SINGAPORE As Chinese policymakers grapple with an expected increase in extreme weather due to global warming, a study has found that periods of cooling between AD 10 to 1900 also caused a wave of disasters, war and upheaval.
Droughts and locust plagues caused by cooler spells probably triggered internal wars, the authors said.
In a modern day parallel, China, the world's top emitter of greenhouse gases blamed for heating up the planet, has taken steps to curb emissions growth fearing growing social unrest from environmental degradation.
Zhibin Zhang of the Chinese Academy of Sciences and his team used historical records and paleoclimatic reconstructions covering nearly 2,000 years.
They found that the frequency of wars, droughts and floods, price of rice, locust plagues and temperatures in China were positively associated within time bands of around 160 and 320 years.
The study was published in the latest issue of the journal Proceedings of the Royal Society.
"Our study suggests that the food production during the last two millennia has been more unstable during cooler periods," the authors said.
This resulted in more social conflict owing to rebellions within dynasties and/or aggression from northern pastoral nomadic societies in ancient China, they said.
The collapses of the agricultural dynasties of the Han (206 BC-AD 220), Tang (681-906), Song (960-1279) and Ming (1368-1643) were more closely associated with low temperature, they said.
"It is very probable that cool temperature may be the driving force in causing high frequencies of meteorological, agricultural disasters and then man-made disasters (wars) in ancient China," they said.
In particular, the results suggested that periodic low temperatures could have increased the frequency of internal wars mainly indirectly through increasing drought and locust plague frequencies between AD 950 and the 1900s.
They said external aggression wars mostly occurred between Chinese dynasties and the pastoral nomadic societies to their north, such as the Manchus who overthrew the Ming dynasty.
A cooling of a few degrees Celsius can shorten the northern growing season of grass by 40 days, adversely affecting grasslands and causing huge losses of domestic livestock. This pushed northern tribes south.
The authors found two predominant periodic bands of around 160 and 320 years during past two millennia.
"These periods may be related to cyclic variations of solar activity, or cyclic changes of orbit position of the Earth," they said, pointing to 87 and 210-year cycles of solar activity based on observations of sunspots.
"It is generally believed that global warming is a threat to human societies in many ways. However, some countries or regions might also benefit from increasing temperatures in some ways," the authors said.
The global climate has natural variations in temperature and rainfall but scientists fear the rapid accumulation of greenhouse gases since the Industrial Revolution could lead to catastrophic climate change unless emissions are sharply reduced.
"However, the present on-going global warming may produce different effect on our industrialised societies which own much higher capacity of dealing with natural disasters than pre-industrial societies," they say in a pointer to China's rapid rise to become the world's number 3 economy.