In the bestiary of powerful climate regimes — El Ninos, La Ninas and other oscillations of one kind and another — the Asian Monsoon is in a class by itself. Its rainfall feeds half the world's population. Its failure can bring down dynasties and, in a stroke, remove several million people from the rolls of living humanity.
Beginning with the British mathematician Sir Gilbert Walker in India in the 19th century, scientists have devoted great time and energy trying to understand what makes the summer rains come and go — or not. Like a lot of things about the climate, however, the problem is more than a little complicated, and the observed record of monsoon behavior is short.
But now the trail is longer, and better illuminated. In the current issue of the journal Science, tree-ring specialists from Columbia University's Lamont-Doherty Earth Observatory in Palisades, NY, after 15 years of field work, unveil a Monsoon Asia Drought Atlas that analyzes data from 300 locations and extends the record back 700 years. (In this photo, researchers Edward Cook, left, and Paul Krusic are taking cores from a 1,000-year-old hemlock in Nepal. The team trekked for nearly two weeks to reach this tree.)
The research captures details of four "megadroughts" provoked by Asian Monsoon failures that accompanied major historic events.
In 1644, China's Ming dynasty collapsed after what the study reveals to be an especially sharp three-year monsoon failure that accompanied peasant rebellions in northeastern China near Beijing. In 1756-1768, severe monsoon failures coincided with the collapse of kingdoms in areas that are now Vietnam, Myanmar and Thailand. In 1790-1796 the so-called East India drought led to famines in India and crop failures and political turmoil in Mexico and Europe. Worst of all, the Great Drought of 1876-78 brought famine and cholera outbreaks across India, China, and Indonesia, killing up to 30 million people.
Scientists have come to realize that arrival of the summer monsoon depends on the interplay of several shifting climate features, including sea surface temperatures in the western Pacific and Indian oceans, the strength of westerly winds and the extent of soil moisture and snow cover in Siberia.
With the greater level of detail contained in the new study, researchers hope computer modelers will be able to improve their ability to predict changes in monsoon precipitation.
IMAGE: Brendan Buckley, Lamont-Doherty Earth Observatory