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Monsoon linkages

An excellent sediment record from the Arabian Sea traces recent patterns in the activity of the Asian monsoon. It reveals both variability in monsoon strength and links with climatic events elsewhere.

The monsoon is the main determinant of environmental conditions over much of Asia, and so affects the most densely populated region on Earth. Differential heating of the north Indian Ocean and the northwest Pacific, and of the Asian land-mass, cause the seasonal reversal of monsoon winds. In summer, these winds blow northwards over the northern Indian Ocean, carrying huge amounts of moisture over the neighbouring land. The ensuing heavy rainfall can have devastating consequences for human life and livelihood. Conversely, agriculture in Asia depends on monsoon rains; and the seasonal upwelling of nutrient-laden subsurface waters, driven by monsoon winds, is essential to the success of coastal fisheries.

Understanding monsoon history and past dynamics is necessary for improving our knowledge of the monsoon system and how it may respond to changing global conditions. On page 354 of this issue1, Gupta et al. take us a step further down that road. They present a fine-scale palaeoceanographic record, from a marine sediment core from the Arabian Sea, that traces the operation of the Asian monsoon 11,000 years back in time — that is, over the Holocene, the epoch that spans the interval from the end of the last glacial period until the present day.

The record is derived from fluctuations in the abundance of a planktonic organism, a foraminifer, that is known to thrive particularly well in waters that provide an ample supply of food. The link with monsoon intensity is through coastal upwelling, induced by monsoon winds, that stimulates marine biological productivity off Oman, including the growth of this foraminiferal species. Gupta and colleagues' record is particularly valuable because the core comes from a depositional environment in which sediments have accumulated swiftly, so limiting the extent to which burrowing organisms have mixed the sediment column. This record, then, is a very precise one, and the variations in foraminiferal abundance indicate that the Holocene was punctuated by a quasi-periodic recurrence of increased monsoon activity on timescales of 1,000 years.

This picture is supported by oxygen isotope analyses2 — a part indicator of water temperature — of the same species in a sediment core from the Somali continental margin to the south. The isotope record documents rapid changes in the summer temperature of surface water during the early Holocene, 11,000–6,500 years ago. In turn, both records fit in nicely with isotope analysis3 of a stalagmite, from Hoti Cave in northern Oman, that likewise indicates much variability in monsoon intensity over that interval.

The work of Gupta et al.1 also takes the record of monsoon variability further into the mid- and late Holocene. They detect continued swings in monsoon intensity, including a notable shift from strong to weak activity during the transition from the Medieval Warm Period (ad 800–1300) to the Little Ice Age (ad 1300–1870). This is confirmed by faunal and organic molecular data from the region4,5: monsoon variation, including an increase in strength during the past century or so, is evidently a robust feature of its climatic history.

To add to the excitement, Gupta and colleagues' record shows some resemblance to that seen in the distribution of haematite in Holocene sediments from the North Atlantic. Haematite is believed to be a key indicator of sand debris that was frozen into icebergs and carried across the North Atlantic, and therefore of the rhythmic recurrence of cold spells in the region. A link between cold episodes in the Atlantic and a weakened Asian monsoon has already been documented for the last glacial period, when cold spells — the so-called Dansgaard/Oeschger and Heinrich events — periodically produced arctic conditions in the North Atlantic region6,7,8. The Dansgaard/Oeschger and Heinrich events caused far more dramatic environmental changes than any of the climatic cycles in the Holocene, so their influence on climatic regimes well beyond the Atlantic region is not surprising. Remarkably, however, the findings of Gupta et al. indicate that this linkage continued into and throughout the current warm period of the past 11,000 years, even though the climatic anomalies have been far smaller.

The monsoon system does not operate in isolation, of course. It is only one of many participants in the global dance of climate oscillators and dipoles (see Box 1), and many of these climatic regimes have evidently undergone rapid swings during the Holocene9,10,11. Following the initial appearance of ice-core palaeoclimate records from Greenland, a belief that climate has been highly stable during the Holocene briefly fluttered through the scientific community. That belief is now long dead12,13, and because of both natural and human-induced change we face an uncertain climate future. It will require a concerted effort from those involved in the observational, modelling and palaeoclimate aspects of climate change to integrate knowledge of the various climatic regimes, past and present, to assess the 'teleconnections' between them, and the stability of their links through time. Only then will we be able to forecast with any confidence potentially hazardous states of climate regimes14,15 — for instance, certain anomalies in sea-surface temperatures in the Pacific and North Atlantic that might promote conditions in which severe flooding occurs in monsoon regions. In providing a more detailed view of monsoon variability in the recent past, and its teleconnections, Gupta and colleagues' contribution is a good example of how further progress can be made on this subject.

References

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Correspondence to Rainer Zahn.

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