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Growing season temperatures play a crucial role in controlling treeline elevation at regional to global scales. However, understanding of treeline dynamics in response to long-term changes in temperature is limited. In this study, we analyze pollen, plant macrofossils, and charcoal preserved in organic layers within a 10,400-year-old ice patch and in sediment from a 6000-year-old wetland located above present-day treeline in the Beartooth Mountains, Wyoming, to explore the relationship between Holocene climate variability and shifts in treeline elevation. Pollen data indicate a lower-than-present treeline between 9000 and 6200 cal yr BP during the warm, dry summer and cold winter conditions of the early Holocene. Increases in arboreal pollen at 6200 cal yr BP suggest an upslope treeline expansion when summers became cooler and wetter. A possible hiatus in the wetland record at ca. 4200–3000 cal yr BP suggests increased snow and ice cover at high elevations and a lowering of treeline. Treeline position continued to fluctuate with growing season warming and cooling during the late-Holocene. Periods of high fire activity correspond with times of increased woody cover at high elevations. The two records indicate that climate was an important driver of vegetation and treeline change during the Holocene. Early Holocene treeline was governed by moisture limitations, whereas late-Holocene treeline was sensitive to increases in growing season temperatures. Climate projections for the region suggest warmer temperatures could decrease effective growing season moisture at high elevations resulting in a reduction of treeline elevation. 

Warming temperatures and prolonged drought periods cause rapid changes of fire frequencies and intensities in high-latitude ecosystems. Associated smoke plumes deposit dark particles from incomplete combustion on the Greenland ice sheet that reduce albedo but also provide a detailed record of paleofire history. Here, we apply an emerging microscopic charcoal technique in combination with established black carbon and lead pollution measurements to an array of 10 ice cores from southern to central Greenland that span recent decades. We found that microscopic charcoal deposition is highly variable among sites, with a few records suggesting recently increasing biomass burning possibly in response to growing fire activity in boreal forest ecosystems. This stands in contrast to decreasing trends in black carbon measured in the same ice cores, consistent with contributions from industrial fossil fuel emissions. Decreasing trends of lead pollution and occurrence of microscopic spheroidal carbonaceous particles (SCP), a microfossil tracer of fossil fuel emissions, further support our interpretation that black carbon in this region is influenced by industrial emissions during recent decades. We conclude that microscopic charcoal analyses in ice may help disentangle biomass burning from fossil-fuel emissions during the industrial period and have potential to contribute to better understanding of regional high-latitude fire regimes. 

Abstract. Volcanic eruptions are a key source of climatic variability, andreconstructing their past impact can improve our understanding of theoperation of the climate system and increase the accuracy of future climateprojections. Two annually resolved and independently dated palaeoarchives –tree rings and polar ice cores – can be used in tandem to assess thetiming, strength and climatic impact of volcanic eruptions over the past∼ 2500 years. The quantification of post-volcanic climateresponses, however, has at times been hampered by differences betweensimulated and observed temperature responses that raised questions regardingthe robustness of the chronologies of both archives. While manychronological mismatches have been resolved, the precise timing and climaticimpact of two major sulfate-emitting volcanic eruptions during the 1450s CE, including the largest atmospheric sulfate-loading event in the last 700 years, have not been constrained. Here we explore this issue through acombination of tephrochronological evidence and high-resolution ice-corechemistry measurements from a Greenland ice core, the TUNU2013 record. We identify tephra from the historically dated 1477 CE eruption of theIcelandic Veiðivötn–Bárðarbunga volcanic system in directassociation with a notable sulfate peak in TUNU2013 attributed to thisevent, confirming that this peak can be used as a reliable and precisetime marker. Using seasonal cycles in several chemical elements and 1477 CEas a fixed chronological point shows that ages of 1453 CE and 1458 CE can beattributed, with high precision, to the start of two other notablesulfate peaks. This confirms the accuracy of a recent Greenland ice-corechronology over the middle to late 15th century and corroborates thefindings of recent volcanic reconstructions from Greenland and Antarctica.Overall, this implies that large-scale Northern Hemisphere climatic coolingaffecting tree-ring growth in 1453 CE was caused by a Northern Hemispherevolcanic eruption in 1452 or early 1453 CE, and then a Southern Hemisphereeruption, previously assumed to have triggered the cooling, occurred laterin 1457 or 1458 CE. The direct attribution of the 1477 CE sulfate peak to the eruption ofVeiðivötn, one of the most explosive from Iceland in the last 1200 years, also provides the opportunity to assess the eruption's climaticimpact. A tree-ring-based reconstruction of Northern Hemisphere summertemperatures shows a cooling in the aftermath of the eruption of −0.35 ∘C relative to a 1961–1990 CE reference period and−0.1 ∘C relative to the 30-year period around the event, as well as arelatively weak and spatially incoherent climatic response in comparison tothe less explosive but longer-lasting Icelandic Eldgjá 939 CE and Laki1783 CE eruptions. In addition, the Veiðivötn 1477 CE eruptionoccurred around the inception of the Little Ice Age and could be used as achronostratigraphic marker to constrain the phasing and spatial variabilityof climate changes over this transition if it can be traced in moreregional palaeoclimatic archives. 

Anthropogenic climate change—combined with increased human-caused ignitions—is leading to increased wildfire frequency, carbon dioxide emissions, and refractory black carbon (rBC) aerosol emissions. This is particularly evident in the Amazon rainforest, where fire activity has been complicated by the synchronicity of natural and anthropogenic drivers of ecological change, coupled with spatial and temporal heterogeneity in past and present land use. One approach to elucidating these factors is through long-term regional fire histories. Using a novel method for rBC determinations, we measured an approximately 3500-year sediment core record from Lake Caranã in the eastern Amazon for rBC influx, a proxy of biomass burning and fossil fuel combustion. Through comparisons with previously published records from Lake Caranã and regional evidence, we distinguished between local and regional rBC emission sources demonstrating increased local emissions of rBC from ~1250 to 500 calendar years before present (cal yr BP), coinciding with increased local-scale fire management during the apex of pre-Columbian activity. This was followed by a regional decline in biomass burning coincident with European contact, pre-Columbian population decline, and regional fire suppression associated with the rubber boom (1850–1910 CE), supporting the minimal influence of climate on regional burning at this time. During the past century, rBC influx has rapidly increased. Our results can serve to validate rBC modeling results, aiding with future predictions of rBC emissions and associated impacts to the climate system. 

The assassination of Julius Caesar in 44 BCE triggered a power struggle that ultimately ended the Roman Republic and, eventually, the Ptolemaic Kingdom, leading to the rise of the Roman Empire. Climate proxies and written documents indicate that this struggle occurred during a period of unusually inclement weather, famine, and disease in the Mediterranean region; historians have previously speculated that a large volcanic eruption of unknown origin was the most likely cause. Here we show using well-dated volcanic fallout records in six Arctic ice cores that one of the largest volcanic eruptions of the past 2,500 y occurred in early 43 BCE, with distinct geochemistry of tephra deposited during the event identifying the Okmok volcano in Alaska as the source. Climate proxy records show that 43 and 42 BCE were among the coldest years of recent millennia in the Northern Hemisphere at the start of one of the coldest decades. Earth system modeling suggests that radiative forcing from this massive, high-latitude eruption led to pronounced changes in hydroclimate, including seasonal temperatures in specific Mediterranean regions as much as 7 °C below normal during the 2 y period following the eruption and unusually wet conditions. While it is difficult to establish direct causal linkages to thinly documented historical events, the wet and very cold conditions from this massive eruption on the opposite side of Earth probably resulted in crop failures, famine, and disease, exacerbating social unrest and contributing to political realignments throughout the Mediterranean region at this critical juncture of Western civilization.