Historical rainfall across the region documented for the first time.
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.
This, says Eugene Wahl, a palaeoclimatologist at the US National Oceanic and Atmospheric Administration's National Climatic Data Center in Boulder, Colorado, reveals how distant ocean conditions might affect Asian weather — again, useful for refining climate models.
But whereas climate models need to be able to duplicate historic weather patterns, there's no guarantee that future climate patterns will mimic past ones, warns Ken Caldeira, a climate scientist at the Carnegie Institution for Science's Department of Global Ecology in Stanford, California. The monsoons are caused, he says, by hot air rising inland over the continents, pulling in moisture from the sea. Future changes in that process will be driven not just by traditional patterns of sea-surface-temperature fluctuations, but by changes in sunlight-reducing haze from air pollution from industrialized areas of China, for example, and differences in the degree to which global warming affects sea and land. "The drivers today are different," Caldeira warns.
Wahl agrees, but notes that Cook's data still give climate modellers a wealth of new information. "It gives you something to start with, and that's really important," he says. 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, Wahl says, "opens the possibility of understanding what nature can throw at us". "That's a big deal," he adds.
The monsoon study is also valuable to historians. In a paper published last month in the Proceedings of the National Academy of Sciences, tree-ring specialist Brendan Buckley, one of Cook's co-authors who is also at the Lamont-Doherty Earth Observatory, used a portion of the same data to show that the demise of Cambodia's Angkor civilization coincided with a previously unrecognized drought from the mid-fourteenth century2.
Historians had speculated that there was an environmental factor involved in the demise of that civilization, he says, "but there's almost no written documentation. Now we've shown that the climate at that time was significantly drier than it's been since."
Additional work published in the study by Cook and his colleagues compared well-documented droughts with their monsoon data to validate the accuracy of their models including one in 1756–68. This was an era of political upheaval and social change in regions as disparate as southeast Asia and Siberia.
Victor Lieberman is a historian at the University of Michigan in Ann Arbor who has written about "strange parallels" in these seemingly unrelated upheavals, speculating that environmental changes might have played a part. Some of his predictions are confirmed and others are contradicted by the monsoon study, Lieberman says. "It's a critically useful study."
Cook, E. R. et al. Science 328, 486-489 (2010).
Buckley, B. N. et al. Proc. Natl Acad. Sci. USA 107, 6748-6752 (2010).