Accelerated warming of the Southern Ocean and its impacts on the hydrological cycle and sea ice

The observed sea surface temperature in the Southern Ocean shows a substantial warming trend for the second half of the 20th century. Associated with the warming, there has been an enhanced atmospheric hydrological cycle in the Southern Ocean that results in an increase of the Antarctic sea ice for the past three decades through the reduced upward ocean heat transport and increased snowfall. The simulated sea surface temperature variability from two global coupled climate models for the second half of the 20th century is dominated by natural internal variability associated with the Antarctic Oscillation, suggesting that the models’ internal variability is too strong, leading to a response to anthropogenic forcing that is too weak. With increased loading of greenhouse gases in the atmosphere through the 21st century, the models show an accelerated warming in the Southern Ocean, and indicate that anthropogenic forcing exceeds natural internal variability. The increased heating from below (ocean) and above (atmosphere) and increased liquid precipitation associated with the enhanced hydrological cycle results in a projected decline of the Antarctic sea ice.     

Broad-scale predictability of carbohydrates and exopolymers in Antarctic and Arctic sea ice

Sea ice can contain high concentrations of dissolved organic carbon (DOC), much of which is carbohydrate-rich extracellular polymeric substances (EPS) produced by microalgae and bacteria inhabiting the ice. Here we report the concentrations of dissolved carbohydrates (dCHO) and dissolved EPS (dEPS) in relation to algal standing stock [estimated by chlorophyll (Chl) a concentrations] in sea ice from six locations in the Southern and Arctic Oceans. Concentrations varied substantially within and between sampling sites, reflecting local ice conditions and biological content. However, combining all data revealed robust statistical relationships between dCHO concentrations and the concentrations of different dEPS fractions, Chl a, and DOC. These relationships were true for whole ice cores, bottom ice (biomass rich) sections, and colder surface ice. The distribution of dEPS was strongly correlated to algal biomass, with the highest concentrations of both dEPS and non-EPS carbohydrates in the bottom horizons of the ice. Complex EPS was more prevalent in colder surface sea ice horizons. Predictive models (validated against independent data) were derived to enable the estimation of dCHO concentrations from data on ice thickness, salinity, and vertical position in core. When Chl a data were included a higher level of prediction was obtained. The consistent patterns reflected in these relationships provide a strong basis for including estimates of regional and seasonal carbohydrate and dEPS carbon budgets in coupled physical-biogeochemical models, across different types of sea ice from both polar regions.Sea ice covers extensive regions of the Arctic and Southern Oceans, as well as some subpolar seas, and exhibits major annual, interannual, and long-term climate-related variability in age, thickness, and structure (1–3). Sea ice is not an inert physical barrier to air–ocean exchange (4), and both microbial activity and physico-chemical reactions within the ice contribute to regional-scale biogeochemical processes at the air–ocean surface interface (5).Sea ice provides a range of habitats for diverse biological assemblages that are characterized by high standing stocks of microalgae and bacteria (6). These microorganisms produce large quantities of dissolved organic carbon (DOC), often in the form of carbohydrate-rich extracellular polymeric substances (EPS) (7). Microbial EPS exist in a dynamic equilibrium from dissolved polysaccharides (dEPS <0.2 µm) to complex particulate EPS that can form gels on the millimeter to centimeter scale (8). Here we focus on the biologically relevant dissolved carbohydrates (dCHO) that constitute a substantial fraction of the DOC in sea ice (9–13) (Fig. 1). dCHO are concentrated from sea ice DOC by dialysis (>8 kDa), with subsequent treatment allowing the definition of four subcomponents of the total dCHO pool: (i) dissolved uronic acids (dUA), produced by ice diatoms and ice bacteria (14–16), that confer strong cross-linkages between polymer chains (8), forming low solubility EPS complexes within brine channels (8, 14, 17); (ii) dEPS, produced by sea ice algae (9, 12, 18, 19) and isolated from dCHO by 70% (vol/vol) alcohol precipitation; (iii) a low solubility fraction of dEPS obtained by 30% (vol/vol) alcohol precipitation, containing complex EPS molecules (dEPS_complex), often produced by algae with reduced biological activity or when under physiological stress (9, 13, 19); and (iv) a fraction of highly soluble carbohydrates that are not considered EPS (dCHO_non-EPS), do not precipitate in alcohol, and are produced by many actively growing ice algae (9, 14).Open in a separate windowFig. 1.Representation of the molecular-size spectrum from large polysaccharides to low molecular-weight components of the total DOC pool (<0.2 µm) in melted sea ice, and partitioning of DOC into dCHO, dUA (by dialysis >8 kDa), dEPS, complex dEPS, and non-EPS carbohydrate fractions (by alcohol precipitation). Dotted boxes indicate a subcomponent of the main category.The bacteria and algae that successfully colonize sea ice habitats have mechanisms that enable them to survive temperatures less than −20 °C and salinities >100 in the sea ice brines (17, 20). However, there is increasing evidence that the processes of seawater freezing can be biologically mediated by ice-binding proteins and EPS secreted by bacteria and algae. These compounds can alter ice structure (14, 15, 21–26) and, in the case of EPS, also form physico-chemical buffers between the organisms and the surrounding brines and ice matrix (9, 14, 17).When sea ice melts, its dissolved and particulate constituents are released into the surface waters (27, 28), contributing to the microbial dynamics in both the melting ice and melt waters (19, 29–31). Physical aggregation of EPS in seawater to form larger particles may promote the sinking of particulate organic matter from the surface waters (19, 32), or produce EPS foams that are a source of aerosol particles, which are thought to have an active role in atmospheric nucleation processes in the Arctic (33).The growing evidence of the important role played by microbial EPS in sea ice, coupled with the changes in the extent and duration of ice cover in the polar seas (3), makes obtaining a quantitative understanding of EPS in sea ice important. Here we present a synthesis of data on dCHO—and its constituents—from sea ice samples from the Arctic and Southern Oceans, covering different seasons and variable microbial standing stocks. Our aim was to identify trends in the relationships between components of dEPS and fundamental parameters, such as temperature, ice thickness, and algal standing stock in sea ice. Using this dataset we present predictive models that can allow the estimation of dCHO and dEPS in sea ice and the potential mass of these carbon-rich organic substances on regional scales.     

Influence of sea ice on Arctic precipitation

Global climate is influenced by the Arctic hydrologic cycle, which is, in part, regulated by sea ice through its control on evaporation and precipitation. However, the quantitative link between precipitation and sea ice extent is poorly constrained. Here we present observational evidence for the response of precipitation to sea ice reduction and assess the sensitivity of the response. Changes in the proportion of moisture sourced from the Arctic with sea ice change in the Canadian Arctic and Greenland Sea regions over the past two decades are inferred from annually averaged deuterium excess (d-excess) measurements from six sites. Other influences on the Arctic hydrologic cycle, such as the strength of meridional transport, are assessed using the North Atlantic Oscillation index. We find that the independent, direct effect of sea ice on the increase of the percentage of Arctic sourced moisture (or Arctic moisture proportion, AMP) is 18.2 ± 4.6% and 10.8 ± 3.6%/100,000 km² sea ice lost for each region, respectively, corresponding to increases of 10.9 ± 2.8% and 2.7 ± 1.1%/1 °C of warming in the vapor source regions. The moisture source changes likely result in increases of precipitation and changes in energy balance, creating significant uncertainty for climate predictions.There is increasing interest in the response of the Arctic hydrologic cycle to changing climate because of its potential to influence, or feedback to, future climate change. Modeling studies have identified enhanced transport of subtropical moisture to the Arctic as well as increased Arctic evaporation as potential mechanisms of augmentation of the water cycle (1–3). The enhanced hydrologic cycle may feedback to climate change either positively or negatively; both the sign and the magnitude are yet to be determined.Observational evidence for hydrological acceleration during the past few decades is limited. Direct measurement of precipitation is difficult in the Arctic because of its cold, windy environments (4). Despite these difficulties, increasing precipitation has been reported for some Arctic locations (5, 6), and it has been hypothesized that changes in sea ice extent may have significantly influenced precipitation both in the past (7) and today (8–10). We report a study of changes in the isotopic composition of precipitation to understand the larger-scale changes of the hydrologic cycle, focusing on moisture source changes. The objective of this work is to assess observationally the effect of sea ice and the moisture transport regime on Arctic precipitation from 1990 to 2012, using the isotopic composition of precipitation from six Arctic stations. In particular, we quantify how the fraction of the total Arctic precipitation that is sourced in the Arctic responds to the sea ice extent. We then use these empirically established sensitivities of precipitation isotope ratios to sea ice change to project potential future precipitation changes and to evaluate impacts of these changes on the energy balance.Our approach is based on the premise that Arctic precipitation is composed mostly of water from two marine evaporation regions or “moisture sources”—one subtropical and one local—and that the relative contributions of the two sources to the precipitation can be determined from the stable isotopic ratios of the precipitation. We partition the two proportions, using the precipitation deuterium excess (d-excess, defined as d = δD − 8δ¹⁸O, where δD and δ¹⁸O are the parts per thousand deviation of deuterium/hydrogen and ¹⁸O/¹⁶O atomic ratios, respectively, from those of the standard mean ocean water), which is an indicator of moisture source conditions, principally the sea surface temperature (SST) and relative humidity (RH) (11–13). Moisture from subtropical regions has high d-excess values, indicative of relatively high SST and low RH at the source, whereas locally evaporated Arctic moisture has low d-excess values (14), indicating low SST and high RH. We hypothesize that precipitation d-excess is positively associated with sea ice area as a consequence of increasing local evaporation and thus increasing proportion of Arctic-sourced moisture with reduction of sea ice.We use the North Atlantic Oscillation (NAO) index as a proxy for general climate conditions to quantify effects that are independent of the sea ice influence on precipitation. Most importantly, the NAO is associated with the strength of meridional transport (15), which in turn affects precipitation d-excess by changing the proportion of subtropical moisture in the total precipitation. For example, if winds from the south strengthen, the proportion of moisture transported from the subtropics would increase, thus increasing the d-excess. In addition to meridional transport, the NAO also influences other variables, such as location of the subtropical moisture source region, temperature and humidity along the storm track, etc., all of which may affect the d-excess of precipitation. When holding the NAO constant (statistically), we also effectively remove the influences of these variables, achieving limited contamination to the signal of the direct precipitation–sea ice relationship.The six sites included in this work were from two regions, the Canadian Arctic (Alert, Eureka, and Cambridge Bay, Canada) and Greenland Sea (Reykjavìk, Iceland, Ny-Ålesend, Norway, and Danmarkshavn, Greenland) (Fig. 1). We consider all sites within a region to share similar local moisture sources. The Canadian Arctic sites receive most of their local moisture from Baffin Bay (16) and the Greenland Sea sites receive it from the Greenland Sea (17).Open in a separate windowFig. 1.Location of sites for monthly precipitation isotope ratio measurements. Shown are Canadian Arctic sites Alert, Eureka, and Cambridge Bay, Canada and Greenland Sea sites Reykjavìk, Iceland; Ny-Ålesend, Norway; and Danmarkshavn, Greenland. Local moisture sources Baffin Bay (BB) and Greenland Sea (GS) are labeled as well.     

The role of double-diffusive convection in basal melting of Antarctic ice shelves

The Antarctic Ice Sheet loses about half its mass through ocean-driven melting of its fringing ice shelves. However, the ocean processes governing ice shelf melting are not well understood, contributing to uncertainty in projections of Antarctica’s contribution to global sea level. We use high-resolution large-eddy simulation to examine ocean-driven melt, in a geophysical-scale model of the turbulent ice shelf–ocean boundary layer, focusing on the ocean conditions observed beneath the Ross Ice Shelf. We quantify the role of double-diffusive convection in determining ice shelf melt rates and oceanic mixed layer properties in relatively warm and low-velocity cavity environments. We demonstrate that double-diffusive convection is the first-order process controlling the melt rate and mixed layer evolution at these flow conditions, even more important than vertical shear due to a mean flow, and is responsible for the step-like temperature and salinity structure, or thermohaline staircase, observed beneath the ice. A robust feature of the multiday simulations is a growing saline diffusive sublayer that drives a time-dependent melt rate. This melt rate is lower than current ice–ocean parameterizations, which consider only shear-controlled turbulent melting, would predict. Our main finding is that double-diffusive convection is an important process beneath ice shelves, yet is currently neglected in ocean–climate models.

Ocean-driven basal melting, which comprises more than half of the mass loss from Antarctica’s ice shelves (1, 2), is thinning the Antarctic Ice Sheet at an accelerating rate (3, 4). The rate of loss of grounded ice mass from West Antarctica alone has increased by 70% since 2002 (3). Consequently, the Antarctic contribution to global sea level is accelerating, presenting a major threat to coastal regions (5). Melting of ice shelves is also contributing a freshwater flux to the ocean, which has a major impact on the production of Antarctic Bottom Water which supplies the lower limb of the global thermohaline circulation (6). Understanding the fine-scale processes by which ocean and ice shelves interact, and how they depend on surrounding water properties, is essential for accurately predicting the response of the Antarctic Ice Sheet to a changing climate.Where the ocean meets the ice shelf, a boundary layer forms which regulates heat and salt exchanges between the “far field” ocean and the ice, and is crucial in determining the rate at which the ice shelf melts. Predicting basal melt rates therefore requires knowledge not only of the ocean properties within the ice shelf cavity but also of the processes controlling transport of heat and salt across the ice–ocean boundary layer (7). Basal melt projections of Antarctic ice shelves rely exclusively on numerical studies using large-scale Reynolds-Averaged Navier–Stokes (RANS) models (8, 9). However, the difficulty in accessing the sub-ice shelf environment to observe fine-scale boundary layer processes means that there are few observational constraints on how basal melting should be represented in these models.RANS models of ice shelf cavities resolve flow at horizontal scales of several hundred meters and vertical scales of meters at best. These scales are too coarse to capture ice–ocean boundary layer processes, which must instead be represented using parameterizations. These parameterizations typically assume that melt is controlled by the shear (velocity gradient) induced by large-scale currents or buoyant plumes (10–12) and set the melt rate as proportional to the plume or current velocity (13). This common approach has not been validated beneath an ice shelf, nor can it be ubiquitously applicable, a counterexample being a case without any mean flow in which melting still takes place (8, 14). Recent laboratory (15), turbulence-resolving numerical (16, 17) and theoretical (18, 19) studies have focused on the melting of sloping or vertical ice, for which meltwater drives a buoyant plume along the ice, finding that the melt rate is controlled by the elevation of ambient temperature above the in situ freezing temperature and is independent of the plume velocity, providing that the velocity is sufficiently low (18, 20).Under certain ocean conditions, the differing diffusivities of heat and salt give rise to a type of convection known as double-diffusive convection (DDC) (21, 22). Melting of ice shelves releases cold, fresh water above warm, salty water which can drive a type of DDC known as diffusive convection (DC), so named to distinguish it from salt fingering (SF)-type DDC in which salinity is the destabilizing component (23). DC can form “diffusive staircases” composed of well-mixed layers separated by sharp interfaces. Diffusive staircases have primarily been observed in polar regions such as the Arctic Ocean (24), the Weddell Sea in Antarctica (25), and beneath George VI Ice Shelf in Antarctica (26). Recent observations from the grounding line of the Ross Ice Shelf (RIS) are also consistent with DC (27), where well-mixed layers in temperature and salinity were observed over the 10-m-thick water column. Both ice shelf sites recorded melt rates that were much lower than common ice–ocean parameterizations, with their assumption of turbulent, unstratified flow, would predict given the observed ocean conditions. DC has been investigated in small-scale experimental (28) and modeling (14) studies of ocean-driven melting of horizontal ice. However, the role of DC remains unclear in the presence of competing processes such as turbulence and mixing due to ocean currents beneath the ice. Importantly, RANS ocean models do not capture the effect of DC on basal melting, leading to uncertainties in basal melting projections. Convection resolving numerical simulations are urgently needed to quantify the effect of DC in ice–ocean interactions and develop parameterizations for use in RANS models.High-resolution modeling techniques such as direct numerical simulation (DNS) and large-eddy simulation (LES) are being recognized for their capabilities in resolving and quantifying ice–ocean boundary layer processes. For example, a recent DNS study demonstrated that DC can drive turbulent fluxes of heat and salt beneath the ice (14). LES has been used to study the effect of current shear and ocean temperature on basal melting, demonstrating that stratification can inhibit the transport of heat and salt across the ice shelf–ocean boundary layer under relatively warm and low-shear conditions (29). However, no studies, to date, have investigated the feedback between mixed layer dynamics and melting for a stratified, geostrophic boundary layer, resolving both the near-ice and mixed layer processes.Here we investigate the processes controlling ice shelf basal melting by modeling the turbulent geostrophic boundary layer beneath an ice shelf. Using LES, we perform experiments that demonstrate the dominant role of DC in ice shelf–ocean interactions at some observed Antarctic conditions. The melting and boundary layer structure in our numerical model agree with field observations of the water column and melting beneath the RIS (27), and allow us to explain the mechanism connecting the ocean state to the basal melt rate.     

Role of proton ordering in adsorption preference of polar molecule on ice surface

Adsorption of polar monomers on ice surface, relevant to the physical/chemical reaction in ice clouds as well as growth of ice, remains an open issue partially due to the unusual surface characteristics with protons at the top layer of ice. Using first-principle calculations, we explore the adsorption properties of ice surface in terms of a surface proton order parameter, which characterizes the inhomogeneity of the dangling atoms on ice surface. We show that, due to an effective electric field created by dangling OH bonds and lone pairs of water molecules not only directly neighboring but also further away from the adsorbed polar molecule on the ice surface, the adsorption energy of polar monomer on ice surface exhibits large variance and a strong correlation with the proton order parameter of ice surface. Our results about the positive correlation between the inhomogeneity of ice surface and adsorption energies suggest that the physical/chemical reactions as well as the growth of ice may prefer to occur firstly on surfaces with larger proton order parameter.     

Divergence of Arctic shrub growth associated with sea ice decline

Arctic sea ice extent (SIE) is declining at an accelerating rate with a wide range of ecological consequences. However, determining sea ice effects on tundra vegetation remains a challenge. In this study, we examined the universality or lack thereof in tundra shrub growth responses to changes in SIE and summer climate across the Pan-Arctic, taking advantage of 23 tundra shrub-ring chronologies from 19 widely distributed sites (56°N to 83°N). We show a clear divergence in shrub growth responses to SIE that began in the mid-1990s, with 39% of the chronologies showing declines and 57% showing increases in radial growth (decreasers and increasers, respectively). Structural equation models revealed that declining SIE was associated with rising air temperature and precipitation for increasers and with increasingly dry conditions for decreasers. Decreasers tended to be from areas of the Arctic with lower summer precipitation and their growth decline was related to decreases in the standardized precipitation evapotranspiration index. Our findings suggest that moisture limitation, associated with declining SIE, might inhibit the positive effects of warming on shrub growth over a considerable part of the terrestrial Arctic, thereby complicating predictions of vegetation change and future tundra productivity.

Arctic sea ice extent (SIE) is decreasing at an accelerating rate (1–5), with a seasonally ice-free Arctic Ocean expected within a few decades (6). Sea ice decline has elicited major changes in local climates and large-scale atmospheric circulation (7), extending beyond the regions of in situ sea ice changes (8). This includes the impact of winter SIE on upper-level atmospheric flow and subsequent summer air temperature, precipitation, and even soil moisture (9). While this rapid change in the physical system is occurring, the mechanisms by which Arctic sea ice interacts with biological systems are still largely unknown, especially in terrestrial systems (10). For this reason, the study of sea ice effects on Arctic biota has recently been classified as a crisis discipline (2).The effects of rapidly diminishing SIE on Arctic terrestrial ecosystems, such as changes in shrub growth and tundra productivity, are highly uncertain and understudied at the biome level (2, 3). This is due to i) the complex nature of sea ice dynamics and its strong coupling with atmospheric circulation patterns (7, 11) and climate variables, such as temperature (12), precipitation (13), and humidity (14); 2) the spatial scale of the processes, which are characterized by strong regional variation (15, 16); and 3) the dynamic nature of interannual changes in SIE (1). It is important that we improve understanding of sea ice effects on tundra ecosystems, because changes in the productivity and composition of Arctic vegetation have the potential to amplify or dampen trends in air temperature and sea ice extent through effects on land surface-atmosphere exchanges of carbon and energy (17).One of the best-documented vegetative responses to Arctic warming is widespread increased productivity and encroachment of deciduous shrubs into lower-statured tundra (18–20). More recently, several studies (21–25) have highlighted the potential for soil moisture to mediate the response of tundra shrub growth to climate warming. A recent synthesis of tundra shrub-ring data showed that shrub growth was more sensitive to interannual climate variability at sites with greater soil moisture (22). Meanwhile, sampling along a moisture gradient within a landscape of northern Alaska revealed a positive correlation between June air temperature and shrub growth at a riparian site and a distinct June air temperature optimum at a drier upland site (23). In Kangerlussuaq, western Greenland, which is a relatively dry area that is experiencing rapid warming, shrub-ring analysis revealed a decline in growth that coincided with decreasing carbon isotope discrimination, low midsummer xylem water potentials, and strong sensitivity of foliar gas exchange to recent rainfall events, suggesting moisture limitation as an underlying cause (25).Loss of sea ice likely promotes warmer conditions in adjacent terrestrial ecosystems because of the associated dramatic decrease in surface albedo (26). Local warming from sea ice loss can extend several hundred kilometers inland (27–31), and tundra responses to declining sea ice are emerging (10, 32). Examining relationships between sea ice conditions and shrub-ring data throughout the Arctic with explicit consideration of the indirect ways by which sea ice variability can ultimately affect vegetation growth may help assess tundra productivity trajectories. However, few studies have investigated sea ice–shrub growth relationships (33) and none have been conducted at the Pan-Arctic scale. In keeping with widespread observations of increasing shrub abundance, we hypothesized that shrub growth across the tundra biome would be promoted by declining SIE through a positive feedback between declining sea ice and increasing near-surface air temperatures. We expect that one important mechanism through which diminishing sea ice amplifies warming in this context is through greater surface solar absorption in expanding open water areas (34), which leads to local heating and thus favors shrub growth across the tundra biome.Here we report on tundra shrub growth responses to changes in SIE using 23 annually resolved shrub-ring chronologies of Betula and Salix shrubs from 19 sites distributed throughout the Arctic from a latitude of 56°N in eastern Canada to 83°N in northern Greenland (Fig. 1B and SI Appendix, Table S1). In total, we analyzed 641 shrubs with 20,336 growth ring measurements in relation to 1) Pan-Arctic and 2) regional SIE (both monthly and seasonal) and 3) the timing of regional sea ice retreat and freeze-up. Each chronology that was significantly correlated with either Pan-Arctic or regional SIE was aggregated into a specific responder group: increasers, chronologies that were negatively correlated with at least one monthly or seasonal SIE variable; and decreasers, chronologies that were positively correlated with at least one monthly or seasonal SIE variable. In order to test for direct and indirect effects of SIE and summer climate (air temperature, precipitation, and standardized precipitation evapotranspiration index (SPEI)) on shrub growth, we construct piecewise structural equation models (35) (SEMs). We verified the link between each sea ice variable and the growth of individual shrubs by analyzing individual shrub-ring series hierarchically in linear mixed effects (LME) models.Open in a separate windowFig. 1.Divergent growth response of Arctic shrubs linked to sea ice decline across the Pan-Arctic region. (A) Mean increaser (blue) and decreaser (red) chronologies (RWI with SD) in comparison with seasonal Arctic sea ice extent (black) time series for previous September–October–November (pSON SIE). Vertical dashed lines indicate the common period (1979 to 2008) studied in the synthesis. (B) Geographical locations of 23 shrub-ring chronologies and 641 shrubs in total included in the synthesis with indication of sea ice extent–shrub growth response type.     

Mechanisms for low-frequency variability of summer Arctic sea ice extent

Satellite observations reveal a substantial decline in September Arctic sea ice extent since 1979, which has played a leading role in the observed recent Arctic surface warming and has often been attributed, in large part, to the increase in greenhouse gases. However, the most rapid decline occurred during the recent global warming hiatus period. Previous studies are often focused on a single mechanism for changes and variations of summer Arctic sea ice extent, and many are based on short observational records. The key players for summer Arctic sea ice extent variability at multidecadal/centennial time scales and their contributions to the observed summer Arctic sea ice decline are not well understood. Here a multiple regression model is developed for the first time, to the author’s knowledge, to provide a framework to quantify the contributions of three key predictors (Atlantic/Pacific heat transport into the Arctic, and Arctic Dipole) to the internal low-frequency variability of Summer Arctic sea ice extent, using a 3,600-y-long control climate model simulation. The results suggest that changes in these key predictors could have contributed substantially to the observed summer Arctic sea ice decline. If the ocean heat transport into the Arctic were to weaken in the near future due to internal variability, there might be a hiatus in the decline of September Arctic sea ice. The modeling results also suggest that at multidecadal/centennial time scales, variations in the atmosphere heat transport across the Arctic Circle are forced by anticorrelated variations in the Atlantic heat transport into the Arctic.Observations reveal multidecadal variations in Arctic surface air temperature (SAT), and amplified Arctic warming similar to that observed in recent decades also occurred during 1930–1940 (1–3). Both observations and climate modeling results suggest that the reduced Arctic sea ice is crucial for the early twentieth century Arctic warming, and internal variability is a very likely cause for that event (3). In recent decades, satellite observations reveal a substantial decline in September Arctic sea ice extent (4). This observed recent Arctic sea ice decline is also found to have played a leading role in causing the observed amplified Arctic surface warming in recent decades (5, 6).The summer Arctic was projected to become ice-free within a few decades by some climate models used in Coupled Model Intercomparison Project Phase 5 (CMIP5) due to the increase in anthropogenic greenhouse gases (7, 8), or even within the next decade if extrapolating the observed trend (9). These future projections imply enormous social and economic impacts, such as the potential for trans-Arctic shipping. However, the most rapid decline in summer Arctic sea ice actually occurred during the recent global warming hiatus period. The CMIP5 multimodel mean response to changes in anthropogenic radiative forcings exhibits much less decline in September Arctic sea ice extent (SIE) but stronger warming in global mean surface temperature than that observed over the recent hiatus period (10), implying that natural variability might have played an important role in the observed recent decline in September Arctic SIE.Various mechanisms have been proposed separately for the observed recent summer Arctic sea ice decline, such as the positive ice infrared feedback, i.e., enhanced downward longwave radiative flux due to increased air temperature, water vapor, cloudiness, and reduced sea ice (11, 12); the positive ice albedo feedback (13–15); the warming of the Atlantic water in the Arctic (16–18); the increase in Bering Strait ocean heat fluxes (19); the influence of wind forcing over the central Arctic associated with the Arctic Oscillation (AO) (20, 21) and the nonlinear positive feedback (22) among Pacific inflow, Beaufort Gyre (23), and AO at interannual time scale; and the interaction between the Arctic Dipole (AD) and transpolar ice drift (24–28). The previous studies are often based on short observational records. Some crucial questions remain unknown, e.g., what are the key players for internal variability of summer Arctic SIE at multidecadal/centennial time scales and how do they contribute to the observed summer Arctic SIE decline?Multidecadal internal variability has been observed in the Atlantic (29), and climate models suggest that the Atlantic Meridional Overturning Circulation (AMOC) variability is a major source for the Atlantic multidecadal variability (AMV) and might be important for the observed opposite trends in Arctic and Antarctica sea ice (30). Both modeling results (31, 32) and multicentury historical records (33) showed that winter Arctic sea ice variability is closely linked to the AMV. The AMOC is suggested to have strengthened since the mid 1970s as implied indirectly by its fingerprints (34, 35). Could a strengthened AMOC have led to an enhanced Atlantic heat transport into the Arctic and thus contributed to the observed recent summer Arctic SIE decline? If the AMOC and the associated Atlantic heat transport into the Arctic were to weaken in the near future due to internal variability, would there be a hiatus in the decline of September Arctic SIE and a delay in attaining a summer ice-free Arctic?Motivated by the above questions, this paper investigates the internal low-frequency variability of summer Arctic SIE, using a 3,600-y segment of a control simulation from a renowned climate model, Geophysical Fluid Dynamics Laboratory (GFDL) Coupled Model version 2.1 (CM2.1) (36). Three key predictors for internal low-frequency variability of summer Arctic SIE are identified, and they cover a broad range of internal variability in the climate system, including both the Atlantic and Pacific ocean heat transport into the Arctic, as well as the atmosphere circulation. A multiple regression model is developed to provide a framework to quantify the contributions of the three key predictors. The advantage of such a long control simulation is the statistical reliability, especially at multidecadal/centennial time scales, which cannot be obtained by short observational records. The estimated contributions of these key predictors to the observed summer Arctic SIE decline are also discussed.     

Quantitative evaluation of the feasibility of sampling the ice plumes at Enceladus for biomarkers of extraterrestrial life

Enceladus, an icy moon of Saturn, is a compelling destination for a probe seeking biosignatures of extraterrestrial life because its subsurface ocean exhibits significant organic chemistry that is directly accessible by sampling cryovolcanic plumes. State-of-the-art organic chemical analysis instruments can perform valuable science measurements at Enceladus provided they receive sufficient plume material in a fly-by or orbiter plume transit. To explore the feasibility of plume sampling, we performed light gas gun experiments impacting micrometer-sized ice particles containing a fluorescent dye biosignature simulant into a variety of soft metal capture surfaces at velocities from 800 m ⋅ s⁻¹ up to 3 km ⋅ s⁻¹. Quantitative fluorescence microscopy of the capture surfaces demonstrates organic capture efficiencies of up to 80 to 90% for isolated impact craters and of at least 17% on average on indium and aluminum capture surfaces at velocities up to 2.2 km ⋅ s⁻¹. Our results reveal the relationships between impact velocity, particle size, capture surface, and capture efficiency for a variety of possible plume transit scenarios. Combined with sensitive microfluidic chemical analysis instruments, we predict that our capture system can be used to detect organic molecules in Enceladus plume ice at the 1 nM level—a sensitivity thought to be meaningful and informative for probing habitability and biosignatures.

Saturn’s moon Enceladus is an ideal location to search for molecular signs of extraterrestrial life within our solar system (1–3). Spectacular water vapor and ice plumes jet into space (4, 5) from a subsurface ocean through cracks in its frozen ice surface (6, 7) providing pristine samples available for analysis. First discovered by the Cassini–Huygens mission in 2005, the plumes are associated with plentiful evidence of habitability: the presence of a rocky core (8), hydrothermal activity at the ocean-core boundary (9–11), geochemical processes (12), and the presence of simple organic compounds (13, 14) as well as complex macromolecular organics (15, 16), which may be present as a thin organic-rich film on top of the oceanic water table. Enceladus therefore satisfies established criteria for habitability, including the presence of liquid water, a source of energy, and organic chemicals that are potential building blocks of life. Cassini’s rich data set provides tantalizing clues to the prevalence and complexity of carbon chemistry deep in Enceladus. These observations justify the development and use of high-performance liquid phase microfluidic organic analysis instruments (17–19) to probe the Enceladus ocean for habitability and for potential biosignatures by analyzing the trace chemical composition of plume ice particles in more sensitive and high-resolution detail.Neveu et al. recently presented a comprehensive review of the possible mission formats and considerations for sampling the Enceladus plume (20). A critical step in designing a mission for trace organic biosignature detection from the plume is developing and deploying a capture system that efficiently gathers intact organic molecules while passing through the plume (21) at velocities from a few hundred m ⋅ s⁻¹ to 3 km ⋅ s⁻¹. It is important to evaluate capture surfaces (CSs) at a variety of velocities representing the relative encounter of a spacecraft transecting with plume ice particles. Because capture efficiency is expected to vary with particle size (22) another challenge is ensuring that the experimental ice particle projectile size range encompasses the Enceladus plume brine ice particle sizes (median radius ∼3 µm) that contribute the most mass density to the plume.To address these challenges, we have evaluated the high-velocity capture of organic doped ice particles, 2 to 10 µm in diameter, utilizing soft metal CSs including Al, Au, and In with mechanical and thermal properties chosen to optimize ice capture and organic survival (18). The idea is to optimize capture efficiency, for a particular particle size-range and encounter velocity, by selecting a CS that is compliant to minimize shock disruption and has high thermal conductivity to minimize ice impact heating. The metals studied were chosen to have low reactivity to avoid catalyzing unwanted compositional changes. The CS must be fabricated and maintained to meet stringent cleanliness requirements for planetary protection, limiting trace organic contamination to parts per trillion levels. The CS must also permit complete release of any captured organics for subsequent liquid phase chemical analysis. For example, Al foil collectors were previously deployed in NASA’s Stardust mission to collect cometary dust, with evidence that traces of entrained amino acids survived the 6.1 km⋅s⁻¹ impact speed (23, 24).Proper evaluation of a capture system requires that experiments quantitatively and realistically represent organic ice particle capture in high-velocity impacts. The “cold” light gas gun (LGG) at the University of Kent, United Kingdom (25, 26), has the ability to perform high-precision, high-velocity shots of particles including ice into targets. Initial studies with poly(methyl-methacrylate) monodisperse microparticle projectiles revealed the relationship between crater size and impacting particle size and velocity for our CS materials (22). This result was needed to quantitate the size of impacting ice particles based on crater size for a distribution of ice fragment sizes produced in an ice shot. Next, we developed and tested a method of freezing ice projectiles doped with organic tracers and firing them at high velocities into targets of interest (27). In the LGG, the rapid acceleration to velocities up to 3.0 km ⋅ s⁻¹ shatters the solid ice projectile, resulting in a wide distribution of micrometer-sized brine ice particle fragments that hit the CS, encompassing the range of particle sizes expected in the Enceladus plume (28), but does not include the smaller water ice particles formed through homogeneous nucleation from water vapor (6), which are less relevant to biosignature searches. Finally, a calibrated epifluorescence microscope method was developed using a fluorescent tracer dye, Pacific Blue (PB), for quantifying the amount of unmodified organic molecules detected on a CS target on a crater-by-crater basis (29). With these calibration experiments in hand, the stage was set for experiments that accurately represent ice impacts in an Enceladus plume encounter.The results presented here allow us to quantitate the organic capture efficiency with respect to impact velocity and particle size in a range that is relevant for a variety of Enceladus plume transits. Combining the quantitative evaluation of the performance of our CS materials with the sensitive analysis capabilities of microfluidic chemical analysis instruments with laser-induced fluorescence (LIF) detection allows us to predict the science measurement potential of a variety of Enceladus plume sampling mission formats. Based on these calculations, we find that there are several scenarios that can provide highly meaningful information about organic chemical signatures of habitability and possible extraterrestrial life at Enceladus.     

Dynamic pressure-induced dendritic and shock crystal growth of ice VI

Crystal growth mechanisms are crucial to understanding the complexity of crystal morphologies in nature and advanced technological materials, such as the faceting and dendrites found in snowflakes and the microstructure and associated strength properties of structural and icy planetary materials. In this article, we present observations of pressure-induced ice VI crystal growth, which have been predicted theoretically, but had never been observed experimentally to our knowledge. Under modulated pressure conditions in a dynamic-diamond anvil cell, rough single ice VI crystal initially grows into well defined octahedral crystal facets. However, as the compression rate increases, the crystal surface dramatically changes from rough to facet, and from convex to concave because of a surface instability, and thereby the growth rate suddenly increases by an order of magnitude. Depending on the compression rate, this discontinuous jump in crystal growth rate or "shock crystal growth" eventually produces 2D carpet-type fractal morphology, and moreover dendrites form under sinusoidal compression, whose crystal morphologies are remarkably similar to those predicted in theoretical simulations under a temperature gradient field. The observed strong dependence of the growth mechanism on compression rate, therefore, suggests a different approach to developing a comprehensive understanding of crystal growth dynamics.     

Coupled impacts of sea ice variability and North Pacific atmospheric circulation on Holocene hydroclimate in Arctic Alaska

Arctic Alaska lies at a climatological crossroads between the Arctic and North Pacific Oceans. The modern hydroclimate of the region is responding to rapidly diminishing sea ice, driven in part by changes in heat flux from the North Pacific. Paleoclimate reconstructions have improved our knowledge of Alaska’s hydroclimate, but no studies have examined Holocene sea ice, moisture, and ocean−atmosphere circulation in Arctic Alaska, limiting our understanding of the relationship between these phenomena in the past. Here we present a sedimentary diatom assemblage and diatom isotope dataset from Schrader Pond, located ∼80 km from the Arctic Ocean, which we interpret alongside synthesized regional records of Holocene hydroclimate and sea ice reduction scenarios modeled by the Hadley Centre Coupled Model Version 3 (HadCM3). The paleodata synthesis and model simulations suggest the Early and Middle Holocene in Arctic Alaska were characterized by less sea ice, a greater contribution of isotopically heavy Arctic-derived moisture, and wetter climate. In the Late Holocene, sea ice expanded and regional climate became drier. This climatic transition is coincident with a documented shift in North Pacific circulation involving the Aleutian Low at ∼4 ka, suggesting a Holocene teleconnection between the North Pacific and Arctic. The HadCM3 simulations reveal that reduced sea ice leads to a strengthened Aleutian Low shifted west, potentially increasing transport of warm North Pacific water to the Arctic through the Bering Strait. Our findings demonstrate the interconnectedness of the Arctic and North Pacific on multimillennial timescales, and are consistent with future projections of less sea ice and more precipitation in Arctic Alaska.

Rapidly rising Arctic air and sea surface temperatures have resulted in the reduced annual duration and extent of Arctic sea ice (1), which in turn drives the ice−albedo feedback leading to amplified warming in the Arctic (2). These reductions in sea ice are projected to continue in future decades (3) and have important implications for Arctic terrestrial hydroclimate, as sea ice extent and duration impact the seasonality, type, and amount of precipitation in this region (4). Recent studies have also suggested teleconnections between the extent and duration of Arctic sea ice and midlatitudinal storm tracks (5, 6), as well as synoptic-scale processes involving the Aleutian Low atmospheric pressure cell (AL) (7, 8) and ocean−atmosphere circulation in the Bering Strait (9–11), which might link North Pacific hydroclimate directly to changes in Arctic sea ice. While recent observations show the influence of North Pacific climate on Arctic sea ice, little is known about their long-term dynamics or their coupled influence on hydroclimate in the western Arctic.Our understanding of past hydroclimate in Arctic Alaska is based in part on stable isotope reconstructions that reflect changes in the oxygen (δ¹⁸O) and hydrogen (δD) isotope composition of water. δ¹⁸O has proven particularly useful for studying both current (12, 13) and past (14–20) hydroclimate in the region, because it is sensitive to climate and environmental variables. As a result, δ¹⁸O has been used as a paleoclimate proxy for precipitation source (16), effective moisture (14), and temperature (20) in Arctic Alaska. Interpretations of these paleoclimate datasets have considered the impact of Holocene changes in AL variability (15, 16, 18), but they have not been used to examine the influence of Holocene Arctic sea ice variability on western Arctic climate, despite well-established sea ice conditions for this time period (e.g., ref. 21). The influence of sea ice extent on δ¹⁸O in various climate archives has been demonstrated in Arctic Alaska during the Pleistocene−Holocene transition (19), as well as in Greenland during the Holocene (22) and the Last Interglacial period (LIG) (23), suggesting that sites adjacent to seasonally ice-free Arctic waters can be sensitive recorders of sea ice conditions.In light of increasing evidence from both data and models for a modern connection between North Pacific circulation and Arctic sea ice (5–8), as well as the demonstrated influence of North Pacific (15, 16, 18) and Arctic (13, 19) ocean−atmosphere systems on past and present terrestrial hydroclimate conditions, it appears that northern Alaska lies at a climatological crossroads within the western Arctic. This means that paleoclimate records from Arctic Alaska are especially well situated for studying the effects of both changing Arctic sea ice and North Pacific circulation. However, existing paleoclimate datasets from this region have not been interpreted in the context of such a coupled system, and little has been done to synthesize possible multimillennial patterns among these and other datasets. Potential teleconnections during the Holocene must be explored, because this paleoclimate context is important for understanding the coevolution of Arctic and Pacific hydroclimate systems on longer timescales, which could help clarify predictions of their continued coevolution in the future.Here we present Holocene diatom assemblage and oxygen isotope (δ¹⁸O_diatom) datasets from Arctic Alaska, which we interpret in terms of past hydroclimatic change. Our results show that Holocene variability in δ¹⁸O_diatom at Schrader Pond (SP) in the northeastern Brooks Range was driven by changes in moisture source associated with fluctuating Arctic sea ice extent. We also present a data−model comparison, featuring a synthesis of Holocene hydroclimate and sea ice reconstructions from regional terrestrial and marine sites, together with coupled atmosphere−ocean model simulations, which supports our interpretation of δ¹⁸O_diatom variability. Our data highlight a prominent shift in terrestrial hydroclimate and sea ice in the region, concomitant with a well-documented shift in North Pacific hydroclimate at ∼4 ka (24). The timing of these near-synchronous shifts suggests an Arctic−Pacific teleconnection has been present over the Middle to Late Holocene, emphasizing the important role of both sea ice and lower-latitude ocean−atmosphere dynamics in the past and future of the Arctic.     

The future of ice sheets and sea ice: Between reversible retreat and unstoppable loss

We discuss the existence of cryospheric “tipping points” in the Earth''s climate system. Such critical thresholds have been suggested to exist for the disappearance of Arctic sea ice and the retreat of ice sheets: Once these ice masses have shrunk below an anticipated critical extent, the ice–albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice. We here give an overview of our current understanding of such threshold behavior. By using conceptual arguments, we review the recent findings that such a tipping point probably does not exist for the loss of Arctic summer sea ice. Hence, in a cooler climate, sea ice could recover rapidly from the loss it has experienced in recent years. In addition, we discuss why this recent rapid retreat of Arctic summer sea ice might largely be a consequence of a slow shift in ice-thickness distribution, which will lead to strongly increased year-to-year variability of the Arctic summer sea-ice extent. This variability will render seasonal forecasts of the Arctic summer sea-ice extent increasingly difficult. We also discuss why, in contrast to Arctic summer sea ice, a tipping point is more likely to exist for the loss of the Greenland ice sheet and the West Antarctic ice sheet.     

Brief Report: The interaction of ice and law in Arctic marine accessibility

Sea ice levies an impost on maritime navigability in the Arctic, but ice cover diminution due to anthropogenic climate change is generating expectations for improved accessibility in coming decades. Projections of sea ice cover retreating preferentially from the eastern Arctic suggest key provisions of international law of the sea will require revision. Specifically, protections against marine pollution in ice-covered seas enshrined in Article 234 of the United Nations Convention on the Law of the Sea have been used in recent decades to extend jurisdictional competence over the Northern Sea Route only loosely associated with environmental outcomes. Projections show that plausible open water routes through international waters may be accessible by midcentury under all but the most aggressive of emissions control scenarios. While inter- and intraannual variability places the economic viability of these routes in question for some time, the inevitability of a seasonally ice-free Arctic will be attended by a reduction of regulatory friction and a recalibration of associated legal frameworks.     

Producing desired ice faces

The ability to prepare single-crystal faces has become central to developing and testing models for chemistry at interfaces, spectacularly demonstrated by heterogeneous catalysis and nanoscience. This ability has been hampered for hexagonal ice, I_h––a fundamental hydrogen-bonded surface––due to two characteristics of ice: ice does not readily cleave along a crystal lattice plane and properties of ice grown on a substrate can differ significantly from those of neat ice. This work describes laboratory-based methods both to determine the I_h crystal lattice orientation relative to a surface and to use that orientation to prepare any desired face. The work builds on previous results attaining nearly 100% yield of high-quality, single-crystal boules. With these methods, researchers can prepare authentic, single-crystal ice surfaces for numerous studies including uptake measurements, surface reactivity, and catalytic activity of this ubiquitous, fundamental solid.Studies of model, single-crystal surfaces have revolutionized understanding of a vast array of heterogeneous catalysts and nanoparticles ranging from pure metals to alloys to semiconductors. Applying the single-crystal surface strategy to ice––arguably one of the most fundamental and ubiquitous hydrogen-bonded interfaces––has been limited due to challenges associated with surface generation. As a result, questions about molecular-level dynamics, surface binding site patterns, and the molecular-level structure remain unanswered (1). Several strategies have been adopted for studying ice: (i) Depositing solid water on a metal or ionic substrate that matches the oxygen lattice (2, 3). However, ice on a substrate often has distinctly different properties from those of neat ice; indeed, such ice can even be hydrophobic (4, 5)! (ii) Uptake measurements often use a Knudsen cell with vapor-deposited ice on a substrate (6) or compacted, finely divided, artificial snow (7) to arrive at a molecular-level picture for gas–particle interaction despite the irregular, highly variable surfaces used. (iii) Small crystallites can be well characterized but, as highlighted by Libbrecht and Rickerby (8), results can be clouded by competition from nearby crystallites; small faces compete with adjacent faces. In addition, crystallites are perturbed by the supporting surface. It is therefore desirable to prepare macroscopic samples with known faces.Interactions at ice surfaces have a particularly profound effect on climate. For example, correlational studies suggest that rain formation depends on ice particles in clouds (9), but not all ice-containing clouds yield rain. It is thought that variation in supersaturation and the mechanism for gathering water molecules by ice particles profoundly affects precipitation. Discrepancies between experiment and theory are often rationalized as a result of irregular shapes, inelastic scattering, or differing binding sites leaving large uncertainties for climate models (10). More reproducible, well-characterized surfaces of I_h––the most stable form of ice at ambient pressure––are needed to bring clarity.Ice is unusual in that the macroscopic sample does not reveal the crystal lattice orientation. Neighboring grain lattice orientation is a critical issue in the ice-core and glaciology communities (11). Hence, previous work (12–14) focused on determining grain orientation with respect to the grain boundary. The most quantitative of these are the two methods of Matsuda (12). The first uses etch pits measuring lengths inside the pit. Large uncertainties in length measurements result in large uncertainties in lattice axis orientation angles; this is not a major issue for grain growth studies but is a serious problem for generating targeted faces. The second method measures only the azimuths, thus incompletely determining orientation. Both methods break down if the optic axis is near-parallel to the surface, and neither provides the tools required to accurately orient a macroscopic sample to generate a targeted face. Lattice orientation could be determined with X-ray methods (15, 16) provided such determination includes a connection to the macroscopic sample. For wide-spread use, a laboratory-based method is preferable. This work describes two methods to fill this important need. The first uses pit perimeter ratio measurements; because the perimeter is sharp, accuracy is greatly improved. The second method locates the optic axis via cross-polarizers (11, 17), then precisely determines the hexagonal orientation via etching. Closed-form, analytical formulas are derived relating lattice orientation to the macroscopic sample. These orientation formulas feed into rotation matrices generating additional analytical formulas enabling precise cutting of any targeted face. The result is illustrated by cutting each of the three major ice faces. These techniques provide researchers with the tools needed to prepare neat ice surfaces.This work specifically describes face preparation from cylindrical boules (18); however, the method is easily adapted to any macroscopic, single-crystal geometry. Due to nearly equal energy faces, ice takes on the shape of the confining container. The near-energy match is demonstrated by growth in the modified Bridgeman apparatus (19). Nucleation occurs on a polycrystalline seed; single-crystal growth is achieved due to competitive growth among the multiple ice–water interfaces (18). Careful thermal management maintains near-equilibrium conditions yielding a large single crystal, but the crystal orientation is not a priori known. [Note: ice seeded by a floating crystal tends to have the optic axis perpendicular to the growth direction but single-crystal yield is low, ~10% (20).] Close energy match among the faces also means that ice does not readily cleave along any lattice plane (21). Thus, successful face preparation for any ice sample begins with characterization of the lattice orientation.     

Stochastic ice stream dynamics

Ice streams are narrow corridors of fast-flowing ice that constitute the arterial drainage network of ice sheets. Therefore, changes in ice stream flow are key to understanding paleoclimate, sea level changes, and rapid disintegration of ice sheets during deglaciation. The dynamics of ice flow are tightly coupled to the climate system through atmospheric temperature and snow recharge, which are known exhibit stochastic variability. Here we focus on the interplay between stochastic climate forcing and ice stream temporal dynamics. Our work demonstrates that realistic climate fluctuations are able to (i) induce the coexistence of dynamic behaviors that would be incompatible in a purely deterministic system and (ii) drive ice stream flow away from the regime expected in a steady climate. We conclude that environmental noise appears to be crucial to interpreting the past behavior of ice sheets, as well as to predicting their future evolution.Ice sheets are large bodies of ice that spread over continents under their own weight, the most prominent contemporary examples being the Greenland and Antarctic ice sheets. Ice sheets are built by snow accumulation over hundreds of thousands of years, and they constantly exchange mass and energy with the ocean and the atmosphere. As such, ice sheets play an active role in the global climate system, to which they are tightly coupled. Ice sheets exhibit rich spatiotemporal dynamics associated with the development of ice streams; these are narrow corridors (tens of kilometers wide) of fast-flowing ice (hundreds to thousands of meters per year) bordered by the slowly moving ice sheet, which stretch from the margin of the ice sheet toward the interior for hundreds of kilometers. Because ice streams convey most ice flux to the ocean, exceeding 80% of the overall discharge in present-day Antarctica (1), their variability has dramatic potential impact on sea level change.Temporal variability of ice stream flow spans decadal to multimillennial time scales. Episodes of large-scale ice discharge from ice streams in the North American Laurentide ice sheet are inferred from deep marine sediment records spanning the last glacial period (2, 3). These Heinrich events, which occurred repeatedly with periodicity of 5,000–10,000 y, are hypothesized to have been large enough to impact ocean and atmospheric circulation (4), and to raise sea level several meters (5). Centennial- and subcentennial-scale variability has taken place in Antarctica over the last 1,000 y (6). Variability on a decadal time scale includes the ongoing deceleration and widening of Whillans Ice Stream (7, 8) and velocity changes in its tributaries (9, 10), as well as subsequent deceleration and speedup of MacAyeal and Bindschadler Ice Streams (11), and centennial-scale stagnation and reactivation cycles have been inferrered for Whillans, Kamb, and MacAyeal Ice Streams (12). The slowdown of Whillans, along with the shutdown of Kamb Ice Stream ca. 160 y ago (13), is responsible for the currently positive mass balance of the Ross region of West Antarctica (14).Ice streams also display a complex spatial dynamics. Self-organization of ice sheet flow into distinct streams, regardless of strong topographic control (6, 15), is evident in the contemporary Ross ice streams, and is suggestive of a flow instability. Moreover, geomorphological evidence supports the occurrence of similar spatiotemporal dynamics in the past during periods of apparently stable climate (16, 17), thus suggesting that significant changes in ice flow patterns can occur even with little or no external forcing, and over short periods of time.The present work focuses on ice sheet temporal variability and, in particular, on the variety of concurrent processes leading to stagnation and activation cycles in ice stream flow. The coupling between ice sheets and the climate system, which features a strong stochastic component (18), motivates our question about whether internal climate variability affects the dynamics of ice stream flow. In fact, a similar pattern of interaction has been identified for mountain glaciers, where fluctuations in the surface mass balance due to climate variability drive fluctuations in glacier length (19–21). However, to our knowledge, no attempt has been made so far to characterize the role of interannual climate variability with respect to ice streams, which are known to undergo variability that can contribute significantly to global sea level.The peculiar flow regime of ice streams originates from a force balance where the bed of the ice stream supports only a small portion of the gravitational driving stress, and the narrow regions that separate slow- and fast-moving ice (the so-called “shear margins”) contribute significantly to the force balance (22, 23). This regime can be explained by thermal feedbacks that enhance basal meltwater production (24–27). How meltwater modulates ice stream velocity depends on the nature of the ice−bed contact: If the ice stream is underlain by a layer of sediment, meltwater infiltrates the sediment, which, in turn, deforms more easily as a result of the shear stress imposed by the ice (27–29). If, instead, ice is in direct contact with bedrock, the lubrication provided by water at the ice−rock interface controls the rate at which ice slips over the bed (25). Water transport and ponding along ice stream beds have been also observed (30–33), and model studies (34–39) also point to the subglacial drainage system as a control on the spatial and temporal dynamics of ice streams.Despite different levels of complexity, a common feature of ice stream models is oscillatory behavior in the form of stagnation and activation cycles (24–27, 29, 40). Recent work (29) has proven that this behavior emerges through a Hopf bifurcation in the dynamics of steady ice stream flow under changes in the control parameters, which noticeably include atmospheric temperature and snow recharge. This kind of bifurcation, which consists of a transition from a stable fixed point to a stable limit cycle, is widespread in environmental systems, and the literature suggests that it is sensitive to stochastic forcing (41, 43).Stochastic forcings were generally associated with additive disordered random fluctuations around the deterministic behavior of dynamical systems. In recent years, it has been shown that random fluctuations can induce temporal and spatial behaviors that do not exist in the presence of purely deterministic dynamics (44, 45). This novel perspective on the significance of randomness has become essential to the environmental sciences, where stochastic components are pervasive (45–48). Even though noise-driven phenomena such as stochastic resonance, noise-controlled patterning, and noise-enhanced shift precursors have been recently identified in this context, unraveling how environmental fluctuations can cause structural changes in the dynamics is still a largely unexplored subject.In this framework, we focus on the random fluctuations occurring in two key external, climate-related, drivers of ice stream dynamics for which time series spanning the whole Holocene are available from climate proxies. We explore the impact of fluctuations in the forcing on the underlying Hopf bifurcation with a modeling approach and find that, in certain parametric regimes, realistic climate fluctuations are able to induce the coexistence of dynamic behaviors that would be incompatible in a purely deterministic system. We investigate the differences between atmospheric temperature forcing and accumulation rate forcing, and find the former to dominate the stochastic dynamics under realistic levels of noise. We also see that stochastic forcings yield temporal variability of ice stream flow with a relatively short characteristic time scale, which could explain some aspects of observations (6, 12) that are reproduced by existing models only partially.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Sea level and global ice volumes from the Last Glacial Maximum to the Holocene

The major cause of sea-level change during ice ages is the exchange of water between ice and ocean and the planet’s dynamic response to the changing surface load. Inversion of ∼1,000 observations for the past 35,000 y from localities far from former ice margins has provided new constraints on the fluctuation of ice volume in this interval. Key results are: (i) a rapid final fall in global sea level of ∼40 m in <2,000 y at the onset of the glacial maximum ∼30,000 y before present (30 ka BP); (ii) a slow fall to −134 m from 29 to 21 ka BP with a maximum grounded ice volume of ∼52 × 10⁶ km³ greater than today; (iii) after an initial short duration rapid rise and a short interval of near-constant sea level, the main phase of deglaciation occurred from ∼16.5 ka BP to ∼8.2 ka BP at an average rate of rise of 12 m⋅ka⁻¹ punctuated by periods of greater, particularly at 14.5–14.0 ka BP at ≥40 mm⋅y⁻¹ (MWP-1A), and lesser, from 12.5 to 11.5 ka BP (Younger Dryas), rates; (iv) no evidence for a global MWP-1B event at ∼11.3 ka BP; and (v) a progressive decrease in the rate of rise from 8.2 ka to ∼2.5 ka BP, after which ocean volumes remained nearly constant until the renewed sea-level rise at 100–150 y ago, with no evidence of oscillations exceeding ∼15–20 cm in time intervals ≥200 y from 6 to 0.15 ka BP.The understanding of the change in ocean volume during glacial cycles is pertinent to several areas of earth science: for estimating the volume of ice and its geographic distribution through time (1); for calibrating isotopic proxy indicators of ocean volume change (2, 3); for estimating vertical rates of land movement from geological data (4); for examining the response of reef development to changing sea level (5); and for reconstructing paleo topographies to test models of human and other migrations (6). Estimates of variations in global sea level come from direct observational evidence of past sea levels relative to present and less directly from temporal variations in the oxygen isotopic signal of ocean sediments (7). Both yield model-dependent estimates. The first requires assumptions about processes that govern how past sea levels are recorded in the coastal geology or geomorphology as well as about the tectonic, isostatic, and oceanographic contributions to sea level change. The second requires assumptions about the source of the isotopic or chemical signatures of marine sediments and about the relative importance of growth or decay of the ice sheets, of changes in ocean and atmospheric temperatures, or from local or regional factors that control the extent and time scales of mixing within ocean basins.Both approaches are important and complementary. The direct observational evidence is restricted to time intervals or climatic and tectonic settings that favor preservation of the records through otherwise successive overprinting events. As a result, the records become increasingly fragmentary backward in time. The isotopic evidence, in contrast, being recorded in deep-water carbonate marine sediments, extends further back in time and often yields near-continuous records of high but imprecise temporal resolution (8). However, they are also subject to greater uncertainty because of the isotope signal’s dependence on other factors. Comparisons for the Holocene for which the direct measures of past sea level are relatively abundant, for example, exhibit differences both in phase and in noise characteristics between the two data [compare, for example, the Holocene parts of oxygen isotope records from the Pacific (9) and from two Red Sea cores (10)].Past sea level is measured with respect to its present position and contains information on both land movement and changes in ocean volume. During glacial cycles of ∼10⁵ y, the most important contribution with a global signature is the exchange of mass between the ice sheets and oceans, with tectonic vertical land movements being important mainly on local and regional scales. Global changes associated with mantle convection and surface processes are comparatively small on these time scales but become important in longer, e.g., Pliocene-scale, periods (11). Changes in ocean volume associated with changing ocean temperatures during a glacial period are also small (12).The sea-level signal from the glacial cycle exhibits significant spatial variability from its globally averaged value because of the combined deformation and gravitational response of the Earth and ocean to the changing ice-water load. During ice-sheet decay, the crust rebounds beneath the ice sheets and subsides beneath the melt-water loaded ocean basins; the gravitational potential and ocean surface are modified by the deformation and changing surface load; and the planet’s inertia tensor and rotation changes, further modifying equipotential surfaces. Together, this response of the earth-ocean system to glacial cycles is referred to as the glacial isostatic adjustment (GIA) (13–17). The pattern of the spatial variability is a function of the Earth''s rheology and of the glacial history, both of which are only partly known. In particular, past ice thickness is rarely observed and questions remain about the timing and extent of the former ice sheets on the continental shelves. The sea-level response within, or close to, the former ice margins (near-field) is primarily a function of the underlying rheology and ice thickness while, far from the former ice margins (far-field), it is mainly a function of earth rheology and the change in total ice volume through time. By an iterative analysis of observational evidence of the past sea levels, it becomes possible to improve the understanding of the past ice history as well as the Earth''s mantle response to forces on a 10⁴ y to 10⁵ y time scale.In this paper, we address one part of the Earth’s response to the glacial cycle: the analysis of far-field evidence of sea-level change to estimate the variation in ice and ocean volumes from the lead into the Last Glacial Maximum (LGM) at ∼35,000 y ago (35 ka BP) to the start of the instrumental records. Such analyses can either be of high-resolution records of a single data type from a single location or of different sea-level indicators from many different locations. We have adopted the latter approach. Most sea-level indicators provide only lower (e.g., fossil coral) or upper (e.g., fossil terrestrial plants) limiting values, and multiple data-type analyses of both upper and lower limiting measurements are less likely to be biased toward one or the other limit. Tectonic displacement of the crust is always a potential contaminator of the sought signal, and a multiplicity of data from tectonically “stable” regions is more likely to average out any undetected tectonic effects as well as any uncertainties in the above-mentioned GIA contributions.     

Nonlinear threshold behavior during the loss of Arctic sea ice

In light of the rapid recent retreat of Arctic sea ice, a number of studies have discussed the possibility of a critical threshold (or “tipping point”) beyond which the ice–albedo feedback causes the ice cover to melt away in an irreversible process. The focus has typically been centered on the annual minimum (September) ice cover, which is often seen as particularly susceptible to destabilization by the ice–albedo feedback. Here, we examine the central physical processes associated with the transition from ice-covered to ice-free Arctic Ocean conditions. We show that although the ice–albedo feedback promotes the existence of multiple ice-cover states, the stabilizing thermodynamic effects of sea ice mitigate this when the Arctic Ocean is ice covered during a sufficiently large fraction of the year. These results suggest that critical threshold behavior is unlikely during the approach from current perennial sea-ice conditions to seasonally ice-free conditions. In a further warmed climate, however, we find that a critical threshold associated with the sudden loss of the remaining wintertime-only sea ice cover may be likely.     

Formation of porous ice frameworks at room temperature

Bulk crystalline ices with ultralow densities have been demonstrated to be thermodynamically metastable at negative pressures. However, the direct formation of these bulk porous ices from liquid water at negative pressures is extremely challenging. Inspired by approaches toward porous media based on host–guest chemistry, such as metal–organic frameworks and covalent organic frameworks, we herein demonstrate via molecular dynamics simulations that a class of ultralow-density porous ices with upright channels can be formed spontaneously from liquid water at 300 K with the assistance of carbon nanotube arrays. We refer to these porous ice polymorphs as water oxygen-vertex frameworks (WOFs). Notably, our simulations revealed that the liquid–WOF phase transition is first-order and occurs at room temperature. All the WOFs exhibited the unique structural feature that they can be regarded as assemblies of nanoribbons of hexagonal bilayer ice (2D ice I) at their armchair or zigzag edges. Based on density functional theory calculations, a comprehensive phase diagram of the WOFs was constructed considering both the thermodynamic and thermal stabilities of the porous ices at negative pressures. Like other types of porous media, these WOFs may be applicable to gas storage, purification, and separation. Moreover, these biocompatible porous ice networks may be exploited as medical-related carriers.

Ice is ubiquitous but mysterious, and no other substance possesses such a rich array of polymorphs to the best of our knowledge (1). Eighteen bulk polymorphs (or phases) of ice (ices I to XVIII) have been discovered over the past 100+ y. For example, ice XVIII was recently revealed to exist at very high pressures of 100 to 400 GPa (2). Although most ice polymorphs exist at positive pressures, negative-pressure regions of the water phase diagram are attracting increasing attention. Recently, the structures of ices XVI and XVII were confirmed in the laboratory, whereas ice XVI is only stable at negative pressures between approximately −0.4 and −1 GPa (3), while ice XVII is a low-density porous ice containing spiral internal channels (4). Numerous zeolite-like porous ices have also recently been predicted to be metastable at negative pressures (5–11). Like many known porous materials, such as zeolites, metal–organic frameworks (MOFs) (12), and covalent organic frameworks (COFs) (13), porous ices may be applicable to gas storage, purification, and separation (14, 15), or even potentially possess medical applications owing to their lack of toxicity. However, in contrast to common ice (I_h), which is easily formed via the homogeneous ice nucleation of liquid water (16), it is extremely challenging to produce porous ices by the direct nucleation of liquid water at negative pressures (3). Recently, it was suggested that porous ice could be formed with the aid of suitable guests within liquid water (17, 18).Motivated by host–guest chemistry, we herein demonstrate via molecular dynamics (MD) simulations that a class of ultralow-density porous ices (hosts) with upright channels can be formed spontaneously from liquid water at 300 K by using carbon nanotube (CNT) arrays as the guests. We refer to these porous ice polymorphs as water oxygen-vertex frameworks (WOFs), by analogy with MOFs and COFs. Extensive structural searches were performed over a wide range of CNT diameters from 0.2 to 1.4 nm while considering various spacings between the CNTs in the array. A series of hexagonal WOFs was observed from the structural searches in this work, and more WOFs could be achieved if the CNT array were arranged in other Brava lattice forms. For example, two distinct tetragonal WOFs were also obtained by using the tetragonal arrangements of the CNT array in our simulations. Importantly, formation of these porous ices does not rely on the use of extreme negative-pressure conditions, in which the liquid–solid phase transitions would happen even at room temperature (300 K). Moreover, the hexagonal WOFs exhibited a unique structural feature in that they can be viewed as assemblies of nanoribbons of AA-stacked hexagonal bilayer ice [2D ice I (19, 20)] at their armchair edges or of AB-stacked bilayer ice. The thermodynamic and structural stabilities of these WOFs are further examined and the associated P–T phase diagram is constructed using a combination of density functional theory (DFT) calculations and MD simulations. Even when the CNTs are removed, the guest-free porous WOFs are found to remain stable, which would be a new type of promising hydrogen storage material.     

Predicting global atmospheric ice nuclei distributions and their impacts on climate

Knowledge of cloud and precipitation formation processes remains incomplete, yet global precipitation is predominantly produced by clouds containing the ice phase. Ice first forms in clouds warmer than -36 °C on particles termed ice nuclei. We combine observations from field studies over a 14-year period, from a variety of locations around the globe, to show that the concentrations of ice nuclei active in mixed-phase cloud conditions can be related to temperature and the number concentrations of particles larger than 0.5 μm in diameter. This new relationship reduces unexplained variability in ice nuclei concentrations at a given temperature from ∼10³ to less than a factor of 10, with the remaining variability apparently due to variations in aerosol chemical composition or other factors. When implemented in a global climate model, the new parameterization strongly alters cloud liquid and ice water distributions compared to the simple, temperature-only parameterizations currently widely used. The revised treatment indicates a global net cloud radiative forcing increase of ∼1 W m^-2 for each order of magnitude increase in ice nuclei concentrations, demonstrating the strong sensitivity of climate simulations to assumptions regarding the initiation of cloud glaciation.     

Greenland ice sheet motion insensitive to exceptional meltwater forcing

Changes to the dynamics of the Greenland ice sheet can be forced by various mechanisms including surface-melt–induced ice acceleration and oceanic forcing of marine-terminating glaciers. We use observations of ice motion to examine the surface melt–induced dynamic response of a land-terminating outlet glacier in southwest Greenland to the exceptional melting observed in 2012. During summer, meltwater generated on the Greenland ice sheet surface accesses the ice sheet bed, lubricating basal motion and resulting in periods of faster ice flow. However, the net impact of varying meltwater volumes upon seasonal and annual ice flow, and thus sea level rise, remains unclear. We show that two extreme melt events (98.6% of the Greenland ice sheet surface experienced melting on July 12, the most significant melt event since 1889, and 79.2% on July 29) and summer ice sheet runoff ∼3.9σ above the 1958–2011 mean resulted in enhanced summer ice motion relative to the average melt year of 2009. However, despite record summer melting, subsequent reduced winter ice motion resulted in 6% less net annual ice motion in 2012 than in 2009. Our findings suggest that surface melt–induced acceleration of land-terminating regions of the ice sheet will remain insignificant even under extreme melting scenarios.Surface melting and runoff from the Greenland ice sheet (GrIS) has increased during the last 30 y (1–3) coincident with Northern Hemisphere warming (4, 5) resulting in unprecedented melt extents (6) and widespread dynamic thinning, which has penetrated up to 120 km into the ice sheet interior (7). One potential dynamic thinning mechanism is surface melt–induced acceleration of ice sheet motion (termed hydrodynamic coupling) during summer (8–11). Observations of GrIS ice motion during the summer show considerable variability over a range of timescales (12). Rapid variations in meltwater input from the ice sheet surface to the glacier bed result in periods when the subglacial drainage system is more highly pressurized, leading to an increase in basal sliding (13, 14). This mechanism explains both multiday increases in ice motion at the beginning of the melt season, analogous to the “spring events” observed at alpine glaciers (15), and increases in velocity at other times when meltwater is delivered to the bed at a rate faster than the subglacial drainage system can expand to accommodate.However, as the drainage-system capacity gradually expands in response to increased melting, the subglacial water pressure falls and higher velocities can therefore only be caused by much larger meltwater pulses than earlier in the melt season (16). This feedback mechanism has been invoked previously to suggest that the ice sheet could flow more slowly in a warmer year, but observations have either been limited to close to the ice sheet margin (17) or have been unable to resolve the seasonal behavior responsible for the velocity variations (18). A recent study, which incorporated seasonal ice flow and melt observations extending beyond the equilibrium line, showed that summer velocity enhancement is negated by subsequent reductions in winter flow rates (19), but the bounding conditions of the study have since been exceeded by the exceptional melting observed in 2012 (20). Moreover, the current paucity of field observations is a significant impediment to modeling the impact of coupled hydrodynamics on net ice-mass loss (21).The recent trend of warmer summers in Greenland is related to an increase in the frequency of anticyclonic conditions (22). Persistent anticyclonic conditions during summer 2012 resulted in extreme runoff volumes from the Greenland ice sheet (23), compounded by unprecedented melt extent in July 2012 associated with low-level liquid clouds (24), which led to flood damage such as the destruction of the Watson River bridge in Kangerlussuaq, west Greenland. These conditions resulted in a year during which ice sheet–wide runoff set a new record at ∼3.9σ above the 1958–2011 mean (23). National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis 1,000-mb temperature anomalies above Kangerlussuaq (25 km west of our site 1) relative to the 1981–2010 mean were +2.2 °C during May–August 2012, compared with ±0.3 °C during May–August 2009. The 2012 melt season is therefore a surrogate for potential future melting and forms a natural test for quantifying the effect of extreme meltwater supply on ice motion compared with the “average” melt year of 2009.We used global positioning system (GPS) records to observe ice motion during 2009 and 2012 at seven sites along a transect on a land-terminating margin of the GrIS, at ∼67°N (Fig. 1). Air temperature and annual ablation were also measured at each site. The lowest three sites on the transect are located within Leverett Glacier’s inferred hydrological catchment, from which we measured bulk runoff (25). Transect dynamics during 2012 (Fig. 2) had similar characteristics to previous years (10, 19): initiation of meltwater-induced acceleration during multiday “spring” speed-up events, followed by shorter duration spikes in velocity superimposed on gradually declining background seasonal velocities, which fell below premelt season velocities by the end of summer.Open in a separate windowFig. 1.Location of the transect on the western margin of the GrIS. Stars indicate sites where ice motion, temperature, and seasonal melting were measured. The triangle indicates where proglacial discharge was measured and the GPS base station located. Circles indicate locations of ‘KAN’ PROMICE/GAP weather stations along the K transect. Contours (in meters) are from a digital elevation model (DEM) of the ice sheet surface produced from interferometric synthetic aperture radar (InSAR) (29). The inferred hydrological catchment of Leverett Glacier, delineated in light gray, was calculated from the ice sheet surface DEM. Inset shows surface and bed elevation along our transect as measured by IceBridge ATM (ILATM2) and MCoRDS (IRMCR2) in 2010 and 2011, respectively (30).Open in a separate windowFig. 2.Transect observations during 2012. (A–E). Daily (24 h) along-track ice velocities (stepped black lines) and positive degree days (gray bars) for each transect site at which daily measurements were made. (F) Discharge hydrograph for Leverett Glacier (in cubic meters per second), with cumulative discharge between May 7 and August 27 (marked by gray box). The associated catchment is shown on Fig. 1. (A–F) Gray shading defines peak velocity response to July 12 and July 29 melt events (see text).Here we concentrate on two specific aspects of hydrodynamic coupling during 2012 to give insight into the likely dynamic behavior of the ice sheet in a warming climate. We examine (i) the ice flow response to the extreme melt events of July 12 and July 29 (20), and (ii) the impact of unprecedented melt volumes (23) on total annual ice motion.Enhanced ice flow lasting ∼2 d was associated with both extreme melt events (shaded periods in Figs. 2–4), with several characteristics common to both events. First, peak velocities occurred in advance of satellite-observed peak ice sheet melt extent (20), while proglacial discharge was still rising—2 d in advance of July 12, and 1 d in advance of July 29. Second, velocities increased at every site along the transect during the enhanced ice flow period. Third, at the majority of sites, velocities were lower after the enhanced ice flow period than before it (Fig. 2).Open in a separate windowFig. 4.Observations around July 29 melt event. See A–F in Fig. 3 for details.Before the July 12 melt event, sites up to 1,482 m above sea level (site 6) experienced positive air temperatures every day from June 10 (Fig. 2). Peak velocities during July 9–10 were coincident with a 2.3 °C increase in mean air temperature at our transect sites and a 73% increase in mean wind speed at Programme for Monitoring the Greenland Ice Sheet (PROMICE)/Greenland Analogue Project (GAP) K-transect sites (Fig. 1) compared with the previous 8 d. The mean daily transect velocity during July 9–10 was 61% greater than during the preceding 8 d, with sites 3 and 4 (794 and 1,061 m a.s.l, respectively) experiencing the highest peak velocities of 103 and 77% greater, respectively, than the previous 8 d (Fig. 3). By July 12, ice velocities were falling despite peaks in both ice sheet–wide melting and proglacial river discharge (∼800 m³ s⁻¹; in excess of double that observed both at the start of the melt event and in previous years; ref. 19). Sites 1 and 2 returned to daily velocities within 10 m y⁻¹ of July 1–8 mean velocities; and sites 3, 4, and 6 decreased to velocities at least 30 m y⁻¹ slower than July 1–8 mean velocities.Open in a separate windowFig. 3.Observations around July 12 melt event. (A–E) Near-surface air temperatures (dashed lines), daily (24 h) along-track ice velocities (stepped black lines) and short-term along-track ice velocities (gray lines) for each site at which daily measurements were made. Periods with inadequate quality observations removed. (F) Discharge hydrograph for Leverett Glacier (in cubic meters per second).(A–F) Gray shading defines the peak velocity response to the melt event (see text).In contrast to the July 12 melt event, a period of falling air temperatures in the previous 15 d leading up to the July 29 melt event (as low as −7 °C at site 6) resulted in falling discharge to a minimum of 240 m³ s⁻¹ on July 25, the lowest since June 18 (Fig. 2). During July 27–28, the mean transect air temperature rose by 4.4 °C compared with the previous 8 d, with associated, although lagged, increases in discharge and velocity (Fig. 4). Mean transect ice velocity on July 27–28 was 116% greater than during the preceding 8 d. At sites 2, 3, and 4, the velocity perturbation was short-lived, lasting ∼2 d before an abrupt drop in velocities that returned to within 20 m y⁻¹ of preevent velocities. Site 6 slowed down more gradually after the July 28 peak. Unlike the July 12 melt event, river discharge remained close to its event peak of ∼400 m³ s⁻¹ for 8 d following the July 29 melt event (Fig. 2F).Increased ice velocities in the lead up to the July 12 and 29 melt events were clearly caused by a rapid increase in the rate of meltwater supply to the ice sheet bed forced by changes in the rate of surface melting. Although antecedent melt conditions and the absolute volumes of meltwater associated with each event were different, the nature and style of meltwater forcing, overwhelming the capacity of the hydrological system and leading to ice acceleration, were very similar, replicating responses observed previously (12, 16).We estimated the potential contribution of each melt event to summer ice displacement by comparison with estimates of the projected ice displacement that would have occurred in the absence of the melt events. We used mean ice velocities at each site during the 8 d preceding each 2-d period of enhanced ice flow to estimate what the total displacement would likely have been through the 2-d enhanced ice flow period and the following 8 d in the absence of the enhanced ice flow period (see Materials and Methods for more information). Observations during the corresponding time periods are shown in Figs. 3 and ​and4.4. On average, the July 12 melt event forced only 7% more ice displacement over the 10-d period, and the July 29 melt event, which was preceded by lower melt rates than the July 12 event, forced 34% more ice displacement over the equivalent 10-d period. These findings reinforce the importance of antecedent melt rates (as opposed to simply meltwater volume) and thus drainage system efficiency in controlling the short-term dynamic response to variations in meltwater supply (26).The second exceptional characteristic of 2012 was ice sheet–wide runoff of ∼3.9σ above the 1958–2011 mean (23). For comparison, Fig. 5 shows observations collected along our transect in the average melt year of 2009. Exceptional melting during 2012 resulted in a mean of 117% more ablation relative to 2009 along our transect (Fig. 5A) with bulk runoff from the local ice sheet margin (2.20 × 10⁹ m³) 113% greater than 2009 (19). Summer velocities (Fig. 5B) at all but the lowest two sites were also higher in 2012 than in 2009. However, winter velocities at all sites were on average 11% lower in 2012 than in 2009, resulting in 6% less net annual ice motion along the transect in 2012–2013 than in 2009–2010 (Fig. 5). These observations support previous findings that stronger melting results in faster summers, but that faster summers are then offset by subsequent slower winter ice flow due to the evolution of a larger, more extensive subglacial drainage system that drains high basal water pressure regions (19). Our findings also support ice sheet modeling results (21), which suggest that enhanced basal lubrication will not cause substantial net mass loss from the ice sheet, and provide the observations which Shannon et al. (21) had stated were currently “insufficient to determine whether changes in subglacial hydraulics will limit the potential for the speedup of flow.”Open in a separate windowFig. 5.(A) Annual (May 1–April 30) ablation in water equivalent meters for sites 2–7 in 2009 and 2012. (B) Summer (Sum, May 1–August 31), winter (Win, September 1–April 30), and annual (Ann, May 1–April 30) velocities for each site in 2009 and 2012.Our findings demonstrate that despite the exceptional melting observed in 2012, annual ice motion along our transect was not enhanced relative to an average melt year (2009). These findings suggest that although hydrologically forced ice motion influences short-term and seasonal ice dynamics, land-terminating margins of the Greenland ice sheet are insensitive dynamically over annual timescales to melt volumes that are commensurate with temperature projections for 2100 (27). Furthermore, our data demonstrate that the importance of hydrologically forced ice motion over annual timescales can only be understood with reference to both summer and winter seasonal velocities due to their significant interannual variability. We also note that the effects of surface melt and oceanic forcing mechanisms on the dynamics of marine terminating glaciers in a warming climate remain unclear and should be a priority for future research.     

Rapid reductions and millennial-scale variability in Nordic Seas sea ice cover during abrupt glacial climate changes

Constraining the past sea ice variability in the Nordic Seas is critical for a comprehensive understanding of the abrupt Dansgaard-Oeschger (D-O) climate changes during the last glacial. Here we present unprecedentedly detailed sea ice proxy evidence from two Norwegian Sea sediment cores and an East Greenland ice core to resolve and constrain sea ice variations during four D-O events between 32 and 41 ka. Our independent sea ice records consistently reveal a millennial-scale variability and threshold response between an extensive seasonal sea ice cover in the Nordic Seas during cold stadials and reduced seasonal sea ice conditions during warmer interstadials. They document substantial and rapid sea ice reductions that may have happened within 250 y or less, concomitant with reinvigoration of deep convection in the Nordic Seas and the abrupt warming transitions in Greenland. Our empirical evidence thus underpins the cardinal role of rapid sea ice decline and related feedbacks to trigger abrupt and large-amplitude climate change of the glacial D-O events.

Sea ice is a critical component of the global climate system as it affects Earth’s albedo, phytoplankton productivity, ocean-atmosphere heat and gas exchange, and ocean circulation (1). Rapid sea ice retreat, as observed in the modern Arctic Ocean, exerts important climate feedbacks that may lead to an accelerated climate warming at northern high latitudes (2). While many climate models have difficulties in reproducing the currently observed Arctic sea ice decline (3), the rates of ongoing atmospheric warming in some Arctic regions are already comparable with those of prominent abrupt climate changes that occurred during the last glacial period (4). The latter are referred to as Dansgaard–Oeschger (D-O) climate events and known from Greenland ice core records as abrupt shifts between cold Greenland stadials (GS) and warmer Greenland interstadials (GI) occurring repeatedly ∼10–110 ka (5, 6). The millennial-scale glacial climate variability was a global phenomenon with different characteristics in the northern and southern hemispheres, but the most striking feature of the D-O events is an extremely abrupt climate transition that includes an atmospheric warming of 5–16.5 °C over the Greenland ice sheet happening in just a few decades (7). Analogous to the modern and future sea ice retreat and resulting warming in the Arctic, the abrupt D-O climate transitions are widely believed to have been amplified by rapid sea ice retreat in the Nordic Seas (8–15).Today, the Nordic Seas are largely ice-free, and warm Atlantic surface waters flow into the Norwegian Sea as far north as Svalbard at ∼80°N (Fig. 1), where the Arctic sea ice cover is being eroded, in particular in the Barents Sea. The warm Atlantic surface waters release heat to the atmosphere as it flows northward, which is accompanied by convective intermediate and deep-water formation between Norway and Greenland, feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) (16). A portion of the Atlantic waters continues flowing into the stratified Arctic Ocean as subsurface waters (17). While the pattern of ocean circulation during GI was fairly comparable to that today, proxy data indicate that the glacial Nordic Seas exhibited a stable surface stratification during GS, similar to the modern Arctic Ocean (13, 18). The AMOC and associated northward surface heat transport into the Nordic Seas were weakened during GS, with most extreme weakening related to Heinrich events signified by massive iceberg discharges to the North Atlantic (19, 20). Intermediate and deep waters in the stadial Nordic Seas were 2–4 °C warmer as compared with GI or modern conditions, resulting from a stable halocline and reduced open-ocean convection (21, 22). Contemporaneously, an extended sea ice cover reaching at least as far south as the Greenland–Scotland Ridge at ∼60°N insulated the high-latitude atmosphere from the deep oceanic heat reservoir (23, 24). Model simulations support a subsurface warming scenario under extended sea ice during GS (22, 25, 26) and suggest that a rapid removal of the sea ice cover might have caused the abrupt and high-amplitude D-O climate warming (11, 12, 14, 15).Open in a separate windowFig. 1.Core sites and regional context of the study area. Yellow diamonds mark the core sites investigated in this study. The map shows the core-top P_BIP₂₅ distribution (42, 43, 63), illustrating the great potential of the biomarker approach for sea ice reconstruction. Orange, yellow, and green dots mark core-top sites north, east, and south of Greenland, respectively, data of which are investigated in this study. Small black dots indicate locations of published core-top data. Purple lines mark the modern sea ice extent during September (dashed) and March (solid), averaged between A.D. 1981 and 2010 (https://nsidc.org/; ref. 64). The thin blue line shows the P_BIP₂₅ = 0.2 isoline, representing best the modern winter/spring sea ice extent. Red arrows illustrate the warm and saline North Atlantic Current (NAC). The map was produced with Ocean Data View software (65).Although there is some evidence of millennial-scale sea ice fluctuations during the last glacial, the few available sea ice proxy records (23, 24, 27–31) are mostly restricted to the southern Norwegian Sea and the Arctic Ocean, often have a limited temporal resolution, and partly reflect opposing trends regarding stadial–interstadial sea ice changes depending on the proxies used. Here we present high-resolution sea ice biomarker records from two key sites that form a North–South transect within the Atlantic inflow region in the Norwegian Sea and are thus ideally suited to record spatiotemporal shifts in sea ice cover in both the entrance and the interior of the ocean basin, oceanic fronts, and Atlantic water inflow during the last glacial (Fig. 1). Furthermore, we combine these marine sea ice proxy records with an independent sea ice record based on bromine-enrichment (Br_enr) values from an East Greenland ice core, which significantly enhances the spatial coverage, the robustness of results, and temporal constraint of the sea ice reconstruction. We focus on five representative glacial D-O cycles between 32 and 41 ka, which comprise long- and short-lasting GI as well as several GS, one of which includes Heinrich Event 4. The application of the cryptotephra-based chronological constraints provides a level of robustness as to the timing, duration, and nature of the events unfolding during abrupt climate changes. Our study provides robust empirical evidence that resolves rapid and widespread sea ice retreat in the Nordic Seas and its role in initiating and amplifying the abrupt climate change of the glacial D-O events.