Andrew D. Moy

A Millennial Proxy Record of ENSO and Eastern Australian Rainfall from the Law Dome Ice Core, East Antarctica

ENSO causes climate extremes across and beyond the Pacific basin; however, evidence of ENSO at high southern latitudes is generally restricted to the South Pacific and West Antarctica. Here, the authors report astatisticallysignificantlinkbetweenENSOandseasaltdepositionduringsummerfromtheLawDome(LD) ice core in East Antarctica. ENSO-related atmospheric anomaliesfrom the central-western equatorialPacific (CWEP) propagate to the South Pacific and the circumpolar high latitudes. These anomalies modulate high- latitude zonal winds, with El Nino (La Nina) conditions causing reduced (enhanced) zonal wind speeds and subsequent reduced (enhanced) summer sea salt deposition at LD. Over the last 1010 yr, the LD summer sea salt(LDSSS)recordhasexhibitedtwobelow-average(ElNino-like)epochs,1000-1260 ADand1920-2009 AD, and a longer above-average (La Nina-like) epoch from 1260 to 1860 AD. Spectral analysis shows the below- average epochs are associated with enhanced ENSO-like variability around 2-5 yr, while the above-average epoch is associated more with variability around 6-7 yr. The LDSSS record is also significantly correlated with annual rainfall in eastern mainland Australia. While the correlation displays decadal-scale variability similar to changes in the interdecadal Pacific oscillation (IPO), the LDSSS record suggests rainfall in the modern instrumental era (1910-2009 AD) is below the long-term average. In addition, recent rainfall declines in some regions of eastern and southeastern Australia appear to be mirrored by a downward trend in the LDSSS record, suggesting current rainfall regimes are unusual though not unknown over the last millennium.

A global multiproxy database for temperature reconstructions of the Common Era

Reproducible climate reconstructions of the Common Era (1 CE to present) are key to placing industrial-era warming into the context of natural climatic variability. Here we present a community-sourced database of temperature-sensitive proxy records from the PAGES2k initiative. The database gathers 692 records from 648 locations, including all continental regions and major ocean basins. The records are from trees, ice, sediment, corals, speleothems, documentary evidence, and other archives. They range in length from 50 to 2000 years, with a median of 547 years, while temporal resolution ranges from biweekly to centennial. Nearly half of the proxy time series are significantly correlated with HadCRUT4.2 surface temperature over the period 1850–2014. Global temperature composites show a remarkable degree of coherence between high- and low-resolution archives, with broadly similar patterns across archive types, terrestrial versus marine locations, and screening criteria. The database is suited to investigations of global and regional temperature variability over the Common Era, and is shared in the Linked Paleo Data (LiPD) format, including serializations in Matlab, R and Python.

Continental-Scale Temperature Variability during the Past Two Millennia: Supplementary Information

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between ad 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period ad 1971–2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.

No coincident nitrate enhancement events in polar ice cores following the largest known solar storms

Knowledge on the occurrence rate of extreme solar storms is strongly limited by the relatively recent advent of satellite monitoring of the Sun. To extend our perspective of solar storms prior to the satellite era and because atmospheric ionization induced by solar energetic particles (SEPs) can lead to the production of odd nitrogen, nitrate spikes in ice cores have been tentatively used to document both the occurrence and intensity of past SEP events. However, the reliability of the use of nitrate in ice records as a proxy for SEP events is strongly debated. This is partly due to equivocal detection of nitrate spikes in single ice cores and possible alternative sources, such as biomass burning plumes. Here we present new continuous high-resolution measurements of nitrate and of the biomass burning species ammonium and black carbon, from several Antarctic and Greenland ice cores. We investigate periods covering the two largest known SEP events of 775 and 994 Common Era as well as the Carrington event and the hard SEP event of February 1956. We report no coincident nitrate spikes associated with any of these benchmark events. We also demonstrate the low reproducibility of the nitrate signal in multiple ice cores and confirm the significant relationship between biomass burning plumes and nitrate spikes in individual ice cores. In the light of these new data, there is no line of evidence that supports the hypothesis that ice cores preserve or document detectable amounts of nitrate produced by SEPs, even for the most extreme events known to date.

Seasonal variations in the sources of natural and anthropogenic lead deposited at the East Rongbuk Glacier in the high-altitude Himalayas

Lead (Pb) isotopic compositions and concentrations, and barium (Ba) and indium (In) concentrations have been analysed at sub-annual resolution in three sections from a <110 m ice core dated to the 18th and 20th centuries, as well as snow pit samples dated to 2004/2005, recovered from the East Rongbuk Glacier in the high-altitude Himalayas. Ice core sections indicate that atmospheric chemistry prior to ~1,953 was controlled by mineral dust inputs, with no discernible volcanic or anthropogenic contributions. Eighteenth century monsoon ice core chemistry is indicative of dominant contributions from local Himalayan sources; non-monsoon ice core chemistry is linked to contributions from local (Himalayan), regional (Indian/Thar Desert) and long-range (North Africa, Central Asia) sources. Twentieth century monsoon and non-monsoon ice core data demonstrate similar seasonal sources of mineral dust, however with a transition to less-radiogenic isotopic signatures that suggests local and regional climate/environmental change. The snow pit record demonstrates natural and anthropogenic contributions during both seasons, with increased anthropogenic influence during non-monsoon times. Monsoon anthropogenic inputs are most likely sourced to South/South-East Asia and/or India, whereas non-monsoon anthropogenic inputs are most likely sourced to India and Central Asia.

Antarctic ice sheet discharge driven by atmosphere-ocean feedbacks at the Last Glacial Termination

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

Modern to glacial age subglacial meltwater drainage at Law Dome, coastal East Antarctica from topography, sediments and jökulhlaup observations

Abstract Rare jökulhlaup events, also known as subglacial lake outburst flood events, have been observed at the Law Dome ice margin and provide an insight into the physical characteristics of subglacial meltwater and drainage. The subglacial topography based on data from the BEDMAP2 and ICECAP projects, together with subsurface transects of the ice margin obtained using ground-penetrating radar, reveal several lakes and lake-like depressions and the drainage pathways of two jökulhlaup events. Oxygen isotope typing of the meltwater during the most recent (2014) jökulhlaup event, combined with ice margin stratigraphy, enable the identification of ice tunnel melt pathways that exploit the 30–90° dipping basal ice layering. The presence of subglacial meltwater beneath Law Dome during the Holocene to Glacial periods is confirmed by the dendritic drainage pattern in the subglacial morphology and extensive layers of basal regelation ice and subglacial carbonate precipitate deposits found within the Løken Moraines sediments. These subglacial carbonates, including ooid layers, formed from the mixing of glacial meltwater and seawater at 72 ka BP. The combined evidence indicates that the ocean discharge of subglacial meltwater may be variable and/or is periodically blocked by basal freezing events near the ice sheet terminus.

Correlation confidence limits for unevenly sampled data

Estimation of correlation with appropriate uncertainty limits for scientific data that are potentially serially correlated is a common problem made seriously challenging especially when data are sampled unevenly in space and/or time. Here we present a new, robust method for estimating correlation with uncertainty limits between autocorrelated series that does not require either resampling or interpolation. The technique employs the Gaussian kernel method with a bootstrapping resampling approach to derive the probability density function and resulting uncertainties. The method is validated using an example from radar geophysics. Autocorrelation and error bounds are estimated for an airborne radio-echo profile of ice sheet thickness. The computed limits are robust when withholding 10%, 20%, and 50% of data. As a further example, the method is applied to two time-series of methanesulphonic acid in Antarctic ice cores from different sites. We show how the method allows evaluation of the significance of correlation where the signal-to-noise ratio is low and reveals that the two ice cores exhibit a significant common signal. HighlightsCorrelation confidence limits can be calculated for unevenly sampled data.Employs Gaussian kernel method used with bootstrapping resampling.Two different studies using highly autocorrelated data validates method.

Individual particle morphology, coatings, and impurities of black carbon aerosols in Antarctic ice and tropical rainfall

Black carbon aerosols (BC) are a large source of climate warming, impact atmospheric chemistry, and are implicated in large-scale changes in atmospheric circulation. Inventories of BC emissions suggest significant changes in the global BC aerosol distribution due to human activity. However, little is known regarding BCs atmospheric distribution or aged particle characteristics before the 20th century. Here, we investigate the prevalence and structural properties of BC particles in Antarctic ice cores from 1759, 1838, and 1930 CE using transmission electron microscopy and energy dispersive x-ray spectroscopy. The study revealed an unexpected diversity in particle morphology, insoluble coatings, and association with metals. In addition to conventionally occurring BC aggregates, we observed single BC monomers, complex aggregates with internally and externally mixed metal and mineral impurities, tar balls, and organo-nitrogen coatings. The results of the study show BC particles in the remote Antarctic atmosphere exhibit complexity that is unaccounted for in atmospheric models of BC.

Application of capillary ion chromatography and capillary ion chromatography coupled with mass spectrometry to determine methanesulfonate and inorganic anions in microliter sample volumes of Antarctic snow and ice

The high costs associated with logistics and the collection of Antarctic ice-cores demands scientists to extract the absolute maximum data from these precious resources. Typically, the chemical analyses of these valuable ice cores, and/or of ice cores from low snow accumulation sites, requires the ice samples to be as small as possible. Despite having a relatively long history within the research lab, recently, capillary ion chromatography (Cap-IC) has become a commercial reality allowing its use as a new analytical capability for the determination of inorganic and organic ions based upon reduced sample volumes. A quantitative study on the simultaneous determination of organic and inorganic anions, including fluoride, methanesulfonate, chloride, sulfate and nitrate anions in Antarctic ice and snow samples was carried out. The new Cap-IC method necessitated only 40 μL of injection volume to attain the analytical performances required, compared to the usual 1–5 mL. In this work, the Cap-IC was also coupled with mass spectrometry, and optimised for the identification and quantification of methanesulfonate. The limit of detection for methanesulfonate was decreased to 0.07 μg L−1 using a hyphenated technique, being the lowest detection limit reported until now in the literature for any ion chromatography based method. To validate the new analytical methods, a comparative study was performed with statistical evaluation of the anion concentrations obtained for snow pit samples from the Aurora Basin North, East Antarctica site, by three separate ion chromatography based methods, namely, standard ion chromatography, and Cap-IC coupled to either suppressed conductivity or mass spectrometry detection.