Tree-ring data from more than 300 sites in Asia have allowed scientists to piece together a year-by-year history of the region's monsoon rains as far back as 1300 AD.
The new database, called the Monsoon Asia Drought Atlas, is important because the summer monsoon, which affects half of the world's population, is little understood by climate modellers. In fact, says tree-ring expert Edward Cook, the models are poor enough that they don't even agree on whether global climate change will strengthen the Asian monsoon or weaken it. "That gives you an idea of just how difficult the problem is," he says.
The problem, says Cook — who is director of the Tree-Ring Laboratory at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, and lead author of the monsoon study — is that the good weather records that are necessary for validating climate models don't exist for much of Asia before about 1950. Filling this gap, he says, is one of the reasons his team compiled the drought atlas, the first analysis of which is published in Science1.
The data were compiled from tree-ring chronologies showing the year-to-year growth of ancient trees at 327 sites. Although these sites are, by necessity, clustered in regions where there are old trees, the rest of the map can be filled in by statistical analyses, explains Cook. These analyses used tree-ring data from recent years, comparing them to existing weather data to find correlations with the older data and so extrapolate to the regions for which no such records were available.
In addition to mapping annual rainfall across thousands of kilometres of Asia, encompassing the Indian, east Asian and Australian monsoon areas, the team also correlated rainfall patterns with nearly 150 years of sea-surface-temperature recordings throughout the Pacific and Atlantic oceans.