publications | Matt Osman

2025

Using reduced-complexity volcanic aerosol and climate models to produce large ensemble simulations of Holocene temperature

M. Verkerk, T. J. Aubry, C. Smith, and 7 more authors

Climate of the Past, 2025

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Volcanic eruptions are one of the most important drivers of climate variability, but climate model simulations typically show stronger surface cooling than proxy-based reconstructions. Uncertainties associated with eruption source parameters, aerosol–climate modelling, and internal climate variability might explain those discrepancies, but their quantification using complex global climate models is computationally expensive. In this study, we combine a reduced-complexity volcanic aerosol model (EVA_H) and a climate model (FaIR) to simulate global-mean surface temperature from 6755 BCE to 1900 CE (8705 to 50 BP) accounting for volcanic forcing, solar irradiance, orbital, ice sheet, greenhouse gases, land-use forcing, and anthropogenic aerosols and ozone forcing for the historical period (1750–1900 CE). The negligible computational cost of the models enables us to use a Monte Carlo approach to propagate uncertainties associated with eruption source parameters, aerosol and climate modelling, and internal climate variability. Averaging over the last 9000 years, we obtain a global-mean volcanic forcing of −0.15 W m−2 and an associated surface cooling of 0.12 K. Averaged over the 14 largest eruptions (injecting more than 20 Tg of SO2) of 1250–1900 CE, the mean temperature response in tree-ring-based reconstructions is in good agreement with the our simulations, scaled to Northern Hemisphere summer temperature. For individual eruptions, discrepancies between the simulated and reconstructed surface temperature response are almost always within uncertainties. At multimillennial timescales, our simulations reproduce the Holocene global warming trend typically derived from simulations and data assimilation products but exhibit some discrepancies on centennial to millennial timescales. In particular, the Medieval Climate Anomaly to Little Ice Age transition is weaker in our simulations, and we also do not capture a relatively cool period between 3000 and 1000 BCE (5000 and 3000 BP), visible in climate reanalyses. We discuss how uncertainties in land-use forcing and model limitations might explain these differences. Our study demonstrates the value of reduced-complexity volcanic aerosol–climate models to simulate climate at annual to multimillennial timescales.
@article{Verkerk2025, author = {Verkerk, M. and Aubry, T. J. and Smith, C. and Hopcroft, P. O. and Sigl, M. and Tierney, J. E. and Anchukaitis, K. and Osman, M. and Schmidt, A. and Toohey, M.}, title = {Using reduced-complexity volcanic aerosol and climate models to produce large ensemble simulations of Holocene temperature}, journal = {Climate of the Past}, year = {2025}, volume = {21}, pages = {1755--1778}, doi = {10.5194/cp-21-1755-2025}, url = {https://doi.org/10.5194/cp-21-1755-2025}, dimensions = {true} }
Early warming over the Southern Ocean during the last deglaciation

P. Zheng, T. Bauska, and M. Osman

Geophysical Research Letters, 2025

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The last deglaciation, ca. 17,000–11,000 years before present (yr BP), marks the most recent period of large-scale climate reorganization on Earth. However, the timing and spatial patterns of the initial warming preceding these changes remain uncertain. Here, we develop a new method using Gaussian Mixture Model clustering to objectively segregate a reanalysis and climate model simulations, respectively, into four distinct patterns of temperature change throughout the last deglaciation. Our findings indicate that the earliest warming signs appeared around 21,000 years BP in the Southern Hemisphere high latitudes. This early warming was accompanied by a cooling in the Northern Hemisphere high latitudes, resulting in a hemispherically asymmetric temperature pattern. Further analysis using single-forcing climate simulations suggests that the early warming and sea ice retreat were likely driven by a weakening in the Atlantic Meridional Overturning Circulation, linked to orbital forcing.
@article{Zheng2025, author = {Zheng, P. and Bauska, T. and Osman, M.}, title = {Early warming over the Southern Ocean during the last deglaciation}, journal = {Geophysical Research Letters}, year = {2025}, volume = {52}, pages = {e2025GL117155}, doi = {10.1029/2025GL117155}, url = {https://doi.org/10.1029/2025GL117155}, dimensions = {true} }
Pliocene Warmth and Patterns of Climate Change Inferred From Paleoclimate Data Assimilation

Jessica E Tierney, Jonathan King, Matthew B Osman, and 5 more authors

AGU Advances, Feb 2025

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Abstract As the last time period when CO2 \textCO_2 concentrations were near 400 ppm, the Pliocene Epoch (5.33?2.58 Ma) is a useful paleoclimate target for understanding future climate change. Existing estimates of global warming and climate sensitivity during the Pliocene rely mainly on model simulations. To reconstruct Pliocene climate and incorporate paleoclimate observations, we use data assimilation to blend sea-surface temperature (SST) proxies with model simulations from the Pliocene Modeling Intercomparison Project 2 and the Community Earth System Models. The resulting reconstruction, ?plioDA,? suggests that the mid-Pliocene (3.25 Ma) was warmer than previously thought (on average 4.1°C warmer than preindustrial, 95% CI = 3.0°C?5.3°C), leading to a higher estimate of climate sensitivity (4.8°C per doubling of CO2 \textCO_2, 90% CI = 2.6°C?9.9°C). In agreement with previous work, the tropical Pacific zonal SST gradient during the mid-Pliocene was moderately reduced (?0.8 -0.8°C, 95% CI = ?2.3 -2.3?0.4°C). However, this gradient was more reduced during the early Pliocene (4.75 Ma, ?2.3 -2.3°C, 95% CI = ?3.9 -3.9??1.1 -1.1°C), a time period that is also warmer than the mid-Pliocene (4.8°C above preindustrial, 95% CI = 3.6°C?6.2°C). PlioDA reconstructs a fresh North Pacific and salty North Atlantic, supporting Arctic gateway closure and contradicting the presence of Pacific Deep Water formation. Overall, plioDA updates our view of global and spatial climate change during the Pliocene, as well as raising questions about the state of ocean circulation and the drivers of differences between the early and mid-Pliocene.
@article{Tierney2025a, author = {Tierney, Jessica E and King, Jonathan and Osman, Matthew B and Abell, Jordan T and Burls, Natalie J and Erfani, Ehsan and Cooper, Vincent T and Feng, Ran}, doi = {https://doi.org/10.1029/2024AV001356}, issn = {2576-604X}, journal = {AGU Advances}, keywords = {climate sensitivity,paleoclimate,pliocene}, month = feb, number = {1}, pages = {e2024AV001356}, publisher = {John Wiley & Sons, Ltd}, title = {{Pliocene Warmth and Patterns of Climate Change Inferred From Paleoclimate Data Assimilation}}, url = {https://doi.org/10.1029/2024AV001356}, volume = {6}, year = {2025}, dimensions = {true} }
Bayesian Calibration for the Arctic Sea Ice Biomarker IP25

C Y Fu, M B Osman, and M A Aquino-López

Paleoceanography and Paleoclimatology, Mar 2025

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Abstract Sea ice plays multiple important roles in regulating the global climate. Rapid sea ice loss in the Arctic has been documented over recent decades, yet our understanding of long-term sea ice variability and its feedbacks remains limited by a lack of quantitative sea ice reconstructions. The sea ice diatom-derived biomarker IP25 \textIP_25 has been combined with sterols produced by open-water phytoplankton in the PIP25 \textPIP_25 index as a sea ice proxy to achieve semi-quantitative reconstructions. Here, we analyze a compilation of over 600 published core-top measurements of IP25 \textIP_25 paired with brassicasterol and/or dinosterol across (sub-)Arctic oceans to calculate a newln(PIP25 \textPIP_25) index that correlates nonlinearly with sea ice concentration. Leveraging sediment trap and sea ice observational studies, we develop a spatially varying Bayesian calibration (BaySIC) for ln(PIP25 \textPIP_25) to account for its non-stationary relationship with sea ice concentration and other environmental drivers (e.g., sea surface salinity). The model is fully invertible, allowing probabilistic forward modeling of the ln(PIP25 \textPIP_25) index as well as inverse modeling of past sea ice concentration with bi-directional uncertainty quantification. BaySIC facilitates direct proxy-model comparisons and palaeoclimate data assimilation, providing the polar proxy constraints currently missing in climate model simulations and enabling, for the first time, fully quantitative Arctic sea ice reconstructions.
@article{Fu2025, author = {Fu, C Y and Osman, M B and Aquino-L{\'{o}}pez, M A}, doi = {https://doi.org/10.1029/2024PA005048}, journal = {Paleoceanography and Paleoclimatology}, keywords = {Arctic,Bayesian statistics,IP25,paleoclimate,sea ice}, month = mar, number = {3}, pages = {e2024PA005048}, publisher = {John Wiley & Sons, Ltd}, title = {{Bayesian Calibration for the Arctic Sea Ice Biomarker IP25}}, url = {https://doi.org/10.1029/2024PA005048}, volume = {40}, year = {2025}, dimensions = {true} }
Advances in Paleoclimate Data Assimilation

Jessica E. Tierney, Emily J. Judd, Matthew B. Osman, and 5 more authors

Annual Review of Earth and Planetary Sciences, Mar 2025

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Reconstructions of past climates in both time and space provide important insight into the range and rate of change within the climate system. However, producing a coherent global picture of past climates is difficult because indicators of past environmental changes (proxy data) are unevenly distributed and uncertain. In recent years, paleoclimate data assimilation (paleoDA), which statistically combines model simulations with proxy data, has become an increasingly popular reconstruction method. Here, we describe advances in paleoDA to date, with a focus on the offline ensemble Kalman filter and the insights into climate change that this method affords. PaleoDA has considerable strengths in that it can blend multiple types of information while also propagating uncertainty. Drawbacks of the methodology include an overreliance on the climate model and variance loss. We conclude with an outlook on possible expansions and improvements in paleoDA that can be made in the upcoming years.
@article{Tierney2025b, author = {Tierney, Jessica E. and Judd, Emily J. and Osman, Matthew B. and King, Jonathan M. and Truax, Olivia J. and Steiger, Nathan J. and Amrhein, Daniel E. and Anchukaitis, Kevin J.}, title = {Advances in Paleoclimate Data Assimilation}, journal = {Annual Review of Earth and Planetary Sciences}, issn = {0084-6597}, publisher = {Annual Reviews}, url = {https://www.annualreviews.org/content/journals/10.1146/annurev-earth-032320-064209}, doi = {https://doi.org/10.1146/annurev-earth-032320-064209}, year = {2025}, dimensions = {true} }

2024

Reconciling ice core CO2 and land-use change following New World-Old World contact

Amy C F King, Thomas K Bauska, Edward. J Brook, and 6 more authors

Nature Communications, Mar 2024

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Ice core records of carbon dioxide (CO2) throughout the last 2000 years provide context for the unprecedented anthropogenic rise in atmospheric CO2 and insights into global carbon cycle dynamics. Yet the atmospheric history of CO2 remains uncertain in some time intervals. Here we present measurements of CO2 and methane (CH4) in the Skytrain ice core from 1450 to 1700 CE. Results suggest a sudden decrease in CO2 around 1610 CE in one widely used record may be an artefact of a small number of anomalously low values. Our analysis supports a more gradual decrease in CO2 of 0.5 ppm per decade from 1516 to 1670 CE, with an inferred land carbon sink of 2.6 PgC per decade. This corroborates modelled scenarios of large-scale reorganisation of land use in the Americas following New World-Old World contact, whereas a rapid decrease in CO2 at 1610 CE is incompatible with even the most extreme land-use change scenarios.
@article{King2024, author = {King, Amy C F and Bauska, Thomas K and Brook, Edward. J and Kalk, Mike and Nehrbass-Ahles, Christoph and Wolff, Eric. W and Strawson, Ivo and Rhodes, Rachael H and Osman, Matthew B}, doi = {10.1038/s41467-024-45894-9}, issn = {2041-1723}, journal = {Nature Communications}, number = {1}, pages = {1735}, title = {{Reconciling ice core CO2 and land-use change following New World-Old World contact}}, url = {https://doi.org/10.1038/s41467-024-45894-9}, volume = {15}, year = {2024}, dimensions = {true} }
Last Glacial Maximum pattern effects reduce climate sensitivity estimates

Vincent T Cooper, Kyle C Armour, Gregory J Hakim, and 11 more authors

Science Advances, Apr 2024

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Here, we show that the Last Glacial Maximum (LGM) provides a stronger constraint on equilibrium climate sensitivity (ECS), the global warming from increasing greenhouse gases, after accounting for temperature patterns. Feedbacks governing ECS depend on spatial patterns of surface temperature (?pattern effects?); hence, using the LGM to constrain future warming requires quantifying how temperature patterns produce different feedbacks during LGM cooling versus modern-day warming. Combining data assimilation reconstructions with atmospheric models, we show that the climate is more sensitive to LGM forcing because ice sheets amplify extratropical cooling where feedbacks are destabilizing. Accounting for LGM pattern effects yields a median modern-day ECS of 2.4°C, 66% range 1.7° to 3.5°C (1.4° to 5.0°C, 5 to 95%), from LGM evidence alone. Combining the LGM with other lines of evidence, the best estimate becomes 2.9°C, 66% range 2.4° to 3.5°C (2.1° to 4.1°C, 5 to 95%), substantially narrowing uncertainty compared to recent assessments. Ice sheet?driven temperature patterns amplified glacial cooling, implying less future warming than previously estimated.
@article{Cooper2025, annote = {doi: 10.1126/sciadv.adk9461}, author = {Cooper, Vincent T and Armour, Kyle C and Hakim, Gregory J and Tierney, Jessica E and Osman, Matthew B and Proistosescu, Cristian and Dong, Yue and Burls, Natalie J and Andrews, Timothy and Amrhein, Daniel E and Zhu, Jiang and Dong, Wenhao and Ming, Yi and Chmielowiec, Philip}, doi = {10.1126/sciadv.adk9461}, journal = {Science Advances}, month = apr, number = {16}, pages = {eadk9461}, publisher = {American Association for the Advancement of Science}, title = {{Last Glacial Maximum pattern effects reduce climate sensitivity estimates}}, url = {https://doi.org/10.1126/sciadv.adk9461}, volume = {10}, year = {2024}, dimensions = {true} }

Using reduced-complexity volcanic aerosol and climate models to produce large ensemble simulations of Holocene temperature

M Verkerk, T J Aubry, C Smith, and 7 more authors

EGUsphere, Dec 2024

DOI Bib

@article{Verkerk2024,
  author = {Verkerk, M and Aubry, T J and Smith, C and Hopcroft, P O and Sigl, M and Tierney, J E and Anchukaitis, K and Osman, M and Schmidt, A and Toohey, M},
  doi = {10.5194/egusphere-2024-3635},
  journal = {EGUsphere},
  month = dec,
  pages = {1--33},
  publisher = {Copernicus Publications},
  title = {{Using reduced-complexity volcanic aerosol and climate models to produce large ensemble simulations of Holocene temperature}},
  url = {https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3635/ https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3635/egusphere-2024-3635.pdf},
  volume = {2024},
  year = {2024},
  dimensions = {true}
}

Calibration of Arctic ice core bromine enrichment records for past sea ice reconstructions

Federico Scoto, Niccolò Maffezzoli, Matthew B Osman, and 13 more authors

Science of The Total Environment, Dec 2024

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Bromine in ice cores has been proposed as a qualitative sea ice proxy to produce sea ice reconstructions for the polar regions. Here we report the first statistical validation of this proxy with satellite sea ice observations by combining bromine enrichment (with respect to seawater, Brenr) records from three Greenlandic ice cores (SIGMA-A, NU and RECAP) with satellite sea ice imagery, over three decades. We find that during the 1984–2016 satellite-era, ice core Brenr values are significantly correlated with first-year sea ice formed in the Baffin Bay and Labrador Sea supporting that the gas-phase bromine enrichment processes, preferentially occurring over the sea ice surface, are the main driver for the Brenr signal in ice cores. Moreover, in assessing Brenr’s capability to record historical sea ice variability, we compare 20th-century Arctic Sea ice historical and proxy records with our reconstructions, based on an autoregressive–moving-average (ARMA) model, finding overall good agreement. While further enhancements are warranted, including site-specific calibrations and a comprehensive investigation into bromine transport-related concerns, this study presents a new method to quantitatively reconstruct past seasonal sea ice variability through bromine enrichment in ice cores.
@article{Scoto2024, author = {Scoto, Federico and Maffezzoli, Niccol{\`{o}} and Osman, Matthew B and Cuevas, Carlos A and Vallelonga, Paul and Matoba, Sumito and Iizuka, Yoshinori and Gagliardi, Alessandro and Varin, Cristiano and Burgay, Fran{\c{c}}ois and Pappaccogli, Gianluca and McConnell, Joseph R and Chellman, Nathan and Barbante, Carlo and Saiz-Lopez, Alfonso and Spolaor, Andrea}, doi = {https://doi.org/10.1016/j.scitotenv.2024.177063}, issn = {0048-9697}, journal = {Science of The Total Environment}, keywords = {Arctic,Bromine,Paleoclimate,Proxy calibration,Sea ice reconstructions}, pages = {177063}, title = {{Calibration of Arctic ice core bromine enrichment records for past sea ice reconstructions}}, url = {https://www.sciencedirect.com/science/article/pii/S0048969724072206}, volume = {955}, year = {2024}, dimensions = {true} }

2023

Collaboration between women helps close the gender gap in ice core science

Bess G Koffman, Matthew B Osman, Alison S Criscitiello, and 1 more author

Nature Geoscience, Dec 2023

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@article{Koffman2023,
  author = {Koffman, Bess G and Osman, Matthew B and Criscitiello, Alison S and Guest, Sofia},
  doi = {10.1038/s41561-023-01315-y},
  issn = {1752-0908},
  journal = {Nature Geoscience},
  number = {12},
  pages = {1088--1091},
  title = {{Collaboration between women helps close the gender gap in ice core science}},
  url = {https://doi.org/10.1038/s41561-023-01315-y},
  volume = {16},
  year = {2023},
  dimensions = {true}
}

Chronology and climate of the Last Glacial Maximum and the subsequent deglaciation in the northern Medicine Bow Mountains, Wyoming, USA

Eric M Leonard, Benjamin J C Laabs, Shaun A Marcott, and 6 more authors

Quaternary Science Advances, Dec 2023

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A combination of 10Be surface-exposure dating of glacially transported boulders and glacially polished bedrock, and numerical modeling of the ∼600 km2 Late Pleistocene icefield complex in the northern Medicine Bow Mountains of Wyoming, USA, constrains the timing and climate forcing of the local last glacial maximum (LLGM) and the subsequent deglaciation in the range. The chronology reported here indicates initial recession of the ∼100 km2 Libby Creek glacier on the east side of the complex from its terminal moraine at 20.7 ± 2.8 ka, followed by length reduction of 38% by 18.0 ± 0.8 ka and 75% by 14.7 ± 0.4 ka. By 14.2 ± 0.3 ka, the icefield had nearly completely disappeared although there is evidence of two subsequent standstills or minor readvances at ∼11.5 ± 0.5 ka and ∼10.5 ± 0.3 ka. Results of numerical glacier-modeling experiments suggest that the LLGM was associated with temperatures 6.0 °C colder than present with an uncertainty of about ±1.7 °C, assuming no change from modern precipitation. If precipitation differed at the LLGM, over a range from half to twice modern, the temperature depression necessary to sustain the icefield complex could have been as much as 8.0 ± 1.7 °C or as little as 3.1 ± 1.7 °C respectively. As most available proxies and climate-model output suggest mildly decreased precipitation at the LLGM, a temperature depression of somewhat more than 6.0 °C is the most likely scenario. Model experiments further suggest that nearly complete deglaciation by 14.2 ± 0.3 ka involved only a ∼1.7 °C rise from LLGM temperature, assuming no change in precipitation. The sensitivity of the icefield complex to such limited warming reflects its plateau-like hypsometry, which makes it particularly sensitive to changing equilibrium-line altitude. While the precise chronology of deglaciation remains open to interpretation, most of the ice loss preceded the North Atlantic Bølling-Allerød interval (∼14.7–12.9 ka), suggesting that global atmospheric CO2 and rising summer insolation were the dominant forcings of ice retreat.
@article{Leonard2023, author = {Leonard, Eric M and Laabs, Benjamin J C and Marcott, Shaun A and Crawford, Edward E and Mackall, Benjamin T and Ibarra, Daniel E and Osman, Matthew B and Plummer, Mitchell A and Caffee, Marc W}, doi = {https://doi.org/10.1016/j.qsa.2023.100109}, issn = {2666-0334}, journal = {Quaternary Science Advances}, keywords = {Cosmogenic surface-exposure dating,Deglaciation,Glacier modeling,Last Glacial Maximum,Paleoclimate,Wyoming}, pages = {100109}, title = {{Chronology and climate of the Last Glacial Maximum and the subsequent deglaciation in the northern Medicine Bow Mountains, Wyoming, USA}}, url = {https://www.sciencedirect.com/science/article/pii/S2666033423000412}, volume = {12}, year = {2023}, dimensions = {true} }
Global warming in the pipeline

James E Hansen, Makiko Sato, Leon Simons, and 15 more authors

Oxford Open Climate Change, Jan 2023

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Improved knowledge of glacial-to-interglacial global temperature change yields Charney (fast-feedback) equilibrium climate sensitivity 1.2 ± 0.3°C (2σ) per W/m2, which is 4.8°C ± 1.2°C for doubled CO2. Consistent analysis of temperature over the full Cenozoic era—including ‘slow’ feedbacks by ice sheets and trace gases—supports this sensitivity and implies that CO2 was 300–350 ppm in the Pliocene and about 450 ppm at transition to a nearly ice-free planet, exposing unrealistic lethargy of ice sheet models. Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols. Equilibrium warming is not ‘committed’ warming; rapid phaseout of GHG emissions would prevent most equilibrium warming from occurring. However, decline of aerosol emissions since 2010 should increase the 1970–2010 global warming rate of 0.18°C per decade to a post-2010 rate of at least 0.27°C per decade. Thus, under the present geopolitical approach to GHG emissions, global warming will exceed 1.5°C in the 2020s and 2°C before 2050. Impacts on people and nature will accelerate as global warming increases hydrologic (weather) extremes. The enormity of consequences demands a return to Holocene-level global temperature. Required actions include: (1) a global increasing price on GHG emissions accompanied by development of abundant, affordable, dispatchable clean energy, (2) East-West cooperation in a way that accommodates developing world needs, and (3) intervention with Earth’s radiation imbalance to phase down today’s massive human-made ‘geo-transformation’ of Earth’s climate. Current political crises present an opportunity for reset, especially if young people can grasp their situation.
@article{Hansen2023, author = {Hansen, James E and Sato, Makiko and Simons, Leon and Nazarenko, Larissa S and Sangha, Isabelle and Kharecha, Pushker and Zachos, James C and von Schuckmann, Karina and Loeb, Norman G and Osman, Matthew B and Jin, Qinjian and Tselioudis, George and Jeong, Eunbi and Lacis, Andrew and Ruedy, Reto and Russell, Gary and Cao, Junji and Li, Jing}, doi = {10.1093/oxfclm/kgad008}, issn = {2634-4068}, journal = {Oxford Open Climate Change}, month = jan, number = {1}, pages = {kgad008}, title = {{Global warming in the pipeline}}, url = {https://doi.org/10.1093/oxfclm/kgad008}, volume = {3}, year = {2023}, dimensions = {true} }

DASH: a MATLAB toolbox for paleoclimate data assimilation

J King, J Tierney, M Osman, and 2 more authors

Geosci. Model Dev., Oct 2023

DOI Bib

@article{King2023,
  author = {King, J and Tierney, J and Osman, M and Judd, E J and Anchukaitis, K J},
  doi = {10.5194/gmd-16-5653-2023},
  issn = {1991-9603},
  journal = {Geosci. Model Dev.},
  month = oct,
  number = {19},
  pages = {5653--5683},
  publisher = {Copernicus Publications},
  title = {{DASH: a MATLAB toolbox for paleoclimate data assimilation}},
  url = {https://gmd.copernicus.org/articles/16/5653/2023/ https://gmd.copernicus.org/articles/16/5653/2023/gmd-16-5653-2023.pdf},
  volume = {16},
  year = {2023},
  dimensions = {true}
}

No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years

Zhiyi Jiang, Chris M Brierley, Jürgen Bader, and 15 more authors

Geophysical Research Letters, Oct 2023

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Abstract The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC’s response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid-to-late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment.
@article{Jiang2023, author = {Jiang, Zhiyi and Brierley, Chris M and Bader, J{\"{u}}rgen and Braconnot, Pascale and Erb, Michael and Hopcroft, Peter O and Jiang, Dabang and Jungclaus, Johann and Khon, Vyacheslav and Lohmann, Gerrit and Marti, Olivier and Osman, Matthew B and Otto-Bliesner, Bette and Schneider, Birgit and Shi, Xiaoxu and Thornalley, David J R and Tian, Zhiping and Zhang, Qiong}, doi = {https://doi.org/10.1029/2023GL103078}, issn = {0094-8276}, journal = {Geophysical Research Letters}, keywords = {AMOC,Holocene}, number = {10}, pages = {e2023GL103078}, title = {{No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years}}, url = {https://doi.org/10.1029/2023GL103078}, volume = {50}, year = {2023}, dimensions = {true} }

2021

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

Matthew B Osman, Sloan Coats, Sarah B Das, and 2 more authors

Proceedings of the National Academy of Sciences, Sep 2021

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The North Atlantic jet stream impacts North American and European societies and is expected to be influenced by ongoing 21st-century warming. To better contextualize recently observed and model-projected jet stream changes, long-term records are required. We use insights from a state-of-the-art water isotope–enabled climate model and a compilation of ice-core records from Greenland to reconstruct mean annual North Atlantic jet stream changes back to the 8th century CE. Our reconstruction suggests that observed jet stream variations are consistent with natural variations, despite dramatic warming across recent decades. Under unabated future warming, however, a progressive migration of the jet stream northward is projected to render it distinct from natural variability by 2060 CE.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 incorporate insights from an ensemble of last-millennium water isotope–enabled climate model simulations and a wide array of mean annual water isotope (\delta18O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.All new ice-core annual accumulation and water isotope time series presented herein, as well as NAJ reconstructions, are publicly available from the NOAA Paleoclimatology Data Archive (https://www.ncdc.noaa.gov/paleo/study/33773). NOAA20C (V3) and ERA20C U-wind and Z500 data are each available from https://www.psl.noaa.gov/data/gridded/data.20thC_ReanV3.html and https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-20c, respectively. All iCESM-LME, CMIP5, and CMIP6 U-wind, Z500, precipitation, and water isotope data are available from the NCAR Climate Data Gateway and the Earth System Grid Federation. The MATLAB code needed to reproduce the main results of this study are available at GitHub, https://github.com/mattosman/NAJ-reconstruction.
@article{Osman2021b, author = {Osman, Matthew B and Coats, Sloan and Das, Sarah B and McConnell, Joseph R and Chellman, Nathan}, doi = {10.1073/pnas.2104105118}, journal = {Proceedings of the National Academy of Sciences}, month = sep, number = {38}, pages = {e2104105118}, title = {{North Atlantic jet stream projections in the context of the past 1,250 years}}, url = {http://www.pnas.org/content/118/38/e2104105118.abstract}, volume = {118}, year = {2021}, dimensions = {true} }
Marine Aerosol Records of Arctic Sea-Ice and Polynya Variability From New Ellesmere and Devon Island Firn Cores, Nunavut, Canada

A S Criscitiello, T Geldsetzer, R H Rhodes, and 6 more authors

Journal of Geophysical Research: Oceans, Sep 2021

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Abstract 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 to 2014. The sub-annual 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.
@article{Criscitiello2021, annote = {https://doi.org/10.1029/2021JC017205}, author = {Criscitiello, A S and Geldsetzer, T and Rhodes, R H and Arienzo, M and McConnell, J and Chellman, N and Osman, M B and Yackel, J J and Marshall, S}, doi = {https://doi.org/10.1029/2021JC017205}, issn = {2169-9275}, journal = {Journal of Geophysical Research: Oceans}, keywords = {Arctic,Nunavut,ice core,marine aerosols,polynya,sea ice}, month = sep, number = {9}, pages = {e2021JC017205}, publisher = {John Wiley & Sons, Ltd}, title = {{Marine Aerosol Records of Arctic Sea-Ice and Polynya Variability From New Ellesmere and Devon Island Firn Cores, Nunavut, Canada}}, url = {https://doi.org/10.1029/2021JC017205}, volume = {126}, year = {2021}, dimensions = {true} }
Globally resolved surface temperatures since the Last Glacial Maximum

Matthew B Osman, Jessica E Tierney, Jiang Zhu, and 4 more authors

Nature, Sep 2021

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Climate changes across the past 24,000 years provide key insights into Earth system responses to external forcing. Climate model simulations1,2 and proxy data3–8 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 assimilation9,10 to produce the first proxy-constrained, full-field reanalysis of surface temperature change spanning the Last Glacial Maximum to present at 200-year resolution. We demonstrate that temperature variability across the past 24 thousand years was linked to two primary climatic mechanisms: radiative forcing from ice sheets and greenhouse gases; and a superposition of changes in the ocean overturning circulation and seasonal insolation. In contrast with previous proxy-based reconstructions6,7 our results show that global mean temperature has slightly but steadily warmed, by \sim0.5 °C, since the early Holocene (around 9 thousand years ago). When compared with recent temperature changes11, our reanalysis indicates that both the rate and magnitude of modern warming are unusual relative to the changes of the past 24 thousand years.
@article{Osman2021, author = {Osman, Matthew B and Tierney, Jessica E and Zhu, Jiang and Tardif, Robert and Hakim, Gregory J and King, Jonathan and Poulsen, Christopher J}, doi = {10.1038/s41586-021-03984-4}, issn = {1476-4687}, journal = {Nature}, number = {7884}, pages = {239--244}, title = {{Globally resolved surface temperatures since the Last Glacial Maximum}}, url = {https://doi.org/10.1038/s41586-021-03984-4}, volume = {599}, year = {2021}, dimensions = {true} }
Abrupt Common Era hydroclimate shifts drive west Greenland ice cap change

Matthew B Osman, Benjamin E Smith, Luke D Trusel, and 5 more authors

Nature Geoscience, Sep 2021

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Ice core archives are well suited for reconstructing rapid past climate changes at high latitudes. Despite this, few records currently exist from coastal Greenlandic ice caps due to their remote nature, limiting our long-term understanding of past maritime and coastal climate variability across this rapidly changing Arctic region. Here, we reconstruct regionally representative glacier surface mass balance and climate variability over the last two thousand years (\sim169–2015 ce) using an ice core collected from the Nuussuaq Peninsula, west Greenland. We find indications of abrupt regional hydroclimate shifts, including an up to 20% decrease in average snow accumulation during the transition from the Medieval Warm Period (950–1250 ce) to Little Ice Age (1450–1850 ce), followed by a subsequent >40% accumulation increase from the early 18th to late 20th centuries ce. These coastal changes are substantially larger than those previously reported from interior Greenland records. Moreover, we show that the strong relationship observed today between Arctic temperature rise and coastal ice cap decay contrasts with that of the last millennium, during which periods of warming led to snowfall-driven glacial growth. Taken together with modern observations, the ice core evidence could indicate a recent reversal in the response of west Greenland ice caps to climate change.
@article{Osman2021a, author = {Osman, Matthew B and Smith, Benjamin E and Trusel, Luke D and Das, Sarah B and McConnell, Joseph R and Chellman, Nathan and Arienzo, Monica and Sodemann, Harald}, doi = {10.1038/s41561-021-00818-w}, issn = {1752-0908}, journal = {Nature Geoscience}, number = {10}, pages = {756--761}, title = {{Abrupt Common Era hydroclimate shifts drive west Greenland ice cap change}}, url = {https://doi.org/10.1038/s41561-021-00818-w}, volume = {14}, year = {2021}, dimensions = {true} }

2019

Industrial-era decline in subarctic Atlantic productivity

Matthew B Osman, Sarah B Das, Luke D Trusel, and 6 more authors

Nature, Sep 2019

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Marine phytoplankton have a crucial role in the modulation of marine-based food webs1, fishery yields2 and the global drawdown of atmospheric carbon dioxide3. 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 warming4, 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 predicted5 collapse of northern Atlantic planktonic stocks in response to a weakened Atlantic Meridional Overturning Circulation6–8. Continued weakening of this Atlantic Meridional Overturning Circulation, as projected for the twenty-first century9,10, may therefore result in further productivity declines across this globally relevant region.
@article{Osman2019a, author = {Osman, Matthew B and Das, Sarah B and Trusel, Luke D and Evans, Matthew J and Fischer, Hubertus and Grieman, Mackenzie M and Kipfstuhl, Sepp and McConnell, Joseph R and Saltzman, Eric S}, doi = {10.1038/s41586-019-1181-8}, issn = {1476-4687}, journal = {Nature}, number = {7757}, pages = {551--555}, title = {{Industrial-era decline in subarctic Atlantic productivity}}, url = {https://doi.org/10.1038/s41586-019-1181-8}, volume = {569}, year = {2019}, dimensions = {true} }

2018

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

L.D. Trusel, S.B. Das, M.B. Osman, and 6 more authors

Nature, Sep 2018

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\textcopyright 2018, Springer Nature Limited. The Greenland ice sheet (GrIS) is a growing contributor to global sea-level rise 1 , with recent ice mass loss dominated by surface meltwater runoff 2,3 . Satellite observations reveal positive trends in GrIS surface melt extent 4 , 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 5,6 and satellite-derived melt duration 4 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.
@article{Trusel2018, author = {Trusel, L.D. and Das, S.B. and Osman, M.B. and Evans, M.J. and Smith, B.E. and Fettweis, X. and McConnell, J.R. and No{\"{e}}l, B.P.Y. and van den Broeke, M.R.}, doi = {10.1038/s41586-018-0752-4}, issn = {14764687}, journal = {Nature}, number = {7734}, title = {{Nonlinear rise in Greenland runoff in response to post-industrial Arctic warming}}, volume = {564}, year = {2018}, dimensions = {true} }

2017

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

M. Osman, S.B. Das, O. Marchal, and 1 more author

Cryosphere, Sep 2017

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\textcopyright 2017 Author(s). 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 NaC (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 (DMS). 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 NaC concentration peaks observed along a high-resolution West Antarctic core is most consistent with 10-12 m2 s-1 DMS 10-11 m-2 s-1, which is 1 order of magnitude greater than the DMS 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.
@article{Osman2017a, author = {Osman, M. and Das, S.B. and Marchal, O. and Evans, M.J.}, doi = {10.5194/tc-11-2439-2017}, issn = {19940424}, journal = {Cryosphere}, number = {6}, title = {{Methanesulfonic acid (MSA) migration in polar ice: Data synthesis and theory}}, volume = {11}, year = {2017}, dimensions = {true} }
Real-time analysis of insoluble particles in glacial ice using single-particle mass spectrometry

M. Osman, M.A. Zawadowicz, S.B. Das, and 1 more author

Atmospheric Measurement Techniques, Sep 2017

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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\mum 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.
@article{Osman2017b, author = {Osman, M. and Zawadowicz, M.A. and Das, S.B. and Cziczo, D.J.}, doi = {10.5194/amt-10-4459-2017}, issn = {18678548}, journal = {Atmospheric Measurement Techniques}, number = {11}, title = {{Real-time analysis of insoluble particles in glacial ice using single-particle mass spectrometry}}, volume = {10}, year = {2017}, dimensions = {true} }