THE idea that climate change will lead to war is often raised by environmental pessimists, and a meeting on the climatic past of South-East Asia, held last month in Dalat, Vietnam, suggests it is not such an unlikely thought. The meeting was organised by the Lamont-Doherty Earth Observatory, part of Columbia University, some of whose researchers have been trying to reconstruct the pattern of South-East Asia's monsoons over the past few centuries. One matter they raised was the possibility that two periods of conflict in the area, in the 15th and 18th centuries, were provoked by droughts.

Historical records of the climate in Asia are lamentable outside India, where the weather-obsessed British collected good data during the 19th and 20th centuries. The observatory's researchers had therefore to rely on tree rings.

This is hard in South-East Asia. Many of the larger, and therefore older specimens in the area's forests have been logged. Even among those that remain, seasonal differences in the rings' growth rates are less noticeable than those that mark summer and winter in temperate climes. Ironically, the diversity of species in tropical forests also presents dendroclimatologists with problems. They would prefer just one or two types of tree, so that they could compare several samples of each more easily.

Nevertheless, Brendan Buckley, one of the observatory's researchers, found that a conifer called Fokienia hodginsii, which can live for more than 1,000 years, gave him the marker he needed. Using it, he has built up a series of tree-ring chronologies from Thailand and Vietnam that indicate a period of severe drought across mainland South-East Asia in the early 1400s.

This was the period when the city of Angkor in present-day Cambodia went into rapid decline, a fact that some historians have blamed on invasions by the rival Siamese and Champa kingdoms. Dr Buckley's data, however, suggest another possibility—that Angkor's canals and reservoirs ran dry and that the invasions were therefore a consequence of decline, rather than its cause. Similarly, another prolonged drought in the 18th century, which was noted by foreign visitors to Siam (modern-day Thailand), coincided with a series of political upheavals that included the sacking of the Siamese capital by Burmese invaders.

The question is what causes such droughts in South-East Asia? El Niño, a periodic warming of the eastern Pacific Ocean, is part of the answer. It corresponds with a weaker south-west monsoon and a longer dry season. It cannot, however, be the whole explanation. Dr Buckley's tree cores show that the 18th-century drought in Siam lasted 30 years or more. That should have corresponded with a warmer Pacific but according to Kevin Anchukaitis, another of the observatory's researchers, data from coral suggest it did not.

The answer may lie in the Indian Ocean, which also influences monsoon patterns, rather than in the Pacific, and there is some evidence that the Indian Ocean was indeed cooler during the 18th-century drought. The details, though, remain obscure—which serves as a reminder of just how much remains to be found out before even the local climate truly becomes explicable.