By analysing tree-ring data from more than 300 sites in Asia, researchers could 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, the models are poor enough that they don’t even agree on whether global climate change will strengthen the Asian monsoon or weaken it, said tree-ring expert Edward Cook, who is director of the Tree-Ring Laboratory at Columbia University .
“That gives you an idea of just how difficult the problem is,” Nature quoted him as saying.
The problem is that the good weather records that are necessary for validating climate models don’t exist for much of Asia before about 1950, said Cook.
Filling this gap, is one of the reasons his team compiled the drought atlas, he said.
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, explained 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.
This reveals how distant ocean conditions might affect Asian weather – again, useful for refining climate models, said Eugene Wahl, a palaeoclimatologist.
Wahl noted that Cook’s data give climate modellers a wealth of new information.
“It gives you something to start with, and that”s really important,” he said.
And by extending climate records back in time, the Asian tree-ring data, like similar studies in North America, have revealed past droughts that were much longer and more severe than anything these regions have had to deal with in modern times.
This “opens the possibility of understanding what nature can throw at us. That”s a big deal,” said Wahl.
The first analysis of the monsoon atlas has been published in Science1.