the origin of the universe and its content. Recently, the field of astrophysics has undergone a technological revolution. In the past, observations were pho- tonic in nature, capturing information across the electromag- netic spectrum. Information now comes from gravitational wave¹⁴²,¹⁴³ and high-energy particle observatories.¹⁴⁴,¹⁴⁵ The “multi-messenger” astrophysics era began with the South Pole IceCube Neutrino Observatory, which identified the first high-energy neutrino source.¹⁴⁶,¹⁴⁷ The Event Horizon Tele- scope, a network that includes the South Pole Telescope, captured the first image of a black hole.¹⁴⁸ This discovery confirmed several elements of general relativity and enables the study of dark matter.

The South Pole Telescope is one of several observa- tories¹⁴⁹,¹⁵⁰ in Antarctica that study the Cosmic Microwave Background radiation, the oldest electromagnetic radiation in the universe. Precise measurements of the Cosmic Micro- wave Background are critical to validate models of the uni- verse. To achieve greater resolution, observatories will increase detector density from hundreds to tens of thousands by the early 2020s. These improvements may lead to the discovery of primordial gravitational waves originating in the early universe, that, if observed, would confirm cosmic infla- tion models. The discovery of B-mode polarization of the Cosmic Microwave Background caused by gravitational lens- ing is testing theories of the formation of the universe.¹⁵¹ The multi-messenger approach is rapidly advancing the search for dark matter. Sources of gravitational waves, such as neutron stars or black hole mergers, release high-energy neutrino emissions. Laser Interferometer Gravitational-Wave Observa- tory and IceCube collaborations have revealed cosmic explosions,¹⁴²,¹⁴⁵,¹⁵² previously unseen by conventional photonic observations. The understanding of the full nature of dark matter remains aspirational. IceCube is designed

to detect neutrinos originating from the decay of dark matter.¹⁴⁷

Antarctica is a key observing platform and a unique window for the study of a broad range of geophysical phenomena, spanning magnetic and geographic latitudes. The high-lati- tude middle and upper atmosphere is a complex system with energetic and dynamical inputs coupled with internal feedback processes. From the outer magnetosphere and so- lar wind, energetic particles and waves follow magnetic field lines into the middle and upper atmospheric system. From the lower atmosphere gravity waves, planetary waves, and tidal waves propagate upward, depositing energy and mo- mentum. Within the system, a mixture of neutral constituents and ionized gases, complex chemical reactions, and magnetic and electric fields generate several processes with local and global feedbacks.

Since the Scan, instrumentation for geospace research has increased, and progress has been made in understanding the sources of atmospheric gravity waves in and around Antarctica,¹⁵³ the effects of energetic particle precipitation,¹⁵⁴ and magnetic and neutral atmospheric “interhemispheric conju- gacy.” For example, atmospheric gravity wave observations from the McMurdo lidar¹⁵⁵ and the Antarctic Gravity Wave Instrument Network all-sky imager¹⁵³ have characterized polar vortex waves. The NASA Balloon Array for Radiation-belt Rela- tivistic Electron Losses mission has observed localized and temporally constrained energetic particle precipitation associ- ated with radio wave activity.¹⁵⁶,¹⁵⁷ The interhemispheric im- pacts of polar vortex variability during stratospheric warmings have now been recognized.

The term “space weather” generally refers to Sun condition, the solar wind, and geospace events that affect the performance and reliability of space-borne and ground-based systems. Besides emitting the solar wind, the Sun periodically releases bil- lions of tons of matter via coronal mass ejections. Immense clouds of material can move toward Earth, causing large
magnetic storms that disrupt radio and Global Positioning Satellite signals, shut down electrical systems, and expose people to radiation. Antarctic observations are critical to devising innovative responses to real-time events and improving space weather forecasts.¹⁵⁸

## Human Presence in Antarctica

A complex network of legal and political regimes and obligations underpin questions about the status of present, and possible future of, the Human Presence in Antarctica (Tables S13, S14, and S16). Antarctic governance, geopolitics, and tourism research has explored the geopolitical configurations of power in the Antarctic and beyond (Q.76 and Q.78).¹⁵⁹ Assessments of pressures on the Antarctic are co-entangled with global resource futures.¹⁶⁰ Antarctic tourism is experiencing rapid growth and diversification, which has led to novel regulatory options being proposed.¹⁶¹ As Antarctica changes and anthropogenic pressures increase, these relationships will require re-examination.

Understanding future human engagement with the Antarctic requires integration of research in political geography, international law, and international relations focusing on concepts of science, peace, and global legal norms as well as barriers to access (Q.77). Recent research on the politics and political importance of science and peace in the Antarctic builds on preceding work.¹⁶²,¹⁶³ The next generation of research will need to address how the politics of anticipation (either environmental or non-environmental anticipation and cultures of forecasting)¹⁶⁴ are tied to the distribution of capacities, the symbolic and material dimensions, and the role played by different actors with interests in Antarctica and beyond.

Geoengineering options are being discussed as a potential solution to mitigate climate-change impacts,¹⁶⁵,¹⁶⁶ although the techniques often do not address ocean acidification or greenhouse gas emissions reduction, especially those that manipulate solar radiation. The assessments of the impacts of geoengineering are only now starting to be made;¹⁶⁶,¹⁶⁷ except for discussions of controversial proposals for iron fertilization of the Southern Ocean.¹⁶⁸⁻¹⁷⁰ Research in the broader Antarctic region on geoengineering impacts has yet to be undertaken (Q.74 and Q.75) and more transdisciplinary methodologies will be required. Equally, the identification and classification of ecosystem services, of political and economic importance in other regions of the planet, have seen little progress in the Antarctic. One exception is the mapping of ecosystem services in the Weddell Sea (Q.79), although the importance of research in this area is now recognized.⁷,¹⁷¹

Many questions of human presence build on future trajectories and scenarios, which, as discussed above, will benefit from refinements. The politics of knowledge production must be connected to political and ideological effects. Scientific knowledge does not necessarily lead to anticipatory action designed to repair or restore ecological states. There is an implicit assumption that if more reliable and robust information is available, then rationality and evidence will underpin actions. In contrast, research on human behavior concludes that “affect-based framings” drive motivations that lead to action. Science-based evidence from the Antarctic is adversely affected by larger societal trends, such as skepticism about

authority figures, data and information overload, paranoia about elites and their agendas, and indifference and apathy due to the “hyper-object” effect—issues (such as climate change) of such vast temporal and spatial dimensions that they challenge traditional ideas about how to motivate people to action.¹⁷²

In the future, lessons from the field of the politics of anticipation in environmental futures will need to be utilized to better communicate the importance of Antarctic research.¹⁷³⁻¹⁷⁷ Lessons learned about how change is communicated through media images, artwork, whalers' logs, diaries, newspapers, and the like¹⁷⁸⁻¹⁸⁰ will be essential to framing complex concepts in understandable ways. These improved framings are needed as knowledge counters long-held popular assumptions that Antarctic environments are pristine, stable, isolated, and reliably frozen. Communicating urgencies to the public is hampered by gaps in our understanding of how people and society are motivated to make better choices, especially if these choices entail sacrifices today to avoid catastrophic outcomes in a distant future.

## Engaging Diverse Audiences

There is an extensive literature on engaging diverse audiences within the field of science communication; however, epistemological and methodological challenges remain.¹⁸¹,¹⁸² Research on climate-change communication has grown over the last decade, partly focusing on the benefits of communicating science via advocates, brokers, and “science translators.”¹⁸³,¹⁸⁴ Public engagement with science is changing, and with it our understanding of the importance of sentiments in climate-change discourse. Emerging concepts include gendered and other intersectional responses;¹⁸⁵,¹⁸⁶ the utilization of augmented reality, virtual reality, and other digital visualization tools in communicating messages;¹⁸⁷,¹⁸⁸ and active engagement based on “establishing trusted two-way communication.”¹⁸⁴,¹⁸⁹ The Antarctic community has much to learn from science communication research, although these lessons are being applied.¹⁹⁰⁻¹⁹⁴ Research is increasing understanding of the linkages between knowledge, sentiment, and action in science as well as governance.

The goal of engaging diverse audiences is informed by the lessons learned from assessing how, by whom, and why the outputs from the Scan and ARC were used. Scan and ARC planning documents explicitly defined the intended audiences (Tables S15 and S16).²⁴,²⁵ One audience was the international Antarctic research community, including the International Science Council's Scientific Committee on Antarctic Research (SCAR).¹⁹⁵ SCAR is the premier international body facilitating scientific research in, from, and about the Antarctic, and under whose auspices the Scan was undertaken. SCAR's dual mission includes providing independent science advice to the Antarctic Treaty System linking science to policy. Another audience was the funders and supporters of Antarctic research, including the coordinating body, the Council of Managers of National Antarctic Programs (COMNAP), under whose auspices ARC was undertaken.¹⁹⁶ A third audience was the Antarctic Treaty System and its array of associated organizations, committees, and observers that depend on scientific advice for decision and policy making (Table S16). And finally, there were audiences
such as non-governmental environmental and advocacy groups, academia and teachers, next-generation scientists, and the public. The delivery of Scan and ARC outputs was assessed to discern the lessons learned and where improvements might be pursued (Table S15).

Scan publications were widely used by the scientific community with citations in the peer-reviewed literature, dissertations and theses, book chapters, and policy papers, spanning all Antarctic disciplines (Table S15). Several post-Scan publications address and/or expand on Scan priorities.<sup>130,197,198</sup> The Scan is referred to in national Antarctic science plans and calls for proposals. Several non-Antarctic horizon scans reference the Scan as an exemplar model. SCAR's strategic plan (2017–2022) describes how the Scan was used “... to guide research priorities and research directions over the next six years and beyond ...”<sup>199</sup> The Scan has served as justification for the formation of new SCAR Scientific Research Programs, existing SCAR groups have framed priorities based on the Scan, and international workshops, meetings, and conferences have been organized within the Scan framework.

The ARC project was the first action attributable to the Scan and was directed at the governmental entities that fund and provide logistical support for Antarctic research. The outcomes of ARC were widely distributed and contributed to a restructure of COMNAP's Expert Groups. ARC provided individual National Antarctic Programs with a better understanding of future science support needs. National Antarctic policies highlight science as a policy goal and tool and often point to the themes identified by the Scan. Several countries have incorporated Scan questions into their strategic planning and have used the outcomes to judge the importance of existing programs and projects, and whether realignments are needed.

It is early to judge how the Scan has informed ideas and practices of Antarctic governance and conservation. SCAR has reported information about the Scan to the Antarctic Treaty Parties. Reports of Antarctic Treaty Consultative Meetings (ATCMs), the Committee on Environmental Protection (CEP), and the Science Committee of the Convention for the Conservation of Antarctic Marine Living Resources (SC-CCAMLR) refer to the Scan and/or ARC (Table S15).

Regarding outreach to the public, the organizers of the Scan conducted many media interviews before and after the Scan event, including print, TV, and radio, and made presentations to multiple organizations. Media ecologies are rapidly changing, posing opportunities and challenges for communicating the co-production of knowledge and understanding of the importance of the Antarctic to the rest of the world.<sup>200</sup>

The Scan and ARC influenced diverse audiences, lending insight into how future research and information can be more effectively mobilized and utilized. Many factors determine the influence, reception, and uptake of futures studies.<sup>22</sup> Judging impact and uptake can be elusive due to limited acknowledgment of usage beyond periodical literature attribution. In the policy arena, the origins and pathways to advice are often obscured as it is blended with other inputs and influenced by non-scientific factors. In the future, the findings of Antarctic research need to be more widely distributed via social media and emerging alternative forms of commu-

nication, such as short videos, hashtag campaigning, and podcasts.

## Conclusions and Outlook

The view from the south is, more than ever, dominated by ominous signs of change. Over the past 5 years, addressing the priorities identified by the Scan have led to new insights of global significance (Tables S1–S14). While much has been accomplished, much remains to be learned, and a renewed commitment to gathering further knowledge will quicken the pace of our understanding of Earth systems and beyond.

Current knowledge of the Antarctic is constrained by a lack of critical observations, due in large part to the vastness, remoteness, and inaccessibility of much of the region and exacerbated by often-severe weather conditions and yearly months of darkness. The ARC identified “access” to the continent and surrounding oceans as one of the major challenges in implementing the Scan Antarctic science roadmap.<sup>21,23</sup> In contrast to direct physical access, the ability to view the globe from satellites, and with airborne sensors, has revolutionized Earth, and at the same time Antarctic, science.<sup>201</sup> Observations support the development, refinement, and calibration of Earth system models for forecasting futures. For the Antarctic, improved models are needed that accurately represent key elements of the Antarctic system, including the atmosphere, the ocean, sea ice, ice sheets and shelves, the solid earth beneath the ice and sea, and the biota and ecosystems within; and the interactions, couplings, and feedbacks between these spheres. Underpinning observations are process studies that further inform model development and parametrization while also serving as validating datasets. There are a range of activities addressing the need for Earth system observations, model development, and process studies, including the Group on Earth Observations (GEO),<sup>202</sup> the Global Climate Observing System (GCOS) of the World Meteorological Organization,<sup>203</sup> the Intergovernmental Oceanographic Commission of UNESCO (IOC-UNESCO),<sup>204</sup> national space agencies (e.g., US National Aeronautics and Space Administration Earth Observations [NEO]<sup>205</sup> and the European Space Agency Observing the Earth<sup>206</sup>), the SCAR programs described above, and others.

As an intrinsic part of the Earth system, a commitment to an ambitious and sustained Antarctic research plan going forward is essential, including:

* Gathering multi-year data via integrated region-wide suites of autonomous instruments monitoring surface to lower stratosphere atmospheric state, turbulent and radiative fluxes, and atmospheric composition to advance atmospheric and climate models.

* Enhancing observations of the Southern Ocean, in tandem with atmospheric observing systems above, via an integrated observing system that illuminates the “blind spot” in present ocean observing efforts below sea ice and floating ice shelves and on the Antarctic continental shelf, delivering the process-level understanding needed to clarify the vulnerabilities of the Antarctic ice sheet and sea ice to ocean change.

* Expanding deployments of instrumental arrays that capitalize on the revolution in ocean sciences brought about
by autonomous instruments (e.g., profiling floats and animal-borne sensors). These arrays will collect continuous observations that cover the full ocean depth equipped with a broad suite of physical and biogeochemical sensors. Observations provide the mechanistic understanding necessary for anticipating the impacts of Southern Ocean change on climate, sea level, and biogeochemical cycles and the cascading effects on biota and ecosystems.

* Combining the broad-scale ocean observations above with detailed process studies and a hierarchy of numerical simulations to better define the sensitivities of Southern Ocean overturning circulation (and therefore the ocean heat and carbon sink) to changes in forcings.

* Systematically and comprehensively surveying the Antarctic continental ice sheet to improve knowledge of ice-sheet dynamics and critical thresholds that accelerate ice loss and sea-level rise.

* Expanding the coverage of high-resolution swath bathymetry surveys of the bed beneath the Antarctic ice sheet by aircraft and/or satellites to develop better representations of grounding line dynamics in models.

* Developing and deploying multi-sensor autonomous (adaptive and self-managing) unmanned underwater vehicles in difficult to sample, biological “hotspots” and rapidly changing environments that complement the observatories above. Observatory networks should also be placed in key coastal regions, such as Wilkes Land and Aurora and Recovery Glaciers. These networks will detect early signs of change, the onset of tipping points, and better define forcings serving as an early warning system of impending, possibly irreversible, change.

* Implementing a coordinated network of systematic, interdisciplinary circumpolar oceanographic campaigns to produce biological time series, coordinated with the ocean and atmosphere observing above, to elucidate biotic-abiotic interactions and to monitor the status and trends of indicators of the health and trajectories of Antarctic ecosystems.

* Establishing a complementary terrestrial circumpolar life observatory network to provide terrestrial/coastal biological time series to detect and monitor species colonization, local extinctions, and invasions.

Beyond the future research directions above, there are a series of important efforts that need to be undertaken in the Antarctic that complement and support enhanced observations and modeling. These additional elements are presented for the Scan themes; however, most are transdisciplinary and cross-cut themes with progress in one area dependent on developments in others, calling for international coordination, cooperation, and integration for greatest effect. A final set of research priorities addresses effective communication with stakeholders that will fortify calls to action.

Ongoing priorities for future Antarctic atmospheric research are:

* Refining the details about teleconnections between low, mid, and high latitudes.²⁰⁷

* Increasing computational capacity to improve atmosphere-ocean coupled climate simulations via large-ensembles treatment of small-scale processes that better isolate the effects of natural variability and highlight anthropogenic signals.

Ongoing priorities for future Southern Ocean, sea ice, ice sheet, and sea-level research are:

* Improving definition of the role of variability in Antarctic sea ice and ocean-ice-shelf interactions as key forces driving future climate and sea-level change.

* Setting the boundary conditions for future trajectories of, and rates of change in, the West and East Antarctic Ice Sheets to better constrain understanding of the societal consequences of sea-level rise.⁵,⁶⁷

* Augmenting understanding of the processes underlying, and the geographical distributions of, basal hydrology, ice damage, calving, ice cliff failure processes, and hydrofracturing, refining ice-sheet model parameterization and improving forecasts.

* Continuing and expanding model intercomparison experiments to further illuminate key processes that are essential to improving coupled ocean/sub-ice-shelf cavity/ice-sheet models within a global system context.⁶,²⁰⁸

Ongoing priorities for future Antarctic solid earth sciences research are:

* Completing comprehensive geophysical exploration of the continent, improving access to subglacial environments, integrating synthesis of existing and future data, and improving numerical models of geological processes.

* Collecting further marine sedimentary records to advance understanding of past changes in ocean circulation by identifying forcings and ice sheet-ocean interactions that are difficult to directly observe; establishing a network of trans-continental sites to complement ongoing and planned oceanic expeditions and recover unique paleoclimate records from Antarctica's interior that will expand knowledge of the past and validate forecasts.

* Adopting holistic systems models to co-determine glacial, subglacial, and oceanic processes and responses to improve forecasts.

Ongoing priorities for future Antarctic life sciences research are:

* Continuing to improve understanding of the factors that lead to loss of biodiversity, how ecosystems respond to changing environments, identifying biological adaptations, defining strategies of resilience, and assessing the efficacy of conservation practices.

* Exploring and better defining the complex downstream implications of ozone hole recovery and its relationship to the Southern Annular Mode for Antarctic and Southern Hemisphere ecosystems.¹⁵,¹⁶

Ongoing priorities for future Antarctic social sciences and humanities research are:

* Improving our understanding of human interactions with the environment, better defining possible governance responses in the face of change and discerning flow-on effects in the Antarctic from external geopolitical influences.
* Defining how changing Antarctic ecosystem services will affect resource regulation as pressures to capitalize on its resources increase.⁷,¹²

* Exploring the role of civic epistemologies in driving interactions within the complex network of organizations in Antarctica to better understand why scientific agreement on climate change fails to stimulate political action.

* Improving messaging of Antarctic urgencies to diverse audiences by exploring the use of augmented and virtual reality and artificial intelligence in communicating complex scientific datasets and concepts to immerse people in a region of the planet most will never visit.

* Utilizing big data via supercomputing and artificial intelligence to better understand human decision making and behavior using agent-based models and how this applies to Antarctic science communication.

* Exploring the role of “surveillance capitalism” and “predictive economies of action” concepts in the context of Antarctic data economies, including how data are collected and used to influence and manipulate human behavior.

* Evaluating the implications of earthly surveillance in geopolitics, economics, and culture as “capital.”

In closing, the slogan of the 2019 UN Climate Action Summit (UNCAS)²⁰⁹ is “a race we can win,” a win that is only attainable if a credible and effective path to the future is defined. Credibility—the capability to persuade others that something will happen or be successful—is fundamental to the global debate on climate change and how societies will, or will not, respond. In this review, the critical role of Antarctic science in “making the case” for concerted action is highlighted. A better understanding of the drivers, underlying processes, feedbacks, transitions, tipping points, and rates of change within the Antarctic region will, in large measure, dictate how “winnable” this race will be. Critical thresholds that foretell changes of state, sometimes irreversible, are needed to provide unambiguous signposts of trouble ahead and indicate necessary course corrections. This review summarizes not only what we know, but more importantly what we do not yet know. As 2019 UNCAS states “... there is a growing recognition that affordable, scalable solutions are available now that will enable us all to leapfrog to cleaner, more resilient economies ....” But the central question remains, is there “... the political will necessary to move forward on ambitious climate action for the benefit of all aspects of society ...”?²⁰⁹ It is abundantly clear that no one nation can accomplish the ambitious Antarctic research roadmap laid out in this review and that climate change calls for an “all-nations” commitment to common cause as the window for action on climate change closes.

## SUPPLEMENTAL INFORMATION

Supplemental Information can be found online at https://doi.org/10.1016/j.oneear.2019.08.014.

## ACKNOWLEDGMENTS

M.C.K. acknowledges the contributions of all those who contributed to the original Horizon Scan and Antarctic Roadmap Challenges projects and to

the research that has since been done to address their outcomes. The Tinker Foundation played a significant role through partial funding of the original projects and support of the Tinker-Muse Prize for Science and Policy in Antarctica. M.C.K. and S.L.C. acknowledge the editing and review assistance of Lucy Klein of Monash University.

## AUTHOR CONTRIBUTIONS

M.C.K. and S.L.C. served in the roles of conceptualization, supervision, project administration, writing original draft, writing review and editing. B.N., C.R., D.B., D.L., L.P., M.J.S., S.R.R., and T.T. served in the roles of section lead author, writing original draft, writing review and editing. A.A., A.M., C.M.B., C.L.B., D.C., D.M.S., D.G.V., H.Y., J.C., K.D., L.T., L.W., M.L., M.N.R., M.R.-F., N.R.G., P.C.N., and S.C. served in the roles of section supporting author, writing original draft, writing review and editing.

## DECLARATION OF INTERESTS

M.C.K is a past-president of SCAR (2008-2012), and S.L.C. is president of SCAR (2016-2020). The remaining authors declare no competing interests other than a life-long appreciation and a personal commitment to preserving the majesty of Antarctica, an obligation to future generations to be good stewards of our planet, and a pledge to strive for a brighter, more sustainable future.

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# Supplemental Information

## Sustained Antarctic Research:

### A 21st Century Imperative

Mahlon C. Kennicutt II, David Bromwich, Daniela Liggett, Birgit Njåstad, Lloyd Peck, Stephen R. Rintoul, Catherine Ritz, Martin J. Siegert, Alan Aitken, Cassandra M. Brooks, John Cassano, Sanjay Chaturvedi, Dake Chen, Klaus Dodds, Nicholas R. Golledge, Céline Le Bohec, Marcelo Leppe, Alison Murray, P. Chandrika Nath, Marilyn N. Raphael, Michelle Rogan-Finnemore, Dustin M. Schroeder, Lynne Talley, Tony Travouillon, David G. Vaughan, Lifan Wang, Allan T. Weatherwax, Huigen Yang, and Steven L. Chown
<table><caption>Table S1. Expert (subjective) assessments of progress toward answering the “Antarctic Atmosphere and Global Connections” questions supported by peer-reviewed literature citations. Comments by experts from crosscutting clusters are identified as ‘Crosscuts [name of cluster]’. The references cited are exemplars and not intended to be an exhaustive literature review.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Conclusions</td></tr></thead><tbody><tr><td>1. How is climate change and variability in the high southern latitudes connected to lower latitudes including the Tropical Ocean and monsoon systems?</td><td>Recent studies of tropical - polar linkages associated with climate variability, on a variety of time scales, highlight the key role that Rossby waves, emanating from the tropics, play in forcing intra-seasonal to decadal variability in Antarctic and Southern Ocean climate.<sup>3</sup> Sea Surface Temperature (SST) anomalies, and both El Niño-Southern Oscillation (ENSO) and Madden-Julian oscillations (MJO), are identified as tropical forcing mechanisms for these waves which impact high southern latitude temperature, sea ice cover and zonal winds. These effects are most marked in West Antarctica and the Antarctic Peninsula.</td></tr><tr><td>2. How do Antarctic processes affect mid-latitude weather and extreme events?</td><td>Research on the impact of Antarctica on mid-latitude weather can be traced to a 1998 paper that documented the impact of Antarctic katabatic winds on atmospheric pressure from the Antarctic to the sub-tropics of the Southern Hemisphere.<sup>4</sup> More recent work found that cold air outbreaks over the Ross, Amundsen and Bellingshausen Seas dominate the turbulent heat fluxes from the ocean to the atmosphere just north of the sea-ice edge and, as a result, the variability of the surface buoyancy forcing of the Southern Ocean.<sup>5</sup> The relationships between Antarctic spring ozone levels and Australian summer temperature has been documented although the role of coupled atmosphere-ocean processes for this Antarctic-Australia linkage remains unresolved.<sup>6</sup></td></tr><tr><td>3. How have teleconnections, feedbacks, and thresholds in decadal and longer-term climate variability affected ice sheet response since the Last Glacial Maximum, and how can this inform future climate projections?</td><td>Recent studies have documented relatively weak El Niño-Southern Oscillation-like variability in the early to mid-Holocene (~11,500-5,000 years ago) accompanied by stronger westerly winds. Opposite conditions have prevailed for the last 5,000 years. Climate model simulations spanning the Last Glacial Maximum must reproduce these conditions if projections are to be credible.<sup>3</sup> It has been concluded that high-latitude Southern Ocean processes may be one of the most important feedbacks to glacial-interglacial cycles with consequences for tropical and global climates.<sup>3</sup></td></tr></tbody></table>
<table><tr><td>4. What drives change in the strength and position of Westerly winds, and what are their effects on ocean circulation, carbon uptake and global teleconnections?</td><td>The Southern Annual Mode is a dominant source of variability in Southern Hemisphere climate impacting the strength of westerly winds and temperature anomalies in the high southern latitudes. Recent trends in Southern Annual Mode have been attributed to forcing from ozone loss opposed by forcing from greenhouse gas increases. Recent work also shows an influence from the tropical Pacific on the Southern Annual Mode.<sup>7,8</sup> The Southern Annual Mode, through changes in westerly wind strength, is thought to impact the circulation of the Southern Ocean, sea ice extent around Antarctica, heat and carbon sequestration in the Southern Ocean, and ocean biogeochemistry. Understanding of the importance of these interconnections continues to evolve. See Q.11.</td></tr><tr><td>5. How did the climate and atmospheric composition vary prior to the oldest ice records?</td><td>The last time the Earth experienced atmospheric CO<sub>2</sub> concentrations like today was during the Pliocene, ~2.6-5.3 million years ago.<sup>9</sup> The global mean temperature was ~2°C higher than today and, at times, global sea level was 10-20 m higher than present. Conditions were variable in Antarctica and the West Antarctic ice sheet may have collapsed.<sup>10</sup> East Antarctica had periods of high summer warmth with low shrubs growing far into the interior. At other times ice covered most of the landscape. Further ice and rock records are needed to better define these linkages and validate models. See Q.9. and Q.33.</td></tr><tr><td>6. What controls regional patterns of atmospheric and oceanic warming and cooling in the Antarctic and Southern Ocean?</td><td>It is now clearer that some temperature changes over Antarctica are attributable to the impacts of the Antarctic Ozone Hole and atmospheric teleconnections from tropical latitudes (see Q.4 and Q.11). The relative role of natural versus anthropogenically forced multi-decadal variability, and its impacts on regional patterns of temperature and sea ice change, is unresolved for Antarctica and the Southern Ocean.<sup>11,12,13</sup> See Q.11.<br/><b>Crosscuts Southern Ocean</b><br/>Understanding of the role of Southern Ocean overturning in sweeping the anthropogenic heat anomaly from the Southern Ocean to intermediate depths at lower latitudes has improved.<sup>14,15,16</sup> Weak oceanic warming, seen at high southern latitudes, is due to wind driven oceanic upwelling while enhanced warming north of the Antarctic circumpolar current is associated with greenhouse gas-induced heat uptake and northward heat transport.<sup>14</sup> These results suggest that high Southern Ocean warming will respond on centennial or longer time scales due to the role of upwelling of deeper waters. See Southern Ocean section questions.</td></tr></table>
<table>
   <tr>
    <td>7. How can coupling and feedbacks between the atmosphere and the surface (land ice, sea ice and ocean) be better represented in weather and climate models?</td>
    <td>Cloud prediction is the largest uncertainty for atmospheric models over land ice, sea ice and the ocean with profound impacts on coupling with the underlying surfaces.<sup>17,18</sup> Capturing the correct balance of cloud liquid water versus cloud ice, that determines whether the downward radiative fluxes from the atmosphere to the surface are accurate, is especially challenging. Sensitivity studies fixing the shortwave radiation biases in the National Center for Atmospheric Research (NCAR) climate model demonstrated the importance of these surface radiation errors for predictions of large-scale atmospheric and oceanic circulation in the Southern Hemisphere.<sup>19</sup> Over the past few years several field projects have studied the role of clouds in the Southern Ocean climate system (e.g. Measurements of Aerosols, Radiation, and Clouds Over the Southern Ocean [MARCUS]).<sup>20</sup> Results from these studies will lead to improvements in the representation of this coupling in weather and climate models.<br/><br/><strong>Crosscuts Southern Ocean</strong><br/>See above<br/><br/><strong>Crosscuts Antarctic Ice Sheet</strong><br/>Coupling the various components of the climate system is a work in progress. This work is primarily achieved through intercomparison experiments.<sup>21,22</sup> See Q.25 and related questions in the Ice Sheet section.</td>
   </tr>
   <tr>
    <td>8. Does past amplified warming of Antarctica provide insight into the effects of future warming on climate and ice sheets?</td>
    <td>Based on coupled ice sheet-climate modeling, and comparisons with conditions during the Pliocene and the last interglacial, it is suggested that hydrofracturing of buttressing ice shelves, caused by atmospheric warming, may create marine ice cliff instability (MICI). This instability may lead to Antarctic ice loss contributing more than a meter to sea-level rise by 2100 and more than 15 m by 2500, if emissions continue unabated.<sup>9,23,24</sup> Others have suggested that up to 0.25 m of sea-level rise from Antarctic ice loss can be expected by 2100.<sup>25</sup> Yet others have argued that MICI is unnecessary to reproduce paleo-sea-levels therefore, extreme estimates of sea-level rise by the end of this century may be overestimated.<sup>26</sup> See Q.11, Q.21, Q.31, Q.32, and Q.33.<br/><br/><strong>Crosscuts Antarctica Ice Sheet</strong><br/>See above and Ice Sheet section questions.</td>
   </tr>
  </table>
<table><tr><td>9. Are there CO<sub>2</sub> equivalent thresholds that foretell collapse of all or part of the Antarctic Ice Sheet?</td><td>Information from the ANDRILL-2A drill core, and complementary ice sheet modeling, show that polar climate and Antarctic ice sheet margins were highly dynamic during the early to mid-Miocene (~23-11 million years ago).<sup>27</sup> Changes in the inferred extent of the Antarctic ice sheet suggests that high southern latitudes were sensitive to relatively small changes in atmospheric CO<sub>2</sub> (between 280 and 500 ppm). Reconstructions through intervals of peak warmth indicate that the Antarctic ice sheet retreated beyond its terrestrial margin under atmospheric CO<sub>2</sub> conditions like those projected for the coming centuries. Past ice sheet response allows calibration and validation of models.<sup>23,24</sup> Understanding changes in past ocean circulation, based on marine sedimentary records, is one way to probe forcings and ice sheet/ocean interactions. Results from marine and ice-sheet drilling projects are expected to provide important new insights (e.g., International Ocean Discovery Program<sup>28</sup>, the International Partnership in Ice Core Sciences<sup>29</sup> and Beyond the European Project for Ice Coring in Antarctica – Oldest Ice<sup>30</sup>).<br/><br/><strong>Crosscuts Antarctic Ice Sheet</strong><br/>According to CO<sub>2</sub> proxies, it has been ~3 million years Before Present since CO<sub>2</sub> values reached those of the present day. The International Partnership in Ice Core Sciences<sup>29</sup> is searching for a 1.5-million-year ice core record. Among the projects with a goal to obtain such an ice core, the European Beyond EPICA project was recently launched and will begin drilling by 2020.<sup>30</sup> See Q.28.</td></tr><tr><td>10. Will there be release of greenhouse gases stored in Antarctic and Southern Ocean clathrates, sediments, soils, and permafrost as climate changes?</td><td>There is little progress on this question, however a SCAR Expert Group, “Antarctic Permafrost and Soils” (AntPaS)<sup>31</sup>, has been formed to address this and related questions. See Q.42.<br/><br/><strong>Crosscuts Dynamic Earth</strong><br/>See above.</td></tr></table>
11. Is the recovery of the ozone hole proceeding as expected and how will its recovery affect regional and global atmospheric circulation, climate and ecosystems?

Ozone recovery is proceeding much as expected with exceptions discussed below.³² The magnitude of the hole has stabilized over the last decade, but the 2018/19 hole was one of the larger in recent years. It is predicted that it will be another 40 years before ozone levels return to those of 50 years ago. Understanding of the impacts of stratospheric ozone on the surface climate in the Southern Hemisphere continues to evolve. Extremes in Antarctic spring ozone have been linked to warmer (more ozone) and cooler (less ozone) than normal summers over large regions of the Southern Hemisphere, especially Australia.³³ It has been postulated that east-west displacements of the location of the springtime Antarctic ozone minimum could be responsible for regional climate variations during the austral springtime.³⁴ Projections of future Southern Hemisphere climate change, due to the recovery of the Ozone Hole, may be more impactful than currently thought. Ozone data for 2018/19 and predictions for the future can be found at "The 2018 Antarctic Ozone Hole Season" web page.³⁵ There is growing evidence that declines in lower stratospheric ozone can offset ozone layer recovery.³⁶ Increases in global emissions of CFC-111 are also being observed.³⁷ Linkages between changes in sea ice and recovering ozone levels have been reported.³⁸

### Crosscuts Antarctic Life and Human Presence

Understanding of the direct effects of ultraviolet radiation (especially UV-B) is well developed for marine and terrestrial systems.³⁹,⁴⁰ Overall the expectation are for minor impacts from UV radiation compared with other drivers of marine systems⁴¹, though some influence on ecosystem structure and function may be expected due to shifts between microbial heterotrophs and larger phytoplankton.³⁹

For terrestrial systems, exposure to relevant levels of UV radiation is generally not damaging. Rather, the largest impacts are expected to come from changes in atopospheric circulation patterns associated with ozone depletion and their influence on Antarctic plant communities, such as the replacement of endemic species which prefer wet conditions with more widespread drought tolerant taxa.⁴²,⁴³,⁴⁴
<table><caption>Table S2. Summary of qualitative (subjective) expert assessments of progress in answering “Antarctic Atmosphere and Global Connection” questions rated as: 1 - no or little progress, 2 - moderate progress, 3 - major progress and 4 - answered. When a question crosscuts other clusters an additional perspective(s) on progress is provided. (New) – indicates a question that was not originally identified as cross-cutting in the Scan. For detailed commentary on progress and supporting references see Table S1.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Qualitative (Subjective) Rating (1-4)</td></tr></thead><tbody><tr><td>1. How is climate change and variability in the high southern latitudes connected to lower latitudes including the Tropical Ocean and monsoon systems?</td><td><strong>2-3</strong><br>Tropical oceans impact Antarctic climate on a variety of time scales but many interactions are yet to be explored.<br><br><strong>Crosscuts Southern Ocean (new): 2</strong></td></tr><tr><td>2. How do Antarctic processes affect mid-latitude weather and extreme events?</td><td><strong>1</strong><br>Understanding of how mid-latitude weather and extreme events affect Antarctic processes, and vice versa has improved. Tools are in place to further advance knowledge on this question.<br><br><strong>Crosscuts Southern Ocean (new): 1</strong></td></tr><tr><td>3. How have teleconnections, feedbacks, and thresholds in decadal and longer-term climate variability affected ice sheet response since the Last Glacial Maximum, and how can this inform future climate projections?</td><td><strong>1-2</strong><br>The absence of annually resolved records limits progress on this question.</td></tr><tr><td>4. What drives change in the strength and position of Westerly winds, and what are their effects on ocean circulation, carbon uptake and global teleconnections?</td><td><strong>3</strong><br>Studies have defined the effect of the ozone hole on Westerly winds but there is much to learn that will improve predictions of future change.</td></tr><tr><td>5. How did the climate and atmospheric composition vary prior to the oldest ice records?</td><td><strong>1-2</strong><br>More studies of the Pliocene in the Antarctic are needed, and collection of sediment and rock records will be required.</td></tr></tbody></table>
6. What controls regional patterns of atmospheric and oceanic warming and cooling in the Antarctic and Southern Ocean?

3

The role of natural multi-decadal variability remains unresolved for both Antarctica and the Southern Ocean, but research is progressing.

**Crosscuts Southern Ocean: 2**

The role of Southern Ocean overturning in sweeping the anthropogenic heat anomaly from the Southern Ocean to intermediate depths at lower latitudes is better understood.

7. How can coupling and feedbacks between the atmosphere and the surface (land ice, sea ice and ocean) be better represented in weather and climate models?

2-3

Cloud prediction is the largest uncertainty for atmospheric models over land ice, sea ice and the ocean but recent field campaigns focused on the study of Southern Ocean clouds should lead to advances on this question.

**Crosscuts Southern Ocean: 2**

Progress is needed in understanding Southern Ocean cloud/radiation bias in climate models.

**Crosscuts Antarctic Ice Sheet: 3**

Coupling the various components of the climate system is a work in progress that will be best achieved through model intercomparison experiments.

8. Does past amplified warming of Antarctica provide insight into the effects of future warming on climate and ice sheets?

2

Research on this question is at its beginnings. Incorporating ozone variability could improve seasonal predictions.

**Crosscuts Antarctic Ice Sheet: 3**

This rating is based on ongoing efforts in ice sheet modelling.

9. Are there CO₂ equivalent thresholds that foretell collapse of all or part of the Antarctic Ice Sheet?

2

It has been 3 million years Before Present since CO₂ values reached those of the present day. Additional geological records are needed.
**Crosscuts Antarctic Ice Sheet: 2**

A better understanding of the temperature-CO₂ relationship is essential for identifying thresholds for ice sheet collapse.

1

Warming is causing changes in ice-free areas that may have ramifications for the release of stored greenhouse gases.

**Cross Cuts Dynamic Earth: 1**

3

Ozone recovery is being affected by recent increases in chlorofluorocarbons that need to be investigated.

**Crosscuts Antarctic Life and Human Presence: 2**

There is limited work on the life sciences aspects of this question including the direct effects of UV radiation. The indirect effects via changing atmospheric circulation are expected to be more pronounced but remain largely unknown.

10. Will there be release of greenhouse gases stored in Antarctic and Southern Ocean clathrates, sediments, soils, and permafrost as climate changes?

11. Is the recovery of the ozone hole proceeding as expected and how will its recovery affect regional and global atmospheric circulation, climate and ecosystems?
<table><caption>Table S3. Expert (subjective) assessments of progress toward answering the “Southern Ocean and Sea Ice in a Warming World” questions supported by peer-reviewed literature citations. Comments by experts from crosscutting clusters are identified as ‘Crosscuts [name of cluster]’. The references cited are exemplars and not intended to be an exhaustive literature review.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Conclusions</td></tr></thead><tbody><tr><td>12. Will changes in the Southern Ocean result in feedbacks that accelerate or slow the pace of climate change?</td><td>Model studies have shown that meltwater from Antarctic ice sheets and shelves (usually absent in climate models) could have widespread implications, including slowing of the increase in average global temperatures and warming of subsurface ocean temperatures near Antarctica, and raising the possibility of a positive feedback driving further ice melt and sea-level rise.<sup>45</sup> Observations suggest that Southern Ocean carbon uptake has been reinvigorated in recent years, indicating variability in feedbacks to global warming.<sup>46</sup> The Southern Ocean dominates global ocean heat uptake, with the overturning circulation acting to transport anthropogenic heat (and carbon dioxide) northward and into the ocean interior.<sup>14,15,47</sup> Consequently, ocean warming is delayed in the Southern Ocean and accelerated to the north of the Antarctic Circumpolar Current. The dominance of southern hemisphere heat uptake in part reflects reduced uptake in the North Atlantic because of the cooling effect of aerosols. As aerosols decrease in the future, both the Southern Ocean and North Atlantic will take up anthropogenic heat.<sup>48</sup> See Q.1, Q.2, Q.4 and Q.6 for atmospheric linkages.</td></tr><tr><td>13. Why are the properties and volume of Antarctic Bottom Water changing, and what are the consequences for global ocean circulation and climate?</td><td>Trends toward warmer, fresher, lighter and less voluminous Antarctic Bottom Water have been documented, contributing to ocean heat storage and sea-level rise.<sup>49,50</sup> The response to episodic events, like iceberg calving, has provided a “natural experiment” with which to assess the sensitivity of bottom water formation to changes in forcings.<sup>51</sup></td></tr><tr><td>14. How does Southern Ocean circulation, including exchange with lower latitudes, respond to climate forcing?</td><td>There is some recent evidence for decadal/multi-decadal variability in the Southern Ocean overturning circulation.<sup>52</sup> While a complete understanding of the sensitivity of the overturning to changes in forcing remains elusive, important progress has been made in understanding the dynamics of the overturning circulation.<sup>53</sup> Notable advances have included evidence that both the upwelling and downwelling limbs of the overturning</td></tr></tbody></table>
<table><tr><td></td><td>circulation are highly localized as a result of interactions of the flow with topography<sup>54,55</sup> and recognition of the importance of sea ice melt in driving water mass transformations that connect the upper and lower branches of the overturning circulation.<sup>56,57,58</sup> See Q.1, Q.2, Q.4 and Q.6 for atmospheric linkages.</td></tr><tr><td>15. What processes and feedbacks drive changes in the mass, properties and distribution of Antarctic sea ice?</td><td>New observations under Antarctic sea ice have revealed how pre-conditioning by ocean stratification can influence rates of sea ice growth.<sup>59</sup> Several potential feedbacks involving interactions between the ocean, atmosphere, sea ice and ice shelves have been identified, but few of the (often non-linear) non-radiative feedbacks have been quantified.<sup>60</sup> The dramatic shift in Antarctic sea ice extent from record high in 2014 to record low in 2017 has been attributed to changes in the atmosphere (linked to strong anomalies in tropical convection and shifts in major climate modes, like the Interdecadal Pacific Oscillation and the Southern Annular Mode) and consequent changes in upper ocean heat content.<sup>61</sup> See Q.17.</td></tr><tr><td>16. How do changes in iceberg numbers and size distribution affect Antarctica and the Southern Ocean?</td><td>The freshwater input to the Southern Ocean from melting icebergs has been quantified and shown to reduce sea surface temperature and increase annual mean sea ice volume, with strong regional variations.<sup>62</sup> Little progress has been made in assessment of changes in iceberg supply and its impact on the Southern Ocean.</td></tr><tr><td>17. How has Antarctic sea ice extent and volume varied over decadal to millennial timescales?</td><td>To date, focus has been largely on decadal scale variability of Antarctic sea ice extent. Regional trends in sea ice extent have been linked to well-known modes of climate variability, including El Niño and the Southern Annular Mode.<sup>63</sup> Model studies suggest that a slow response to open ocean convection events, like the Weddell Polynya, may also explain trends in sea ice extent.<sup>64</sup> Antarctic sea ice extent has decreased from a record high in 2014 to a record low in 2017, a rate of ice loss that surpasses the widely publicized decline in Arctic sea ice.<sup>65</sup> The precipitous decline has been linked to forcing from the tropics and the stratosphere, with a possible contribution from anthropogenic factors.<sup>38,61</sup> Little progress has been made on sea ice volume, although an updated dataset of sea ice thickness (European Space Agency Climate Change Initiative project on Sea Ice<sup>66</sup>) has promise for describing variability on timescales of a decade and shorter. See Q.1, Q.2, Q.11, Q.15 and Q.19.</td></tr><tr><td>18. How will changes in ocean surface waves influence Antarctic sea ice and floating glacial ice?</td><td>Substantial progress has been made in understanding the influence of ocean surface waves on sea ice and ice shelves.<sup>67,68,69,70,71</sup> Waves cause sea ice break-up, rafting and</td></tr></table>
<table><tr><td></td><td>variations in sea ice properties; altering ice floe size, distribution and concentrations. Loss of sea ice can allow ocean swell to reach ice shelves and cause fracturing.</td></tr><tr><td>19. How do changes in sea ice extent, seasonality and properties affect Antarctic atmospheric and oceanic circulation?</td><td>The importance of brine rejection during sea ice formation in the creation of dense Antarctic Bottom Water has been recognized for many years, but less attention was paid to the impacts of sea ice melt. Several recent studies have shown that freshwater added by sea ice melt is critical for transforming dense water to lighter water in the upper branch of the Southern Ocean overturning circulation.<sup>57,58,72</sup> With respect to the atmosphere, modelling studies have shown that increased Antarctic sea ice can shift the Southern Hemisphere mid-latitude jet poleward in winter.<sup>73</sup> Similarly, Antarctic sea ice loss will act to shift the tropospheric jet equatorward, an internal negative feedback to the poleward shift associated with increased greenhouse gases. Antarctic sea ice loss is expected to have little effect on regional Antarctic air temperatures.<sup>74</sup> See Q.1, Q.2 and Q.17.<br/><strong>Crosscuts Antarctic Atmosphere</strong><br/>On local scales, Antarctic coastal air temperatures have increased (decreased) over the last 60 years as the Antarctic sea ice extent decreased (increased) with the trend reversing in 1979.<sup>11</sup></td></tr><tr><td>20. How do extreme events affect the Antarctic cryosphere and Southern Ocean?</td><td>In 2017, the Maud Rise polynya in the Weddell Sea opened for an extended period for the first time since the 1970s. Recurrence of this uncommon "extreme event" has been linked to the combined effect of unusually strong atmospheric storms and oceanographic preconditioning.<sup>75,76</sup> Anomalous winds over the Southern Ocean linked to an extreme El Niño Southern Oscillation event, a change in phase of the Interdecadal Pacific Oscillation, and internal variability associated with the Southern Annular Mode contributed to anomalous sea ice decay in 2016,<sup>61,77,78,79</sup> and to extensive summer melt on the West Antarctic Ice Sheet.<sup>80</sup><br/><strong>Crosscuts Antarctic ice sheet</strong><br/>All processes leading to ice melt (at the surface or below the ice shelves) are strongly nonlinear. This nonlinearity may increase the impact of extreme warm events, especially on ice shelf collapse. Periods of above-normal precipitation can be thought of as extreme events that increase surface accumulation.<sup>81,82</sup></td></tr></table>
<table><tr><td>21. How did the Antarctic cryosphere and the Southern Ocean contribute to glacial/inter-glacial cycles?</td><td>Changes in Southern Ocean overturning circulation contribute to glacial/inter-glacial cycles by regulating the release or storage of carbon dioxide in the deep ocean, with sea ice and upper ocean stratification also influencing exchange with the atmosphere.<sup>83,84,85,86</sup><br/><strong>Crosscuts Antarctic ice sheet</strong><br/>The Antarctic ice sheet contributed to sea level changes during glacial-interglacial cycles.<sup>87</sup> During the Last Glacial Maximum (LGM) Antarctic ice-sheet contribution to sea-level lowering (120 m in total) is estimated to be between 5-30 m with the most recent estimates in the lower part of this range (5-15 m).<sup>87</sup></td></tr><tr><td>22. How will climate change affect the physical and biological uptake of CO<sub>2</sub> by the Southern Ocean?</td><td>The mechanisms responsible for heat and carbon dioxide uptake and storage in the Southern Ocean, both in the present day and in the future, are now better understood. Both passive advection (e.g. transport of anomalous input of heat and carbon by an unchanged circulation) and changes in circulation (e.g. a change in strength of the overturning circulation) contribute to uptake and storage of CO<sub>2</sub>.<sup>14,15,53,88,89</sup> Early results from the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project suggest that less CO<sub>2</sub> is taken up by the Southern Ocean than previously thought.<sup>90</sup> The response of Southern Ocean phytoplankton to climate change will vary with species, due to varying physiology and response to changes in iron supply.<sup>91</sup> See below.<br/><strong>Crosscuts Antarctic life</strong><br/>This is an active area of research that has produced advances over the past five years via modeling and controlled experiments to evaluate Southern Ocean biotic impact on dissolved inorganic carbon uptake. Ecosystem modeling under the RCP8.5 scenario with at least two phytoplankton functional types predicted significant increases in the importance of the biological pump in the Southern Ocean as a result of anthropogenic CO<sub>2</sub>.<sup>92</sup> For example, experimental studies demonstrate that the interactive effects of carbon dioxide, light, temperature, and changing iron availability can impact the growth, production, and morphology in a species-specific manner when different phytoplankton species are considered.<sup>93</sup> The multi-factor effects of climate change; including increased carbon dioxide concentrations, acidification, changes in the temperature and structure of the water column, and the resulting implications for nutrient regimes; result in complex interactions. The influences of acidification are less clear than previously suggested. For pelagic systems, pteropods are more capable of shell repair than previously thought, but juvenile stages may be at risk.<sup>94,95,96,97</sup></td></tr></table>
23. How will changes in freshwater inputs affect ocean circulation and ecosystem processes?

New results affirm the importance of the freshwater flux added by sea ice melt to water mass transformations in the upper limb of overturning circulation.<sup>45,57,58</sup> See Q.12.

### Crosscuts Antarctic Life

Changing freshwater inputs will have differing effects on ecosystem processes depending on the site, the ecosystem and the magnitude of the inputs. On land and beneath ice shelves, warming increases melt which affects ice shelf stability<sup>98</sup> and produces new open water in summer where it did not previously exist. This allows new aquatic ecosystems to be established. In marine environments, impacts from increased freshwater inputs will vary from the loss of some shallow coastal water assemblages to increased phytoplankton productivity and higher nutrient input to the ecosystem from terrestrial sources.<sup>99</sup> The spatial distribution of runoff around Antarctica influences sea ice cover and the stratification of water masses.<sup>100</sup> Strong freshwater forcing, leading to accelerations of the coastal current and sea ice removal, may result in a decrease in sea ice extent and volume effecting the feeding behaviors and reproduction of numerous species.
<table><caption>Table S4. Summary of qualitative (subjective) expert assessments of progress in answering “Southern Ocean and Sea Ice in a Warming World” questions rated as: 1 - no or little progress, 2 - moderate progress, 3 - major progress and 4 - answered. When a question crosscuts other clusters an additional perspective(s) on progress is provided. (New) – indicates a question that was not originally identified as cross-cutting in the Scan. For detailed commentary on progress and supporting references see Table S3.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Qualitative (Subjective) Rating (1-4)</td></tr></thead><tbody><tr><td>12. Will changes in the Southern Ocean result in feedbacks that accelerate or slow the pace of climate change?</td><td><strong>2</strong><br/>The Southern Ocean dominates global ocean heat uptake, with the overturning circulation acting to transport anthropogenic heat (and carbon dioxide) northward and into the ocean interior, so changes in circulation may result in climate feedbacks.<br/><strong>Crosscuts Antarctic Ice Sheet (new): 2</strong><br/>Changes in the Southern Ocean due to input of ice melt have been shown to reduce global mean temperatures.</td></tr><tr><td>13. Why are the properties and volume of Antarctic Bottom Water changing, and what are the consequences for global ocean circulation and climate?</td><td><strong>2</strong><br/>Trends toward warmer, fresher, lighter and less voluminous Antarctic Bottom Water have been documented, contributing to ocean heat storage and sea level rise.<br/><strong>Crosscuts Antarctic Ice Sheet (new): 2</strong><br/>See above.</td></tr><tr><td>14. How does Southern Ocean circulation, including exchange with lower latitudes, respond to climate forcing?</td><td><strong>2</strong><br/>There is some recent evidence for decadal/multi-decadal variability in the Southern Ocean overturning circulation but understanding of the sensitivity of the circulation to changes in forcing remains incomplete.</td></tr></tbody></table>
<table><tr><td>15. What processes and feedbacks drive changes in the mass, properties and distribution of Antarctic sea ice?</td><td><b>2</b><br/>New observations under Antarctic sea ice have revealed how pre-conditioning by ocean stratification can influence rates of sea ice growth.<br/><b>Crosscuts Antarctic Ice Sheet (new): 2</b><br/>See above.</td></tr><tr><td>16. How do changes in iceberg numbers and size distribution affect Antarctica and the Southern Ocean?</td><td><b>2</b><br/>The freshwater input to the Southern Ocean from melting icebergs has been quantified and shown to reduce sea surface temperatures and increase annual mean sea ice volume.</td></tr><tr><td>17. How has Antarctic sea ice extent and volume varied over decadal to millennial time scales?</td><td><b>2</b><br/>A multi-decadal trend of slow expansion of Antarctic sea ice was interrupted by a sharp decline to record low levels in 2016 and has remained low, while regional trends in sea ice extent have been linked to well-known modes of climate variability.</td></tr><tr><td>18. How will changes in ocean surface waves influence Antarctic sea ice and floating glacial ice?</td><td><b>3</b><br/>Substantial progress has been made in understanding the influence of ocean surface waves on sea ice and ice shelves.<br/><b>Crosscuts Antarctic Ice Sheet (new): 2</b><br/>For floating glacial ice indirect impacts from sea ice are known. This process is seldom simulated in models.</td></tr><tr><td>19. How do changes in sea ice extent, seasonality and properties affect Antarctic atmospheric and oceanic circulation?</td><td><b>2</b><br/>Freshwater added by sea ice melt is critical for transforming dense water to lighter water in the upper branch of the Southern Ocean overturning circulation<br/><b>Crosscuts Antarctic Atmosphere: 2-3</b><br/>Changes in sea ice extent influence the location of the mid-latitude jet and air temperatures near the Antarctic coast.</td></tr></table>
20. How do extreme events affect the Antarctic cryosphere and Southern Ocean?

2

Recurrence of an uncommon "extreme event" (e.g. opening of the Weddell Polynya) has been linked to the combined effect of unusually strong atmospheric storms and oceanographic preconditioning, while the recent shift from record high to record low sea ice extent provides another example of an extreme event in the coupled system.

**Crosscuts Antarctic Ice Sheet: 2**

The nonlinearity of processes leading to ice melt may increase the impact of extreme warming events.

21. How did the Antarctic cryosphere and the Southern Ocean contribute to glacial-interglacial cycles?

3

There are new insights into the contribution of changes in Southern Ocean overturning circulation to glacial/inter-glacial cycles.

**Crosscuts Antarctic Ice Sheet: 3**

The Antarctic ice sheet contributes to sea level changes during glacial cycles.

22. How will climate change affect the physical and biological uptake of CO₂ by the Southern Ocean?

2

Early results from profiling biogeochemical floats suggest that less CO₂ is taken up by the Southern Ocean than previously thought, and there is growing evidence that decadal variability in the Southern Ocean carbon sink is more pronounced than expected, but projections of future heat and carbon uptake remain uncertain.

**Crosscuts Antarctic Life: 2**
See above.

23. How will changes in freshwater inputs affect ocean circulation and ecosystem processes?

2

The freshwater flux added by sea ice melt contributes to water mass transformations in the upper limb of overturning circulation.
**Crosscuts Antarctic Life: 1-2**

Changing freshwater inputs will have differing effects on ecosystem processes depending on the site, the ecosystem and the magnitude of the inputs. A better understanding of biotic-abiotic (physical) process interactions is needed.
<table><caption>Table S5. Expert (subjective) assessments of progress toward answering “Antarctic Ice Sheets and Sea Level” questions supported by peer-reviewed literature citations. Comments by experts from crosscutting clusters are identified as ‘Crosscuts [name of cluster]’. The references cited are exemplars and not intended to be an exhaustive literature review.</caption><thead><tr><th>Antarctic Science Horizon Scan Question<sup>1,2</sup></th><th>Conclusions</th></tr></thead><tbody><tr><td>24. How does small-scale morphology in subglacial and continental shelf bathymetry affect Antarctic Ice Sheet response to changing environmental conditions?</td><td>Knowledge of bed morphology is essential in assessing where Marine Ice Sheet Instability (MISI) and eventually Marine Ice Cliff Instability (MICI) may occur in response to climate change. Major improvements have been made in the coverage of ice thickness measurements (Ice Bridge).<sup>101</sup> Integration of all data, for instance in Bedmap 3, and modelling approaches like BedMachine are expected to improve predictions.<sup>102</sup> The roughness of the bed is important and the most advanced ice sheet models would benefit from access to detailed data rather than from coarse-scale grids in grounding-line regions.<sup>103</sup> Bathymetry plays an important role in the access of warm water beneath ice shelves that triggers MISI. The knowledge of small-scale morphology is a limiting factor for ocean models. Progress has been made in mapping bathymetry.<sup>104</sup><br/><br/><strong>Crosscuts Dynamic Earth</strong><br/>Targeted high-resolution Radio Echo Soundings (RES) in ice stream troughs have been conducted.<sup>104,105,106</sup> There is an awareness of along/crossflow roughness in ice stream trunks. There is now good knowledge of Totten, Thwaites, and Pine Island Glacier and other ice sheets/shelves that are at risk. More data are needed, especially in grounding zones, to better define sub-ice shelf and sub-sea-ice bathymetry. Bedmap3 and Bed Machine will be important in addressing this question (see above). Related to Q.25, Q.26 and Q. 27</td></tr></tbody></table>
<table><tr><td>25. What are the processes and properties that control the form and flow of the Antarctic Ice Sheet?</td><td>The form and flow of the Antarctic Ice Sheet are governed by:<br/>1) the material (rheological) properties of ice [There is a consensus on the deformation law (Glen flow law) but this law is modulated by ice anisotropy that develops with depth affecting ice deformation. The deformation properties are also dependent on the 3-D temperature field that affects viscosity. Finally, ice can be damaged due to intense deformation, for instance, at the shear margin of ice streams and this can affect deformation properties], and 2) the properties of the bed; i.e. roughness, hard rock, sediment (saturated or not) and water pressure at the ice-bed interface. [These properties govern the basal friction experienced by the slab of ice, and whether the ice-bed interface is at the melting point (see also geothermal flux). Radio echo sounding, primarily done to measure ice thickness, can provide information on the status of the ice-bed interface.<sup>107</sup>]<br/><br/>Among improvements in recent years, inverse methods in ice sheet models allow for retrieval of many of the characteristics (or at least the vertically integrated impact) based on observed velocity fields.<sup>108</sup> The main question remains – how do these fields (essentially basal friction and damage) vary in time, especially in the context of changes in ice geometry?<br/><br/>Efforts in modelling processes (friction law or basal hydrology) are being pursued but will need time-dependent observations of velocity fields for calibration.<sup>109</sup> Numerical ice sheet models have incorporated more rigorous simulations of grounding line retreat. Several model intercomparisons have established benchmarks for simulations of marine ice sheet instability (MISI).<sup>110,111</sup> The numerical schemes on which ice sheet models are based have advanced considerably since the Scan. Multi-centennial simulations are now possible using anisotropic grids<sup>112,113</sup> and adaptive mesh refinements.<sup>114,115,116</sup> Similarly, inverse modelling of basal drag properties has improved simulations of present-day ice flow.<sup>117</sup> Ice sheet and ocean model parameterization and coupling must be improved. Intercomparison experiments based on idealized geometries<sup>118</sup> have investigated processes and feedbacks between ice sheet-ice shelves and the ocean. At continental-scales model intercomparisons are also being pursued.<sup>21,119</sup> See Q.7.</td></tr><tr><td>26. How does subglacial hydrology affect ice sheet dynamics, and how important is it?</td><td>The Antarctic subglacial water system is active as mainly revealed by repeated surface altimetry.<sup>120</sup> This dynamic behavior improves knowledge of hydrological processes.<sup>121</sup> Subglacial hydrology affects ice dynamics through basal friction (see Q.25). There is some evidence that basal water moving in and out of subglacial lakes causes fluctuations in ice velocity.<sup>122</sup> This movement can be estimated by radar echo sounding and radar</td></tr></table>
<table>
   <tr>
    <td></td>
    <td>scattering behavior at the ice-bed interface.<sup>123</sup> Models for subglacial water flow have evolved in recent years with a variety of routing processes now considered.<sup>120</sup> The distribution of basal water, and its connectivity, is heterogenous and new observations, including sediment thickness, are necessary at a continental scale. Basal water production, which is an input to hydrology models, can be simulated by ice sheet models but depends on the value used for geothermal flux which remains poorly known (see Q.27).<br/><br/><strong>Crosscuts Dynamic Earth</strong><br/>There is a better understanding of the location of basal water.<sup>124,125</sup> Radio Echo Sounding (RES) data of "modern quality", or from better technologies, is needed. Determination of water-saturated sediments is also needed. The presence of groundwater, and its influence, remains largely unknown.</td>
   </tr>
   <tr>
    <td>27. How do the characteristics of the ice sheet bed, such as geothermal heat flux and sediment distribution, affect ice flow and ice sheet stability?</td>
    <td>Geothermal flux controls temperature within the ice, as well as where ice reaches the melting point, driving basal water production. Geothermal flux is poorly known. Up to now, several maps of geothermal flux have been proposed using different methods and the differences between these maps are important.<sup>126</sup> Within the program 'Solid Earth Response and influence on Cryosphere Evolution (SERCE)', an effort is being made to reconcile different ways of inferring geothermal flux.<sup>127</sup> Information about the geothermal flux value is also crucial in the search for sites where ice core records reach 1.5 million years Before Present.<sup>128</sup> Sediment distribution is important to ice sheet dynamics because deformation adds to ice velocity and can be an important influence, especially when the sediment is water-saturated. Short-term glacial isostatic adjustment can slow ice mass loss. This is known to be operative at least in the Amundsen Sea region.<sup>116</sup> There is an improved understanding of the processes involved in the evolution of the Antarctic Ice Sheet.<sup>129</sup><br/><br/><strong>Crosscuts Dynamic Earth</strong><br/>On heat flux, estimates are proxy-based and include assumptions that lead to inconsistent results. On sediment - radar data is available but has not been properly integrated into Bedmap. Ice sheet models do not use detailed geothermal distributions and the West and East Antarctic Ice Sheet may need to be differently treated.</td>
   </tr>
   <tr>
    <td>28. What are the thresholds that lead to irreversible loss of all or part of the Antarctic ice sheet?</td>
    <td>Several authors estimate that the only way to avoid serious ice shelf retreat is to follow RCP2.6.<sup>23,130</sup> At centuries to millennium timescales, it has been estimated that a tipping point exists at, or around, 2°C of global warming.<sup>131</sup> However, for Thwaites Glacier there</td>
   </tr>
  </table>
<table>
   <tr>
    <td></td>
    <td>are concerns that a threshold for unstoppable grounding line retreat has been, or is about to be, crossed.<sup>132,133,134</sup></td>
   </tr>
   <tr>
    <td>29. How will changes in surface melt over the ice shelves and ice sheet evolve, and what will be the impact of these changes?</td>
    <td>Surface melt is important for its impact on calving, ice shelf break-up and marine ice cliff instability processes (although the MICI process is being debated). Projections reveal a scenario-independent doubling of Antarctic-wide melt by 2050 with projections diverging afterwards. By 2100, under the high emissions pathway, melt on several ice shelves could lead to ice shelf collapse, at least along the northeast Antarctic Peninsula.<sup>134</sup> However, run-off from surface rivers might limit the impact on ice-shelf breakup.<sup>135</sup></td>
   </tr>
   <tr>
    <td>30. How do oceanic processes beneath ice shelves vary in space and time, how are they modified by sea ice, and do they affect ice loss and ice sheet mass balance?</td>
    <td>Ice loss in Antarctica is mainly due to an acceleration of outlet glaciers and grounding line retreat driven by the thinning of floating ice shelves. High melt at the ice-shelves base results from the influx of relatively warm circumpolar deep water into cavities beneath ice shelves. The access of these cavities to the ocean varies from place-to-place depending on small-scale bathymetric channels.<sup>136</sup> For observed collapse events, the absence of sea ice makes the ice shelves more vulnerable to swell.<sup>69</sup><br/><b>Crosscuts Southern Ocean</b><br/>Ocean-ice shelf interactions are influenced by teleconnections to the tropics.<sup>137,138</sup></td>
   </tr>
   <tr>
    <td>31. How will large-scale processes in the Southern Ocean and atmosphere affect the Antarctic Ice Sheet, particularly the rapid disintegration of ice shelves and ice sheet margins?</td>
    <td>In the Amundsen Sea area, the decadal variability of the ocean is found to control the variability of the basal melt via a non-linear relationship with ocean temperature.<sup>139,140</sup> The International Thwaites Glacier Collaboration<sup>141</sup> began in 2019 and will address specific questions about Thwaites Glacier and its future, and general questions about the West Antarctic ice sheet, with a goal of reducing uncertainty in future sea-level rise contributions. See. Q.8, Q.21, Q.32 and Q.33.<br/><b>Crosscuts Antarctic Atmosphere</b><br/>See above.<br/><b>Crosscuts Southern Ocean</b><br/>There is emerging evidence that ocean heat flux drives rapid basal melt in some parts of East Antarctica.<sup>142</sup> Work is being done on the importance of variability that is driven by tropical teleconnections (see Q.30).</td>
   </tr>
  </table>
<table><tr><td>32. How fast has the Antarctic Ice Sheet changed in the past and what does that tell us about the future?</td><td>Improved understanding of how the Antarctic Ice Sheet responded to forcings in the past is crucial to improving the reliability of forecasts (see Q.8, Q.21, Q.31, and Q.33).<sup>24</sup> Past responses allow calibration and validation of ice sheet models.<sup>23</sup> Understanding changes in past ocean circulation, based on marine sedimentary records, is one way to identify forcings and ice sheet/ocean interactions. Results from marine<sup>28</sup> and ice-sheet drilling projects<sup>30</sup> will provide important advances in understanding (see Q.9). The last deglaciation (~20 - 10 thousand years Before Present) is the most documented rapid decrease of the Antarctic ice sheet in the past. Ice sheet mass balance estimates have been recently summarized.<sup>143</sup> Grounding line retreat rates have been estimated for the Ross sea sector.<sup>144</sup> Contributions to past sea level rise have been modelled.<sup>23</sup> The ice sheet mass balance inter-comparison exercise (IMBIE) project began pre-scan (2011) and produces estimates of ice sheet mass balance as a contribution the Intergovernmental Panel on Climate Change.<sup>145</sup></td></tr><tr><td>33. How did marine-based Antarctic ice sheets change during previous inter-glacial periods?</td><td>Information on past interglacial periods is mainly based on sea-level records from ice sheets in both the north and south. Greenland and Antarctic ice sheets are considered, and individual contributions are difficult to disentangle. For the last interglacial, the Greenland ice sheet is estimated to have contributed 0.6 to 3.5 m to sea level and West Antarctic Ice Sheet collapse could explain the remaining global sea-level rise of 6-9 m above present.<sup>9</sup> For Marine Isotope Stage11 and the Mid-Pliocene, with even higher sea-level stand, it is suggested that some parts of the East Antarctic Ice Sheet may have been involved.<sup>23</sup> See Q.8, Q.21, Q.31 and Q.32.</td></tr><tr><td>34. How will the sedimentary record beneath the ice sheet inform our knowledge of the presence or absence of continental ice?</td><td>Stratigraphic changes identified through seismic surveys suggest former ice margin retreat positions and variable erosion rates. Nearshore sediment sequences provide information on the continuity of sedimentation from outlet glaciers which can be used to infer the presence or absence of terrestrial ice.<br/><strong>Crosscuts Dynamic Earth</strong><br/>There is some progress in identifying targets for research, such as subglacial lakes and basins, but reliable stratigraphic sequences have not been the subject of detailed seismic surveys.<sup>146,147</sup> A shallow sediment core has been retrieved from Lake Mercer, but disturbance is suspected. Much remains unknown. Progress in developing the Rapid Access Ice Drill (RAID)/ Fast drill has been limited.<sup>148,149</sup></td></tr></table>
<table><caption>Table S6. Summary of qualitative (subjective) expert assessments of progress in answering “Antarctic Ice Sheets and Sea Level” questions rated as: 1 - no or little progress, 2 - moderate progress, 3 - major progress and 4 - answered. When a question crosscuts other clusters an additional perspective(s) on progress is provided. (New) – indicates a question that was not originally identified as cross-cutting in the Scan. For detailed commentary on progress and supporting references see Table S5.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Qualitative (Subjective) Rating (1-4)</td></tr></thead><tbody><tr><td>24. How does small-scale morphology in subglacial and continental shelf bathymetry affect Antarctic Ice Sheet response to changing environmental conditions?</td><td><strong>2-3</strong><br>Knowledge of bed morphology is essential to assessing ice sheet instability in response to climate change.<br><br><strong>Crosscuts Dynamic Earth: 2-3</strong><br>In terms of understanding how to do it - 3. In terms of getting the data and using the methods available - 2. Grounding lines remain an unresolved issue.</td></tr><tr><td>25. What are the processes and properties that control the form and flow of the Antarctic Ice Sheet?</td><td><strong>3</strong><br>There is good knowledge of the present state of the ice sheet.<br><br><strong>2</strong><br>It remains difficult to simulate the time dependency of bed properties and water pressure at the ice bed interface.</td></tr><tr><td>26. How does subglacial hydrology affect ice sheet dynamics, and how important is it?</td><td><strong>2</strong><br>There is some evidence that basal water moving in and out of subglacial lakes causes fluctuations in ice velocity. However, there are few observations and the underlying processes remain largely unknown.<br><br><strong>Crosscuts Dynamic Earth: 2</strong><br>There are important gaps in knowledge of important processes as above.</td></tr></tbody></table>
27. How do the characteristics of the ice sheet bed, such as geothermal heat flux and sediment distribution, affect ice flow and ice sheet stability?

2

Geothermal flux controls temperature within the ice as well as where ice reaches the melting point driving basal water production. However, maps are inconsistent and incomplete.

**Crosscuts Dynamic Earth: 2**

On heat flux, measurements are proxy-based and make assumptions that have led to inconsistent results. On sediment – radar, it has not been integrated into Bedmap or ice sheet models.

28. What are the thresholds that lead to irreversible loss of all or part of the Antarctic ice sheet?

2

The existence of various tipping points has been proposed.

29. How will changes in surface melt over the ice shelves and ice sheet evolve, and what will be the impact of these changes?

2

Surface melt is important for its impact on calving, ice shelf break-up and marine ice cliff instability processes however, knowledge is limited on the underlying processes and forcings.

30. How do oceanic processes beneath ice shelves vary in space and time, how are they modified by sea ice, and do they affect ice loss and ice sheet mass balance?

2

Ice loss in Antarctica is mainly due to an acceleration of outlet glaciers and grounding line retreat driven by the thinning of floating ice shelves. Most knowledge to date is based on modelling.

**Crosscuts Southern Ocean**

See above.

31. How will large-scale processes in the Southern Ocean and atmosphere affect the Antarctic Ice Sheet, particularly the rapid disintegration of ice shelves and ice sheet margins?

2

In the Amundsen Sea area, the decadal variability of the ocean was found to control the variability of basal melt via a non-linear relationship with ocean temperature.

**Crosscuts Antarctic Atmosphere: 2**

See above.
**Crosscuts Southern Ocean**

See above.

32. How fast has the Antarctic Ice Sheet changed in the past and what does that tell us about the future?

2

Improved understanding of how the Antarctic Ice Sheet responded to forcings in the past is crucial to improving the reliability of forecasts. Most studies to date are for the last deglaciation.

33. How did marine-based Antarctic ice sheets change during previous inter-glacial periods?

2

Information on past interglacial periods is mainly based on sea-level records in both the north and south. In these records it is difficult to disentangle contributions from the Greenland and Antarctic ice sheets.

34. How will the sedimentary record beneath the ice sheet inform our knowledge of the presence or absence of continental ice?

1

Even if robust indicators of ice-free conditions can be extracted from sedimentary records, the difficulties of subglacial access means that only sparse point data could be recovered, making large-scale inferences about ice sheet presence/absence challenging.

**Crosscuts Dynamic Earth: 1**

Geological drilling on land ice has yet to be initiated.
<table><caption>Table S7. Expert (subjective) assessments of progress toward answering “Dynamic Earth - Probing Beneath Antarctic Ice” questions supported by peer-reviewed literature citations. Comments by experts from crosscutting clusters are identified as ‘Crosscuts [name of cluster]’. The references cited are exemplars and not intended to be an exhaustive literature review.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Conclusions</td></tr></thead><tbody><tr><td>35. How does the bedrock geology under the Antarctic Ice Sheet inform our understanding of supercontinent assembly and break-up through Earth history?</td><td>There is some progress on this question at local (fine resolution) and continental (low resolution) scales.<sup>150</sup> ADMAP2 offers an updated continent-wide view. However, resolution remains low in some places, and no drilling to bedrock (and sampling) limits interpretations. More quantitative geological approaches, including machine learning are not well developed for subglacial geology.</td></tr><tr><td>36. Do variations in geothermal heat flux in Antarctica provide a diagnostic signature of sub-ice geology?</td><td>There has been extensive work on this question, but there remains a lack of consensus because of the diversity of proxy methods (e.g. from radar, magnetics, and seismic surveys) used to estimate heat flux variations.<sup>126,151,152,153</sup> A systematic attempt to reconcile differing proxy methods is needed, and further progress tied to geology is limited by a lack of direct sampling. The role of groundwater in modulating heat flux at the base of the ice sheet is largely unexplored but could potentially be a significant factor. There is an informative blog related to Q.36.<sup>154</sup></td></tr><tr><td>37. What is the crust and mantle structure of Antarctica and the Southern Ocean, and how do they affect surface motions due to glacial isostatic adjustment?</td><td>Progress has been made for several regions through seismic arrays<sup>155</sup>, airborne programs<sup>156</sup> and continental-scale models.<sup>151</sup> There remain substantial gaps in high quality data coverage, especially in East Antarctica, though this is improving. Direct measurements of crustal motions have not been made for areas covered with ice due to the challenge of removing the ice flow signal. Including higher resolution data in Glacial isostatic adjustment (GIA) models is far from straightforward – but capabilities for complex models are improving. Modelling has assisted in understanding the sensitivity of isostatic changes to lithospheric structures.<sup>157</sup></td></tr></tbody></table>
<table>
   <tr>
    <td>38. How does volcanism affect the evolution of the Antarctic lithosphere, ice sheet dynamics, and global climate?</td>
    <td>There has been some progress on the identification of volcanos using bed morphology<sup>158</sup>, although the mapping accuracy is limited by false positives and data artifacts. There have also been detections of englacial ash layers<sup>159</sup>, though this has not progressed as a theme, despite some efforts by the Scientific Committee on Antarctic Research (Antvolc).<sup>160</sup> Currently there is a lack of understanding of feedbacks of volcanism with ice sheets and their significance, except perhaps locally. There is an informative blog related to Q.38.<sup>161</sup><br/><b>Crosscuts Antarctic Atmosphere</b><br/>See above.<br/><b>Crosscuts Antarctic Ice Sheet</b><br/>There is some evidence that subglacial volcanism may have occurred during the last glacial maximum, leading to subglacial melting.<sup>162</sup> Whether meltwater led to flow acceleration, and/or freshwater discharge, to the ocean remains unknown.</td>
   </tr>
   <tr>
    <td>39. What are and have been the rates of geomorphic change in different Antarctic regions, and what are the ages of preserved landscapes?</td>
    <td>Recently limited work has been undertaken on landscape development using Radio Echo Sounding (RES) data, as well as thermochronological data. Most work has been regional, with some attempts at continental-scale assessments.<sup>163,164,165</sup> The Bedmap3 project, and the data used to build it, will address Q.39.</td>
   </tr>
   <tr>
    <td>40. How do tectonics, dynamic topography, ice loading and isostatic adjustment affect the spatial pattern of sea level change on all time scales?</td>
    <td>Substantial conceptual progress has been made in recognizing the potential effects of dynamic topography and tectonics on paleoceanography, the ice sheet and past estimates of sea level.<sup>129,166</sup> Understanding is limited by a lack of consistency between models and a lack of data to constrain models leading to a high degree of uncertainty in predictions.<br/><b>Crosscuts Antarctic Ice Sheet</b><br/>See above.</td>
   </tr>
   <tr>
    <td>41. Will increased deformation and volcanism characterize Antarctica when ice mass is reduced in a warmer world, and if so, how will glacial- and ecosystems be affected?</td>
    <td>There is limited progress on this question. For volcanism see Q38. Feedbacks between ice mass loss and tectonic activity need further study. It is unclear how to assess the role of deformation without good hydrological and hydrogeological models, as deformation-potential is controlled by fluid pressures. Groundwater dynamics may be important but there are few studies.</td>
   </tr>
  </table>
<table><tr><td></td><td><strong>Crosscuts Antarctic Life</strong><br>There is little progress on the life sciences aspects of this question. General analyses of species richness and its spatial variability show that biodiversity is higher at geothermal sites for many groups; and the effect extends beyond the immediate geothermal, largely volcanic, areas; but little evidence is available.<sup>167</sup></td></tr><tr><td>42. How will permafrost, the active layer and water availability in Antarctic soils and marine sediments change in a warming climate, and what are the effects on ecosystems and biogeochemical cycles?</td><td>There is limited research mainly focused on the Antarctic Peninsula (see Q.10). SCAR has formed an expert group to investigate this topic, Antarctic Permafrost and Soils (Antpas), and advances are expected in next few years.<sup>31</sup></td></tr></table>
<table><caption>Table S8. Summary of qualitative (subjective) expert assessments of progress in answering “Dynamic Earth - Probing Beneath Antarctic Ice” questions rated as: 1 - no or little progress, 2 - moderate progress, 3 - major progress and 4 - answered. When a question crosscuts other clusters an additional perspective(s) on progress is provided. (New) – indicates a question that was not originally identified as cross-cutting in the Scan. For detailed commentary on progress and supporting references see Table S7.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Qualitative (Subjective) Rating (1-4)</td></tr></thead><tbody><tr><td>35. How does the bedrock geology under the Antarctic Ice Sheet inform our understanding of supercontinent assembly and break-up through Earth history?</td><td><strong>2</strong><br>Machine learning approaches have not been adapted. Samples from the bed are key – hence fast drill/ Rapid Access Ice Drill are important technologies, but little progress has been made in perfecting these tools.</td></tr><tr><td>36. Do variations in geothermal heat flux in Antarctica provide a diagnostic signature of sub-ice geology?</td><td><strong>1-2</strong><br>Available geothermal maps differ from each other making it difficult to resolve geology. In situ samples are key.</td></tr><tr><td>37. What is the crust and mantle structure of Antarctica and the Southern Ocean, and how do they affect surface motions due to glacial isostatic adjustment?</td><td><strong>2-3</strong><br>Significant progress on this question but gaps in knowledge persist. Major advances have been accomplished by the Polar Observing Network (POLENET). Detailed models exist for lithospheric thickness and other variables, but data are needed for validations.</td></tr><tr><td>38. How does volcanism affect the evolution of the Antarctic lithosphere, ice sheet dynamics, and global climate?</td><td><strong>1</strong><br>Many questions remain – knowing where volcanoes are is a first step – but understanding of the resultant processes is largely unknown [except perhaps Ross Island]. Ash layers in ice sheet layers may lend important clues.<br><br><strong>Crosscuts Antarctic Atmosphere: 1</strong><br>See above.</td></tr></tbody></table>
Crosscuts Antarctic Ice Sheet

Field evidence shows that subglacial eruptions have occurred throughout the Cenozoic, but the impacts of these events on ice dynamics has not, and perhaps cannot, be determined from observations.

2

There are ideas on how to do this, but they need to be expanded continent-wide. Links to modelling need improvements.

2

Dynamic topography may be important. Pore pressures may contribute to a better understanding of aseismicity.

Crosscuts Antarctic Ice Sheet: 3

Conceptual progress but there is a lack of data to constrain the models.

1

No known ongoing work on this question.

1

There is limited research that is mainly focused on the Antarctic Peninsula however, a Scientific Committee on Antarctic Research Expert Group has been recently formed to address this and other related questions.

Crosscuts Antarctic Life

See above.

39. What are and have been the rates of geomorphic change in different Antarctic regions, and what are the ages of preserved landscapes?

40. How do tectonics, dynamic topography, ice loading and isostatic adjustment affect the spatial pattern of sea level change on all time scales?

41. Will increased deformation and volcanism characterize Antarctica when ice mass is reduced in a warmer world, and if so, how will glacial- and ecosystems be affected?

42. How will permafrost, the active layer and water availability in Antarctic soils and marine sediments change in a warming climate, and what are the effects on ecosystems and biogeochemical cycles?
<table><caption>Table S9. Expert (subjective) assessments of progress toward answering “Antarctic Life on the Precipice” questions supported by peer-reviewed literature citations. Comments by experts from crosscutting clusters are identified as ‘Crosscuts [name of cluster]’. The references cited are exemplars and not intended to be an exhaustive literature review.</caption><thead><tr><td>Antarctic Science Horizon Scan Question<sup>1,2</sup></td><td>Conclusions</td></tr></thead><tbody><tr><td>43. What is the genomic basis of adaptation in Antarctic and Southern Ocean organisms and communities?</td><td>Several important advances in understanding the genomic basis of adaptation in Antarctic single and multi-celled organisms have been made in the last five years. Comparative genomics with lower latitude species has provided key insights. For example, nearly 25% of the genome, of an Antarctic diatom (<i>Fragilariopsis cylindrus</i>), was comprised of significantly divergent alleles that were differentially expressed in high latitude conditions including low temperature, darkness, limiting iron and high CO<sub>2</sub>.<sup>168</sup> Likewise, important new information on Antarctic fishes has been produced via high quality genome sequencing. This work has shed light on evolutionary history of the cold-tolerant Antarctic Notothenoidei<sup>169</sup>, rapid adaptation of cellular respiration<sup>170</sup> and subsequent gene loss of hemoglobin, including details of antifreeze glycoproteins evolution in addition to gene family adaptations to high oxygen in subzero Antarctic waters.<sup>169</sup> Full genome sequence of the iconic midge, <i>Belgica antarctica</i>, indicates that its genome is the smallest of any insect. Environmental extremes have constrained genome architecture but not gene content, leading to small genome size. Many genes related to external stimuli, development and metabolism reflect adaptations to the Antarctic environment.<sup>171</sup> Genomic studies have indicated that trace gas scavenging supports primary productivity in Antarctic soils. Atmospheric H<sub>2</sub>, CO<sub>2</sub> and CO provide dependable sources of energy and carbon which support soil microbial communities. Thus, atmospheric energy sources could provide a basis for ecosystem function without solar or geological energy sources. The work has profoundly altered ideas about soil microbial adaptations in the Antarctic and has implications for exobiology.<sup>172</sup> See Q.47.</td></tr><tr><td>44. How fast are mutation rates and how extensive is gene flow in the Antarctic and the Southern Ocean?</td><td>Oceanic fronts around Antarctica and the Southern Ocean are the main drivers of speciation and diversification processes playing a central role in limiting gene flow and generating genetic and phylogeographic structure. New insights regarding postglacial (e.g. Last Glacial Maximum) expansion, recolonization, recent diversification - in both terrestrial and marine environments - from a less ice-impacted refugium, and especially their rapidity,</td></tr></tbody></table>
<table><tr><td></td><td>have been reported in vertebrates and invertebrates. Others are reconstructing past-demographic histories.<sup>173,174</sup> Changes in the permeability of the barriers, such as the Polar Front, due to global warming have also been highlighted.<sup>175</sup> High gene flows and rapid mutation rates have been identified, e.g. in sponges.<sup>176</sup> Rapid progress is being made in understanding the extent of gene flow and dispersal around the continent and in the Southern Ocean.<sup>177,178</sup> Many groups of organisms show extensive historic dispersal and connectivity with limited current dispersal, though for others significant patterns of current connectivity are evident.</td></tr></table>

45. How have ecosystems in the Antarctic and the Southern Ocean responded to warmer climate conditions in the past?

Studies on the impacts on Antarctic ecosystems of past conditions fall into three main areas: 1. the recent past (decades); 2. the last 1 million years (the glacial cycles); and 3. the deep past (millions of years). Recent past research focusses on ongoing human-driven climate change and its impacts on biodiversity. There have been no warmer periods in this time. During the last million years there has been at least one warm period where Antarctica was very different, including the collapse of the West Antarctic Ice Sheet and the presence of an open seaway between the Ross and Weddell Seas.<sup>179</sup> This link allowed species to travel between sites now cut off and the signals of the link are present in the genetics of bryozoans.<sup>179</sup> Over the last million years there have been glacial cycles of warming and cooling. The contraction of available habitat during glacial extremes restricted biodiversity to refugia in both marine and terrestrial habitats.<sup>180,181</sup> In marine systems this repeated contraction and expansion of habitat has been termed the biodiversity pump, leading to increased biodiversity over time.<sup>182</sup> Significant progress has been made on this subject in terrestrial refugia.<sup>183</sup> Range extension and contractions of numerous species have been investigated mainly from the Last Glacial Maximum and the present, in order to project future distributions out to 2100 (see Q.44 regarding postglacial expansion, recolonization, and diversification from a less ice-impacted refugium).<sup>173,174,184,185,186,187</sup> Strong but lasting impacts of past global warming on baleen whales have been noted.<sup>188</sup> Transformations of landscape and ecosystems in response to past warmer climate have been documented in moss species compositions.<sup>189</sup> More recently, over the past 90 years, the distribution of the krill population in the southwest Atlantic sector has contracted southward.<sup>190</sup> Recent warming cycles in the Antarctic Peninsula Region, over the last 50 years, have had rapid and large effects on lichen vegetation.<sup>191</sup> In the deeper-time periods over millions of years there were warmer periods. These do not inform how current ecosystems will respond to climate change but could give insight into long-term changes in biodiversity if future warming continues unabated for many decades. In the last five years much research has elucidated adaptations to the cold (e.g. on the loss of hemoglobin in
<table>
   <tr>
    <td></td>
    <td>Channichthyid icefish) and the associated adaptations with this condition.<sup>98,192</sup> The reverse evolution of these conditions will require very long times and will not be relevant to current climate change, but groups currently not present in Antarctica that were present when sea temperatures were significantly warmer, such as shallow water sharks, could return in the medium- to longer-term.<br/><b>Crosscuts Antarctic Atmosphere and Southern Ocean</b><br/>See above.</td>
   </tr>
   <tr>
    <td>46. How has life evolved in the Antarctic in response to dramatic events in the Earth's history?</td>
    <td>Several major adaptations of Antarctic species have been identified in relation to dramatic events in the Earth's history.<sup>98,169,173,193,194</sup> These include the evolution of antifreezes and cryoprotectants in terrestrial invertebrates, the emergence of Notothenioids as a species flock, antifreeze in fishes and the loss of hemoglobin in the Channichthyid ice fishes. These adaptations arose because of events such as the separation of Antarctica from South America and the subsequent cooling. All of these were known before the Horizon Scan and no new large-scale adaptations related to major events have been identified since. However, recent advances have been made in relation to the survival during past glacial maxima using phylogenetic approaches to identify likely refugia in both terrestrial and marine environments.<sup>179,193,196</sup> In both terrestrial and marine systems, geothermal heating has been identified as a potential heat source producing refugia during glacial maxima. Two recent global to semi-global analyses have shown the importance of the Antarctic for the evolution of global marine life, one showing extensive diversification of fishes and the other of brittle stars in the Antarctic in response to past changes in the Earth system.<sup>195,196</sup></td>
   </tr>
   <tr>
    <td></td>
    <td><b>Crosscuts Dynamic Earth</b><br/>See above</td>
   </tr>
   <tr>
    <td>47. How do subglacial systems inform models for the development of life on Earth and elsewhere?</td>
    <td>The discovery of functioning microbial ecosystems underlying the Antarctic ice sheet in sublacial Lake Whillans<sup>197</sup> provided a critical advance in understanding of the requirements of ecosystem sustainability in the absence of light. In addition, extensive subglacial brine networks were revealed in the Taylor<sup>198</sup> and Victoria valleys<sup>199</sup>, both of which have the potential to support subsurface microbial life at sustained subzero temperature and high salt - conditions that could be present in the subsurface of Mars or the icy moons of Jupiter (Europa) and Saturn (Enceladus) where ice shell-covered oceans exist. See Q. 42.<br/><b>Crosscuts Eyes on the Sky</b><br/>See above.</td>
   </tr>
  </table>
<table><tr><td>48. Which ecosystems and food webs are most vulnerable in the Antarctic and Southern Ocean, and which organisms are most likely to go extinct?</td><td>Threats and stressors differ between terrestrial and marine environments. On land recent research has demonstrated significant regionalization, with sixteen ecoregions identified on the continent and eight more on the surrounding islands.<sup>200</sup> Some groups, such as mites and springtails have highly differentiated species groups between regions but data for others (e.g. rotifers, tardigrades) are too limited to make assessments. The main perceived threats are invasive species that will outcompete local species and the movement of species within Antarctica breaking down isolation.<sup>201</sup> Tourists and scientists have been identified as significant vectors for both processes. For plants recent data are sparse but some lichen species have been shown to be poor at resisting environmental change.<sup>202</sup> The most significant data thus far are for mosses in East Antarctica, demonstrating how drought resistant species are replacing species with preferences for moister environments, associated with increased drying due to the Southern Annular Mode.<sup>203</sup> Responses to warming have also been demonstrated in the moss communities of the Antarctic Peninsula with increases in growth rate and accumulation rate of mosses and increases in microbial productivity recorded from the early to mid-1900s but with records extending back to ca. 1850.<sup>204</sup> Indications are that systems will change profoundly. In the marine environment several threats have been identified.<sup>98</sup> King Crabs migrating from deeper water in shallow sites have the potential to alter communities.<sup>205</sup> Significant new research has been published in the last five years. Antarctic marine species have been shown to be vulnerable to warming with some especially vulnerable species such as the brittle star <i>Ophionotus victoriae</i>.<sup>206</sup> Many species seem resistant to acidification but some species, such as pteropods, are at risk from acidification.<sup>207,208</sup> Glacier retreat has been highlighted in recent years as being a major threat to assemblages living in nearshore sites where smothering by sediment can remove species on a large scale.<sup>209,210</sup> Research on the effects of warming have been done using <i>in situ</i> heating of surfaces to temperatures were 1°C or 2°C above ambient.<sup>206</sup> These experiments showed that a 1°C increase dramatically improved performance in marine invertebrates, doubling growth rates, but a 2°C rise pushed some species to, or beyond, their limits. A recent, Antarctic-wide investigation has projected declines in the abundance of the emperor penguin of ≥ 50%.<sup>211</sup></td></tr><tr><td>49. How will threshold transitions vary over different spatial and temporal scales, and how will they impact ecosystem functioning under future environmental conditions?</td><td>At the time of the Scan, the SCAR program, "Antarctic Thresholds – Ecosystem Resilience and Adaptation (Ant-ERA) began aiming to identify tolerance limits and threshold effects and resistance and resilience to environmental change.<sup>212,213</sup> For the Antarctic region little is known about whether thresholds are detectable but the field in general is rapidly progressing.<sup>214-217</sup> Much remains to be done to understand the scales that transitions operate on, and what are the effects on the biota consistent with general ecological</td></tr></table>
<table><tr><td></td><td>interpretations of thresholds.<sup>214</sup> Some progress has been made on the theoretical aspects of threshold transitions<sup>217</sup> but empirical work is only beginning. Current work is exploring what the impacts might be of large events, such as the breakup of ice-shelves, the absence of fast ice in the ocean and melt events in terrestrial areas.<sup>216,217</sup> Positive thresholds may be encountered when coastal glaciers and ice shelves have been lost or retreated making new areas available for the establishment of new ecosystems. Negative thresholds may be passed when ice shelves disintegrate and unique under ice shelf assemblages are lost (see Q.48). A major factor likely to produce threshold changes in terrestrial assemblages is water availability. Recent work has shown changes in moss communities in East Antarctica and in ecosystems in the Dry Valleys.<sup>203,216,217</sup> Research over decades, using cloche systems or under soil heating, has also demonstrated that the transition from a frozen to a liquid environment has large effects on terrestrial ecosystems (see Q.50). Yet in all cases large or extreme events, and events with large impacts, have not been clearly differentiated from threshold transitions.</td></tr><tr><td>50. What are the synergistic effects of multiple stressors and environmental change drivers on Antarctic and Southern Ocean biota?</td><td>There have been significant efforts in identifying the effects of multiple stressors.<sup>218, 219, 220, 221, 222, 223</sup> These have been mainly focused on the combined effects of temperature and acidification and have generally demonstrated larger effects from predicted warming than from acidification. Significant research has been done on phytoplankton and a range of marine invertebrates. Research on macroalgae showed synergistic positive effects of warming and acidification on growth in two species of Desmarestia.<sup>222</sup> However, progress towards robust evaluations of the effects of multiple stressors is limited as temperature, acidification, salinity, turbidity, sedimentation, oxygen and physical disturbance from ice are all recognized as environmental stressors and few studies, if any, have investigated more than two factors at a time in Antarctic systems. Little is known about the impact of important stressors such as the combined impacts of climate change and fishing, though there is growing awareness that such change may be important. The impacts of multiple stressors in terrestrial systems are also poorly known. Interactions between climate change and disease may have been responsible for significant changes to populations of a cushion plant on sub-Antarctic Macquarie Island.<sup>224</sup> A large-scale study of springtails indicates that invasive species, including in the Antarctic region, have greater tolerance to warming than their indigenous counterparts.<sup>225</sup></td></tr><tr><td>51. How will organism and ecosystems respond to a changing soundscape in the Southern Ocean?</td><td>Research on how marine species endemic to Antarctica respond to anthropogenic noise is sparse.<sup>226,227</sup> How anthropogenic noise affects species at the population level or how effects vary depending on the life-stage of the organism in question are poorly understood.</td></tr></table>