以棲蘭山區臺灣扁柏樹輪最大密度重建過往800年溫度

Abstract

氣候變遷為當前重要的研究議題，但相關研究受限於氣象觀測資料的時間長度，重建與瞭解古氣候變異為探討現今氣候變遷不可或缺的步驟。樹輪最大密度已廣泛應用於樹輪氣候學，並用以重建許多地區過往之溫度變化。 本研究以棲蘭山區的臺灣扁柏(Chamaecyparis obtusa var. formosana)的樹輪密度為材料，使用集成經驗模態分解法(Ensemble empirical mode decomposition, EEMD)建立樹輪平均密度(ring density, RD)、最大密度(maximum density, MXD)、最小密度(minimum density, MND)年表。之後將樹輪年表與美國NCER/NCAR再分析資料進行相關性分析，瞭解樹輪密度與氣候之關係，並藉此重建氣候。 結果顯示，RD、MXD、MND皆與溫度呈顯著正相關，與相對濕度則呈顯著負相關，其中MXD與前一年6月至當年10月的臺灣北半部高空溫度(氣壓高度850mb)之相關性最高，因此使用MXD重建1200–2006年的前一年6月至當年10月之溫度。重建溫度顯示4個明顯暖期：1290–1350, 1390–1420, 1515–1535, 1990–2006年，以及3個明顯冷期：1460–1480, 1680–1720, 1810–1850年。 根據場域相關性分析，本研究的重建溫度可代表東亞和西太平洋的大範圍溫度變化。將本研究的重建溫度與北半球、東亞、青藏高原樹輪年表重建溫度相比，同樣發現有相似的溫度變化。另外，或許是受太陽輻射量影響，棲蘭山區臺灣扁柏的樹輪密度與太陽活動有密切關聯。

Limited instrumental records have hindered the efforts to understand long-term climate variability better. Using reliable paleoclimate proxies to reconstruct long-term climates is crucial to evaluate whether the current climate change is outside the range of natural climate variability. Tree-ring maximum density (MXD) has been extensively applied in dendroclimatology to reconstruct past climate variations. The main objectives of this study were to understand the relationships between the variations of climate and the tree-ring density of Taiwan yellow false cypress (Chamaecyparis obtusa var. formosana) from Chilan Shan, northeastern Taiwan, and to reconstruct the region’s paleoclimate based on the relationships between tree-ring density and climate. Ensemble empirical mode decomposition was used to develop tree-ring density chronologies, including ring density, maximum density (MXD), and minimum density. Climate data was from NCEP/NCAR Reanalysis 1 data. The three tree-ring density variables were all positively correlated with temperature and negatively correlated with relative humidity. The correlation of MXD with previous June to current October monthly mean temperature at 850mb pressure level over northern Taiwan was the highest. Therefore, this study used MXD chronology to reconstruct previous June to current October monthly mean temperature from CE 1200 to 2006. The reconstruction revealed four warm periods, namely, 1290–1350, 1390–1420, 1515–1535, 1990–2006, and three cold periods, namely, 1460–1480, 1680–1720, 1810–1850. Spatial correlation analysis revealed that the temperature reconstruction could represent temperature variation in East Asia and West Pacific. The reconstructed temperature also had similar variations with East Asia and the North Hemisphere temperature reconstructions based on tree-ring width. Correspondences between the variability of solar activity and tree-ring density of Taiwan yellow false cypress were also found.