Three alternate China‐wide temperature composites covering the last 2000 years were established by combining multiple paleoclimate proxy records obtained from ice cores, tree rings, lake sediments and historical documents. Five periods of temperature variation can be identified: a warm stage in AD 0–240, a cold interval between AD 240 and 800, a return to warm conditions from AD 800–1400, including the Medieval Warm Period between AD 800–1100, the cool Little Ice Age period between 1400–1920, and the present warm stage since 1920. Regional temperature variation is found during AD 800–1100, when warm conditions occurred in Eastern China and in the northeastern Tibetan Plateau and in AD 1150–1380, when the southern Tibetan Plateau experienced a warm interval. In contrast, evidence for cool conditions during the LIA is more consistent among the proxy records. The temperature reconstructions for China and the Northern Hemisphere show good agreement over the past millennium.

An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region.

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)

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.