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Glacier-archived records are powerful windows into past climates and ecosystems, but variation in co-preserved microbiota is rarely characterized over long time periods or connected to changing climates. In a 310-meter ice core from the Guliya ice cap, Tibetan Plateau, we linked microbial communities to concomitant climatic conditions across 33 depths, spanning at least the last 150,000 years. Communities differed significantly between cold and warm periods, and among the three major climate epochs Holocene, Last Glacial Stage (LGS), and Pre-LGS. Although source inputs varied during these periods, the microbial changes appeared independently impacted by climate as well. Co-occurrence network analyses suggested the importance of glacial surface-growing communities, and their phototrophs, to the preserved microbial record. The inferred microbial growth (by cell densities, diversity, and potential doubling times pre-compaction into solid ice) on Guliya’s surfaces was higher during cold periods than warm periods, likely associated with deeper snow and firn layers under colder conditions. Three Cyanobacteria “blooms” were captured in the record, and were significantly correlated to overall microbial concentration and diversity, as well as the abundance of two heterotrophic photosynthetic clades (the genus Geodermatophilus and the class Chloroflexia). Taxonomically, 6 genera were historically persistent and dominant throughout the record (Polaromonas, Flavobacterium, Massilia, Aquaspirillum, Pedobacter, and Cryobacterium), one of which (Cryobacterium) exhibited a shift in dominant strains from the Pre-LGS to the LGS, indicating a possible speciation event. Collectively, these findings advance our understanding of ancient glacier-archived microbial communities and the ecological forces they experienced, provide evidence for microbiological responses to the prevailing climate regime, and may shed new light on the microbial evolution across a long-term history over at least the last >150,000 years.

Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric Osub2/subin air bubbles (δsup18/supOsubatm/sub), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δsup18/supOsubice/suband d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δsup18/supOsubatm/sub, although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (Osub2/sub) by micro-organisms in the ice, which fractionates the δsup18/supO of Osub2/subfrom the atmospheric value. Here, we make corrections for melt and respiration to address these complications. The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δsup18/supOsubice/subare complex and temporally variable; however, Guliya δsup18/supOsubice/subvalues since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature.

Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure changes weakly disrupt gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). The 86Krxs may therefore reflect the time-averaged synoptic pressure variability over several years (site “storminess”), but it likely cannot record individual synoptic events as ice core gas samples typically average over several years. We validate 86Krxs using late Holocene ice samples from 11 Antarctic ice cores and 1 Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong spatial correlation (r=-0.94, p) between site average 86Krxs and time-averaged synoptic variability from reanalysis data. The main uncertainties in the analysis are the corrections for gas loss and thermal fractionation and the relatively large scatter in the data. Limited scientific understanding of the firn physics and potential biases of 86Krxs require caution in interpreting this proxy at present. We show that Antarctic 86Krxs appears to be linked to the position of the Southern Hemisphere eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 kyr from the West Antarctic Ice Sheet (WAIS) Divide ice core. Based on the empirical spatial correlation of synoptic activity and 86Krxs at various Antarctic sites, we interpret this record to show that West Antarctic synoptic activity is slightly below modern levels during the Last Glacial Maximum (LGM), increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods, weakens abruptly at the Holocene onset, remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the Intertropical Convergence Zone (ITCZ) on orbital and millennial timescales such that West Antarctic storminess is weaker when the ITCZ is displaced northward and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño–Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate, and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain speculative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs. A list of suggested future studies is provided.

Currently in review. Methods are described in the full text. Please consult the README.mdfile for information about the included data.&nbsp

Ice cores from the northwestern Tibetan Plateau (NWTP) contain long records of regional climate variability, but refrozen meltwater and dust in these cores has hampered development of robust timescales. Here, we introduce an approach to dating the ice via the isotopic composition of atmospheric O 2 in air bubbles (δ 18 O atm ), along with annual layer counting and radiocarbon dating. We provide a robust chronology for water isotope records (δ 18 O ice and d-excess) from three ice cores from the Guliya ice cap in the NWTP. The measurement of δ 18 O atm , although common in polar ice core timescales, has rarely been used on ice cores from low-latitude, high-altitude glaciers due to (1) low air pressure, (2) the common presence of refrozen melt that adds dissolved gases and reduces the amount of air available for analysis, and (3) the respiratory consumption of molecular oxygen (O 2 ) by micro-organisms in the ice, which fractionates the δ 18 O of O 2 from the atmospheric value. Here, we make corrections for melt and respiration to address these complications. The resulting records of water isotopes from the Guliya ice cores reveal climatic variations over the last 15,000 y, the timings of which correspond to those observed in independently dated lake and speleothem records and confirm that the Guliya ice cap existed before the Holocene. The millennial-scale drivers of δ 18 O ice are complex and temporally variable; however, Guliya δ 18 O ice values since the mid-20th century are the highest since the beginning of the Holocene and have increased with regional air temperature.

Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure variability weakly disrupts gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). We validate 86Krxs using late Holocene ice samples from eleven Antarctic and one Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong correlation (r = -0.94, p < 0.01) between site-average 86Krxs and site synoptic variability from reanalysis data. The main uncertainties in the method are the corrections for gas loss and thermal fractionation, and the relatively large scatter in the data. We show 86Krxs is linked to the position of the eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 ka from the WAIS Divide ice core. West Antarctic synoptic activity is slightly below modern levels during the last glacial maximum (LGM); increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods; weakens abruptly at the Holocene onset; remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the intertropical convergence zone (ITCZ) on orbital and millennial timescales, such that West Antarctic storminess is weaker when the ITCZ is displaced northward, and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal-mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain tentative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs.

Background Glacier ice archives information, including microbiology, that helps reveal paleoclimate histories and predict future climate change. Though glacier-ice microbes are studied using culture or amplicon approaches, more challenging metagenomic approaches, which provide access to functional, genome-resolved information and viruses, are under-utilized, partly due to low biomass and potential contamination. Results We expand existing clean sampling procedures using controlled artificial ice-core experiments and adapted previously established low-biomass metagenomic approaches to study glacier-ice viruses. Controlled sampling experiments drastically reduced mock contaminants including bacteria, viruses, and free DNA to background levels. Amplicon sequencing from eight depths of two Tibetan Plateau ice cores revealed common glacier-ice lineages including Janthinobacterium, Polaromonas, Herminiimonas, Flavobacterium, Sphingomonas, and Methylobacterium as the dominant genera, while microbial communities were significantly different between two ice cores, associating with different climate conditions during deposition. Separately, ~355- and ~14,400-year-old ice were subject to viral enrichment and low-input quantitative sequencing, yielding genomic sequences for 33 vOTUs. These were virtually all unique to this study, representing 28 novel genera and not a single species shared with 225 environmentally diverse viromes. Further, 42.4% of the vOTUs were identifiable temperate, which is significantly higher than that in gut, soil, and marine viromes, and indicates that temperate phages are possibly favored in glacier-ice environments before being frozen. In silico host predictions linked 18 vOTUs to co-occurring abundant bacteria (Methylobacterium, Sphingomonas, and Janthinobacterium), indicating that these phages infected ice-abundant bacterial groups before being archived. Functional genome annotation revealed four virus-encoded auxiliary metabolic genes, particularly two motility genes suggest viruses potentially facilitate nutrient acquisition for their hosts. Finally, given their possible importance to methane cycling in ice, we focused on Methylobacterium viruses by contextualizing our ice-observed viruses against 123 viromes and prophages extracted from 131 Methylobacterium genomes, revealing that the archived viruses might originate from soil or plants. Conclusions Together, these efforts further microbial and viral sampling procedures for glacier ice and provide a first window into viral communities and functions in ancient glacier environments. Such methods and datasets can potentially enable researchers to contextualize new discoveries and begin to incorporate glacier-ice microbes and their viruses relative to past and present climate change in geographically diverse regions globally.

Using an assemblage of four ice cores collected around the Pacific basin, one of the first basinwide histories of Pacific climate variability has been created. This ice core–derived index of the interdecadal Pacific oscillation (IPO) incorporates ice core records from South America, the Himalayas, the Antarctic Peninsula, and northwestern North America. The reconstructed IPO is annually resolved and dates to 1450 CE. The IPO index compares well with observations during the instrumental period and with paleo-proxy assimilated datasets throughout the entire record, which indicates a robust and temporally stationary IPO signal for the last ~550 years. Paleoclimate reconstructions from the tropical Pacific region vary greatly during the Little Ice Age (LIA), although the reconstructed IPO index in this study suggests that the LIA was primarily defined by a weak, negative IPO phase and hence more La Niña–like conditions. Although the mean state of the tropical Pacific Ocean during the LIA remains uncertain, the reconstructed IPO reveals some interesting dynamical relationships with the intertropical convergence zone (ITCZ). In the current warm period, a positive (negative) IPO coincides with an expansion (contraction) of the seasonal latitudinal range of the ITCZ. This relationship is not stationary, however, and is virtually absent throughout the LIA, suggesting that external forcing, such as that from volcanoes and/or reduced solar irradiance, could be driving either the ITCZ shifts or the climate dominating the ice core sites used in the IPO reconstruction.

Records of recent climate from ice cores drilled in 2015 on the Guliya ice cap in the western Kunlun Mountains of the Tibetan Plateau, which with the Himalaya comprises the Third Pole (TP), demonstrate that this region has become warmer and moister since at least the middle of the 19th century. Decadal-scale linkages are suggested between ice core temperature and snowfall proxies, North Atlantic oceanic and atmospheric processes, Arctic temperatures, and Indian summer monsoon intensity. Correlations between annual-scale oxygen isotopic ratios (δ18O) and tropical western Pacific and Indian Ocean sea surface temperatures are also demonstrated. Comparisons of climate records during the last millennium from ice cores acquired throughout the TP illustrate centennial-scale differences between monsoon and westerlies dominated regions. Among these records, Guliya shows the highest rate of warming since the end of the Little Ice Age, but δ18O data over the last millennium from TP ice cores support findings that elevation-dependent warming is most pronounced in the Himalaya. This, along with the decreasing precipitation rates in the Himalaya region, is having detrimental effects on the cryosphere. Although satellite monitoring of glaciers on the TP indicates changes in surface area, only a few have been directly monitored for mass balance and ablation from the surface. This type of ground-based study is essential to obtain a better understanding of the rate of ice shrinkage on the TP.

While glacier ice cores provide climate information over tens to hundreds of thousands of years, study of microbes is challenged by ultra-low-biomass conditions, and virtually nothing is known about co-occurring viruses. Here we establish ultra-clean microbial and viral sampling procedures and apply them to two ice cores from the Guliya ice cap (northwestern Tibetan Plateau, China) to study these archived communities. This method reduced intentionally contaminating bacterial, viral, and free DNA to background levels in artificial-ice-core control experiments, and was then applied to two authentic ice cores to profile their microbes and viruses. The microbes differed significantly across the two ice cores, presumably representing the very different climate conditions at the time of deposition that is similar to findings in other cores. Separately, viral particle enrichment and ultra-low-input quantitative viral metagenomic sequencing from ∼520 and ∼15,000 years old ice revealed 33 viral populations (i.e., species-level designations) that represented four known genera and likely 28 novel viral genera (assessed by gene-sharing networks).In silicohost predictions linked 18 of the 33 viral populations to co-occurring abundant bacteria, includingMethylobacterium,Sphingomonas, andJanthinobacterium, indicating that viruses infected several abundant microbial groups. Depth-specific viral communities were observed, presumably reflecting differences in the environmental conditions among the ice samples at the time of deposition. Together, these experiments establish a clean procedure for studying microbial and viral communities in low-biomass glacier ice and provide baseline information for glacier viruses, some of which appear to be associated with the dominant microbes in these ecosystems.ImportanceThis study establishes ultra-clean microbial and viral sampling procedures for glacier ice, which complements priorin silicodecontamination methods and expands, for the first time, the clean procedures to viruses. Application of these methods to glacier ice confirmed prior common microbiological findings for a new ice core climate record, and provides a first window into viral genomes and their ecology from glacier ice across two time horizons, and emphasizes their likely impact on abundant microbial groups. Together these efforts provide clean sampling approaches and foundational datasets that should enable simultaneous access to an archived virosphere in glacier ice.

The glaciers near Puncak Jaya in Papua, Indonesia, the highest peak between the Himalayas and the Andes, are the last remaining tropical glaciers in the West Pacific Warm Pool (WPWP). Here, we report the recent, rapid retreat of the glaciers near Puncak Jaya by quantifying the loss of ice coverage and reduction of ice thickness over the last 8 y. Photographs and measurements of a 30-m accumulation stake anchored to bedrock on the summit of one of these glaciers document a rapid pace in the loss of ice cover and a ∼5.4-fold increase in the thinning rate, which was augmented by the strong 2015–2016 El Niño. At the current rate of ice loss, these glaciers will likely disappear within the next decade. To further understand the mechanisms driving the observed retreat of these glaciers, 2 ∼32-m-long ice cores to bedrock recovered in mid-2010 are used to reconstruct the tropical Pacific climate variability over approximately the past half-century on a quasi-interannual timescale. The ice core oxygen isotopic ratios show a significant positive linear trend since 1964 CE (0.018 ± 0.008‰ per year;P< 0.03) and also suggest that the glaciers’ retreat is augmented by El Niño–Southern Oscillation processes, such as convection and warming of the atmosphere and sea surface. These Papua glaciers provide the only tropical records of ice core-derived climate variability for the WPWP.

A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (Ta‾) and net snow accumulation (c˙‾). We estimate a total ice-sheet-wide FAC of 26 800±1840 km3, of which 6500±450 km3 resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area (Ta‾≤-19 ∘C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (Ta‾>-19 ∘C and c˙‾≤600 mm w.e. yr−1), FAC has decreased by 23±16 % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between 150±100 Gt and 540±440 Gt, respectively, from the top 10 m and entire firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and firn retention capacity.

A new ice core drilled in 2010 to bedrock from the Bruce Plateau (BP) on the Antarctic Peninsula (AP) provides a high temporal resolution record of environmental conditions in this region. The extremely high annual accumulation rate at this site facilitates analysis of the relationships between annual net accumulation An on the BP and large-scale atmospheric oscillations. Over the last ~45 years, An on the BP has been positively correlated with both the southern annular mode (SAM) and Southern Oscillation index (SOI). Extending this analysis back to 1900 reveals that these relationships are not temporally stable, and they exhibit major shifts in the late-1940s and the mid-1970s that are contemporaneous with phase changes in the Pacific decadal oscillation (PDO). These varying multidecadal characteristics of the An–oscillation relationships are not apparent when only data from the post-1970s era are employed. Analysis of the longer ice core record reveals that the influence of the SAM on An depends not only on the phase of the SAM and SOI but also on the phase of the PDO. When the SAM’s influence on BP An is reduced, such as under negative PDO conditions, BP An is modulated by variability in the tropical and subtropical atmosphere through its impacts on the strength and position of the circumpolar westerlies in the AP region. These results demonstrate the importance of using longer-term ice core–derived proxy records to test conventional views of atmospheric circulation variability in the AP region.

The firn covering the Greenland ice sheet interior can retain part of the surface melt, buffering the ice sheet’s contribution to sea level, but its characteristics are still little known. Using remote-sensing observations from 2000–2017, we estimate that firn covers 1,405,500 ± 17,250 km2 of the ice sheet. We present 344 firn-core-derived observations of the top 10 m firn air content (FAC10), indicative of the firn’s meltwater retention capacity. FAC10 remained stable in the coldest 74 % of the firn area during 1953–2017, while FAC10 decreased in the warmest and driest 12 % of the firn area between 1997–2008 and 2011–2017, resulting in a loss of 180 ± 78 km3 (−26 ± 11 %) of air from the near-surface firn.

Data collected between 1974 and 2016 from snow pits and core samples from two Peruvian ice fields demonstrate the effect of the recent warming over the tropical Andes, augmented by El Nino, on the preservation of the climate record. As the 0°C isotherm is approaching the summit of the Quelccaya ice cap in the Andes of southern Peru (5,670 meters above sea level (masl)), the distinctive seasonal δ18O oscillations in the fresh snow deposited within each thermal year are attenuated at depth due to melting and percolation through the firn. This has become increasingly pronounced over 43 years. In the Andes of northern Peru, the ice field on the col of Nevado Huascaran (6050 masl) has retained its seasonal δ18O variations at depth due to its higher elevation. During the 2015/2016 El Nino, snow on Quelccaya and Huascaran was isotopically (δ18O) enriched and the net sum of accumulation over the previous year (NSA) was below the mean for non–El Nino years, particularly on Quelccaya (up to 64% below the mean) which was more pronounced than the NSA decrease during the comparable 1982/1983 El Nino. Interannual large-scale oceanic and middle to upper-level atmospheric temperatures influence δ18O in precipitation on both ice fields, although the influences are variably affected by strong El Nino–Southern Oscillation events, especially on Quelccaya. The rate of ice wastage along Quelccaya's margin was dramatically higher during 2015/2016 compared with that of the previous 15 years, suggesting that warming from future El Ninos may accelerate mass loss on Peruvian glaciers.

En los Andes peruanos, el calentamiento de la troposfera media, potenciado por el reciente fuerte El Niño, está destruyendo las señales climáticas preservadas en los campos de hielo y acelerando la retirada de los glaciares. En ninguna parte está mejor documentada la pérdida de los glaciares tropicales y es más importante que en los Andes del Perú. El registro más largo de retirada de glaciares proviene de un estudio de 44 años realizado en el casquete de hielo Quelccaya en el sur de los Andes, que prueba la pérdida de un archivo climático muy importante y la pérdida acelerada de un recurso hídrico que alimenta el río Amazonas y el lago Titicaca. En la Cordillera Blanca, los glaciares por debajo de los 5400 msnm sufren tanto un deshielo estacional como el movimiento del agua de deshielo a través de las capas superiores porosas. Debido a su gran altitud, el nevado Huascarán es uno de los pocos sitios tropicales donde aún se conserva una historia climática prácticamente inalterada, que se extiende hasta la última etapa glacial. Sin embargo, todos los glaciares de Cordillera Blanca documentados por INAIGEM (en prensa) estar en retroceso. Dadas las tasas actuales de calentamiento en los Andes tropicales, solo es cuestión de tiempo para que los registros climáticos del hielo del Huascarán también se pierdan. El retroceso de los glaciares a lo largo de los Andes peruanos está contribuyendo a las crisis emergentes de los recursos hídricos y los peligros ambientales tanto para las poblaciones urbanas como rurales. Aunque actualmente la descarga de la estación seca está aumentando, no se mantendrá así a largo plazo. La mayoría de la población del Perú vive en el desierto de la costa oeste, que depende de los ríos alimentados por los glaciares para la agricultura y los medios de subsistencia. El derretimiento de los glaciares también agrava los peligros geológicos en esta región propensa a terremotos, formando lagunas glaciares con represas de hielo o morrenas, lo que puede dar como resultado estallidos de lagunas e inundaciones y flujos de escombros. Comprender el impacto de esta aceleración de la pérdida de glaciares en los recursos hídricos futuros requiere informaciónsobre los cambios del pasado en el balance de masas de los glaciares de alta elevación.

The nitrogen and oxygen isotopic composition of nitrate in ice cores offers unique potential for examining reactive nitrogen oxide (NOx) budgets and oxidation chemistry of past atmospheres. A low-latitude record is of particular interest given that the dominant natural sources of NOx and production of hydroxyl radical are most prevalent in the tropics. Any interpretation of nitrate in ice cores, however, must first consider that nitrate in snow is vulnerable to postdepositional loss and isotopic alteration. We report and assess the integrity of nitrate–δ15N, –δ18O, and –Δ17O in a 30 m ice core from a high-elevation site in the central Andes. Clear seasonality in δ15N, δ18O, and nitrate concentration exists throughout most of the record and cannot be explained by photolysis or evaporation based on our current understanding of these processes. In contrast, nitrate in the upper ~12 m of the core and in a snowpit shows very different behavior. This may reflect alteration facilitated by recent melting at the surface. The relationships between δ15N, δ18O, Δ17O, and concentration in the unaltered sections can be interpreted in terms of mixing of nitrate from discrete sources. Transport effects and an englacial contribution from nitrification cannot be ruled out at this time, but the observed isotopic compositions are consistent with expected signatures of known NOx sources and atmospheric oxidation pathways. Specifically, nitrate deposited during the wet season reflects biogenic soil emissions and hydroxyl/peroxy radical chemistry in the Amazon, while dry season deposition reflects a lightning source and ozone chemistry at higher levels in the troposphere.

The seasonality of accumulation in west central Greenland is investigated to determine whether a summer bias exists in a multi-century ice core recovered from Crawford Point (CP). Such a bias would negatively affect the ice core’s potential for reconstructing the history of winter circulation patterns including the North Atlantic Oscillation. An automatic weather station (AWS) installed at the CP site in 1995 records sub-daily surface heights and affords a unique opportunity to assess the seasonal distribution of accumulation and test the performance of five gridded reanalysis datasets and a regional climate model. Simulated accumulation compares remarkably well with in situ measurements from both the AWS and CP ice core, demonstrating their potential to accurately represent accumulation in this region. Seasonal accumulation exhibits no summer maximum, indicating that any concurrent precipitation maximum is likely offset by melt and/or sublimation effects. The lack of a strong seasonal accumulation bias implies that the CP ice core is well suited for future investigations of the history of winter circulation patterns.

Quelccaya Ice Cap Ice cores drilled in the ice sheets of Greenland and Antarctica are some of the most important sources of information about the paleoclimate of high latitudes. Comparable records from the tropics are rare, however, because there are so few locations at which long-lived, undisturbed ice can be found. Thompson et al. (p. 945 , published online 4 April) report results obtained from one of the few such sites, the Quelccaya ice cap in the Peruvian Andes. The annually resolved data, extending back 1800 years, provide a detailed chronicle of changes in the isotopic composition of the oxygen in the ice, which are related to the sea surface temperature of the water's source. Analyses of a collection of major ions such as ammonium and nitrate reveal how atmospheric circulation in the region varied over that period. Finally, the radiocarbon content of ancient plants—recently exposed by the retreat of the ice sheet—reveals that Quelccaya has not been smaller for at least six thousand years.

Annual net accumulation (A(sub n)) from the Bruce Plateau (BP) ice core retrieved from the Antarctic Peninsula exhibits a notable relationship with sea ice extent (SIE) in the Bellingshausen Sea. Over the satellite era, both BP A(sub n) and Bellingshausen SIE are influenced by large-scale climatic factors such as the Amundsen Sea Low, Southern Annular Mode, and Southern Oscillation. In addition to the direct response of BP A(sub n) to Bellingshausen SIE (e.g., more open water as a moisture source), these large-scale climate phenomena also link the BP and the Bellingshausen Sea indirectly such that they exhibit similar responses (e.g., northerly wind anomalies advect warm, moist air to the Antarctic Peninsula and neighboring Bellingshausen Sea, which reduces SIE and increases A(sub n)). Comparison with a time series of fast ice at South Orkney Islands reveals a relationship between BP A(sub n) and sea ice in the northern Weddell Sea that is relatively consistent over the twentieth century, except when it is modulated by atmospheric wave patterns described by the Trans-Polar Index. The trend of increasing accumulation on the Bruce Plateau since approximately 1970 agrees with other climate records and reconstructions in the region and suggests that the current rate of sea ice loss in the Bellingshausen Sea is unrivaled in the twentieth century.

Approximately half of Greenland's current annual mass loss is attributed to runoff from surface melt(1). At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn2. Two recent studies suggest that all(3) or most(3,4) of Greenland's firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come(3). Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012 (refs 5,6), revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than similar to 1,900m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient(7) surface discharge system and intensifying ice sheet mass loss earlier than previously suggested(3).

Two ice core boreholes of 143.18 m and 447.73 m (bedrock) were drilled during the 2009–2010 austral summer on the Bruce Plateau at a location named LARISSA Site Beta (66°02' S, 64°04' W, 1975.5 m a.s.l.). Both boreholes were logged with thermistors shortly after drilling. The shallow borehole was instrumented for 4 months with a series of resistance thermometers with satellite uplink. Surface temperature proxy data derived from an inversion of the borehole temperature profiles are compared to available multi-decadal records from weather stations and ice cores located along a latitudinal transect of the Antarctic Peninsula to West Antarctica. The LARISSA Site Beta profiles show temperatures decreasing from the surface downward through the upper third of the ice, and warming thereafter to the bed. The average temperature for the most recent year is −14.78°C (measured at 15 m depth, abbreviated T15). A minimum temperature of −15.8°C is measured at 173 m depth, and basal temperature is estimated to be −10.2°C. Current mean annual temperature and the gradient in the lower part of the measured temperature profile have a best fit with an accumulation rate of 1.9×103 kg m−2 a−1 and basal heat flux (q) of 88 mW m−2, if steady-state conditions are assumed. However, the mid-level temperature variations show that recent temperature has varied significantly. Reconstructed surface temperatures (Ts=T15) over the last 200 yr are derived by an inversion technique (Tikhonov and Samarskii, 1990). From this, we find that cold temperatures (minimum Ts=−16.2°C) prevailed from ~1920 to ~1940, followed by a gradual rise of temperature to −14.2°C around 1995, then cooling over the following decade and warming in the last few years. The coldest period was preceded by a relatively warm 19th century at T15≥−15°C. To facilitate regional comparisons of the surface temperature history, we use our T15 data and nearby weather station records to refine estimates of lapse rates (altitudinal, adjusted for latitude: Γa(l)). Good temporal and spatial consistency of Γa(l) over the last 35 yr are observed, implying that the climate trends observed here are regional and consistent over a broad altitude range.

Assessing the significance of current glacier loss on Kilimanjaro, Tanzania, demands a well-constrained temporal perspective. That context is provided by direct measurements, ancillary observations of the ice fields and the analyses of the ice cores collected from them. Ice retreat mechanisms observed there today are consistent with the preservation of the oldest ice, ~11.7 ka, in the central deepest part of the Northern Ice Field (NIF). This ice-core derived paleoclimate history published by Thompson and others (2002) is further confirmed by more recent paleoclimate records from tropical East Africa. Mounting evidence suggests that the (anticipated) loss of the entire NIF will be unprecedented within the past 10 000 years. New evidence bears directly on the mechanisms driving the current ice loss. Measurements made in 2000 on the NIF document that air temperature at 0.5 and 1.5 m above the surface remained below 5°C, while a surface temperature of 0.0°C was sustained for up to 8 hours d-1 under clear conditions, consistent with observations of melting on all Kilimanjaro summit ice fields. The linear relationship between oxygen and hydrogen isotopic ratios for all six ice cores drilled in 2000 lies very close to the global meteoric waterline and does not support sublimation (evaporation) as a major driver of ice loss today or in the past on Kilimanjaro.

In this paper we review the interaction of El Niño Southern Oscillation (ENSO) variability and warming trends recorded in ice-core records from high-altitude tropical glaciers, discuss the implications of the warming trends for the glaciers and consider the societal implications of glacier retreat. ENSO has strong impacts on meteorological phenomena that directly or indirectly affect most regions on the planet and their populations. Many tropical ice fields have provided continuous annually resolved proxy records of climatic and environmental variability preserved in measurable parameters, especially oxygen and hydrogen isotopic ratios (δ18O, δD) and the net mass balance (accumulation). These records present an opportunity to examine the nature of tropical climate variability in greater detail and to extract new information on linkages between rising temperatures on tropical glaciers and equatorial Pacific sea surface temperatures in critical ENSO indicator regions. The long-term climate records from a collection of high-altitude tropical ice cores provide the longer-term context essential for assessing the significance of the magnitude and rate of current climate changes that are in large measure driving glacier retreat. The well-documented ice loss on Quelccaya in the Peruvian Andes, Naimona’nyi in the Himalaya, Kilimanjaro in eastern Africa and the ice fields near Puncak Jaya in Papua, Indonesia, presents a grim future for low-latitude glaciers. The ongoing melting of these ice fields (response) is consistent with model predictions for a vertical amplification of temperature in the tropics (driver) and has serious implications for the people who live in these areas.

The dramatic loss of Kilimanjaro's ice cover has attracted global attention. The three remaining ice fields on the plateau and the slopes are both shrinking laterally and rapidly thinning. Summit ice cover (areal extent) decreased approximately 1% per year from 1912 to 1953 and approximately 2.5% per year from 1989 to 2007. Of the ice cover present in 1912, 85% has disappeared and 26% of that present in 2000 is now gone. From 2000 to 2007 thinning (surface lowering) at the summits of the Northern and Southern Ice Fields was approximately 1.9 and approximately 5.1 m, respectively, which based on ice thicknesses at the summit drill sites in 2000 represents a thinning of approximately 3.6% and approximately 24%, respectively. Furtwängler Glacier thinned approximately 50% at the drill site between 2000 and 2009. Ice volume changes (2000-2007) calculated for two ice fields reveal that nearly equivalent ice volumes are now being lost to thinning and lateral shrinking. The relative importance of different climatological drivers remains an area of active inquiry, yet several points bear consideration. Kilimanjaro's ice loss is contemporaneous with widespread glacier retreat in mid to low latitudes. The Northern Ice Field has persisted at least 11,700 years and survived a widespread drought approximately 4,200 years ago that lasted approximately 300 years. We present additional evidence that the combination of processes driving the current shrinking and thinning of Kilimanjaro's ice fields is unique within an 11,700-year perspective. If current climatological conditions are sustained, the ice fields atop Kilimanjaro and on its flanks will likely disappear within several decades.

Radiocarbon dating of well-preserved, in-place vegetation exposed by the retreating Quelccaya Ice Cap of southeastern Peru constrains the last time the ice cap's extent was smaller than at present. Seventeen plant samples from two sites along the central western margin collectively date to 4700 and 5100 cal yr BP and strongly indicate that current ice cap retreat is unprecedented over the past ∼ 5 millennia. Seventeen vegetation samples interbedded in a nearby clastic sedimentary sequence suggest ice-free conditions at this site from ∼ 5200 to at least ∼ 7000 cal yr BP, and place minimum constraint on early- to mid-Holocene ice cap extent.

Observed changes in the surface elevation of the Greenland ice sheet are caused by ice dynamics, basal elevation change, surface mass balance (SMB) variability, and by compaction of the overlying firn. The latter two contributions are quantified here using a firn model that includes compaction, meltwater percolation, and refreezing. The model is forced with surface mass fluxes and temperature from a regional climate model for the period 1960–2013. The model results agree with observations of surface density, density profiles from 62 firn cores, and altimetric observations from regions where ice-dynamical surface height changes are likely small. We find that the firn layer in the high interior is generally thickening slowly (1–5 cm yr−1). In the percolation and ablation areas, firn and SMB processes account for a surface elevation lowering of up to 20–50 cm yr−1. Most of this firn-induced marginal thinning is caused by an increase in melt since the mid-1990s, and partly compensated by an increase in the accumulation of fresh snow around most of the ice sheet. The total firn and ice volume change between 1980 and 2013 is estimated at −3900 ± 1030 km3 due to firn and SMB, corresponding to an ice-sheet average thinning of 2.32 ± 0.61 m. Most of this volume decrease occurred after 1995. The computed changes in surface elevation can be used to partition altimetrically observed volume change into surface mass balance and ice-dynamically related mass changes.

Over the last two decades, ice core records have been systematically recovered from low-latitude, high-elevation ice fields from across the Tibetan Plateau (TP) and each core has provided new information about regional climate and environmental change. When viewed collectively, these ice core histories provide compelling evidence that the growth (glaciation) and decay (deglaciation) of large ice fields in the lower latitudes are often asynchronous with high-latitude glaciation and deglaciation that occur on Milankovitch time scales. Here, we examine the evidence for asynchronous glaciation on the TP as recorded in ice cores from the Guliya ice cap on the northwestern margin, and the Puruogangri ice cap and Dasuopu glacier in central Tibet and along the southern margin of the TP, respectively. We contend that although stable isotopic records from Antarctica, Greenland, Tanzania, Peru, and Bolivia suggest global-scale cooling during the last glacial stage, over much of the TP precipitation is a stronger driver of glaciation. Here, we present evidence suggesting that glacier expansion on the southern and central TP is driven mainly by variations in monsoonal precipitation that is modulated by precession-driven insolation changes. The basal (oldest) ice in the Puruograngri and Dasuopu ice fields was deposited in the early to mid-Holocene, while ice near the base of Guliya is more than 500,000 yr old.

To investigate the effects of both non-meltwater and meltwater-related post-depositional processes on chemical species within the snow–firn pack, a research program, the Program for Glacier Processes Investigation, was initiated in July 2002 by the Tien Shan Glaciological Station, Chinese Academy of Sciences. The seasonal variability of the ionic concentrations in surface snow samples and ion elution behavior in the snow–firn pack were assessed from surface samples collected year-round and 1011 samples collected from a snow pit at weekly intervals from September 2003 through September 2004. The results indicate that elevated ionic concentrations in spring and summer result from Asian dust-storm-derived aerosol input and other aerosols entrained in precipitation. Potential sources of these chemical species are explored using correlation and factor analyses. The elution sequence through the snow–firn pack was determined to be SO42– >Ca2+> Na+>NO3− >Cl−>K+> Mg2+ >NH4+. The elution of ions at the sampling site was found to be driven primarily by air temperature and became evident when a diurnal mean temperature of –3.6˚C was attained. At 0.3˚C all of the year-round new ionic input was leached from the snow.

A global collection of high temporally resolved ice-core-derived δ18O records is examined to assess whether the proxy records are consistent with contemporaneous observed temperature variations in their respective regions. This is prerequisite to using the older parts of the proxy (δ18O) records to assess whether 20th-century temperatures remain within the range of longer-term natural variability. Excluding the high plateaus in East and West Antarctica where 20th-century temperatures show modest cooling, the ice-core records from other regions suggest modest to strong 20th-century warming. The recent warming over Greenland has been modest and spatially variable. The 20th-century warming over both the Barents Sea region and the Tibetan Plateau now falls well outside the range of prior longer-term temperature variability. Similarly, over the South American Andes and the Antarctic Peninsula the recent warming exceeds the long-term mean for the last 1000 and 500 years, respectively. The ice fields in these regions are in danger of being compromised or lost if the present warming trend in these regions persists.

Two ice cores (118.4 and 214.7 m in length) were collected in 2000 from the Puruogangri ice cap in the center of the Tibetan Plateau (TP) in a joint US-Chinese collaborative project. These cores yield paleoclimatic and environmental records extending through the Middle Holocene, and complement previous ice-core histories from the Dunde and Guliya ice caps in northeast and northwest Tibet, respectively, and Dasuopu glacier in the Himalaya. The high-resolution Puruogangri climate record since AD 1600 details regional temperature and moisture variability. The post-1920 period is characterized by above-average annual net balance, contemporaneous with the greatest 18O enrichment of the last 400 years, consistent with the isotopically inferred warming observed in other TP ice-core records. On longer timescales the aerosol history reveals large and abrupt events, one of which is dated ∼4.7 kyr BP and occurs close to the time of a drought that extended throughout the tropics and may have been associated with centuries-long weakening of the Asian/Indian/African monsoon system. The Puruogangri climate history, combined with the other TP ice-core records, has the potential to provide valuable information on variations in the strength of the monsoon across the TP during the Holocene.

Observations of the δ18O in precipitation from four ice cores (Puruogangri, Dasuopu, Guliya and Dunde) from the Tibetan Plateau (TP) provide additional important perspectives on climatic warming during the 20th century in a region where there is a lack of instrumental and observational climate data. The average δ18O and surface air temperature over the TP show very similar fluctuations since 1955, which provides new evidence that the δ18O in the ice cores is at least in part a temperature signal. Nevertheless differences and similarities exist among the four records. Some climatic events, particularly the major cooling episodes, are synchronously recorded in Puruogangri and Dasuopu and in the Bange meteorological air-temperature record. The major features of the ice cores allow them to be classified into two groups, the northern TP group (Dunde and Guliya) and southern TP group (Puruogangri and Dasuopu). This classification is determined by the different processes driving climate change between the northern and southern regions of the TP. Moreover, the δ18O variability between the ice cores within each region further documents the smaller-scale regional variability.

Over the last 28 years ice core records have been systematically recovered from ten high-elevation ice fields, nine of which are located in the low latitudes. Each core has provided new information about the regional climate and environmental change, and together their records challenge existing paradigms about the Earth's climate system. When viewed collectively, these ice core histories provide compelling evidence that the growth (glaciation) and decay (deglaciation) of large ice fields in the lower latitudes are often asynchronous, both between the hemispheres and with high latitude glaciation that occurs on Milankovitch timescales. Although stable isotopic records suggest that global-scale cooling occurred during the Last Glacial Stage (LGS), we contend that precipitation is the primary driver of glaciation in the low latitudes. This is consistent with the time-transgressive nature of precession-driven changes in insolation (and hence precipitation) such that glaciers advance/retreat in the tropics north of the equator while glaciers retreat/advance in the tropics south of the equator. Thus, the coeval inter-hemispheric retreat of glaciers in the 20th century and the beginning of the 21st century is atypical. Copyright © 2005 John Wiley & Sons, Ltd.

Ice that forms the bottom 18 m of a 308 m ice core drilled from the Guliya ice cap on the Qinghan-Tibetan plateau in Western China is over 750000 years old and is the oldest glacial ice known to date. Fourteen bacterial isolates have been recovered from samples of this ice from approximately 296 m below the surface (mbs). Based on 16S rDNA sequences, these are members of the alpha- and beta-proteobacterial, actinobacterial and low-G + C Gram-positive bacterial lineages. 16S rDNA molecules have also been amplified directly, cloned and sequenced from the ice-core melt water. These originated from Pseudomonas and Acinetobacter gamma-proteobacterial species. These results demonstrate that bacteria can be recovered from water ice that has frozen for time periods relevant to biological survival through terrestrial ice ages or during interplanetary transport.

The activity was designed to provide guidance on future research questions in the Arctic over the next 10-20 years, identifying the key scientific questions that are emerging in different realms of Arctic science and exploring both disciplinary realms (e.g., marine, terrestrial, atmosphere, cryosphere, and social sciences) and cross cutting realms (e.g., integrated systems science and sustainability science). Based on the emerging research questions, the study also helps identify research infrastructure needs and collaboration opportunities. Attention was given to assessing needs where there may be a mismatch between rates of change and the pace of scientific research. Although it is understood that there is no one answer, the committee was asked to explore how agency decision makers might achieve balance in their research portfolios and associated investments. This report is meant to guide future directions in U.S. Arctic research so that research is targeted on critical scientific and societal questions and conducted as effectively as possible. This activity was sponsored by USARC, DOE, NASA, NOAA, NSF, and the Smithsonian Institution.

Water accumulation, defined as precipitation minus evaporation, was estimated over all of Greenland as part of a program to understand changes in ice sheet mass and elevation. Over 360 historical and recent point accumulation estimates on the Greenland ice sheet were evaluated, and 276 estimates that were judged to be high quality were used to develop the accumulation map. The data set includes 99 points developed as part of four investigations of the past 5–15 years; these are judged to have the greatest accuracy. Using kriging, the average accumulation over Greenland is estimated to be ∼30 g cm−2 yr−1. For the interior part of the ice sheet above 1800 m elevation, where most of the data were acquired, the average accumulation is also estimated to be ∼30 g cm−2 yr−1. There are still many areas on the ice sheet, including northwest, southeast, and southern Greenland, where accumulation is highly uncertain, exceeding the mean ice sheet uncertainty at a point of ∼20–25%. In these regions, further sampling will be required to reduce uncertainty in both regional and ice-sheet-wide accumulation.

The spatial pattern of elevation change estimates derived from satellite radar altimeter data for the period from 1978 to 1988 for the southern Greenland ice sheet is examined. As reported previously, the results from 12 ice cores widely distributed in the study area indicate that much of the spatial variability in the elevation change estimates can be explained by temporal variations in accumulation. Most notably, the areas of largest thickening and thinning, east and west of the ice divide around 66°N, recently experienced substantial decadal fluctuations in accumulation sufficient to explain the observed elevation change rates that span the range of ±24 cm yr -1 . Elevation change estimates in the NW of the study area around the 2000-m elevation contour were found to be inconsistent with both short- and long-term changes in accumulation fluctuations. Examination of stratigraphy in several shallow ice cores in the NW indicates that significant melting took place in 1977 and 1978, and this was followed by many years of very little melt activity. This likely resulted in a significant downward bias in the 1978-1988 elevation change estimates in the NW. Comparison of measured surface velocities with those calculated assuming steady state balance show very good agreement overall, but there are many instances in the southeast where the steady state velocities are significantly smaller than measured surface velocities. We cannot rule out the possibility that increased ice flow in lower-elevation outlet glaciers has migrated farther upstream thereby causing negative long-term mass imbalance in southeast Greenland.