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This work provides evidence of the influence of large volcanic eruptions on Sahel rainfall relying on PMIP4/past1000 multi‐model simulations, covering the last millennium. A classification of volcanic eruptions in the last millennium according to the meridional symmetry of the associated radiative forcing reveals different mechanisms of the West African Monsoon response at inter‐annual timescale. In all cases, these simulated changes result in Sahel drying up to 2 years after an eruption. Besides, we add evidence of a role of varying volcanic activity across the past millennium in the Sahel precipitation variability at multi‐decadal to secular timescales. Plain Language Summary Relying on climate simulations of the past millennium, this work shows that the largest volcanic eruptions documented induce different mechanisms in the West African Monsoon response depending on whether the eruption occurs at extra‐tropical or tropical latitudes. In both cases, such volcanic impacts can induce droughts the two following rainy seasons in the West African Sahel region. Moreover, we show first evidence of an influence of the frequency of volcanic eruptions on the Sahel precipitation regime over decades to centuries across the past millennium. Key Points Climate model simulations of the past millennium show Sahel drought in response to large volcanic eruptions up to the following 2 years The mechanisms leading to the Sahel drying are different if it responds to extra‐tropical or tropical eruptions The increasing frequency of eruptions throughout the past millennium modulates Sahel precipitation variability on multi‐decadal timescales

Asian precipitation changes over multiple time scales have been extensively studied, yet the relative roles of external forcing and internal variability in shaping the large‐scale Asian precipitation pattern over the past millennium remain underexplored. Here, we demonstrate that the tripolar pattern of decadal precipitation variability across South Asia, southeastern Asia, and northern East Asia in the past millennium is primarily driven by the Interdecadal Pacific Oscillation (IPO) but is modulated and synchronized by external volcanic forcing. Volcanic aerosol forcing stimulates IPO‐like sea surface temperature (SST) anomalies that influence Asian precipitation through mechanisms similar to those from the internal IPO. Nonetheless, volcanic forcing effect is distinguishable from the IPO due to subtle differences in the resulting large‐scale SST and atmospheric circulation anomalies. These anomalies, induced by interhemispherically asymmetric external forcing, differ from the IPO's more symmetric patterns and provide a pathway to differentiate the internally generated and the externally forced climate variations.

Volcanic forcing, a major natural source of climate variability, represents a challenge for current climate modeling because of the unpredictability and specificity of individual eruptions, and because of the complexity of processes linking the eruption to the climate response. Volcanic forcing is largely underrepresented in available future climate projections, which is a critical problem. The study by Man Mei Chim and Colleagues (Chim et al., 2023, https://doi.org/10.1029/2023GL103743) tackles this known unknown and reveals how climatically relevant volcanic activity may be stronger than currently thought in a future warmer climate, enhancing uncertainty of climate projections. The study exemplifies the profound implications of inaccuracies within simplified climate scenarios and motivates new research on volcanically forced climate variability. It also arouses some thoughts on climate uncertainty communication. Plain Language Summary Volcanic eruptions are a critical part of the earth system, capable of causing large climatic fluctuations over periods from years to decades. Available scenario experiments typically neglect volcanic forcing, which may cause systematic errors in future climate projections. Man Mei Chim and Colleagues (Chim et al., 2023, https://doi.org/10.1029/2023GL103743) use a series of statistical and mathematical models to comprehensively simulate how volcanic eruptions may affect climate during the 21st century. The results indicate that a more realistic representation of volcanic eruptions yields slightly less future warming and larger uncertainty than standard projections. In other words, we are less confident of future climate changes once we account for the (unpredictable) volcanic forcing. By considering future volcano‐climate interactions in their whole complexity and revealing a substantial role for small‐to‐moderate eruptions, the study places a new important piece in the volcano‐climate puzzle. It will motivate new coordinated research to improve the simulated representation of volcano‐climate interactions not only in the ambit of future climate scenarios but in paleoclimatic investigations and idealized volcanic experiments well. The research also reminds us of limitations inherent in climate model experiments and should foster improved communication of natural climate variability and associated uncertainties. Key Points The study by Man Mei Chim and Colleagues shows that volcanic forcing can strongly affect future climate projections The study reaffirms the importance of natural variability for future climates, stimulating research on volcano‐climate interactions Volcanic forcing remains unpredictable, which rises the question of how to communicate irreducible climate projection uncertainties

Climate simulations of the industrial era typically start in 1850, using the first fifty years as a baseline for ‘pre-industrial’ climate. However, the period immediately prior to 1850 is of particular interest due to early human influence and heightened volcanic activity, the latter of which led to cooler global temperatures than those observed in the subsequent historical period. In this study, we present a suite of Earth system model simulations (using UKESM1.1) that start in 1750 and span the entire industrial period. We compare these simulations to a new instrumental observation-based dataset, GloSATref, which provides global surface air temperature variations from 1781 onwards. We investigate the climatic changes during the early industrial period, separating the effects of natural and anthropogenic forcings. Model-simulated early-19th-century temperature patterns show substantial cooling relative to the long-term mean, particularly in low latitudes, which agree well with observed patterns. We find significant long-term differences between simulations initialized in 1750 and 1850, with lasting effects well into the 20th century, consistent with differences in vegetation and the substantial ocean cooling driven by high volcanic activity in the 1750 simulations. Our results indicate that an earlier start to historical simulations could lead to more representative climate simulations over the historical period, and deepen our understanding of early anthropogenic warming, natural climate variability, and the climate responses to future volcanic eruptions.

Holocene climate variability is punctuated by episodic climatic events such as the Little Ice Age (LIA) predating the industrial-era warming. Their dating and forcing mechanisms have however remained controversial. Even more crucially, it is uncertain whether earlier events represent climatic regimes similar to the LIA. Here we produce and analyse a new 7500-year long palaeoclimate record tailored to detect LIA-like climatic regimes from northern European tree-ring data. In addition to the actual LIA, we identify LIA-like ca. 100–800 year periods with cold temperatures combined with clear sky conditions from 540 CE, 1670 BCE, 3240 BCE and 5450 BCE onwards, these LIA-like regimes covering 20% of the study period. Consistent with climate modelling, the LIA-like regimes originate from a coupled atmosphere–ocean–sea ice North Atlantic-Arctic system and were amplified by volcanic activity (multiple eruptions closely spaced in time), tree-ring evidence pointing to similarly enhanced LIA-like regimes starting after the eruptions recorded in 1627 BCE, 536/540 CE and 1809/1815 CE. Conversely, the ongoing decline in Arctic sea-ice extent is mirrored in our data which shows reversal of the LIA-like conditions since the late nineteenth century, our record also correlating highly with the instrumentally recorded Northern Hemisphere and global temperatures over the same period. Our results bridge the gaps between low- and high-resolution, precisely dated proxies and demonstrate the efficacy of slow and fast components of the climate system to generate LIA-like climate regimes.

The Pacific decadal oscillation (PDO) is the leading mode of decadal climate variability over the North Pacific. However, it remains unknown to what extent external forcings can influence the PDO’s periodicity and magnitude over the past 2000 years. We show that the paleo-assimilation products (LMR) and proxy data suggest a 20–40 year PDO occurred during both the Mediaeval Climate Anomaly (MCA, ~ 750–1150) and Little Ice Age (LIA, ~ 1250–1850) while a salient 50–70 year variance peak emerged during the LIA. These results are reproduced well by the CESM simulations in the all-forcing (AF) and single volcanic forcing (Vol) experiments. We show that the 20–40 year PDO is an intrinsic mode caused by internal variability but the 50–70 year PDO during the LIA is a forced mode primarily shaped by volcanic forcing. The intrinsic mode develops in tandem with tropical ENSO-like anomalies, while the forced mode develops from the western Pacific and unrelated to tropical sea surface temperature anomalies. The volcanism-induced land–sea thermal contrast may trigger anomalous northerlies over the western North Pacific (WNP), leading to reduced northward heat transport and the cooling in the Kuroshio–Oyashio Extension (KOE), generating the forced mode. A 50–70 year Atlantic multidecadal oscillation founded during the LIA under volcanic forcing may also contribute to the forced mode. These findings shed light on the interplay between the internal variability and external forcing and the present and future changes of the PDO.

Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

Decades of research have focused on establishing the exact year and climatic impact of the Minoan eruption of Thera, Greece (c.1680 to 1500 BCE). Ice cores offer key evidence to resolve this controversy, but attempts have been hampered by a lack of multivolcanic event synchronization between records. In this study, Antarctic and Greenland ice-core records are synchronized using a double bipolar sulfate marker, and calendar dates are assigned to each eruption revealed within the ‘Thera period’. From this global-scale sequence of volcanic sulfate loading, we derive indications toward each eruption’s latitude and potential to disrupt the climate system. Ultrafine sampling for sulfur isotopes and tephra conclusively demonstrate a colossal eruption of Alaska’s Aniakchak II as the source of stratospheric sulfate in the now precisely dated 1628 BCE ice layer. These findings end decades of speculation that Thera was responsible for the 1628 BCE event, and place Aniakchak II (52 ± 17 Tg S) and an unknown volcano at 1654 BCE (50 ± 13 Tg S) as two of the largest Northern Hemisphere sulfur injections in the last 4,000 years. This opens possibilities to explore widespread climatic impacts for contemporary societies and, in pinpointing Aniakchak II, confirms that stratospheric sulfate can be globally distributed from eruptions outside the tropics. Dating options for Thera are reduced to a series of precisely dated, constrained stratospheric sulfur injection events at 1611 BCE, 1561/1558/1555BCE, and c.1538 BCE, which are all below 14 ± 5 Tg S, indicating a climatic forcing potential for Thera well below that of Tambora (1815 CE).

The distributions of forest, ice and snow in the Hengduan Mountains of China have undergone significant changes due to ongoing climatic warming. To better understand the spatiotemporal pattern of temperature changes in the Hengduan Mountains, we used tree-ring cores collected from multiple individuals of Larix speciosa Cheng et Law at five sites to develop a regional chronology and to establish the relationship between tree-ring radial growth and warm-season (May–September) mean temperature. The regional chronology accounts for 46.1% of the observed variance in the warm season and was used to reconstruct regional temperature levels back to 1420. Four cool intervals (1490–1570, 1590–1660, 1700–1790, and 1800–1880) indicate that the Western Hengduan Mountains experienced the Little Ice Age, and the changes were synchronous with cooling on the Tibetan Plateau and in the Northern Hemisphere, demonstrating a well-defined Little Ice Age signal in the South Asian monsoon region. Air–sea interactions and solar activity affected the variability of the warm-season mean temperature variations on interannual or interdecadal scales. Our temperature reconstruction improves the understanding of multi-centennial climate change in the Western Hengduan Mountains and has implications for advancing high-resolution paleoclimate science in the region.

The impact of volcanic forcing on tropical precipitation is investigated in a new set of sensitivity experiments within the Max Planck Institute Grand Ensemble framework. Five ensembles are created, each containing 100 realizations for an idealized ‘Pinatubo-like’ equatorial volcanic eruption with emissions covering a range of 2.5-40 Tg sulfur (S). The ensembles provide an excellent database to disentangle the influence of volcanic forcing on monsoons and tropical hydroclimate over the wide spectrum of the climate’s internal variability. Monsoons are generally weaker for two years after volcanic eruption and their weakening is a function of emissions. However, only a stronger than Pinatubo-like eruption ( ⩾ 10 Tg S) leads to significant and substantial monsoon changes, and some regions (such as North and South Africa, South America and South Asia) are much more sensitive to this kind of forcing than the others. The decreased monsoon precipitation is strongly tied to the weakening of the regional tropical overturning. The reduced atmospheric net energy input and increased gross moist stability at the Hadley circulation updraft due to the equatorial volcanic eruption, require a slowdown of the circulation as a consequence of less moist static energy exported away from the intertropical convergence zone.