papers

An up-to-date list is also available on Google Scholar.

in prep

1. Rapid west Greenland climate shift foreshadows Norse disappearance by decades

   Osman, M.B.

   in prep
2. Northern Hemisphere jet stream changes since the Last Glacial Maximum

   Osman, M.B.

   in prep
3. Hadley and Walker circulation centennial variability since the Last Glacial Maximum

   Hakim, et al.

   in prep

in press

1. Nature

   Globally resolved surface temperatures since the Last Glacial Maximum

   Osman, M.B., Tierney, J.E., Zhu, J., Tardif, R., Hakim, G.J., King, J., and Poulsen, C.J.

   in press

   Abs

   Climate changes across the last 24,000 years provide key insights into Earth system responses to external forcing. Climate model simulations and proxy data have independently allowed for study of this crucial interval; however, they have at times yielded disparate conclusions. Here, we leverage both types of information using paleoclimate data assimilation to produce the first observationally constrained, full-field reanalysis of surface temperature change spanning the Last Glacial Maximum to present. We demonstrate that temperature variability across the last 24 kyr was linked to two modes - radiative forcing from ice sheets and greenhouse gases; and a superposition of changes in thermohaline circulation and seasonal insolation. In contrast with previous proxy-based reconstructions our reanalysis results show that global mean temperatures warmed between the early and middle Holocene and were stable thereafter. When compared with recent temperature changes, our reanalysis indicates that both the rate and magnitude of modern observed warming are unprecedented relative to the changes of the last 24 kyr.

2. JGR

   Marine Aerosol Records of Arctic Sea-ice and Polynya Variability from new Ellesmere and Devon Island Firn Cores, Nunavut, Canada

   Criscitiello, A., Geldsetzer, T., Rhodes, R., Arienzo, M., McConnell, J.R., Chellman, N., Osman, M.B., Yackel, J.J., and Marshall, S.

   in press

   Abs

   Sea ice plays a critical role in the Earth’s climate system, including influencing ocean heat uptake, reflecting solar radiation, and contributing to dense water formation. Instrumental records of polar sea ice extent are only available since 1979, however. The short length of such records also limits our knowledge of polynya variability, which can reflect large-scale atmospheric and climate changes. Ice core proxy records can extend these observations, but require further development and regional validation. We compare chloride and methanesulfonic acid concentrations from two new firn cores from the Canadian Arctic with satellite-derived observations of regional sea-ice concentration and polynya variability from 2002–2014. The monthly resolution of these cores allows for detailed investigation of how regional sea-ice concentration is recorded in the ice at Prince of Wales Icefield (POW), Ellesmere Island and Devon Ice Cap (DIC), Devon Island, Nunavut. Over the period 2002–2014 we find that the primary sources of marine aerosols to POW are polynyas within Arctic Canada and the Canada basin of the Arctic Ocean, whereas the primary sources of marine aerosols to DIC are a broader region of the Queen Elizabeth Islands, Baffin Bay, and the Arctic Ocean. Marine aerosol sources to the two core sites are distinct, reflecting different moisture source regions and, likely, differing transport pathways. Air mass back trajectory results support the satellite-derived results. Glaciochemical records from this dynamic, warming region may provide a proxy for reconstructing North Water polynya and other regional polynya and shore-lead variability prior to the satellite era.

2021

1. Nature Geo.

   Enhanced sensitivity of west Greenland ice caps to last millennium climate change

   Osman, M.B., Smith, B.E., Trusel, L.D., Das, S.B., McConnell, J.R., Chellman, N., Arienzo, M., and Sodemann, H.

   Nature Geoscience 2021

   Abs

   Ice cores provide high-resolution archives of past climatic conditions making them uniquely well-suited for reconstructing rapid climate change at high latitudes. Despite this potential, few records exist from coastal Greenlandic ice caps due to their remote nature, high melt, and potentially complex advance/retreat histories, limiting our long-term understanding of past maritime and coastal climate conditions across this rapidly changing Arctic region. Here, we reconstruct glacier surface mass balance and climate variability from a two-thousand-year (~169 – 2015 CE), regionally representative ice core collected from Nuussuaq Peninsula, west Greenland. We find abrupt and previously unknown regional hydroclimate shifts, including up to ~20{\%} decreases in average snow accumulation during the Medieval Warm Period (MWP; 950-1250 CE) to Little Ice Age (LIA; 1450-1850 CE) transition, followed by subsequent ~40{\%} accumulation increases toward present. These changes are significantly larger than those previously reported from interior Greenland records. Moreover, we illuminate a robust positive association between west Greenland ice cap growth and regional temperature variability during much of the Common Era that diverges from the strongly negative relationship observed across most Greenland glaciers today and predicted under future warming scenarios. Taken together, the evidence could indicate a recent reversal in the response of west Greenland ice caps to climate change, with implications for contemporary and future sea level rise.

2. PNAS

   North Atlantic jet stream projections in the context of the past 1,250 years

   Osman, M.B., Coats, S., McConnell, J.R., Chellman, N., and Das, S.B.

   PNAS 2021

   Abs

   Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely-unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we compile a Greenland-wide array of water isotope (δ18O) and annually accumulated snowfall ice core records that, in tandem with novel insights from a well-validated ensemble of water isotope-enabled global climate model simulations, enable us to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide the first pre-observational constraints on both NAJ position and intensity to show that late 20th and early 21st century NAJ variations were likely not unique relative to natural variability, contrasting suggestions from a recent tree-ring-based reconstruction and earlier observational studies. Rather, new insights from our thirteen-century reconstruction highlight the overwhelming role of internal variability in thus far masking the response of mid-latitude atmospheric dynamics to anthropogenic forcing6, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with modeled NAJ position emerging as distinct (p < 0.05) from the range of reconstructed natural variability by as early as 2060 CE.

2019

1. Nature

   Industrial-era decline in subarctic Atlantic productivity

   Osman, M.B., Das, S.B., Trusel, L.D., Evans, M.J., Fischer, H., Grieman, M.M., Kipfstuhl, S., McConnell, J.R., and Saltzman, E.S.

   Nature 2019

   Abs HTML PDF

   Marine phytoplankton have a crucial role in the modulation of marine-based food webs, fishery yields and the global drawdown of atmospheric carbon dioxide. However, owing to sparse measurements before satellite monitoring in the twenty-first century, the long-term response of planktonic stocks to climate forcing is unknown. Here, using a continuous, multi-century record of subarctic Atlantic marine productivity, we show that a marked 10 ± 7{\%} decline in net primary productivity has occurred across this highly productive ocean basin over the past two centuries. We support this conclusion by the application of a marine-productivity proxy, established using the signal of the planktonic-derived aerosol methanesulfonic acid, which is commonly identified across an array of Greenlandic ice cores. Using contemporaneous satellite-era observations, we demonstrate the use of this signal as a robust and high-resolution proxy for past variations in spatially integrated marine productivity. We show that the initiation of declining subarctic Atlantic productivity broadly coincides with the onset of Arctic surface warming, and that productivity strongly covaries with regional sea-surface temperatures and basin-wide gyre circulation strength over recent decades. Taken together, our results suggest that the decline in industrial-era productivity may be evidence of the predicted collapse of northern Atlantic planktonic stocks in response to a weakened Atlantic Meridional Overturning Circulation. Continued weakening of this Atlantic Meridional Overturning Circulation, as projected for the twenty-first century, may therefore result in further productivity declines across this globally relevant region.

2018

1. Nature

   {Nonlinear rise in Greenland runoff in response to post-industrial Arctic warming}

   Trusel, L.D., Das, S.B., Osman, M.B., Evans, M.J., Smith, B.E., Fettweis, X., McConnell, J.R., No{\"{e}}l, B.P.Y., and Broeke, M.R.

   Nature 2018

   Abs HTML PDF

   The Greenland ice sheet (GrIS) is a growing contributor to global sea-level rise, with recent ice mass loss dominated by surface meltwater runoff. Satellite observations reveal positive trends in GrIS surface melt extent, but melt variability, intensity and runoff remain uncertain before the satellite era. Here we present the first continuous, multi-century and observationally constrained record of GrIS surface melt intensity and runoff, revealing that the magnitude of recent GrIS melting is exceptional over at least the last 350 years. We develop this record through stratigraphic analysis of central west Greenland ice cores, and demonstrate that measurements of refrozen melt layers in percolation zone ice cores can be used to quantifiably, and reproducibly, reconstruct past melt rates. We show significant (P < 0.01) and spatially extensive correlations between these ice-core-derived melt records and modelled melt rates and satellite-derived melt duration across Greenland more broadly, enabling the reconstruction of past ice-sheet-scale surface melt intensity and runoff. We find that the initiation of increases in GrIS melting closely follow the onset of industrial-era Arctic warming in the mid-1800s, but that the magnitude of GrIS melting has only recently emerged beyond the range of natural variability. Owing to a nonlinear response of surface melting to increasing summer air temperatures, continued atmospheric warming will lead to rapid increases in GrIS runoff and sea-level contributions.

2017

1. AMP

   {Real-time analysis of insoluble particles in glacial ice using single-particle mass spectrometry}

   Osman, M.B., Zawadowicz, M.A., Das, S.B., and Cziczo, D.J.

   Atmospheric Measurement Techniques 2017

   Abs HTML

   Insoluble aerosol particles trapped in glacial ice provide insight into past climates, but analysis requires information on climatically relevant particle properties, such as size, abundance, and internal mixing. We present a new analytical method using a time-of-flight single-particle mass spectrometer (SPMS) to determine the composition and size of insoluble particles in glacial ice over an aerodynamic size range of ∼ 0.2-3.0$\mu$m diameter. Using samples from two Greenland ice cores, we developed a procedure to nebulize insoluble particles suspended in melted ice, evaporate condensed liquid from those particles, and transport them to the SPMS for analysis. We further determined size-dependent extraction and instrument transmission efficiencies to investigate the feasibility of determining particle-class-specific mass concentrations. We find SPMS can be used to provide constraints on the aerodynamic size, composition, and relative abundance of most insoluble particulate classes in ice core samples. We describe the importance of post-aqueous processing to particles, a process which occurs due to nebulization of aerosols from an aqueous suspension of originally soluble and insoluble aerosol components. This study represents an initial attempt to use SPMS as an emerging technique for the study of insoluble particulates in ice cores.

2. The Cryos.

   Methanesulfonic acid (MSA) migration in polar ice - Data synthesis and theory

   Osman, M.B., Das, S.B., Marchal, O., and Evans, M.J.

   Cryosphere 2017

   Abs HTML

   Methanesulfonic acid (MSA; CH3SO3H) in polar ice is a unique proxy of marine primary productivity, synoptic atmospheric transport, and regional sea-ice behavior. However, MSA can be mobile within the firn and ice matrix, a post-depositional process that is well known but poorly understood and documented, leading to uncertainties in the integrity of the MSA paleoclimatic signal. Here, we use a compilation of 22 ice core MSA records from Greenland and Antarctica and a model of soluble impurity transport in order to comprehensively investigate the vertical migration of MSA from summer layers, where MSA is originally deposited, to adjacent winter layers in polar ice. We find that the shallowest depth of MSA migration in our compilation varies over a wide range (∼2 to 400 m) and is positively correlated with snow accumulation rate and negatively correlated with ice concentration of Na (typically the most abundant marine cation). Although the considered soluble impurity transport model provides a useful mechanistic framework for studying MSA migration, it remains limited by inadequate constraints on key physicochemical parameters-most notably, the diffusion coefficient of MSA in cold ice (D({MS}). We derive a simplified version of the model, which includes DMS as the sole parameter, in order to illuminate aspects of the migration process. Using this model, we show that the progressive phase alignment of MSA and Na concentration peaks observed along a high-resolution West Antarctic core is most consistent with 10^{-12} m^{2} s^{-1} - 10^{-11} m^{-2} s^{-1}, which is 1 order of magnitude greater than the D_{MS} values previously estimated from laboratory studies. More generally, our data synthesis and model results suggest that (i) MSA migration may be fairly ubiquitous, particularly at coastal and (or) highaccumulation regions across Greenland and Antarctica; and (ii) can significantly change annual and multiyear MSA concentration averages. Thus, in most cases, caution should be exercised when interpreting polar ice core MSA records, although records that have undergone severe migration could still be useful for inferring decadal and lower-frequency climate variability.