Extreme climates and current analogues : from the last millennia to the Holocene optimum (3)

Abstract

China's climate is dominated by the monsoon phenomenon, characterized by a marked seasonality of wind and rain. In summer, humid air masses from the Indian Ocean and eastern Pacific generate intense rainfall in the south and east of the country. Since its beginnings some 6 000 years ago, Chinese agriculture has followed the geographical distribution of temperature and rainfall. In particular, the monsoon was instrumental in the invention and development of irrigated rice-growing, necessary to feed a large population.

The history of China begins around 1500 BC with the advent of writing, making it possible to trace the succession of kingdoms and dynasties that shared Chinese and Tibetan territory. Dynastic transitions correspond to warlike conflicts and social vicissitudes, mentioned in numerous Chinese texts. Recent palaeographic and philological studies have made it possible to calculate indices of prosperity, taking account in particular of the abundance of harvests, as well as indices of social unrest marked by revolts and wars and punctuated by dynastic changes.

These historical indicators can be compared with records of a tracer of summer monsoon intensity (^18O/16Oratio measured in the stalagmites of many Chinese caves). These paleoclimatic series, well dated by the U-Th method, show fluctuations over the last millennia, with episodes corresponding to the Little Ice Age, the Medieval Optimum, the Little Ice Age of Late Antiquity and the Roman Climatic Optimum. Abrupt drops in monsoon intensity also correspond to dynastic transitions and warlike conflicts. The ^18O/16Ovariations in Chinese stalagmites have been confirmed by other paleoclimatic series based on ice cores from Tibetan glaciers and lake sediments dating back to over 2000 BC (Huguang Maar).

Recent research has made it possible to study the relationship with climatic variability on the periphery of China. For the last millennium, there are clear correspondences with ^18O/16Ostalagmite series from eastern India and dendroclimatic series from the Indochinese peninsula. Episodes of drought are thought to have been responsible for the collapse of the " hydraulic " city of Angkor in the Khmer kingdom (modern-day Cambodia). On the other hand, a wetter climate in the desert areas of Central Asia during the Little Ice Age would have favoured the rise of the Mongol Empire and led to the invasion of China, which in turn led to the Yuan dynasty.

To identify the causes of these climate changes, Chinese climate series were compared with variations in external forcings (Sun and volcanoes), as well as those observed for the El Nino/La Nina oscillation originating in the Pacific. The inclusion and explicit simulation of these phenomena by climate models has confirmed the importance of ENSO, volcanic and solar forcings in explaining drought episodes and precipitation gradients on the Asian continent.