Lessons from the past
The monsoon is Asia's curse and blessing, bringing long-awaited rain and sometimes deadly masses of water in equal measure. But if it does not come at all, drought, hunger and fire threaten. A look into the past therefore helps to better predict one's future. Fire and water have dominated Southeast Asia in recent months, bringing many inconveniences to the local population. First, thousands of smaller and larger deforestation fires in Sumatra and Borneo with their smoke wrapped Singapore, Kuala Lumpur and other metropolises in the region in a dense smog. And then the wintry rainy season twice flooded southern Malaysia and northern Sumatra, forcing hundreds of thousands of people to flee and destroying local infrastructure.
Both are closely related, because the destruction of the rainforests on site for paper mills, precious wood exports, oil palm and rubber plantations - the latter mainly by arson - left bare slopes and agitated soils. However, they have nothing to oppose the rain masses of the annual monsoon, and consequently the flood reports are increasing. The current fires and floods may also be a foretaste of things to come, according to past data from Nerilie Abram of Australia's National University in Canberra and her colleagues.
Abram's team studied the rich chemical information deposited in fast-growing yet ancient coral stocks off the island of Mentawai off Sumatra's west coast. In their growth rings - similar to the annual rings of trees - different isotope ratios of oxygen (especially 18O) and of strontium to calcium reflect the regional climate history of the past 6500 years. For example, if the ratio of strontium to calcium is decreasing, the Indian Ocean was relatively supercooled at the time of deposition, while at the same time lower 18O values indicate aridity.
These drought phases are caused by changes in the so-called Indian Ocean Dipole (IOD) – a change in the oceanic and atmospheric conditions of the world ocean comparable to the El Niño fluctuations (ENSO) in the Pacific. During the positive phase, the south-east trade wind intensifies in the area of the Indonesian islands and drives larger water masses westwards. Cooler deep water is now able to rise in the waters of Indonesia and Australia, causing sea surface temperatures to fall, while off Africa and around Madagascar they are rising. The air tends to sink over the cooler water, more high pressure areas are forming, and the rainfall over the western islands of Indonesia and in western Australia is becoming weaker. The western Indian Ocean, on the other hand, experiences more frequent lows, sometimes heavy rainfall hits East Africa, and the Indian monsoon intensifies - the situation reverses in the negative phase.
During the last few centuries, according to the findings of paleoclimatologists, the droughts triggered by the IOD in Sumatra or Java often coincide with a pronounced precipitation deficit on the eastern islands such as Borneo caused by El Niño. A connection between the two climatic phenomena is therefore obvious, at least for today's conditions, but that was not always the case. Because during the early to middle Holocene, according to the coral data, even stronger and longer-lasting droughts appeared on Sumatra and its offshore islands, although El Niño was only very weakly noticeable at the time and usually had to give way to its cool sister La Niña in the Pacific. However, it generally brings very productive rainy seasons to Southeast Asia - and therefore actually rules out water shortages.
But if El Niño was not to blame, what triggered the drought calamities in western Indonesia? This is where the big brother of the little Southeast Asian monsoon comes into play: the Indian monsoon, whose humid summer air current decides the weal and woe of Indian and Pakistani harvests today. The more intense it turns out to be, the stronger the south-east trade wind will blow across the Indo-Australian region - with the well-known drought-promoting consequences. This was exactly what happened 6500 to 4000 years ago, because larger seasonal differences in solar radiation intensified the Indian monsoon at that time.
Even in winter, when the wind conditions reversed and the air masses from the South Asian subcontinent flowed out to sea, Sumatra didn't bring any precipitation back then. Then foothills of the now offshore Indian monsoon currents coming from the northeast ensured that cold water continued to rise and dry air to descend in front of the island. The result: a prolonged and worsening drought.
This doesn't bode well for Indonesia's future, because many climate models expect the Indian monsoon to intensify again. Global warming is heating up mainland Asia, which in turn draws in even more moist air from the Indian Ocean, while Southeast Asia's islands are having to contend with more severe dry spells. More fires are likely to come with the drought - a nightmare for environmentalists, physicians and economists alike.