Societal upheaval occurred across Eurasia in the sixth and seventh centuries. Tree-ring reconstructions suggest a period of pronounced cooling during this time associated with several volcanic eruptions. Climatic changes during the first half of the Common Era have been suggested to play a role in societal reorganizations in Europe1,2 and Asia3,4. In particular, the sixth century coincides with rising and falling civilizations1,2,3,4,5,6, pandemics7,8, human migration and political turmoil8,9,10,11,12,13. Our understanding of the magnitude and spatial extent as well as the possible causes and concurrences of climate change during this period is, however, still limited. Here we use tree-ring chronologies from the Russian Altai and European Alps to reconstruct summer temperatures over the past two millennia. We find an unprecedented, long-lasting and spatially synchronized cooling following a cluster of large volcanic eruptions in 536, 540 and 547 AD (ref. 14), which was probably sustained by ocean and sea-ice feedbacks15,16, as well as a solar minimum17. We thus identify the interval from 536 to about 660 AD as the Late Antique Little Ice Age. Spanning most of the Northern Hemisphere, we suggest that this cold phase be considered as an additional environmental factor contributing to the establishment of the Justinian plague7,8, transformation of the eastern Roman Empire and collapse of the Sasanian Empire1,2,5, movements out of the Asian steppe and Arabian Peninsula8,11,12, spread of Slavic-speaking peoples9,10 and political upheavals in China13.

/pdf/cooling-and-societal-change-during-the-late-antique-little-13i6w1txn2.pdf

Cooling and societal change during the Late Antique Little Ice Age from 536 to around 660 AD

East Asia has experienced strong warming since the 1960s accompanied by an increased frequency of heat waves and shrinking glaciers over the Tibetan Plateau and the Tien Shan. Here, we place the recent warmth in a long-term perspective by presenting a new spatially resolved warm-season (May-September) temperature reconstruction for the period 1–2000 CE using 59 multiproxy records from a wide range of East Asian regions. Our Bayesian Hierarchical Model (BHM) based reconstructions generally agree with earlier shorter regional temperature reconstructions but are more stable due to additional temperature sensitive proxies. We find a rather warm period during the first two centuries CE, followed by a multi-century long cooling period and again a warm interval covering the 900–1200 CE period (Medieval Climate Anomaly, MCA). The interval from 1450 to 1850 CE (Little Ice Age, LIA) was characterized by cooler conditions and the last 150 years are characterized by a continuous warming until recent times. Our results also suggest that the 1990s were likely the warmest decade in at least 1200 years. The comparison between an ensemble of climate model simulations and our summer reconstructions since 850 CE shows good agreement and an important role of internal variability and external forcing on multi-decadal time-scales.

/pdf/east-asian-warm-season-temperature-variations-over-the-past-26nsu1r644.pdf

East Asian warm season temperature variations over the past two millennia.

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

/pdf/european-summer-temperatures-since-roman-times-3vonb9dkli.pdf

European summer temperatures since Roman times

Earth system impacts of the European arrival and Great Dying in the Americas after 1492

Explosive volcanism can alter global climate, and hence trigger economic, political and demographic change1, 2. The climatic impact of the largest volcanic events has been assessed in numerous modelling studies and tree-ring-based hemispheric temperature reconstructions3, 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 cooling7, 8; and/or the modelled forcing is unrealistically high9. 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 processes3, 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.

Estimates of volcanic-induced cooling in the Northern Hemisphere over the past 1,500 years

Volcanic eruptions contribute to climate variability, but quantifying these contributions has been limited by inconsistencies in the timing of atmospheric volcanic aerosol loading determined from ice cores and subsequent cooling from climate proxies such as tree rings. Here we resolve these inconsistencies and show that large eruptions in the tropics and high latitudes were primary drivers of interannual-to-decadal temperature variability in the Northern Hemisphere during the past 2,500 years. Our results are based on new records of atmospheric aerosol loading developed from high-resolution, multi-parameter measurements from an array of Greenland and Antarctic ice cores as well as distinctive age markers to constrain chronologies. Overall, cooling was proportional to the magnitude of volcanic forcing and persisted for up to ten years after some of the largest eruptive episodes. Our revised timescale more firmly implicates volcanic eruptions as catalysts in the major sixth-century pandemics, famines, and socioeconomic disruptions in Eurasia and Mesoamerica while allowing multi-millennium quantification of climate response to volcanic forcing.

/pdf/timing-and-climate-forcing-of-volcanic-eruptions-for-the-sw6kak6482.pdf

Timing and climate forcing of volcanic eruptions for the past 2,500 years

This books provides a detailed analysis of the social structure of the First Crusade (1096 – 1099) along with an original assessment of the sociology of its contemporary or near contemporary sources.

The social structure of the First Crusade

The years from the Easter Rising in 1916 to the ending of the Civil War in 1923 were years full of drama and of fast-changing events, whose outcome shaped Irish society for generations. The conventional narrative of this period is one that focuses on the efforts of the nationalist politicians of various hues and the military activities of the IRA in their struggle to escape the British Empire. But these were also years of extraordinary popular initiatives, directed against both the British administration of the Ireland and indigenous employers. Over a hundred 'soviets' were proclaimed in the period. This book studies the militancy of working people throughout urban and rural Ireland during these revolutionary years. It examines how this popular militancy impacted on the struggle for independence: how it shaped the policy of the British authorities; how leaders of the national movement, such as de Valera and Collins, were affected by the popular struggle; and how the ebbs and flows of the popular movement were closely intertwined with the issue of partition. This book provides a great deal of detail on working-class militancy between 1917 and 1923. It stresses the large scale and considerable impact of mass action and labor activism, challenging traditional interpretations that focus almost exclusively on the role of armed groups. There is a full account of the strikes, factory occupations and land seizures that shook Irish society. It details popular involvement in the struggles of those years, paying particular attention to the socialist and trade union movement. This revised edition consists of a comprehensive rewrite that updates the work in the light of recent publications and the release of new archive materials. In particular, this new edition includes new eyewitness accounts from the archives of the Bureau of Military History including the testimony of notable trade union activists, an extension of the account of the mutiny of the Connaught Rangers (28 June 1920), and extra information on Michael Collins' intelligence system.

Conor Kostick

Papers

Timing and climate forcing of volcanic eruptions for the past 2,500 years

The social structure of the First Crusade

Revolution in Ireland: Popular Militancy, 1917 to 1923

Reconciling multiple ice-core volcanic histories: The potential of tree-ring and documentary evidence, 670–730 CE

The Crusades and the Near East : Cultural Histories