青海杂多圆柏年轮指示的近700年旱涝变化

根据采自青海南部高原杂多地区树木年轮样本, 建立了该地717年树木年轮年表序列。依据响应函数、 相关与偏相关方法, 分析了该年表与树木生长时温度和湿度的关系\.结果表明： 该年表对杂多5～6月的气温、 蒸发、 降水量、 相对湿度和干燥度指数反映敏感, 由其重建了高原春末夏初的干燥度指数序列, 并应用交叉检验方法对校准方程进行了检验, 证明重建方程稳定, 所重建的旱涝变化比较可靠, 具有一定的区域代表性。通过对重建序列的进一步分析得出, 重建序列在一定程度上反映了青海南部高原春末夏初干燥度指数大小的历史变化。1360年以来在年代际尺度上, 持续时间较长的干旱时段有4个, 即1441-1450年、 1691-1700年、 1741-1750年和1871-1880年; 持续时间较长的湿润时段有5个, 即1361-1370年、 1561-1570年、 1761-1770年、 1831-1850年和1891-1910年。     

Bristlecone pine tree rings and volcanic eruptions over the last 5000 yr

Many years of low growth identified in a western USA regional chronology of upper forest border bristlecone pine (Pinus longaeva and Pinus aristata) over the last 5000 yr coincide with known large explosive volcanic eruptions and/or ice core signals of past eruptions. Over the last millennium the agreement between the tree-ring data and volcano/ice-core data is high: years of ring-width minima can be matched with known volcanic eruptions or ice-core volcanic signals in 86% of cases. In previous millennia, while there is substantial concurrence, the agreement decreases with increasing antiquity. Many of the bristlecone pine ring-width minima occurred at the same time as ring-width minima in high latitude trees from northwestern Siberia and/or northern Finland over the past 4000-5000 yr, suggesting climatically-effective events of at least hemispheric scale. In contrast with the ice-core records, the agreement between widely separated tree-ring records does not decrease with increasing antiquity. These data suggest specific intervals when the climate system was or was not particularly sensitive enough to volcanic forcing to affect the trees, and they augment the ice core record in a number of ways: by providing confirmation from an alternative proxy record for volcanic signals, by suggesting alternative dates for eruptions, and by adding to the list of years when volcanic events of global significance were likely, including the mid-2nd-millennium BC eruption of Thera.     

Climatic significance of D/H and13C/12C ratios in Irish oak cellulose

δD and δ¹³C analyses of cellulose nitrate from two modern Irish oak trees that form part of the 7400 year long chronology were carried out, covering a period of 123 years (1861–1983 A.D.) with a 5 year resolution so as to assess the potential of this long chronology for retrieval of palaeoenvironmental data. One of the trees (Q5293) showed significant correlations of δD, δ¹³ C and ring width with mean annual temperatures as recorded at the Armagh weather station nearby and the mean fall temperatures of Central England. The other tree (Q5296) did not exhibit any significant climatic correlations either because it grew utilizing a nearby permanent source of ground water or because the intra-ring isotopic variations in Irish oak are significant enough to mask the climatic signal. Whilst our results have given a positive indication of the usefulness of these trees for palaeoenvironmental information, more trees need to be analysed to confirm our findings. Even though one of the trees did not exhibit climatic correlations, both trees show a significant positive correlation of δ¹³C and a negative correlation of δD with ring width variations. Furthermore, two tree samples that grew during the 1620s B.C., when a volcano is thought to have erupted on the Aegean island of Santorini, show increased δD and decreased δ¹³C for one to two decades following the eruption, though the magnitudes of change seem to vary with site and trees. We have proposed a possible mechanism based on tree phenology to explain both the above effects.     

A bimillennial-length tree-ring reconstruction of precipitation for the Tavaputs Plateau, Northeastern Utah

Despite the extensive network of moisture-sensitive tree-ring chronologies in western North America, relatively few are long enough to document climatic variability before and during the Medieval Climate Anomaly (MCA) ca. AD 800-1300. We developed a 2300-yr tree-ring chronology extending to 323 BC utilizing live and remnant Douglas-fir (Pseudotsuga menziesii) from the Tavaputs Plateau in northeastern Utah. A resulting regression model accounts for 70% of the variance of precipitation for the AD 1918-2005 calibration period. Extreme wet and dry periods without modern analogues were identified in the reconstruction. The MCA is marked by several prolonged droughts, especially prominent in the mid AD 1100s and late 1200s, and a lack of wet or dry single-year extremes. The frequency of extended droughts is not markedly different, however, than before or after the MCA. A drought in the early AD 500s surpasses in magnitude any other drought during the last 1800 yr. A set of four long high-resolution records suggests this drought decreased in severity toward the south in the western United States. The spatial pattern is consistent with the western dipole of moisture anomaly driven by El Niño and is also similar to the spatial footprint of the AD 1930s “Dust Bowl” drought.     

Does the El Niño-Southern Oscillation control the interhemispheric radiocarbon offset?

Since the 1970s it has been recognised that Southern Hemisphere samples have a lower radiocarbon content than contemporaneous material in the Northern Hemisphere. This interhemispheric radiocarbon offset has traditionally been considered to be the result of a greater surface area in the southern ocean and high-latitude deepwater formation. This is despite the fact that the El Niño-Southern Oscillation (ENSO) is known to play a significant role in controlling the interannual variability of atmospheric carbon dioxide by changing the flux of ‘old’ CO₂ from the tropical Pacific. Here we demonstrate that over the past millennium, the Southern Hemisphere radiocarbon offset is characterised by a pervasive 80-yr cycle with a step shift in mean values coinciding with the transition from the Medieval Warm Period to the Little Ice Age. The observed changes suggest an ENSO-like role in influencing the interhemispheric radiocarbon difference, most probably modulated by the Interdecadal Pacific Oscillation, and supports a tropical role in forcing centennial-scale global climate change.