再探戰國到兩漢的氣候變遷
陳良佐
一年生或二年生作物，完成其全生育期需要恆定數量的積溫。當氣溫低，達不到作物需要的積溫，該作物生育期延長，成熟期延後。中國古文獻曾留下有關麥和粟的播種期以及收穫期。根據這類資枓可以確定歷史上某個時代的氣候。文獻中記錄下來的農事以及物候資料也是本文重要的依據。
兩漢書曾留下了一些低溫以及「陰陽失序」的記錄。所謂「陰陽失序」絕大多數指氣候異常。這兩種資料與上述積溫、農事以及物候所顯示的氣候狀況完全一致。
關于戰國到兩漢的氣候變化，本文研究到的結論如下：戰國到文景時代的氣候是溫暖期，《管子》與《孟子》時代的氣溫比戰國末期高。戰國末期到文景時代的氣候，基本上與今日相同，是屬於溫暖期。武帝時期是氣候溫暖期轉入小冰期的過渡期。昭宣時代的氣候似乎比較穩定。到了元帝時期正式進入小冰期。王莽時代低溫和旱災達到高峰。東漢初期的氣候是西漢小冰期的延續，即使有所改善，也極為有限。和帝以後，氣候的波動比之東漢初期大。東漢中晚期的年平均溫低於西漢，麥的成熟期晚於成帝時代。桓靈時代氣候惡劣的程度不下於王莽時期。
中國古代氣候變化的趨勢，從戰國到兩漢是降溫的走向，到了東漢末達到最高峰。這期間當然有多次反覆，但實際情形則難以考察。
關鍵詞：戰國 兩漢 氣侯變遷 小冰期
Climatic Change from the Warring States Period through the Eastern Han: A Re-investigation
Cheng Liang-tso
Institute of History, National Tsing-hua University
This article investigates climatic changes in China from the warring states period through the Eastern Han.Both single crops and double crops require a stable cumulative temperature to complete their full growth cycle. Temperatures that remain far below those required will prolong the growth period of a crop and delay its ripening period. Sources from China’s distant past contain records of the sowing and harvest periods of wheat and millet. Such materials allow the determination of climatic conditions during different historical periods. Records of farming activities and materials concerning growth periods also provide important evidence for the present argument.
Both the History of the Han and the History of the Eastern Han contain records of low temperatures and of “disruption of yin and yang” In the majority of cases, the phrase “disruption of yin and yang” refers to climatic irregularities. These two types of records are fully consistent with the climatic conditions shown by the materials about cumulative temperatures, farming activities, and growth period.
The present article reached the following conclusions concerning climatic changes through the Western Han (206 BCE-8CE). The era stretching from the Warring States period (403-222 BCE) to the reigns of Wen-ti (180-157 BCE) and Ching-ti (157-141 BCE) was a warm period. Temperatures during times of the Kuan-tzu and the Mencius were higher than those during the late Warring States period. The temperatures during the third and the early half of the second century BCE were, on the whole, similar to those of today, and they belong to a warm period. The reign of Wu-ti (141-87 BCE) saw the transition from this warm period into a small ice age. Temperatures under Chao-ti (87-74 BCE) and Hsuan-ti (73-49 BCE) appear to have been relatively stable, but under yuan-ti (49-33 BCE) a little ice age set in. Low temperatures and droughts reached their climax during Wang Mang’s reign (9-23 CE). This little ice age continued in the early years of the Eastern Han with very limited improvement, if any. After Ho-ti (88-105 CE), climatic fluctuations increased. Average temperatures during the middle and late Eastern Han were lower than those under the Western Han, and the ripening period of wheat was longer than under Ch’ent-ti (33-7 BCE). The process of climatic deterioration during Huan-ti (146-167 BE) and Ling-ti (1680-189 CE) was comparable to that under Wang mang.
The climatic trend in ancient China was a downward one, from the Warring States through the Han Eastern Han dynasty, reaching its lowest point during the last years of the Eastern Han. Needless to say, a great number of reversals occurred during this period, but these are difficult to investigate.