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2019
Mühle, J, Trudinger CM, Western LM, Rigby M, Vollmer MK, Park S, Manning AJ, Say D, Ganesan A, Steele LP, Ivy DJ, Arnold T, Li S, Stohl A, Harth CM, Salameh PK, McCulloch A, O'Doherty S, Park MK, Jo CO, Young D, Stanley KM, Krummel PB, Mitrevski B, Hermansen O, Lunder C, Evangeliou N, Yao B, Kim J, Hmiel B, Buizert C, Petrenko VV, Arduini J, Maione M, Etheridge DM, Michalopoulou E, Czerniak M, Severinghaus JP, Reimann S, Simmonds PG, Fraser PJ, Prinn RG, Weiss RF.  2019.  Perfluorocyclobutane (PFC-318, c-C4F8) in the global atmosphere. Atmos. Chem. Phys.. 19:10335-10359.: Copernicus Publications   10.5194/acp-19-10335-2019   Abstract

We reconstruct atmospheric abundances of the potent greenhouse gas c-C4F8 (perfluorocyclobutane, perfluorocarbon PFC-318) from measurements of in situ, archived, firn, and aircraft air samples with precisions of ∼1 %–2 % reported on the SIO-14 gravimetric calibration scale. Combined with inverse methods, we found near-zero atmospheric abundances from the early 1900s to the early 1960s, after which they rose sharply, reaching 1.66 ppt (parts per trillion dry-air mole fraction) in 2017. Global c-C4F8 emissions rose from near zero in the 1960s to 1.2±0.1 (1σ) Gg yr−1 in the late 1970s to late 1980s, then declined to 0.77±0.03 Gg yr−1 in the mid-1990s to early 2000s, followed by a rise since the early 2000s to 2.20±0.05 Gg yr−1 in 2017. These emissions are significantly larger than inventory-based emission estimates. Estimated emissions from eastern Asia rose from 0.36 Gg yr−1 in 2010 to 0.73 Gg yr−1 in 2016 and 2017, 31 % of global emissions, mostly from eastern China. We estimate emissions of 0.14 Gg yr−1 from northern and central India in 2016 and find evidence for significant emissions from Russia. In contrast, recent emissions from northwestern Europe and Australia are estimated to be small (≤1 % each). We suggest that emissions from China, India, and Russia are likely related to production of polytetrafluoroethylene (PTFE, “Teflon”) and other fluoropolymers and fluorochemicals that are based on the pyrolysis of hydrochlorofluorocarbon HCFC-22 (CHClF2) in which c-C4F8 is a known by-product. The semiconductor sector, where c-C4F8 is used, is estimated to be a small source, at least in South Korea, Japan, Taiwan, and Europe. Without an obvious correlation with population density, incineration of waste-containing fluoropolymers is probably a minor source, and we find no evidence of emissions from electrolytic production of aluminum in Australia. While many possible emissive uses of c-C4F8 are known and though we cannot categorically exclude unknown sources, the start of significant emissions may well be related to the advent of commercial PTFE production in 1947. Process controls or abatement to reduce the c-C4F8 by-product were probably not in place in the early decades, explaining the increase in emissions in the 1960s and 1970s. With the advent of by-product reporting requirements to the United Nations Framework Convention on Climate Change (UNFCCC) in the 1990s, concern about climate change and product stewardship, abatement, and perhaps the collection of c-C4F8 by-product for use in the semiconductor industry where it can be easily abated, it is conceivable that emissions in developed countries were stabilized and then reduced, explaining the observed emission reduction in the 1980s and 1990s. Concurrently, production of PTFE in China began to increase rapidly. Without emission reduction requirements, it is plausible that global emissions today are dominated by China and other developing countries. We predict that c-C4F8 emissions will continue to rise and that c-C4F8 will become the second most important emitted PFC in terms of CO2-equivalent emissions within a year or two. The 2017 radiative forcing of c-C4F8 (0.52 mW m−2) is small but emissions of c-C4F8 and other PFCs, due to their very long atmospheric lifetimes, essentially permanently alter Earth's radiative budget and should be reduced. Significant emissions inferred outside of the investigated regions clearly show that observational capabilities and reporting requirements need to be improved to understand global and country-scale emissions of PFCs and other synthetic greenhouse gases and ozone-depleting substances.

Baggenstos, D, Haberli M, Schmitt J, Shackleton SA, Birner B, Severinghaus JP, Kellerhals T, Fischer H.  2019.  Earth's radiative imbalance from the Last Glacial Maximum to the present. Proceedings of the National Academy of Sciences of the United States of America. 116:14881-14886.   10.1073/pnas.1905447116   AbstractWebsite

The energy imbalance at the top of the atmosphere determines the temporal evolution of the global climate, and vice versa changes in the climate system can alter the planetary energy fluxes. This interplay is fundamental to our understanding of Earth's heat budget and the climate system. However, even today, the direct measurement of global radiative fluxes is difficult, such that most assessments are based on changes in the total energy content of the climate system. We apply the same approach to estimate the long-term evolution of Earth's radiative imbalance in the past. New measurements of noble gas-derived mean ocean temperature from the European Project for Ice Coring in Antarctica Dome C ice core covering the last 40,000 y, combined with recent results from the West Antarctic Ice Sheet Divide ice core and the sea-level record, allow us to quantitatively reconstruct the history of the climate system energy budget. The temporal derivative of this quantity must be equal to the planetary radiative imbalance. During the deglaciation, a positive imbalance of typically +0.2 W.m(-2) is maintained for similar to 10,000 y, however, with two distinct peaks that reach up to 0.4 Wm(-2) during times of substantially reduced Atlantic Meridional Overturning Circulation. We conclude that these peaks are related to net changes in ocean heat uptake, likely due to rapid changes in North Atlantic deep-water formation and their impact on the global radiative balance, while changes in cloud coverage, albeit uncertain, may also factor into the picture.

2018
Wagner, TJW, Dell RW, Eisenman I, Keeling RF, Padman L, Severinghaus JP.  2018.  Wave inhibition by sea ice enables trans-Atlantic ice rafting of debris during Heinrich events. Earth and Planetary Science Letters. 495:157-163.   10.1016/j.epsl.2018.05.006   AbstractWebsite

The last glacial period was punctuated by episodes of massive iceberg calving from the Laurentide Ice Sheet, called Heinrich events, which are identified by layers of ice-rafted debris (IRD) in ocean sediment cores from the North Atlantic. The thickness of these IRD layers declines more gradually with distance from the iceberg sources than would be expected based on present-day iceberg drift and decay. Here we model icebergs as passive Lagrangian particles driven by ocean currents, winds, and sea surface temperatures. The icebergs are released in a comprehensive climate model simulation of the last glacial maximum (LGM), as well as a simulation of the modern climate. The two simulated climates result in qualitatively similar distributions of iceberg meltwater and hence debris, with the colder temperatures of the LGM having only a relatively small effect on meltwater spread. In both scenarios, meltwater flux falls off rapidly with zonal distance from the source, in contrast with the more uniform spread of IRD in sediment cores. To address this discrepancy, we propose a physical mechanism that could have prolonged the lifetime of icebergs during Heinrich events. The mechanism involves a surface layer of cold and fresh meltwater formed from, and retained around, large densely packed armadas of icebergs. This leads to wintertime sea ice formation even in relatively low latitudes. The sea ice in turn shields the icebergs from wave erosion, which is the main source of iceberg ablation. We find that sea ice could plausibly have formed around the icebergs during four months each winter. Allowing for four months of sea ice in the model results in a simulated IRD distribution which approximately agrees with the distribution of IRD in sediment cores. (C) 2018 Elsevier B.V. All rights reserved.

Bereiter, B, Kawamura K, Severinghaus JP.  2018.  New methods for measuring atmospheric heavy noble gas isotope and elemental ratios in ice core samples. Rapid Communications in Mass Spectrometry. 32:801-814.   10.1002/rcm.8099   AbstractWebsite

RationaleThe global ocean constitutes the largest heat buffer in the global climate system, but little is known about its past changes. The isotopic and elemental ratios of heavy noble gases (krypton and xenon), together with argon and nitrogen in trapped air from ice cores, can be used to reconstruct past mean ocean temperatures (MOTs). Here we introduce two successively developed methods to measure these parameters with a sufficient precision to provide new constraints on past changes in MOT. MethodsThe air from an 800-g ice sample - containing roughly 80mL STP air - is extracted and processed to be analyzed on two independent dual-inlet isotope ratio mass spectrometers. The primary isotope ratios (N-15, Ar-40 and Kr-86 values) are obtained with precisions in the range of 1 per meg (0.001) per mass unit. The three elemental ratio values Kr/N-2, Xe/N-2 and Xe/Kr are obtained using sequential (non-simultaneous) peak-jumping, reaching precisions in the range of 0.1-0.3. ResultsThe latest version of the method achieves a 30% to 50% better precision on the elemental ratios and a twofold better sample throughput than the previous one. The method development uncovered an unexpected source of artefactual gas fractionation in a closed system that is caused by adiabatic cooling and warming of gases (termed adiabatic fractionation) - a potential source of measurement artifacts in other methods. ConclusionsThe precisions of the three elemental ratios Kr/N-2, Xe/N-2 and Xe/Kr - which all contain the same MOT information - suggest smaller uncertainties for reconstructed MOTs (+/- 0.3-0.1 degrees C) than previous studies have attained. Due to different sensitivities of the noble gases to changes in MOT, Xe/N-2 provides the best constraints on the MOT under the given precisions followed by Xe/Kr, and Kr/N-2; however, using all of them helps to detect methodological artifacts and issues with ice quality.

Bertler, NAN, Conway H, Dahl-Jensen D, Emanuelsson DB, Winstrup M, Vallelonga PT, Lee JE, Brook EJ, Severinghaus JP, Fudge TJ, Keller ED, Baisden WT, Hindmarsh RCA, Neff PD, Blunier T, Edwards R, Mayewski PA, Kipfstuhl S, Buizert C, Canessa S, Dadic R, Kjaer HA, Kurbatov A, Zhang DQ, Waddington ED, Baccolo G, Beers T, Brightley HJ, Carter L, Clemens-Sewall D, Ciobanu VG, Delmonte B, Eling L, Ellis A, Ganesh S, Golledge NR, Haines S, Handley M, Hawley RL, Hogan CM, Johnson KM, Korotkikh E, Lowry DP, Mandeno D, McKay RM, Menking JA, Naish TR, Noerling C, Ollive A, Orsi A, Proemse BC, Pyne AR, Pyne RL, Renwick J, Scherer RP, Semper S, Simonsen M, Sneed SB, Steig EJ, Tuohy A, Venugopal AU, Valero-Delgado F, Venkatesh J, Wang FT, Wang SM, Winski DA, Winton VHL, Whiteford A, Xiao CD, Yang J, Zhang X.  2018.  The Ross Sea Dipole - temperature, snow accumulation and sea ice variability in the Ross Sea region, Antarctica, over the past 2700 years. Climate of the Past. 14:193-214.   10.5194/cp-14-193-2018   AbstractWebsite

High-resolution, well-dated climate archives provide an opportunity to investigate the dynamic interactions of climate patterns relevant for future projections. Here, we present data from a new, annually dated ice core record from the eastern Ross Sea, named the Roosevelt Island Climate Evolution (RICE) ice core. Comparison of this record with climate reanalysis data for the 1979-2012 interval shows that RICE reliably captures temperature and snow precipitation variability in the region. Trends over the past 2700 years in RICE are shown to be distinct from those in West Antarctica and the western Ross Sea captured by other ice cores. For most of this interval, the eastern Ross Sea was warming (or showing isotopic enrichment for other reasons), with increased snow accumulation and perhaps decreased sea ice concentration. However, West Antarctica cooled and the western Ross Sea showed no significant isotope temperature trend. This pattern here is referred to as the Ross Sea Dipole. Notably, during the Little Ice Age, West Antarctica and the western Ross Sea experienced colder than average temperatures, while the eastern Ross Sea underwent a period of warming or increased isotopic enrichment. From the 17th century onwards, this dipole relationship changed. All three regions show current warming, with snow accumulation declining in West Antarctica and the eastern Ross Sea but increasing in the western Ross Sea. We interpret this pattern as reflecting an increase in sea ice in the eastern Ross Sea with perhaps the establishment of a modern Roosevelt Island polynya as a local moisture source for RICE.

Bereiter, B, Shackleton S, Baggenstos D, Kawamura K, Severinghaus J.  2018.  Mean global ocean temperatures during the last glacial transition. Nature. 553:39-+.   10.1038/nature25152   AbstractWebsite

Little is known about the ocean temperature's long-term response to climate perturbations owing to limited observations and a lack of robust reconstructions. Although most of the anthropogenic heat added to the climate system has been taken up by the ocean up until now, its role in a century and beyond is uncertain. Here, using noble gases trapped in ice cores, we show that the mean global ocean temperature increased by 2.57 +/- 0.24 degrees Celsius over the last glacial transition (20,000 to 10,000 years ago). Our reconstruction provides unprecedented precision and temporal resolution for the integrated global ocean, in contrast to the depth-, region-, organism-and season-specific estimates provided by other methods. We find that the mean global ocean temperature is closely correlated with Antarctic temperature and has no lead or lag with atmospheric CO2, thereby confirming the important role of Southern Hemisphere climate in global climate trends. We also reveal an enigmatic 700-year warming during the early Younger Dryas period (about 12,000 years ago) that surpasses estimates of modern ocean heat uptake.

2017
McConnell, JR, Burke A, Dunbar NW, Kohler P, Thomas JL, Arienzo MM, Chellman NJ, Maselli OJ, Sigl M, Adkins JF, Baggenstos D, Burkhart JF, Brook EJ, Buizert C, Cole-Dai J, Fudge TJ, Knorr G, Graf HF, Grieman MM, Iverson N, McGwire KC, Mulvaney R, Paris G, Rhodes RH, Saltzman ES, Severinghaus JP, Steffensen JP, Taylor KC, Winckler G.  2017.  Synchronous volcanic eruptions and abrupt climate change similar to 17.7 ka plausibly linked by stratospheric ozone depletion. Proceedings of the National Academy of Sciences of the United States of America. 114:10035-10040.   10.1073/pnas.1705595114   AbstractWebsite

Glacial-state greenhouse gas concentrations and Southern Hemisphere climate conditions persisted until similar to 17.7 ka, when a nearly synchronous acceleration in deglaciation was recorded in paleoclimate proxies in large parts of the Southern Hemisphere, with many changes ascribed to a sudden poleward shift in the Southern Hemisphere westerlies and subsequent climate impacts. We used high-resolution chemical measurements in the West Antarctic Ice Sheet Divide, Byrd, and other ice cores to document a unique, similar to 192-y series of halogen-rich volcanic eruptions exactly at the start of accelerated deglaciation, with tephra identifying the nearby Mount Takahe volcano as the source. Extensive fallout from these massive eruptions has been found >2,800 km from Mount Takahe. Sulfur isotope anomalies and marked decreases in ice core bromine consistent with increased surface UV radiation indicate that the eruptions led to stratospheric ozone depletion. Rather than a highly improbable coincidence, circulation and climate changes extending from the Antarctic Peninsula to the subtropics-similar to those associated with modern stratospheric ozone depletion over Antarctica-plausibly link the Mount Takahe eruptions to the onset of accelerated Southern Hemisphere deglaciation similar to 17.7 ka.

Orsi, AJ, Kawamura K, Masson-Delmotte V, Fettweis X, Box JE, Dahl-Jensen D, Clow GD, Landais A, Severinghaus JP.  2017.  The recent warming trend in North Greenland. Geophysical Research Letters. 44:6235-6243.   10.1002/2016gl072212   AbstractWebsite

The Arctic is among the fastest warming regions on Earth, but it is also one with limited spatial coverage of multidecadal instrumental surface air temperature measurements. Consequently, atmospheric reanalyses are relatively unconstrained in this region, resulting in a large spread of estimated 30 year recent warming trends, which limits their use to investigate the mechanisms responsible for this trend. Here we present a surface temperature reconstruction over 1982-2011 at NEEM (North Greenland Eemian Ice Drilling Project, 51 degrees W, 77 degrees N), in North Greenland, based on the inversion of borehole temperature and inert gas isotope data. We find that NEEM has warmed by 2.7 +/- 0.33 degrees C over the past 30 years, from the long-term 1900-1970 average of -28.55 +/- 0.29 degrees C. The warming trend is principally caused by an increase in downward longwave heat flux. Atmospheric reanalyses underestimate this trend by 17%, underlining the need for more in situ observations to validate reanalyses.

2016
Cuffey, KM, Clow GD, Steig EJ, Buizert C, Fudge TJ, Koutnik M, Waddington ED, Alley RB, Severinghaus JP.  2016.  Deglacial temperature history of West Antarctica. Proceedings of the National Academy of Sciences of the United States of America. 113:14249-14254.   10.1073/pnas.1609132113   AbstractWebsite

The most recent glacial to interglacial transition constitutes a remarkable natural experiment for learning how Earth's climate responds to various forcings, including a rise in atmospheric CO2. This transition has left a direct thermal remnant in the polar ice sheets, where the exceptional purity and continual accumulation of ice permit analyses not possible in other settings. For Antarctica, the deglacial warming has previously been constrained only by the water isotopic composition in ice cores, without an absolute thermometric assessment of the isotopes' sensitivity to temperature. To overcome this limitation, we measured temperatures in a deep borehole and analyzed them together with ice-core data to reconstruct the surface temperature history of West Antarctica. The deglacial warming was 11.3 +/- 1.8 degrees C, approximately two to three times the global average, in agreement with theoretical expectations for Antarctic amplification of planetary temperature changes. Consistent with evidence from glacier retreat in Southern Hemisphere mountain ranges, the Antarctic warming was mostly completed by 15 kyBP, several millennia earlier than in the Northern Hemisphere. These results constrain the role of variable oceanic heat transport between hemispheres during deglaciation and quantitatively bound the direct influence of global climate forcings on Antarctic temperature. Although climate models perform well on average in this context, some recent syntheses of deglacial climate history have underestimated Antarctic warming and the models with lowest sensitivity can be discounted.

Petrenko, VV, Severinghaus JP, Schaefer H, Smith AM, Kuhl T, Baggenstos D, Hua Q, Brook EJ, Rose P, Kulin R, Bauska T, Harth C, Buizert C, Orsi A, Emanuele G, Lee JE, Brailsford G, Keeling R, Weiss RF.  2016.  Measurements of 14C in ancient ice from Taylor Glacier, Antarctica constrain in situ cosmogenic 14CH4 and 14CO production rates. Geochimica et Cosmochimica Acta. 177:62-77.   10.1016/j.gca.2016.01.004   Abstract

Carbon-14 (14C) is incorporated into glacial ice by trapping of atmospheric gases as well as direct near-surface in situ cosmogenic production. 14C of trapped methane (14CH4) is a powerful tracer for past CH4 emissions from “old” carbon sources such as permafrost and marine CH4 clathrates. 14C in trapped carbon dioxide (14CO2) can be used for absolute dating of ice cores. In situ produced cosmogenic 14C in carbon monoxide (14CO) can potentially be used to reconstruct the past cosmic ray flux and past solar activity. Unfortunately, the trapped atmospheric and in situ cosmogenic components of 14C in glacial ice are difficult to disentangle and a thorough understanding of the in situ cosmogenic component is needed in order to extract useful information from ice core 14C. We analyzed very large (≈1000 kg) ice samples in the 2.26–19.53 m depth range from the ablation zone of Taylor Glacier, Antarctica, to study in situ cosmogenic production of 14CH4 and 14CO. All sampled ice is >50 ka in age, allowing for the assumption that most of the measured 14C originates from recent in situ cosmogenic production as ancient ice is brought to the surface via ablation. Our results place the first constraints on cosmogenic 14CH4 production rates and improve on prior estimates of 14CO production rates in ice. We find a constant 14CH4/14CO production ratio (0.0076 ± 0.0003) for samples deeper than 3 m, which allows the use of 14CO for correcting the 14CH4 signals for the in situ cosmogenic component. Our results also provide the first unambiguous confirmation of 14C production by fast muons in a natural setting (ice or rock) and suggest that the 14C production rates in ice commonly used in the literature may be too high.

2015
Orsi, AJ, Kawamura K, Fegyveresi JM, Headly MA, Alley RB, Severinghaus JP.  2015.  Differentiating bubble-free layers from melt layers in ice cores using noble gases. Journal of Glaciology. 61:585-594.   10.3189/2015JoG14J237   AbstractWebsite

Melt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Buizert, C, Adrian B, Ahn J, Albert M, Alley RB, Baggenstos D, Bauska TK, Bay RC, Bencivengo BB, Bentley CR, Brook EJ, Chellman NJ, Clow GD, Cole-Dai J, Conway H, Cravens E, Cuffey KM, Dunbar NW, Edwards JS, Fegyveresi JM, Ferris DG, Fitzpatrick JJ, Fudge TJ, Gibson CJ, Gkinis V, Goetz JJ, Gregory S, Hargreaves GM, Iverson N, Johnson JA, Jones TR, Kalk ML, Kippenhan MJ, Koffman BG, Kreutz K, Kuhl TW, Lebar DA, Lee JE, Marcott SA, Markle BR, Maselli OJ, McConnell JR, McGwire KC, Mitchell LE, Mortensen NB, Neff PD, Nishiizumi K, Nunn RM, Orsi AJ, Pasteris DR, Pedro JB, Pettit EC, Price PB, Priscu JC, Rhodes RH, Rosen JL, Schauer AJ, Schoenemann SW, Sendelbach PJ, Severinghaus JP, Shturmakov AJ, Sigl M, Slawny KR, Souney JM, Sowers TA, Spencer MK, Steig EJ, Taylor KC, Twickler MS, Vaughn BH, Voigt DE, Waddington ED, Welten KC, Wendricks AW, White JWC, Winstrup M, Wong GJ, Woodruff TE, Members WDP.  2015.  Precise interpolar phasing of abrupt climate change during the last ice age. Nature. 520:661-U169.   10.1038/nature14401   AbstractWebsite

The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeodimate archives'. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa''', suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw(4-6). Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events'. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision''''". Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 +/- 92 years (2 sigma a) for DansgaardOeschger events, including the Bolling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 +/- 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

2014
Marcott, SA, Bauska TK, Buizert C, Steig EJ, Rosen JL, Cuffey KM, Fudge TJ, Severinghaus JP, Ahn J, Kalk ML, McConnell JR, Sowers T, Taylor KC, White JWC, Brook EJ.  2014.  Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature. 514:616-+.   10.1038/nature13799   AbstractWebsite

Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles(1,2). The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination(1,3) (23,000 to 9,000 years ago), however, remains uncertain(1-3). Establishing the timing and rate of CO2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO2 and methane (CH4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO2 variations parallel changes in Antarctic temperature, abrupt CO2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination(4), given the strong covariance of CO2 levels and Antarctic temperatures(5). Our data establish a contribution from an abrupt, centennial-scale mode of CO2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.

Buizert, C, Gkinis V, Severinghaus JP, He F, Lecavalier BS, Kindler P, Leuenberger M, Carlson AE, Vinther B, Masson-Delmotte V, White JWC, Liu ZY, Otto-Bliesner B, Brook EJ.  2014.  Greenland temperature response to climate forcing during the last deglaciation. Science. 345:1177-1180.   10.1126/science.1254961   AbstractWebsite

Greenland ice core water isotopic composition (delta O-18) provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns. We investigate Greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean-atmosphere climate model. Contrary to the traditional delta O-18 interpretation, the Younger Dryas period was 4.5 degrees +/- 2 degrees C warmer than the Oldest Dryas, due to increased carbon dioxide forcing and summer insolation. The magnitude of abrupt temperature changes is larger in central Greenland (9 degrees to 14 degrees C) than in the northwest (5 degrees to 9 degrees C), fingerprinting a North Atlantic origin. Simulated changes in temperature seasonality closely track changes in the Atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon.

Buizert, C, Baggenstos D, Jiang W, Purtschert R, Petrenko VV, Lu ZT, Muller P, Kuhl T, Lee J, Severinghaus JP, Brook EJ.  2014.  Radiometric Kr-81 dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica. Proceedings of the National Academy of Sciences of the United States of America. 111:6876-6881.   10.1073/pnas.1320329111   AbstractWebsite

We present successful Kr-81-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four similar to 350-kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The Kr-81 radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 +/- 2.5 ka. Our experimental methods and sampling strategy are validated by (i) Kr-85 and Ar-39 analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the Kr-81 ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA Kr-81 analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, Kr-81 dating of ice cores is a future possibility.

2013
Arnold, T, Harth CM, Mühle J, Manning AJ, Salameh PK, Kim J, Ivy DJ, Steele PL, Petrenko VV, Severinghaus JP, Baggenstos D, Weiss RF.  2013.  Nitrogen trifluoride global emissions estimated from updated atmospheric measurements. Proceedings of the National Academy of Sciences.   10.1073/pnas.1212346110   AbstractWebsite

Nitrogen trifluoride (NF3) has potential to make a growing contribution to the Earth’s radiative budget; however, our understanding of its atmospheric burden and emission rates has been limited. Based on a revision of our previous calibration and using an expanded set of atmospheric measurements together with an atmospheric model and inverse method, we estimate that the global emissions of NF3 in 2011 were 1.18 ± 0.21 Gg⋅y−1, or ∼20 Tg CO2-eq⋅y−1 (carbon dioxide equivalent emissions based on a 100-y global warming potential of 16,600 for NF3). The 2011 global mean tropospheric dry air mole fraction was 0.86 ± 0.04 parts per trillion, resulting from an average emissions growth rate of 0.09 Gg⋅y−2 over the prior decade. In terms of CO2 equivalents, current NF3 emissions represent between 17% and 36% of the emissions of other long-lived fluorinated compounds from electronics manufacture. We also estimate that the emissions benefit of using NF3 over hexafluoroethane (C2F6) in electronics manufacture is significant—emissions of between 53 and 220 Tg CO2-eq⋅y−1 were avoided during 2011. Despite these savings, total NF3 emissions, currently ∼10% of production, are still significantly larger than expected assuming global implementation of ideal industrial practices. As such, there is a continuing need for improvements in NF3 emissions reduction strategies to keep pace with its increasing use and to slow its rising contribution to anthropogenic climate forcing.

Petrenko, VV, Martinerie P, Novelli P, Etheridge DM, Levin I, Wang Z, Blunier T, Chappellaz J, Kaiser J, Lang P, Steele LP, Hammer S, Mak J, Langenfelds RL, Schwander J, Severinghaus JP, Witrant E, Petron G, Battle MO, Forster G, Sturges WT, Lamarque JF, Steffen K, White JWC.  2013.  A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air. Atmospheric Chemistry and Physics. 13:7567-7585.   10.5194/acp-13-7567-2013   AbstractWebsite

We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140-150 nmol mol(-1), which is higher than today's values. CO mole fractions rose by 10-15 nmol mol(-1) from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a approximate to 30 nmol mol(-1) decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.

Rasmussen, SO, Abbott PM, Blunier T, Bourne AJ, Brook E, Buchardt SL, Buizert C, Chappellaz J, Clausen HB, Cook E, Dahl-Jensen D, Davies SM, Guillevic M, Kipfstuhl S, Laepple T, Seierstad IK, Severinghaus JP, Steffensen JP, Stowasser C, Svensson A, Vallelonga P, Vinther BM, Wilhelms F, Winstrup M.  2013.  A first chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core. Climate of the Past. 9:2713-2730.   10.5194/cp-9-2713-2013   AbstractWebsite

A stratigraphy-based chronology for the North Greenland Eemian Ice Drilling (NEEM) ice core has been derived by transferring the annual layer counted Greenland Ice Core Chronology 2005 (GICC05) and its model extension (GICC05modelext) from the NGRIP core to the NEEM core using 787 match points of mainly volcanic origin identified in the electrical conductivity measurement (ECM) and dielectrical profiling (DEP) records. Tephra horizons found in both the NEEM and NGRIP ice cores are used to test the matching based on ECM and DEP and provide five additional horizons used for the timescale transfer. A thinning function reflecting the accumulated strain along the core has been determined using a Dansgaard-Johnsen flow model and an isotope-dependent accumulation rate parameterization. Flow parameters are determined from Monte Carlo analysis constrained by the observed depth-age horizons. In order to construct a chronology for the gas phase, the ice age-gas age difference (Delta age) has been reconstructed using a coupled firn densification-heat diffusion model. Temperature and accumulation inputs to the Delta age model, initially derived from the water isotope proxies, have been adjusted to optimize the fit to timing constraints from delta N-15 of nitrogen and high-resolution methane data during the abrupt onset of Greenland interstadials. The ice and gas chronologies and the corresponding thinning function represent the first chronology for the NEEM core, named GICC05modelext-NEEM-1. Based on both the flow and firn modelling results, the accumulation history for the NEEM site has been reconstructed. Together, the timescale and accumulation reconstruction provide the necessary basis for further analysis of the records from NEEM.

Kawamura, K, Severinghaus JP, Albert MR, Courville ZR, Fahnestock MA, Scambos T, Shields E, Shuman CA.  2013.  Kinetic fractionation of gases by deep air convection in polar firn. Atmospheric Chemistry and Physics. 13:11141-11155.   10.5194/acp-13-11141-2013   AbstractWebsite

A previously unrecognized type of gas fractionation occurs in firn air columns subjected to intense convection. It is a form of kinetic fractionation that depends on the fact that different gases have different molecular diffusivities. Convective mixing continually disturbs diffusive equilibrium, and gases diffuse back toward diffusive equilibrium under the influence of gravity and thermal gradients. In near-surface firn where convection and diffusion compete as gas transport mechanisms, slow-diffusing gases such as krypton (Kr) and xenon (Xe) are more heavily impacted by convection than fast diffusing gases such as nitrogen (N-2) and argon (Ar), and the signals are preserved in deep firn and ice. We show a simple theory that predicts this kinetic effect, and the theory is confirmed by observations using a newly-developed Kr and Xe stable isotope system in air samples from the Megadunes field site on the East Antarctic plateau. Numerical simulations confirm the effect's magnitude at this site. A main purpose of this work is to support the development of a proxy indicator of past convection in firn, for use in ice-core gas records. To this aim, we also show with the simulations that the magnitude of the kinetic effect is fairly insensitive to the exact profile of convective strength, if the overall thickness of the convective zone is kept constant. These results suggest that it may be feasible to test for the existence of an extremely deep (similar to 30-40 m) convective zone, which has been hypothesized for glacial maxima, by future ice-core measurements.

Fischer, H, Severinghaus J, Brook E, Wolff E, Albert M, Alemany O, Arthern R, Bentley C, Blankenship D, Chappellaz J, Creyts T, Dahl-Jensen D, Dinn M, Frezzotti M, Fujita S, Gallee H, Hindmarsh R, Hudspeth D, Jugie G, Kawamura K, Lipenkov V, Miller H, Mulvaney R, Parrenin F, Pattyn F, Ritz C, Schwander J, Steinhage D, van Ommen T, Wilhelms F.  2013.  Where to find 1.5 million yr old ice for the IPICS "Oldest-Ice" ice core. Climate of the Past. 9:2489-2505.   10.5194/cp-9-2489-2013   AbstractWebsite

The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500m would be required to find 1.5 Myr old ice (i.e., more than 700m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.

2012
Buizert, C, Petrenko VV, Kavanaugh JL, Cuffey KM, Lifton NA, Brook EJ, Severinghaus JP.  2012.  In situ cosmogenic radiocarbon production and 2-D ice flow line modeling for an Antarctic blue ice area. Journal of Geophysical Research-Earth Surface. 117   10.1029/2011jf002086   AbstractWebsite

Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic C-14 production in ice, and this component must be well understood before useful information can be extracted from C-14 data. We combine cosmic ray scaling and production estimates with a two-dimensional ice flow line model to study cosmogenic C-14 production at Taylor Glacier, Antarctica. We find (1) that C-14 production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface C-14 activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two-dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future C-14 and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric C-14 content of methane.

Buizert, C, Martinerie P, Petrenko VV, Severinghaus JP, Trudinger CM, Witrant E, Rosen JL, Orsi AJ, Rubino M, Etheridge DM, Steele LP, Hogan C, Laube JC, Sturges WT, Levchenko VA, Smith AM, Levin I, Conway TJ, Dlugokencky EJ, Lang PM, Kawamura K, Jenk TM, White JWC, Sowers T, Schwander J, Blunier T.  2012.  Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland. Atmospheric Chemistry and Physics. 12:4259-4277.   10.5194/acp-12-4259-2012   AbstractWebsite

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1 sigma Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (Delta age) at the firn-ice transition is calculated to be 182(-9)(+3) yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

2011
Kobashi, T, Kawamura K, Severinghaus JP, Barnola JM, Nakaegawa T, Vinther BM, Johnsen SJ, Box JE.  2011.  High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters. 38   10.1029/2011gl049444   AbstractWebsite

Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise. Yet, it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability. To address this need, we reconstruct Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilises argon and nitrogen isotopic ratios from occluded air bubbles. The estimated average Greenland snow temperature over the past 4000 years was -30.7 degrees C with a standard deviation of 1.0 degrees C and exhibited a long-term decrease of roughly 1.5 degrees C, which is consistent with earlier studies. The current decadal average surface temperature (2001-2010) at the GISP2 site is -29.9 degrees C. The record indicates that warmer temperatures were the norm in the earlier part of the past 4000 years, including century-long intervals nearly 1 C warmer than the present decade (20012010). Therefore, we conclude that the current decadal mean temperature in Greenland has not exceeded the envelope of natural variability over the past 4000 years, a period that seems to include part of the Holocene Thermal Maximum. Notwithstanding this conclusion, climate models project that if anthropogenic greenhouse gas emissions continue, the Greenland temperature would exceed the natural variability of the past 4000 years sometime before the year 2100. Citation: Kobashi, T., K. Kawamura, J. P. Severinghaus, J.-M. Barnola, T. Nakaegawa, B. M. Vinther, S. J. Johnsen, and J. E. Box (2011), High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core, Geophys. Res. Lett., 38, L21501, doi:10.1029/2011GL049444.

2010
Severinghaus, JP, Albert MR, Courville ZR, Fahnestock MA, Kawamura K, Montzka SA, Muhle J, Scambos TA, Shields E, Shuman CA, Suwa M, Tans P, Weiss RF.  2010.  Deep air convection in the firn at a zero-accumulation site, central Antarctica. Earth and Planetary Science Letters. 293:359-367.   10.1016/j.epsl.2010.03.003   AbstractWebsite

Ice cores provide unique archives of past atmospheres and climate, but interpretation of trapped-gas records and their climatic significance has been hampered by a poor knowledge of the prevalence of air convection in the firn layer on top of polar ice sheets. In particular, the phasing of greenhouse gases and climate from ice cores has been obscured by a discrepancy between empirical and model-based estimates of the age difference between trapped gases and enclosing ice, which may be due to air convection. Here we show that deep air convection (>23 m) occurs at a windy, near-zero-accumulation rate site in central Antarctica known informally as the Megadunes site (80.77914 degrees S, 124.48796 degrees E). Deep convection is evident in depth profiles of air withdrawn from the firn layer, in the observed pattern of the nitrogen isotope ratio (15)N/(14)N, the argon isotope ratio (40)Ar/(36)Ar, and in the mixing ratios of the anthropogenic halocarbons methyl chloroform (CH(3)CCl(3)) and HFC-134a (CH(2)FCF(3)). Transport parameters (diffusivities) were inferred and air was dated using measured carbon dioxide (CO(2)) and methane (CH(4)) mixing ratios, by comparing with the Law Dome atmospheric record, which shows that these are the oldest firn air samples ever recovered (CO(2) mean age = 1863 AD). The low accumulation rate and the consequent intense metamorphism of the firn (due to prolonged exposure to seasonal temperature cycling) likely contribute to deep air convection via large grain size and vertical cracks that act as conduits for vigorous air motion. The Megadunes site provides a possible modern analog for the glacial conditions in the Vostok, Dome Fuji, and Dome C ice core records and a possible explanation for lower-than-expected (15)N/(14)N ratios in trapped air bubbles at these times. A general conclusion is that very low accumulation rate causes deep air convection via its effect on firn structural characteristics. (C) 2010 Elsevier B.V. All rights reserved.

Kobashi, T, Severinghaus JP, Barnola JM, Kawamura K, Carter T, Nakaegawa T.  2010.  Persistent multi-decadal Greenland temperature fluctuation through the last millennium. Climatic Change. 100:733-756.   10.1007/s10584-009-9689-9   AbstractWebsite

Future Greenland temperature evolution will affect melting of the ice sheet and associated global sea-level change. Therefore, understanding Greenland temperature variability and its relation to global trends is critical. Here, we reconstruct the last 1,000 years of central Greenland surface temperature from isotopes of N(2) and Ar in air bubbles in an ice core. This technique provides constraints on decadal to centennial temperature fluctuations. We found that northern hemisphere temperature and Greenland temperature changed synchronously at periods of similar to 20 years and 40-100 years. This quasi-periodic multi-decadal temperature fluctuation persisted throughout the last millennium, and is likely to continue into the future.