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Friday, 25th June 2010

China's population and climate change

There was a news article some time ago saying that Chinese civilization flourished when the climate was warm and plummeted when the climate was cooler. Specifically, there were three extended periods of warm weather roughly coinciding with, respectively, the Shang (1600 - 1046 BC), Eastern Zhou (770 - 256 BC) and Tang period (618 - 907 CE).

Figure 1     Temperature trend in the history of China

Figure 1      Temperature trend in the history of China


The above plot was based on the work of the famous Chinese meteorologist, Zhu Kezhen (1890 - 1974). There was no instrument record for most of the long history of China. In fact, measurement did not start until mid -19th century. However, official Chinese accounts dating back to over 3000 years contain detailed records such as the first frost or snow, river freezing and melting, budding and flowering of plants and trees, as well as arrival and departure of migratory birds in the year. This allows an assessment of the climate and its variations over time.

The above evidence suggests that compared with today, it was a lot warmer and more humid at different times in history. For instance, the make-up of some Chinese characters (such as clothing, utensils, books, furniture and musical instruments) used in the early Zhou period enabled scholars to deduce that bamboos were widespread in the Yellow River basin at the time. There was also an account of people using bamboo crates filled with stones to combat floods when the Yellow River overflew its banks in 110 BC. The Records of the Grand Historian, written during 109 - 91 BC, mentioned bamboos as an economic staple in the northwest province of Shaanxi. Nowadays bamboos are no longer seen in these places.

The Records also described the production of such sub-tropical plants as mandarin orange in Sichuan and mulberry in Shandong, both places being quite north for these species nowadays. During the Shang-Zhou period, the arrival time of swallows was used by people in Shandong to determine the Spring Equinox. By the 1970s, it was not until late March before they arrived at the lower reaches of the Yangtze. There were also accounts of plums being widely used in food during the one thousand years BC, as vinegar was still unknown for most part of this period. An early poem cited growth of the fruit near present-day Xian, which no longer exists these days.

The warmer and more humid climate back then in general favoured agriculture and population growth, as people concentrated mostly between and around the Yellow River and Yangtze River. The diagram below depicts estimations of the Chinese population over the past 2000 years. Several national censuses or household surveys had been conducted in history, the earliest occurring in the Shang period (16th to 11th century BC). The earliest existing records of national household survey dated back to 2 BC in the Han period. Except for those after the 1950s, all the numbers depicted were at best rough figures and probably underestimates, as the emphasis then was on people who had to pay tax.

Figure 2     China's population (estimated) over the past 2000 years

Figure 2      China's population (estimated) over the past 2000 years


From the above plot, it can be seen that even though it is generally on the rise, China's population can change quite wildly. It was not unusual to see population being reduced by half. This often happened during periods of warfare, famine and diseases. Some of the time warfare preceded famine and diseases, but vice-versa at some other times when there might be uprisings because of rapidly deteriorating livelihood arising from drought or extensive floods.

Figure 3     Map of China showing the area occupied by the states during the Warring States Period (475 - 221 BC)
Figure 3      Map of China showing the area occupied by the
states during the Warring States Period (475 - 221 BC)


The past two thousand years or so also saw gradual spreading of the population to the south, from the two rivers' region towards south China. One obvious reason is the incessant invasion of horsemen from the north (hence the construction of the Great Wall since the Qin period and its repair and reinforcement at different times in subsequent dynasties). Another reason is that it has become generally cooler over the past 1000 years (Figure 1), which favoured southward migration as people sought more habitable climate. Nowadays, the population in the south China province of Guangdong alone already tops 100 million, a figure that exceeds China's total population at any time before 1500 AD. Like other parts of the world, the population increase would not have been possible without advances in such societal aspects as administration, technology and medical care.

Climate change especially in the past 30 years adds further complications. Apart from increased drought and floods, heat waves, severe storms and continuous rise in the sea level, it is necessary to point out the accelerating retreat of glaciers in recent years (see the blog of 24 February 2010), including those in the Himalayas. It has been said that the Himalayan glaciers supply freshwater to at least 50 million people in China, south Asia and southeast Asia. The impact of the Himalayan glacier retreat on water resources has already raised concerns in a number of countries in Asia.

Another complication is the general temperature increase in many places. While this may bring short-term benefits (e.g. more crops in a year), too much of a rise may not be good thing. Temperature rise, coupled with reduced rainfall or water supply in some areas, would result in loss of soil moisture in the absence of irrigation. This exacerbates the demand for water if the aim is to maintain the present level of agriculture.

Viewed from the population angle, therefore, there are pros and cons to a warmer climate in China, with probably more disadvantages than advantages. Active planning for adaptation and mitigation of the effects of climate change may be necessary.


B.Y. Lee


References
1. 'Preliminary studies on climate changes in China in the past 5000 years', Zu Kezhen, extracted from People's Daily, 19 June 1973 (in Chinese).
2. 'How do the 1.3 billion Chinese come about', by Ge Jianxiong, Historical Geography, published by City University of Hong Kong, 2002 (in Chinese).


Last revision date: <17 Jan 2013>