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Model and proxy-based estimates of climate cooling from volcanic eruptions have disagreed. Refined simulations and tree-ring time series converge on a total of 0.8 to 1.3 °C of cooling in the Northern Hemisphere from the 1257 and 1815 eruptions. Explosive volcanism can alter global climate, and hence trigger economic, political and demographic change 1 , 2 . The climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree-ring-based hemispheric temperature reconstructions 3 , 4 , 5 , 6 . However, volcanic surface cooling derived from climate model simulations is systematically much stronger than the cooling seen in tree-ring-based proxies, suggesting that the proxies underestimate cooling 7 , 8 ; and/or the modelled forcing is unrealistically high 9 . Here, we present summer temperature reconstructions for the Northern Hemisphere from tree-ring width and maximum latewood density over the past 1,500 years. We also simulate the climate effects of two large eruptions, in AD 1257 and 1815, using a climate model that accounts explicitly for self-limiting aerosol microphysical processes 3 , 10 . Our tree-ring reconstructions show greater cooling than reconstructions with lower spatial coverage and based on tree-ring width alone, whereas our simulations show less cooling than previous simulations relying on poorly constrained eruption seasons and excluding nonlinear aerosol microphysics. Our tree-ring reconstructions and climate simulations are in agreement, with a mean Northern Hemisphere extra-tropical summer cooling over land of 0.8 to 1.3 °C for these eruptions. This reconciliation of proxy and model evidence paves the way to improved assessment of the role of both past and future volcanism in climate forcing.

The Arctic is warming faster than any other region on Earth. Putting this rapid warming into perspective is challenging because instrumental records are often short or incomplete in polar regions and precisely-dated temperature proxies with high temporal resolution are largely lacking. Here, we provide this long-term perspective by reconstructing past summer temperature variability at Yamal Peninsula – a hotspot of recent warming – over the past 7638 years using annually resolved tree-ring records. We demonstrate that the recent anthropogenic warming interrupted a multi-millennial cooling trend. We find the industrial-era warming to be unprecedented in rate and to have elevated the summer temperature to levels above those reconstructed for the past seven millennia (in both 30-year mean and the frequency of extreme summers). This is undoubtedly of concern for the natural and human systems that are being impacted by climatic changes that lie outside the envelope of natural climatic variations for this region. A 7,638 yr summer temperature reconstruction based on subfossil trees buried in the Siberian Arctic shows that recent warming is unprecedented and interrupted a multi-millennial cooling trend.

Debris-flow activity in the Alps is anticipated to undergo pronounced changes in response to a warming climate. Yet, a fundamental challenge in comprehensively assessing changes in process activity is the systematic lack of long-term observational debris-flow records. Here, we reconstruct the longest, continuous time series (1626-2020) of debris flows at Multetta, a supply-limited torrential system in the Eastern Swiss Alps. Relying on growth-ring records of trees that were damaged by debris flows, we do not detect significant changes in the frequency or magnitude over time. This seeming absence of a direct climatic influence on debris-flow initiation aligns with the regular distribution of repose time patterns, indicating a dependence of local process activity on sediment discharge and recharge. This stark difference in process behavior between our supply-limited site and transport-limited catchments has implications for assessing torrential hazard and risk mitigation in a context of global warming. Reconstruction of debris flows in a supply-limited system shows that process activity is controlled by sediment supply over multi-decadal to centennial timescales. Debris flows recur less frequently here and are, unlike transport-limited systems, not affected by climate change.

Polar ice core records attest to a colossal volcanic eruption that took place ca. A.D. 1257 or 1258, most probably in the tropics. Estimates based on sulfate deposition in these records suggest that it yielded the largest volcanic sulfur release to the stratosphere of the past 7,000 y. Tree rings, medieval chronicles, and computational models corroborate the expected worldwide atmospheric and climatic effects of this eruption. However, until now there has been no convincing candidate for the mid-13th century “mystery eruption.” Drawing upon compelling evidence from stratigraphic and geomorphic data, physical volcanology, radiocarbon dating, tephra geochemistry, and chronicles, we argue the source of this long-sought eruption is the Samalas volcano, adjacent to Mount Rinjani on Lombok Island, Indonesia. At least 40 km ³ (dense-rock equivalent) of tephra were deposited and the eruption column reached an altitude of up to 43 km. Three principal pumice fallout deposits mantle the region and thick pyroclastic flow deposits are found at the coast, 25 km from source. With an estimated magnitude of 7, this event ranks among the largest Holocene explosive eruptions. Radiocarbon dates on charcoal are consistent with a mid-13th century eruption. In addition, glass geochemistry of the associated pumice deposits matches that of shards found in both Arctic and Antarctic ice cores, providing compelling evidence to link the prominent A.D. 1258/1259 ice core sulfate spike to Samalas. We further constrain the timing of the mystery eruption based on tephra dispersal and historical records, suggesting it occurred between May and October A.D. 1257.

Explosive volcanism is a key contributor to climate variability on interannual to centennial timescales 1 . Understanding the far-field societal impacts of eruption-forced climatic changes requires firm event chronologies and reliable estimates of both the burden and altitude (that is, tropospheric versus stratospheric) of volcanic sulfate aerosol 2 , 3 . However, despite progress in ice-core dating, uncertainties remain in these key factors 4 . This particularly hinders investigation of the role of large, temporally clustered eruptions during the High Medieval Period (HMP, 1100–1300  ce ), which have been implicated in the transition from the warm Medieval Climate Anomaly to the Little Ice Age 5 . Here we shed new light on explosive volcanism during the HMP, drawing on analysis of contemporary reports of total lunar eclipses, from which we derive a time series of stratospheric turbidity. By combining this new record with aerosol model simulations and tree-ring-based climate proxies, we refine the estimated dates of five notable eruptions and associate each with stratospheric aerosol veils. Five further eruptions, including one responsible for high sulfur deposition over Greenland circa 1182  ce , affected only the troposphere and had muted climatic consequences. Our findings offer support for further investigation of the decadal-scale to centennial-scale climate response to volcanic eruptions. Analysis of contemporary reports of total lunar eclipses, combined with aerosol model simulations and tree-ring-based climate proxies, allowed greater precision in dating of the occurrence of stratospheric volcanic eruptions during the High Medieval Period.

The eruption of Samalas in Indonesia in 1257 ranks among the largest sulfur-rich eruptions of the Common Era with sulfur deposition in ice cores reaching twice the volume of the Tambora eruption in 1815. Sedimentological analyses of deposits confirm the exceptional size of the event, which had both an eruption magnitude and a volcanic explosivity index of 7. During the Samalas eruption, more than 40 km 3 of dense magma was expelled and the eruption column is estimated to have reached altitudes of 43 km. However, the climatic response to the Samalas event is debated since climate model simulations generally predict a stronger and more prolonged surface air cooling of Northern Hemisphere summers than inferred from tree-ring-based temperature reconstructions. Here, we draw on historical archives, ice-core data and tree-ring records to reconstruct the spatial and temporal climate response to the Samalas eruption. We find that 1258 and 1259 experienced some of the coldest Northern Hemisphere summers of the past millennium. However, cooling across the Northern Hemisphere was spatially heterogeneous. Western Europe, Siberia and Japan experienced strong cooling, coinciding with warmer-than-average conditions over Alaska and northern Canada. We suggest that in North America, volcanic radiative forcing was modulated by a positive phase of the El Niño–Southern Oscillation. Contemporary records attest to severe famines in England and Japan, but these began prior to the eruption. We conclude that the Samalas eruption aggravated existing crises, but did not trigger the famines. The climatic response to the 1257 Samalas eruption is unclear. Analyses of proxy data and medieval archives suggest that the eruption triggered some of the coldest summers of the past millennium, but only in some Northern Hemisphere regions.

The trade winds cause strong upwelling in the eastern equatorial Pacific, and create the eastern Pacific Cold Tongue (EPCT) that has far‐reaching impacts on global climate. However, large discrepancies persist in quantifying 20th‐century EPCT sea surface temperature (SST) changes across different instrumental data sets. Here we synthesize four coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a Cold Tongue Index (CTI) reconstruction for 1887–1997. The coral CTI record shows a rapid 20th century warming of the EPCT, suggesting an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in reconstructed EPCT SST show an association with the Walker Circulation. Our reconstruction indicates that recent EPCT cooling during the global warming hiatus is not unusual in the context of the 20th century. Our results provide new evidence for 20th century EPCT SST changes and an observational constraint for predicting future tropical climate changes. Plain Language Summary The tropical Pacific exhibits an asymmetric pattern of sea surface temperatures (SST) along the equator, with a warm pool in the west and a cold tongue in the east. The Eastern Pacific Cold Tongue (EPCT) plays a crucial role in influencing global climate. To date, however, considerable disagreement persists about changes and causes of the EPCT SST over the 20th century due to the sparse instrumental observations before ∼1960. Here we synthesize published coral Sr/Ca‐SST records from the tropical central‐eastern Pacific to develop a coral Cold Tongue Index (CTIcoral) for the period 1887–1997. The CTIcoral record correlates significantly with instrumental data since 1960, and thus allows reliable assessment of 20th century EPCT SST changes. Our record exhibits a rapid 20th century warming trend of the EPCT, indicating an underestimation of warming trends in instrumental CTI records. The decadal to multidecadal changes in CTIcoral also show an association with the Walker Circulation. Comparing the magnitude of the 1992–2011 trend in instrumental CTI with our reconstruction reveals that recent EPCT cooling during the global warming hiatus is not anomalous in the context of 20th century. Our findings have implications for predicting future EPCT SST changes by offering a constraint for model simulations. Key Points Synthesizing coral Sr/Ca‐SST records yields robust estimates of 20th century warming trend in the eastern Pacific Cold Tongue (EPCT) A coral‐based reconstruction indicates an underestimation of 20th century EPCT warming trends in instrumental SST records Recent EPCT cooling during the global warming hiatus is not anomalous in the context of the 20th century

The Eldgjá lava flood is considered Iceland’s largest volcanic eruption of the Common Era. While it is well established that it occurred after the Settlement of Iceland (circa 874 CE), the date of this great event has remained uncertain. This has hampered investigation of the eruption’s impacts, if any, on climate and society. Here, we use high-temporal resolution glaciochemical records from Greenland to show that the eruption began in spring 939 CE and continued, at least episodically, until at least autumn 940 CE. Contemporary chronicles identify the spread of a remarkable haze in 939 CE, and tree ring-based reconstructions reveal pronounced northern hemisphere summer cooling in 940 CE, consistent with the eruption’s high yield of sulphur to the atmosphere. Consecutive severe winters and privations may also be associated with climatic effects of the volcanic aerosol veil. Iceland’s formal conversion to Christianity dates to 999/1000 CE, within two generations or so of the Eldgjá eruption. The end of the pagan pantheon is foretold in Iceland’s renowned medieval poem, Vǫluspá (‘the prophecy of the seeress’). Several lines of the poem describe dramatic eruptive activity and attendant meteorological effects in an allusion to the fiery terminus of the pagan gods. We suggest that they draw on first-hand experiences of the Eldgjá eruption and that this retrospection of harrowing volcanic events in the poem was intentional, with the purpose of stimulating Iceland’s Christianisation over the latter half of the tenth century.

The mid-17th century is characterized by a cluster of explosive volcanic eruptions in the 1630s and 1640s, climatic conditions culminating in the Maunder Minimum, and political instability and famine in regions of western and northern Europe as well as China and Japan. This contribution investigates the sources of the eruptions of the 1630s and 1640s and their possible impact on contemporary climate using ice core, tree-ring, and historical evidence but will also look into the socio-political context in which they occurred and the human responses they may have triggered. Three distinct sulfur peaks are found in the Greenland ice core record in 1637, 1641–1642, and 1646. In Antarctica, only one unambiguous sulfate spike is recorded, peaking in 1642. The resulting bipolar sulfur peak in 1641–1642 can likely be ascribed to the eruption of Mount Parker (6∘ N, Philippines) on 26 December 1640, but sulfate emitted from Komaga-take (42∘ N, Japan) volcano on 31 July 1641 has potentially also contributed to the sulfate concentrations observed in Greenland at this time. The smaller peaks in 1637 and 1646 can be potentially attributed to the eruptions of Hekla (63∘ N, Iceland) and Shiveluch (56∘ N, Russia), respectively. To date, however, none of the candidate volcanoes for the mid-17th century sulfate peaks have been confirmed with tephra preserved in ice cores. Tree-ring and written sources point to cold conditions in the late 1630s and early 1640s in various parts of Europe and to poor harvests. Yet the early 17th century was also characterized by widespread warfare across Europe – and in particular the Thirty Years' War (1618–1648) – rendering any attribution of socio-economic crisis to volcanism challenging. In China and Japan, historical sources point to extreme droughts and famines starting in 1638 (China) and 1640 (Japan), thereby preceding the eruptions of Komaga-take (31 July 1640) and Mount Parker (4 January 1641). The case of the eruption cluster between 1637 and 1646 and the climatic and societal conditions recorded in its aftermath thus offer a textbook example of difficulties in (i) unambiguously distinguishing volcanically induced cooling, wetting, or drying from natural climate variability and (ii) attributing political instability, harvest failure, and famines solely to volcanic climatic impacts. This example shows that while the impacts of past volcanism must always be studied within the contemporary socio-economic contexts, it is also time to move past reductive framings and sometimes reactionary oppositional stances in which climate (and environment more broadly) either is or is not deemed an important contributor to major historical events.

Recently revised ice core chronologies for Greenland have newly identified one of the largest sulfate deposition signals of the last millennium as occurring between 1108 and 1113 CE. Long considered the product of the 1104 CE Hekla (Iceland) eruption, this event can now be associated with substantial deposition seen in Antarctica under a similarly revised chronology. This newly recognized bipolar deposition episode has consequently been deemed to reveal a previously unknown major tropical eruption in 1108 CE. Here we show that a unique medieval observation of a “dark” total lunar eclipse attests to a dust veil over Europe in May 1110 CE, corroborating the revised ice-core chronologies. Furthermore, careful evaluation of ice core records points to the occurrence of several closely spaced volcanic eruptions between 1108 and 1110 CE. The sources of these eruptions remain unknown, but we propose that Mt. Asama, whose largest Holocene eruption occurred in August 1108 CE and is credibly documented by a contemporary Japanese observer, is a plausible contributor to the elevated sulfate in Greenland. Dendroclimatology and historical documentation both attest, moreover, to severe climatic anomalies following the proposed eruptions, likely providing the environmental preconditions for subsistence crises experienced in Western Europe between 1109 and 1111 CE.