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The Impact of Climate Change on the Southeast Asian Monsoon

Final Report Summary - ASIAN MONSOON (The Impact of Climate Change on the Southeast Asian Monsoon)

This project investigated the variability of the monsoon in Southeast Asia based on stable isotopes measured in stalagmites from Lao PDR. The population in Southeast Asia is one of the most vulnerable to climate change, yet little is known about the natural climate variability and the magnitude of past climate changes in this area. During two field seasons we investigated more than 20 caves in northern Laos and collected stalagmites from Tham Thia Cave in Houaphan province and Chom Ong cave in Oudomxai province. Uranium-series ages of these stalagmites reveals that the majority of growth occurred during the last 10 000 years (the Holocene epoch), but some stalagmites also grew during other intervals, such as at the peak of the last glaciation (~20 000 years ago) or during marine isotope stages 3 and 4. Since the Holocene is the most relevant period to society, the main focus was on developing a high-resolution record of climate change in SE Asia during this interval. We used a stalagmite from Tham Thia Cave, which grew during the last 700 years. The oxygen and carbon isotopes from this stalagmite suggest that its formation occurred in isotopic equilibrium with the dripwaters and it is therefore likely that this sample records the climate variability outside the cave.

Although we sampled at sub-annual resolution, the magnitude of the signal is not large enough to capture intra-annual climate variability. However, comparison of the oxygen isotopes in northern Laos with the Pacific decadal oscillations (PDO) and the Southern oscillation index (SOI) indicates that the highest values in the speleothem record coincide with a period of sustained positive PDO overlaping with two strong ENSO events. In addition, the Lao speleothem record shows similarities with the monsoon variability captured in Chinese speleothems, suggesting that the climate in Indochina is coupled on inter-annual timescales to large-scale atmospheric circulation in the Pacific Ocean. We are refining the chronology in order to attempt to develop a quantitative reconstruction of past precipitation based on comparisons with other stalagmites situated along the same moisture transport pathway.

The insights gained from this study will have socio-economical implications by improving our understanding of factors controlling monsoon variability in this area, and therefore our ability to understand future climate changes in this area. Furthermore, by providing a record of monsoon variability that overlaps with, and extends beyond the instrumental records will allow an assessment of the full range of natural climate variability in Indochina.