بررسی تغییرات اقلیم بر اساس مدارک تاریخی چین

Chinese historical documents and climate change

Bibliography

As late as the nineteenth century, it was commonly believed that climate represented a kind of averaged weather over time at a geographical location and hardly changed, at least for the period comparable to the length of human history. It has been demonstrated that climate changes at millennial, centennial, and even decadal scales, as many studies in the twentieth century have revealed. Such studies were largely done by examining environmental data such as tree rings, pollen assemblages, lake sediment, and ice cores. These data are objective and usually continuous, but they are often difficult to interpret. For example, a narrow tree ring could mean either a dry spell or cold spring, or both. Consequently, conclusions obtained this way are often associated with considerable uncertainties or ambiguities. Furthermore, environmental data usually have low time resolution and hence are not suitable for high-resolution analysis.

Historical climate records

Many historical documents contain records of direct human observations of climatic conditions (cold, warm, wet, and dry) that can be used directly for studying climatic changes. In addition, human activities are influenced by climate, and by carefully scrutinizing the descriptions in historical documents detailing these activities, it is possible to decipher what the climate must have been. These documents, if available, can serve as a very useful source of data for climate studies.

The main advantage of human-recorded climate descriptions, as opposed to those inferred from environmental data, is that the climatic conditions are presumably direct observations, which are easier to interpret, at least qualitatively. For example, it is hard to imagine that an extremely cold winter would be recorded as warm. Of course, missing or discontinuous records may be a serious problem as documents could become lost or destroyed due to dislocation, war, and other causes (such as earthquake and floods). To be useful for climatic studies, it is necessary to have long and continuous records.

China is one of the few countries that has kept a huge amount of historical documents, thanks to its long and continuous history and antiquarian culture. There were governmental offices that were responsible for watching unusual environmental phenomena and recording them in daily logs, and many of these phenomena related to weather and climate. Furthermore, China's main economical activity has been agriculture, which is greatly influenced by climate. As a result, a large amount of agriculture-related records of floods, droughts, famine, harvest, price of grain, damages to fruit trees, and so on have been kept in archives that can be used as a data source for climate studies. Then there are phenological records, such as the freezing and thawing dates of lakes and rivers, blossoming dates of certain flowers, and arrival dates of certain species of migrating birds, which are indicators of climatic conditions. In addition, there are some unusual weather phenomena (for example, thunderstorms in winter), which were considered as bad omens out of superstition and were recorded systematically in official histories that can also serve as sources of climate data. Indeed, all these records have been used successfully for reconstructing past climate conditions in China.

Given that there are so many different kinds of data, one would expect to see many discrepancies among conclusions derived from different data series. Instead, there is a surprising degree of consistency among them, at least in the broad sense. One typical example is shown in the illustration. Here two different series are plotted: a winter temperature series and a winter thunderstorm frequency series. It is obvious that the two series have good correlation, despite the fact that they are of completely different nature.

Winter temperature (dashed curve) and winter thunderstorm frequency (solid curve) of China in historical...

Winter temperature (dashed curve) and winter thunderstorm frequency (solid curve) of China in historical time. The winter temperature series was derived from phenological records (K. C. Chu, 1972), and the winter thunderstorm series was compiled from the Twenty-Four Official Histories of China (P. K. Wang, 1980). The winter temperature series is plotted as the deviation from the modern (1900 to 1960s) mean temperature (the peaks represent cold deviation). Winter thunderstorm frequency represents the number of winter thunderstorm reports in a 30-year interval.

Temperature series

In the following, we will focus on discussing the temperature series only. This series, deduced mostly from phenological phenomena as described in historical documents, shows immediately that there had been obvious warm and cold periods in China in the historical time. Several notable broad periods are discussed, for which the ranges are approximate.

A.D. 200–600

This was a cold period (as compared to the current situation) roughly corresponding to the Three-Kingdoms, Wei, Jin and North-South Dynasties in Chinese history. This was a time of relative turmoil when many kingdoms coexisted. Wars within each kingdom and between different kingdoms were frequent. Previous to this period during the Han dynasty, bamboo, a semitropical plant, was still spread widely in northern China. During this colder period, bamboo became less populated, and it is virtually extinct in nature in northern China today. Similarly, the once widespread tangerine orchards became nearly extinct because of the colder climate. One report at the beginning of the third century says that the tangerine trees merely flowered but failed to bear fruit in Henan Province. Cold spells in winter occurred more frequently. In 225, a military exercise at Guangling (33.5°N, 119°E), a town near Huai River, was halted due to a sudden freeze of the river. During the later part of this period, called the South-North Dynasties, China was divided into north and south empires, separated by the Yangtze River. An imperial “icehouse” was established in Nanjing (32N°, 119°E) by the southern empires to store ice obtained during the winter for the imperial family's use. This was the first time in history that the imperial icehouse was built at such a southern latitude. This feat would be impossible today because the winter would not be cold enough to obtain thick chunks of ice there.

600–1000

A relatively warm climate characterized this period and, coincidentally or perhaps consequentially, China enjoyed a relatively stable political climate as well. The Tang Dynasty, often regarded as the golden age of Chinese culture, began at 618 and ended in 907. In 650, 669, and 678, it was reported that there was no snow or ice in winter in the national capital Changan (today's Xian, 34.2°N, 109°E). Orange and tangerine trees were again widely planted in Changan, and “the fruits tasted as good as those tributes from southern China.” Another indication of the warm climate was the wide existence of plum trees at the time, as indicated in many poems.

Near the end of this period, however, the warm climate turned into a colder period. The once stable Tang Empire fragmented into several kingdoms and entered the Five Dynasties and Ten Countries period. This again was a fairly chaotic time, and wars between kingdoms ensued. As a rule in Chinese history, it appears that the cold period usually coincided with more frequent wars between kingdoms. (This coincidence is not explained here.)

After 1000 and until at least the beginning of the twentieth century, it appears that the climate of China fluctuated more rapidly than in previous periods. Cold and warm periods seemed to alternate in intervals of roughly 100–200 years, but in general the mean winter temperature appears to be lower than that in the 600–1000 period and moderately lower than that in the twentieth century. At present it is unclear whether this rapid fluctuation is real or due to artifacts in the data.

1000–1200

This generally cold period culminated around 1150. An initially affluent Song Empire (960–1279) was gradually weakened by the rising nomad rivals in the north—the Liao, Jin, and eventually the Yuan (Mongolian) empires. The Song Empire shrank to become the Southern Song Empire in 1127, occupying only lands to the south of the Yangtze River. Kublai Khan defeated Jin and then the Southern Song Empire in 1279. The Lake Tai [Tai Hu, area 2250 km2 (870 mi2), located at 31.2°N] was reported to freeze for the first time in history in 1111, and the cold spell wiped out all citrus trees in this area. Snowfall records indicated that the average last snow dates in 1131–1260 was the ninth day of the fourth month (roughly corresponding to May in the Gregorian calendar), nearly a full month later than that in 1100–1110, indicating a colder trend. There were also many reports of damage to other fruit plants (notably, Nephelium litchi, commonly known as lichee) in parts of China even farther south because of the cold climate in the twelfth century.

1200–1300

This was a relatively warm period. An indication was the reinstating of government offices overseeing the bamboo production in Shanxi and Henan provinces under the Mongolian rule. These offices were disbanded previously in the twelfth century by Song rulers because the cold climate greatly reduced production.

1300–1400

In this cold period, according to the Mongolian poet Nai-Xian (1309–1352), icing in the Yellow River occurred earlier by as much as a full month as compared to the present. The poet also lamented that swallows “were merely short visitors” due to the colder climate.

1400–1900

The climate became somewhat warmer near 1400, although still colder than the present. In general, this was a colder period but seemed to approach the present warm condition near 1900. Ironically, the records in this period become too numerous to easily decipher the climatic conditions without ambiguities. Many records are probably noise rather than signals, which will take much research effort to sort out. But certain severe climate-caused disasters stand out as clear signals. One such example was the adverse climatic conditions (severe winters and droughts) near the end of Ming Dynasty (1368–1644), which were often attributed to the collapse of the Ming Empire and succession by the Qing (Manchurian) Dynasty (1644–1911).

See also: Asia; Bamboo; Climate history; Climate modification; Climatic prediction; Global climate change; Lychee; Plant geography

Pao K. Wang

Bibliography

* K. C. Chu, A preliminary study on the climatic fluctuations during the last 5000 years in China, Scientia Sinica , 16:226–256, 1973