. A new temperature reconstruction with decadal resolution, covering the last two millennia, is presented for the extratropical Northern Hemisphere (90–30°N), utilizing many palaeo‐temperature proxy records never previously included in any large‐scale temperature reconstruction. The amplitude of the reconstructed temperature variability on centennial time‐scales exceeds 0.6°C. This reconstruction is the first to show a distinct Roman Warm Period c. ad 1–300, reaching up to the 1961–1990 mean temperature level, followed by the Dark Age Cold Period c. ad 300–800. The Medieval Warm Period is seen c. ad 800–1300 and the Little Ice Age is clearly visible c. ad 1300–1900, followed by a rapid temperature increase in the twentieth century. The highest average temperatures in the reconstruction are encountered in the mid to late tenth century and the lowest in the late seventeenth century. Decadal mean temperatures seem to have reached or exceeded the 1961–1990 mean temperature level during substantial parts of the Roman Warm Period and the Medieval Warm Period. The temperature of the last two decades, however, is possibly higher than during any previous time in the past two millennia, although this is only seen in the instrumental temperature data and not in the multi‐proxy reconstruction itself. Our temperature reconstruction agrees well with the reconstructions by Moberg et al. (2005) and Mann et al. (2008) with regard to the amplitude of the variability as well as the timing of warm and cold periods, except for the period c. ad 300–800, despite significant differences in both data coverage and methodology.

Atlantic Multidecadal Variability impacts the climate around the North Atlantic and in many other parts of the world. There are ongoing discussions on the extent to which Atlantic Multidecadal Variability is driven by external (e.g., solar, volcanic, and/or aerosol) forcing versus internal variability. Here, we provide new evidence for persistent multidecadal variability during AD 800-2010. We produce a summer Atlantic Multidecadal Variability reconstruction using a network of annually-resolved terrestrial proxy records from the circum-North Atlantic region. We find that both large volcanic eruptions and solar irradiance minima induce cool phases of Atlantic Multidecadal Variability and that both forcings together explain approximately 30% of the reconstruction variance (on timescales > 30 years). We define the Atlantic Multidecadal Oscillation as the internally-generated component of Atlantic Multidecadal Variability, and calculate it by empirically removing externally-forced variations. The Atlantic Multidecadal Oscillation reveals persistent multidecadal variability throughout the past twelve centuries, making the largest contribution to Atlantic Multidecadal Variability, and also shows coherence with Northern Hemisphere temperature variations. This attempt to quantify the internally-generated and externally-forced components of Atlantic Multidecadal Variability over more than a millennium supports further understanding of its past behavior and its role in potential decadal-scale climate predictability. B.Y. and J.W. are supported by the National Science Foundation of China (NSFC)

Past global climate changes had strong regional expression. To elucidate their spatio-temporal pattern, we reconstructed past temperatures for seven continental-scale regions during the past one to two millennia. The most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the nineteenth century. At multi-decadal to centennial scales, temperature variability shows distinctly different regional patterns, with more similarity within each hemisphere than between them. There were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age, but all reconstructions show generally cold conditions between AD 1580 and 1880, punctuated in some regions by warm decades during the eighteenth century. The transition to these colder conditions occurred earlier in the Arctic, Europe and Asia than in North America or the Southern Hemisphere regions. Recent warming reversed the long-term cooling; during the period AD 1971-2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years

Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.