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Climate-driven changes in high-elevation forest distribution and reductions in snow and ice cover have major implications for ecosystems and global water security. In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of a high-elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pinus albicaulis) trees, located ~180 m in elevation above modern treeline, that date to the mid-Holocene (c. 5,950 to 5,440 cal y BP). Here, we used this subfossil wood record to develop tree-ring-based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice-patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 °C (±0.2 °C), until a multicentury cooling anomaly suppressed temperatures below 5.8 °C, resulting in stand mortality by c. 5,440 y BP. Transient climate model simulations indicate that regional cooling was driven by changes in summer insolation and Northern Hemisphere volcanism. The initial cooling event was followed centuries later (c. 5,100 y BP) by sustained Icelandic volcanic eruptions that forced a centennial-scale 1.0 °C summer cooling anomaly and led to rapid ice-patch growth and preservation of the trees. With recent warming (c. 2000–2020 CE), warm-season temperatures now equal and will soon exceed those of the mid-Holocene period of high treeline. It is likely that perennial ice cover will again disappear from the region, and treeline may expand upslope so long as plant-available moisture and disturbance are not limiting. 

Abstract. The injection of sulfur into the stratosphere by volcanic eruptions is thedominant driver of natural climate variability oninterannual to multidecadal timescales. Based on a set of continuous sulfateand sulfur records from a suite of ice cores from Greenland and Antarctica,the HolVol v.1.0 database includes estimates of the magnitudes andapproximate source latitudes of major volcanic stratospheric sulfurinjection (VSSI) events for the Holocene (from 9500 BCE or 11 500 years BP to1900 CE), constituting an extension of the previous record by 7000 years.The database incorporates new-generation ice-core aerosol records with asub-annual temporal resolution and a demonstrated sub-decadal dating accuracyand precision. By tightly aligning and stacking the ice-core records on theWD2014 chronology from Antarctica, we resolve long-standing inconsistenciesin the dating of ancient volcanic eruptions that arise from biased (i.e.,dated too old) ice-core chronologies over the Holocene for Greenland. Wereconstruct a total of 850 volcanic eruptions with injections in excess of 1 teragram of sulfur (Tg S); of these eruptions, 329 (39 %) are located in the low latitudes with bipolarsulfate deposition, 426 (50 %) are located in the Northern Hemisphere extratropics (NHET) and 88 (10 %) are located in the Southern Hemisphere extratropics (SHET). The spatial distribution of the reconstructed eruption locationsis in agreement with prior reconstructions for the past 2500 years. Intotal, these eruptions injected 7410 Tg S into thestratosphere: 70 % from tropical eruptions and 25 % from NHextratropical eruptions. A long-term latitudinally and monthly resolvedstratospheric aerosol optical depth (SAOD) time series is reconstructed fromthe HolVol VSSI estimates, representing the first Holocene-scalereconstruction constrained by Greenland and Antarctica ice cores. These newlong-term reconstructions of past VSSI and SAOD variability confirm evidencefrom regional volcanic eruption chronologies (e.g., from Iceland) in showingthat the Early Holocene (9500–7000 BCE) experienced a higher number ofvolcanic eruptions (+16 %) and cumulative VSSI (+86 %) compared withthe past 2500 years. This increase coincides with the rapid retreat of icesheets during deglaciation, providing context for potential future increasesin volcanic activity in regions under projected glacier melting in the 21stcentury. The reconstructed VSSI and SAOD data are available at https://doi.org/10.1594/PANGAEA.928646 (Sigl et al., 2021). 

Abstract. Volcanic fallout in polar ice sheets provides important opportunities to date and correlate ice-core records as well as to investigate theenvironmental impacts of eruptions. Only the geochemical characterization of volcanic ash (tephra) embedded in the ice strata can confirm the sourceof the eruption, however, and is a requisite if historical eruption ages are to be used as valid chronological checks on annual ice layercounting. Here we report the investigation of ash particles in a Greenland ice core that are associated with a volcanic sulfuric acid layer previouslyattributed to the 79 CE eruption of Vesuvius. Major and trace element composition of the particles indicates that the tephra does not derive fromVesuvius but most likely originates from an unidentified eruption in the Aleutian arc. Using ash dispersal modeling, we find that only an eruptionlarge enough to include stratospheric injection is likely to account for the sizable (24–85 µm) ash particles observed in the Greenlandice at this time. Despite its likely explosivity, this event does not appear to have triggered significant climate perturbations, unlike some otherlarge extratropical eruptions. In light of a recent re-evaluation of the Greenland ice-core chronologies, our findings further challenge the previousassignation of this volcanic event to 79 CE. We highlight the need for the revised Common Era ice-core chronology to be formally accepted by the widerice-core and climate modeling communities in order to ensure robust age linkages to precisely dated historical and paleoclimate proxy records. 

Abstract. The 852/3 CE eruption of Mount Churchill, Alaska, was one of the largestfirst-millennium volcanic events, with a magnitude of 6.7 (VEI 6) and atephra volume of 39.4–61.9 km3 (95 % confidence). The spatial extent of the ash fallout from this event is considerable and the cryptotephra (White River Ash east; WRAe) extends as far as Finland and Poland. Proximal ecosystem and societal disturbances have been linked with this eruption; however, wider eruption impacts on climate and society are unknown. Greenland ice core records show that the eruption occurred in winter 852/3 ± 1 CE and that the eruption is associated with a relatively moderate sulfate aerosol loading but large abundances of volcanic ash and chlorine. Here we assess the potential broader impact of this eruption using palaeoenvironmental reconstructions, historical records and climate model simulations. We also use the fortuitous timing of the 852/3 CE Churchill eruption and its extensively widespread tephra deposition of the White River Ash (east) (WRAe) to examine the climatic expression of the warm Medieval Climate Anomaly period (MCA; ca. 950–1250 CE) from precisely linked peatlands in the North Atlantic region. The reconstructed climate forcing potential of the 852/3 CE Churchill eruptionis moderate compared with the eruption magnitude, but tree-ring-inferredtemperatures report a significant atmospheric cooling of 0.8 ∘Cin summer 853 CE. Modelled climate scenarios also show a cooling in 853 CE, although the average magnitude of cooling is smaller (0.3 ∘C). The simulated spatial patterns of cooling are generally similar to those generated using the tree-ring-inferred temperature reconstructions. Tree-ring-inferred cooling begins prior to the date of the eruption suggesting that natural internal climate variability may have increased the climate system's susceptibility to further cooling. The magnitude of the reconstructed cooling could also suggest that the climate forcing potential of this eruption may be underestimated, thereby highlighting the need for greater insight into, and consideration of, the role of halogens and volcanic ash when estimating eruption climate forcing potential. Precise comparisons of palaeoenvironmental records from peatlands acrossNorth America and Europe, facilitated by the presence of the WRAe isochron,reveal no consistent MCA signal. These findings contribute to the growingbody of evidence that characterises the MCA hydroclimate astime-transgressive and heterogeneous rather than a well-defined climaticperiod. The presence of the WRAe isochron also demonstrates that nolong-term (multidecadal) climatic or societal impacts from the 852/3 CEChurchill eruption were identified beyond areas proximal to the eruption.Historical evidence in Europe for subsistence crises demonstrate a degree of temporal correspondence on interannual timescales, but similar events were reported outside of the eruption period and were common in the 9thcentury. The 852/3 CE Churchill eruption exemplifies the difficulties ofidentifying and confirming volcanic impacts for a single eruption, even whenthe eruption has a small age uncertainty.