Accelerated sea-level rise from West Antarctica

Remotely sensing potential climate change tipping points across scales

Potential climate tipping points pose a growing risk for societies, and policy is calling for improved anticipation of them. Satellite remote sensing can play a unique role in identifying and anticipating tipping phenomena across scales. Where satellite records are too short for temporal early warning of tipping points, complementary spatial indicators can leverage the exceptional spatial-temporal coverage of remotely sensed data to detect changing resilience of vulnerable systems. Combining Earth observation with Earth system models can improve process-based understanding of tipping points, their interactions, and potential tipping cascades. Such fine-resolution sensing can support climate tipping point risk management across scales.

Seasonal variability of ocean circulation near the Dotson Ice Shelf, Antarctica

Recent rapid thinning of West Antarctic ice shelves are believed to be caused by intrusions of warm deep water that induce basal melting and seaward meltwater export. This study uses data from three bottom-mounted mooring arrays to show seasonal variability and local forcing for the currents moving into and out of the Dotson ice shelf cavity. A southward flow of warm, salty water had maximum current velocities along the eastern channel slope, while northward outflows of freshened ice shelf meltwater spread at intermediate depth above the western slope. The inflow correlated with the local ocean surface stress curl. At the western slope, meltwater outflows followed the warm influx along the eastern slope with a ~2–3 month delay. Ocean circulation near Dotson Ice Shelf, affected by sea ice distribution and wind, appears to significantly control the inflow of warm water and subsequent ice shelf melting on seasonal time-scales.

The intrusion of relatively warm water is causing the recent rapid thinning of the Dotson ice shelf, West Antarctica. Here, the authors analyzed two-years of mooring data from the Dotson ice shelf front and found that seasonal variability of the ocean circulation and ocean surface stress are the main causes of variability in heat transport.

Spatial Habitat Suitability Models of Mangroves with Kandelia obovata

Mangrove forests provide important estuarine ecosystem services but are threatened by rising sea levels and anthropogenic impacts. Understanding the habitat characteristics required for mangrove growth is significant for mangrove restoration and integrated management. This study aims to build spatial habitat suitability index (HSI) models for Kandelia obovata mangrove trees. Biological and habitat-related environmental data were collected in the Wazwei and Guandu wetlands in northern Taiwan. We adopted inundation frequency, soil sorting coefficient, and water salinity as the key environmental factors to build HSI models. The dependent variable of these environmental factors was the mangrove biomass per unit area. Significant differences were found for the mangrove biomass on different substrata and shore elevations. The tidal creek had the lowest elevation, and mangrove areas were found at the highest elevations. The oxidization level of the substrate under mangrove forests was high, indicating that the root system of mangroves could carry oxygen into the soil and result in oxidation. Human activities were found to lead to the reduced growth conditions of mangroves. The validation of the HSI model, considering the inundation frequency and soil sorting coefficient, proved to be reliable, with an accuracy ranging from 78% to 90%. A better simulation was found after revising the model by incorporating the factor of water salinity. The model forecast of the mangrove responses to the sea-level rise indicated an increase in the inundation frequency and thus an induced shift and shrinkage of the mangrove area. The increased HSI values of the bare mudflat area demonstrate an option for the potential restoration of mangrove trees. Given the findings of this study, we concluded that mangroves could spread from estuaries to upstream areas due to rising sea levels and might be limited by humanmade impacts. Restoring degraded floodplains is suggested for mangrove habitat rehabilitation.

The Southern Ocean and its interaction with the Antarctic Ice Sheet

The Southern Ocean exerts a major influence on the mass balance of the Antarctic Ice Sheet, either indirectly, by its influence on air temperatures and winds, or directly, mostly through its effects on ice shelves. How much melting the ocean causes depends on the temperature of the water, which in turn is controlled by the combination of the thermal structure of the surrounding ocean and local ocean circulation, which in turn is determined largely by winds and bathymetry. As climate warms and atmospheric circulation changes, there will be follow-on changes in the ocean circulation and temperature. These consequences will affect the pace of mass loss of the Antarctic Ice Sheet.

Trends in Antarctic Ice Sheet Elevation and Mass

Fluctuations in Antarctic Ice Sheet elevation and mass occur over a variety of time scales, owing to changes in snowfall and ice flow. Here we disentangle these signals by combining 25 years of satellite radar altimeter observations and a regional climate model. From these measurements, patterns of change that are strongly associated with glaciological events emerge. While the majority of the ice sheet has remained stable, 24% of West Antarctica is now in a state of dynamical imbalance. Thinning of the Pine Island and Thwaites glacier basins reaches 122 m in places, and their rates of ice loss are now five times greater than at the start of our survey. By partitioning elevation changes into areas of snow and ice variability, we estimate that East and West Antarctica have contributed -1.1 ± 0.4 and +5.7 ± 0.8 mm to global sea level between 1992 and 2017.

Element concentrations of environmental concern in surface sediment samples from a broad marine area of 25 de Mayo (King George) Island, South Shetland Islands

Western Antarctica (WA) constitutes the area with the highest human presence in the white continent and also the region where the effects of global warming are more evident worldwide. Such human presence represents a potential risk of pollution with both, organic and inorganic contaminants. Global warming also could modify dynamics and transport of the pollutants, increasing summer water runoff, ice melting and iceberg scouring. Under this fast-changing scenario, knowledge about the concentration of contaminants is essential to evaluate the environmental status of this ecologically relevant area. In this work, we performed the first regional-scale monitoring of 9 trace elements (Cr, Co, Ni, Cu, Zn, As, Pb, Cd and Hg), as well as Fe and Mn, in surface sediment from 64 sites comprising six different areas in Maxwell Bay, 25 de Mayo (King George) Island. Target elements were quantified in surface sediment samples (20-30 m depth) obtained during two summer Antarctic expeditions: 2010/11 and 2011/12 by inductively coupled plasma linked to a quadrupole mass spectrometer (ICP-MS). Based on the average values observed for the reference areas, baseline values were defined for the studied region. A regional enrichment in Cu (compared with the global mean upper crust) was observed and related to the widespread mineralization of volcanic rocks. The most anthropized area (South Fildes) mainly showed sediment class 3 (moderately polluted) for Pb, Cd and Hg with a number samples revealing some highly contaminated hot spots. Although elemental contamination in some samples close to scientific stations or sites where logistic operations were evidenced, a pollution pattern was not clearly identified. The present work represents the first regional-scale attempt to define the baseline values and the anthropic impacts in this region of the WA and also provides the first data about Hg concentration in surface sediment of the study area.

First data on uranium uptake in three nototheniid fishes from Antarctica (James Ross Island)

Recent studies have confirmed historic atmospheric deposition of uranium in Antarctica, with a steep and significant increase in levels deposited since the 1980s in Antarctic Peninsula ice core samples. To date, however, there has been little or no attention paid to uranium in the Antarctic food web. Here, we present results for uranium content in scales of three common nototheniid species (Trematomus bernacchii, Gobionotothen gibberifrons, Notothenia coriiceps) from coastal waters off James Ross Island (Antarctic Peninsula). While mean total uranium levels (mean ± SD) were low and similar between species (N. coriiceps 0.08 μg g^-1 ± 0.01, T. bernacchii 0.17 μg g^-1 ± 0.10; G. gibberifrons 0.11 μg g^-1 ± 0.04), linear regressions against standard length indicated bioaccumulation in T. bernacchii (ANOVA, F = 7.8349, P = 0.0076). We suggest this may be the result of dietary specialisation on prey with calcareous shells that accumulate uranium. To the best of our knowledge, this paper provides the first quantitative baseline data on uranium levels in coastal Antarctic nototheniids. While the low levels recorded are unlikely to represent a threat within the food chain, we suggest that further long-term trophic studies (including stable isotope analysis) are needed, recognising that the feeding ecology of individual species (and even individuals) can have a strong effect on overall trends.

Response of a vulnerable barrier island to multi-year storm impacts: LiDAR-data-inferred morphodynamic changes on Ship Island, Mississippi, USA

Barrier systems around the world are experiencing accelerated sea-level rise, reduced sediment supply, and frequent hurricane impacts. However, detailed quantitative field-based studies concerning the response to these external forcing mechanisms are scarce, particularly on the scale of entire islands. The Mississippi - Alabama barrier island chain, located along the U.S. Gulf of Mexico coastline has lost land on the order of hectares per year since records began in the 1840s, putting mainland coastal communities and important ecosystems at risk. Here we present an analysis of Light Detection and Ranging (LiDAR) digital elevation models, revealing erosional/depositional patterns and geomorphologic changes around the most vulnerable of these islands, Ship Island. Four LiDAR datasets (2004, 2007, 2010, and 2012), capturing the complete topography of the island and some bathymetry in the inlet and surrounding shallows to depths of up to 8 m, are used to investigate subaerial and subaqueous sediment volume changes between these years. The impact of Hurricane Katrina, which produced the highest storm surge ever recorded in the United States, is captured in the 2004-2007 dataset. During this time, sediment comparable to 1.5 times the 2004 subaerial island volume was lost from the area included in the topographic/bathymetric dataset. Only 1/5 of this volume was recovered to this area between 2007 and 2010. The island returned to a state of sediment loss between 2010 and 2012, albeit within the error bounds, while the areal extent of the islands continued to increase. This study examines the impact severe storm events can have on vulnerable barrier islands. It highlights the importance of utilizing 3D datasets that include both topographic and bathymetric data for morphodynamic analyses of barrier island systems.

Rapid thinning of Pine Island Glacier in the early Holocene

Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid thinning and retreat for the past two decades. We demonstrate, using glacial-geological and geochronological data, that Pine Island Glacier (PIG) also experienced rapid thinning during the early Holocene, around 8000 years ago. Cosmogenic (10)Be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 centimeters per year, which is comparable with contemporary thinning rates. The most likely mechanism was a reduction in ice shelf buttressing. Our findings reveal that PIG has experienced rapid thinning at least once in the past and that, once set in motion, rapid ice sheet changes in this region can persist for centuries.

Frequency response of ice streams

Changes at the grounding line of ice streams have consequences for inland ice dynamics and hence sea level. Despite substantial evidence documenting upstream propagation of frontal change, the mechanisms by which these changes are transmitted inland are not well understood. In this vein, the frequency response of an idealized ice stream to periodic forcing in the downstream strain rate is examined for basally and laterally resisted ice streams using a one-dimensional, linearized membrane stress approximation. This reveals two distinct behavioural branches, which we find to correspond to different mechanisms of upstream velocity and thickness propagation, depending on the forcing frequency. At low frequencies (centennial to millennial periods), slope and thickness covary hundreds of kilometres inland, and the shallow-ice approximation is sufficient to explain upstream propagation, which occurs through changes in grounding-line flow and geometry. At high frequencies (decadal to sub-decadal periods), penetration distances are tens of kilometres; while velocity adjusts rapidly to such forcing, thickness varies little and upstream propagation occurs through the direct transmission of membrane stresses. Propagation properties vary significantly between 29 Antarctic ice streams considered. A square-wave function in frontal stress is explored by summing frequency solutions, simulating some aspects of the dynamical response to sudden ice-shelf change.

Recent 137Cs deposition in sediments of Admiralty Bay, Antarctica

Cesium-137, radium-226 and lead-210 profiles of a 25 cm sediment core give an indication of recent changes in land-ocean interactions at a polar coastal environment (Admiralty Bay, King George Island, Antarctica). The linear sedimentation accumulation rate at the study site calculated from the unsupported (210)Pb profile was 6.7 mm/year from 1965 to 2005. A 3.5-fold increase in (137)Cs concentrations was observed in the top layer of this sediment core. This sharp increase seems to indicate a recent redistribution of fallout radionuclides previously deposited on soil, vegetation and snow. These results imply enhanced land-ocean interactions at this site likely as a result of climate change. Because our results are based on a single core, additional investigations are needed to confirm our observations.

Stability of ice-sheet grounding lines

Recent observations of the West Antarctic Ice Sheet document rapid changes in the mass balance of its component glaciers. These observations raise the question of whether changing climatic conditions have triggered a dynamical instability in the ice-sheet–ice-shelf system. The dynamics of marine ice sheets are sensitive to grounding-line position and variation, characteristics that are poorly captured by most current models. We present a theory for grounding-line dynamics in three spatial dimensions and time. Our theory is based on a balance of forces across the grounding line; it is expressed as a differential equation that is analogous to the canonical Stefan condition. We apply this theory to the question of grounding-line stability under conditions of retrograde bed slope in a suite of calculations with different basal topography. A subset of these have basal topography inspired by the Pine Island glacier, where basal depth varies in both the along-flow and across-flow directions. Our results indicate that unstable retreat of the grounding line over retrograde beds is a robust feature of models that evolve based on force balance at the grounding line. We conclude, based on our simplified model, that unstable grounding-line recession may already be occurring at the Pine Island glacier.

Contemporary sea level rise

Measuring sea level change and understanding its causes has considerably improved in the recent years, essentially because new in situ and remote sensing observations have become available. Here we report on most recent results on contemporary sea level rise. We first present sea level observations from tide gauges over the twentieth century and from satellite altimetry since the early 1990s. We next discuss the most recent progress made in quantifying the processes causing sea level change on timescales ranging from years to decades, i.e., thermal expansion of the oceans, land ice mass loss, and land water-storage change. We show that for the 1993-2007 time span, the sum of climate-related contributions (2.85 +/- 0.35 mm year(-1)) is only slightly less than altimetry-based sea level rise (3.3 +/- 0.4 mm year(-1)): approximately 30% of the observed rate of rise is due to ocean thermal expansion and approximately 55% results from land ice melt. Recent acceleration in glacier melting and ice mass loss from the ice sheets increases the latter contribution up to 80% for the past five years. We also review the main causes of regional variability in sea level trends: The dominant contribution results from nonuniform changes in ocean thermal expansion.

Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets

Many glaciers along the margins of the Greenland and Antarctic ice sheets are accelerating and, for this reason, contribute increasingly to global sea-level rise. Globally, ice losses contribute approximately 1.8 mm yr(-1) (ref. 8), but this could increase if the retreat of ice shelves and tidewater glaciers further enhances the loss of grounded ice or initiates the large-scale collapse of vulnerable parts of the ice sheets. Ice loss as a result of accelerated flow, known as dynamic thinning, is so poorly understood that its potential contribution to sea level over the twenty-first century remains unpredictable. Thinning on the ice-sheet scale has been monitored by using repeat satellite altimetry observations to track small changes in surface elevation, but previous sensors could not resolve most fast-flowing coastal glaciers. Here we report the use of high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry to map change along the entire grounded margins of the Greenland and Antarctic ice sheets. To isolate the dynamic signal, we compare rates of elevation change from both fast-flowing and slow-flowing ice with those expected from surface mass-balance fluctuations. We find that dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt. In Greenland, glaciers flowing faster than 100 m yr(-1) thinned at an average rate of 0.84 m yr(-1), and in the Amundsen Sea embayment of Antarctica, thinning exceeded 9.0 m yr(-1) for some glaciers. Our results show that the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins.

Elevation changes in Antarctica mainly determined by accumulation variability

Antarctic Ice Sheet elevation changes, which are used to estimate changes in the mass of the interior regions, are caused by variations in the depth of the firn layer. We quantified the effects of temperature and accumulation variability on firn layer thickness by simulating the 1980-2004 Antarctic firn depth variability. For most of Antarctica, the magnitudes of firn depth changes were comparable to those of observed ice sheet elevation changes. The current satellite observational period ( approximately 15 years) is too short to neglect these fluctuations in firn depth when computing recent ice sheet mass changes. The amount of surface lowering in the Amundsen Sea Embayment revealed by satellite radar altimetry (1995-2003) was increased by including firn depth fluctuations, while a large area of the East Antarctic Ice Sheet slowly grew as a result of increased accumulation.

Tipping elements in the Earth's climate system

The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

Terminal Gold-Oxo Complexes

In contradiction to current bonding paradigms, two terminal Au-oxo molecular complexes have been synthesized by reaction of AuCl3 with metal oxide-cluster ligands that model redox-active metal oxide surfaces. Use of K10[alpha2-P2W17O61].20H2O and K2WO4 (forming the [A-PW9O34]9- ligand in situ) produces K15H2[Au(O)(OH2)P2W18O68].25H2O (1); use of K10[P2W20O70(OH2)2].22H2O (3) produces K7H2[Au(O)(OH2)P2W20O70(OH2)2].27H2O (2). Complex 1 crystallizes in orthorhombic Fddd, with a=28.594(4) A, b=31.866(4) A, c=38.241(5) A, V=34844(7) A3, Z=16 (final R=0.0540), and complex 2 crystallizes in hexagonal P6(3)/mmc, with a=16.1730(9) A, b=16.1730(9) A, c=19.7659(15) A, V=4477.4(5) A3, Z=2 (final R=0.0634). The polyanion unit in 1 is disorder-free. Very short (approximately 1.76 A) Au-oxo distances are established by both X-ray and 30 K neutron diffraction studies, and the latter confirms oxo and trans aqua (H2O) ligands on Au. Seven findings clarify that Au and not W is present in the Au-oxo position in 1 and 2. Five lines of evidence are consistent with the presence of d8 Au(III) centers that are stabilized by the flanking polytungstate ligands in both 1 and 2: redox titrations, electrochemical measurements, 17 K optical spectra, Au L2 edge X-ray absorption spectroscopy, and Au-oxo bond distances. Variable-temperature magnetic susceptibility data for crystalline 1 and 2 establish that both solids are diamagnetic, and 31P and 17O NMR spectroscopy confirm that both remain diamagnetic in solution. Both complexes have been further characterized by FT-IR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and other techniques.

Climate change and trace gases

Palaeoclimate data show that the Earth's climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the 'albedo flip' property of ice/water, provides a powerful trigger mechanism. A climate forcing that 'flips' the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Inertia of ice sheet and ocean provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are also important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years. The most important of the non-CO2 forcings is methane (CH4), as it causes the second largest human-made GHG climate forcing and is the principal cause of increased tropospheric ozone (O3), which is the third largest GHG forcing. Nitrous oxide (N2O) should also be a focus of climate mitigation efforts. Black carbon ('black soot') has a high global warming potential (approx. 2000, 500 and 200 for 20, 100 and 500 years, respectively) and deserves greater attention. Some forcings are especially effective at high latitudes, so concerted efforts to reduce their emissions could preserve Arctic ice, while also having major benefits for human health, agricultural productivity and the global environment.

Free-drifting icebergs: hot spots of chemical and biological enrichment in the Weddell Sea

The proliferation of icebergs from Antarctica over the past decade has raised questions about their potential impact on the surrounding pelagic ecosystem. Two free-drifting icebergs, 0.1 and 30.8 square kilometers in aerial surface area, and the surrounding waters were sampled in the northwest Weddell Sea during austral spring 2005. There was substantial enrichment of terrigenous material, and there were high concentrations of chlorophyll, krill, and seabirds surrounding each iceberg, extending out to a radial distance of approximately 3.7 kilometers. Extrapolating these results to all icebergs in the same size range, with the use of iceberg population estimates from satellite surveys, indicates that they similarly affect 39% of the surface ocean in this region. These results suggest that free-drifting icebergs can substantially affect the pelagic ecosystem of the Southern Ocean and can serve as areas of enhanced production and sequestration of organic carbon to the deep sea.

Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets

After a century of polar exploration, the past decade of satellite measurements has painted an altogether new picture of how Earth's ice sheets are changing. As global temperatures have risen, so have rates of snowfall, ice melting, and glacier flow. Although the balance between these opposing processes has varied considerably on a regional scale, data show that Antarctica and Greenland are each losing mass overall. Our best estimate of their combined imbalance is about 125 gigatons per year of ice, enough to raise sea level by 0.35 millimeters per year. This is only a modest contribution to the present rate of sea-level rise of 3.0 millimeters per year. However, much of the loss from Antarctica and Greenland is the result of the flow of ice to the ocean from ice streams and glaciers, which has accelerated over the past decade. In both continents, there are suspected triggers for the accelerated ice discharge-surface and ocean warming, respectively-and, over the course of the 21st century, these processes could rapidly counteract the snowfall gains predicted by present coupled climate models.

Assessment of mangrove response to projected relative sea-level rise and recent historical reconstruction of shoreline position

We predict the decadal change in position of three American Samoa mangroves from analysis of a time series of remotely sensed imagery, a geographic information system, tide gauge data, and projections for change in sea-level relative to the mangrove surface. Accurate predictions of changes to coastal ecosystem boundaries, including in response to projected relative sea-level rise, enable advanced planning to minimize and offset anticipated losses and minimize social disruption and cost of reducing threats to coastal development and human safety. The observed mean landward migration of three mangroves' seaward margins over four decades was 25, 64, and 72 mma(-1), 12 to 37 times the observed relative sea-level rise rate. Two of the sites had clear trends in reductions in mangrove area, where there was a highly significant correlation between the change in position of the seaward mangrove margin and change in relative sea-level. Here it can be inferred that the force of sea-level rise relative to the mangrove surface is causing landward migration. Shoreline movement was variable at a third site and not significantly correlated with changing sea-level, where it is likely that forces other than change in relative sea-level are predominant. Currently, 16.5%, 23.4%, and 68.0% of the three mangroves' landward margins are obstructed by coastal development from natural landward migration. The three mangroves could experience as high as a 50.0% reduction in area by the year 2100. A 12% reduction in mangrove area by the year 2100 is possible in the Pacific islands region.

A General Synthetic Strategy for Oxide-Supported Metal Nanoparticle Catalysts

Despite recent exciting progress in catalysis by supported gold nanoparticles, there remains the formidable challenge of preparing supported gold catalysts that collectively incorporate precise control over factors such as size and size-distribution of the gold nanoparticles, homogeneous dispersion of the particles on the support, and the ability to utilize a wide range of supports that profoundly affect catalytic performance. Here, we describe a synthetic methodology that achieves these goals. In this strategy, weak interface interactions evenly deposit presynthesized organic-capped metal nanoparticles on oxide supports. The homogeneous dispersion of nanoparticles on oxides is then locked in place, without aggregation, through careful calcination. The approach takes advantage of recent advances in the synthesis of metal and oxide nanomaterials and helps to bring together these two classes of materials for catalysis applications. An important feature is that the strategy allows metal nanoparticles to be well dispersed on a variety of oxides with few restrictions on their physical and chemical properties. Following this synthetic procedure, we have successfully developed efficient gold catalysts for green chemistry processes, such as the production of ethyl acetate from the selective oxidation of ethanol by oxygen at 100 degrees C.

Catalytically active gold: from nanoparticles to ultrathin films

Ordered gold (Au) mono- and bilayer structures have been synthesized on a highly reduced titania surface. The Au bilayer exhibits a significantly higher catalytic activity for carbon monoxide oxidation than does the Au monolayer structure. This is the first report of Au completely wetting an oxide surface and demonstrates that ultrathin Au films on an oxide surface have exceptionally high catalytic activity, comparable to the activity observed for Au nanoparticles. This discovery is a key to understanding the nature of the active site of supported Au catalysts.

One-Dimensional Colloidal Gold and Silver Nanostructures

One-dimensional (1-D) metallic nanoscale materials have long been of interest to many groups of scientists. Within the last 2 decades, great advances in the synthesis of metallic nanorods and nanowires have been made, with a variety of templating methods. More recently, bottom-up chemical syntheses of these materials have become increasingly reported in the literature. This Forum Article describes the synthesis, physical properties, and potential applications of 1-D metals, with an emphasis on silver and gold derived from studies in the authors' laboratories.

Insignificant Change in Antarctic Snowfall Since the International Geophysical Year

Antarctic snowfall exhibits substantial variability over a range of time scales, with consequent impacts on global sea level and the mass balance of the ice sheets. To assess how snowfall has affected the thickness of the ice sheets in Antarctica and to provide an extended perspective, we derived a 50-year time series of snowfall accumulation over the continent by combining model simulations and observations primarily from ice cores. There has been no statistically significant change in snowfall since the 1950s, indicating that Antarctic precipitation is not mitigating global sea level rise as expected, despite recent winter warming of the overlying atmosphere.

Observations of change in the Southern Ocean

The Southern Ocean has been in a state of disequilibrium with its atmosphere and cryosphere during recent decades. Ocean station and drifting float observations have revealed rising temperatures in the upper 3000m. Salinity has declined in intermediate waters and more rapidly in the sparsely sampled high latitudes. Dissolved oxygen levels may also have decreased, but measurement accuracy is inconsistent. Sea ice area increased from 1979 to 1998, particularly in the Ross Sea, while a decline in ice extent since the early 1970s has been led by the Amundsen-Bellingshausen sector. Fresher waters with lower oxygen isotope content on the Pacific-Antarctic continental shelf are consistent with increased melting of continental ice. Newly forming bottom water has become colder and less salty downstream from that region, but generally warmer in the Weddell Sea. Many ice shelves have retreated or thinned, but others have grown and no trend is apparent in the large iceberg calving rate. Warming and isotherm shoaling within the polar gyres may result in part from changes in the Southern Annular Mode, which could facilitate deep-water access to the continental shelves. Sea-level rise over the past half century has a strong eustatic component and has recently accelerated. Observations over longer periods and with better spatial coverage are needed to better understand the processes causing these changes and their links to the Antarctic ice sheet.

Recent trends in Antarctic snow accumulation from Polar MM5 simulations

Polar MM5, a mesoscale atmospheric model optimized for use over polar ice sheets, is employed to simulate Antarctic accumulation in recent decades. Two sets of simulations, each with different initial and boundary conditions, are evaluated for the 17yr period spanning 1985-2001. The initial and boundary conditions for the two sets of runs are provided by the (i) European Centre for Medium-Range Weather Forecasts 40 year Reanalysis, and (ii) National Centres for Environmental Prediction-Department of Energy Atmospheric Model Intercomparison Project Reanalysis II. This approach is used so that uncertainty can be assessed by comparing the two resulting datasets. There is broad agreement between the two datasets for the annual precipitation trends for 1985-2001. These generally agree with ice core and snow stake accumulation records at various locations around the continent, indicating broad areas of both upward and downward trends. Averaged over the continent the annual trends are small and not statistically different from zero, suggesting that recent Antarctic snowfall changes do not mitigate current sea-level rise. However, this result does not suggest that Antarctica is isolated from the recent climate changes occurring elsewhere on Earth. Rather, these are expressed by strong seasonal and regional precipitation changes.

Changes in ice dynamics and mass balance of the Antarctic ice sheet

The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 degrees C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.

The environment and evolution of the West Antarctic ice sheet: setting the stage

The West Antarctic ice sheet is the last ice sheet of the type cradled in a warm, marine geologic basin. Its perimeter stretches into the surrounding seas allowing warmer ocean waters to reach the undersides of its floating ice shelves and its relatively low surface elevation permits snow-carrying storms to extend well into its interior. This special environment has given rise to theories of impending collapse and for the past quarter-century has challenged researchers who seek a quantitative prediction of its future behaviour and the corresponding effect on sea level. Observations confirm changes on a variety of time scales from the quaternary to less than a minute. The dynamics of the ice sheet involve the complex interaction of ice that is warm at its base and cold along the margins of ice streams; subglacial till that is composed of a combination of marine sediment and eroded sedimentary rocks; and water that moves primarily between the ice and bed, but whose flow direction can differ from the direction of ice motion. The pressure of the water system is often sufficient to float the ice sheet locally and small changes in the amount of water in the till can cause it to rapidly switch from very weak to very stiff.

Measurements of time-variable gravity show mass loss in Antarctica

Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment satellites, we determined mass variations of the Antarctic ice sheet during 2002-2005. We found that the mass of the ice sheet decreased significantly, at a rate of 152 +/- 80 cubic kilometers of ice per year, which is equivalent to 0.4 +/- 0.2 millimeters of global sea-level rise per year. Most of this mass loss came from the West Antarctic Ice Sheet.

Enhanced CO Tolerance for Hydrogen Activation in Au−Pt Dendritic Heteroaggregate Nanostructures

Au-Pt heteroaggregate nanostructures were prepared by sequential reduction methods. The structures have approximately 11 nm Au cores with Pt "tendrils" attached to the Au surface. The heteroaggregates are active H2 oxidation catalysts and show high activity at 90 degrees C in the presence of 1000 ppm CO. The surprising CO-tolerant behavior arises from the composition and unusual architecture of the particles.

Catalyzed CO Oxidation at 70 K on an Extended Au/Ni Surface Alloy

A Au/Ni(111) surface alloy catalyzes the oxidation of CO at low temperature by at least three distinct mechanisms. At the lowest temperature of 70 K, molecularly adsorbed O2, spectroscopically characterized as peroxo or superoxo species bound at multiple sites with vibrational frequencies of 865 and 950 cm-1, is the reactant with CO. Between 105 and 125 K, CO2 production coincides with O2 dissociation, suggesting a "hot atom" mechanism. Above 125 K, adsorbed CO reacts with atomically adsorbed O atoms. These results show that nanosize Au clusters bound to oxide supports are not a necessary condition for Au-catalyzed, low-temperature CO oxidation.

Defining dangerous anthropogenic interference: the role of science, the limits of science

Defining "dangerous anthropogenic interference with the climate system" in the context of Article 2 of the UN Framework Convention on Climate Change (UNFCCC) presents a complex challenge for those developing long-term climate policy. Natural science has a key role to play in quantifying vulnerabilities of elements of the Earth system and estimating the risks from a changing climate. But attempts to interpret Article 2 will inevitably draw on understanding from social science, psychology, law, and ethics. Here I consider the limits of science in defining climate "danger" by focusing on the potential disintegration of the major ice sheets as an example of an extreme impact. I show that considerations of timescale, uncertainty, and learning preclude a definition of danger drawn purely from natural science. Decision makers will be particularly challenged by one characteristic of global problems: answers to some scientific questions become less accurate over decadal timescales, meandering toward the wrong answer, a feature I call negative learning. I argue for a precautionary approach to Article 2 that would be based initially on current, limited scientific understanding of the future of the ice sheets.