NOAA/WDS Paleoclimatology - D'Arrigo - Central Alaska Composite - PCGL - ITRDB AK111

Previous tree-ring-based Northern Hemisphere temperature reconstructions portray a varying amplitude range between the "Medieval Warm Period" (MWP), "Little Ice Age" (LIA) and present. We describe a new reconstruction, developed using largely different methodologies and additional new data compared to previous efforts. Unlike earlier studies, we quantify differences between more traditional (STD) and Regional Curve Standardization (RCS) methodologies, concluding that RCS is superior for retention of low-frequency trends. Continental North American versus Eurasian RCS series developed prior to merging to the hemispheric scale cohere surprisingly well, suggesting common forcing, although there are notable deviations (e.g., fifteenth to sixteenth century). Results indicate clear MWP (warm), LIA (cool), and recent (warm) episodes. Direct interpretation of the RCS reconstruction suggests that MWP temperatures were nearly 0.7C cooler than in the late twentieth century, with an amplitude difference of 1.14C from the coldest (1600-1609) to warmest (1937-1946) decades. However, we advise caution with this analysis. Although we conclude, as found elsewhere, that recent warming has been substantial relative to natural fluctuations of the past millennium, we also note that owing to the spatially heterogeneous nature of the MWP, and its different timing within different regions, present palaeoclimatic methodologies will likely "flatten out" estimates for this period relative to twentieth century warming, which expresses a more homogenous global "fingerprint." Therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences, at hemispheric scales, about MWP warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local/regional scale.

既往基于树轮的北半球温度重建结果表明，中世纪暖期 (Medieval Warm Period, MWP)、小冰期 (Little Ice Age, LIA) 与现代时段的温度振幅存在显著差异。本研究构建了一套全新的温度重建序列，与既往研究相比，其采用了大幅差异化的研究方法，并补充了新增观测数据。与早期研究不同，本研究量化了传统方法 (Standard, STD) 与区域曲线标准化 (Regional Curve Standardization, RCS) 两种重建路径的差异，并得出结论：区域曲线标准化方法在保留气候序列低频趋势方面表现更优。在合并为半球尺度序列之前，北美大陆与欧亚大陆的区域曲线标准化序列展现出意外高度的一致性，暗示二者受共同的气候强迫机制驱动，但二者也存在显著偏差（如15至16世纪时段）。研究结果清晰识别出中世纪暖期（暖期）、小冰期（冷期）以及现代（暖期）三个典型气候时段。对区域曲线标准化重建序列的直接解读显示，中世纪暖期的温度较20世纪末低约0.7℃，而最冷十年（1600-1609年）与最热十年（1937-1946年）之间的温度振幅差达1.14℃。但本研究提醒，对该分析结果应持谨慎态度。尽管本研究与其他同类研究结论一致，即相较于过去千年的自然气候波动，现代变暖幅度已十分显著，但我们也指出：由于中世纪暖期的空间分布具有非均质性，且不同区域的暖期出现时间存在差异，当前的古气候学 (paleoclimatic) 方法在对比中世纪暖期与20世纪变暖时，可能会“抹平”前者的温度估算结果——而20世纪变暖呈现出更为均一的全球气候“指纹”特征。因此，本研究强调：当前已有的古气候重建序列，不足以在半球尺度上推断中世纪暖期相较于现代人为变暖期的暖化程度，此类对比仅可在局地或区域尺度上开展。