Laos is now wet and verdant, but new findings suggest it experienced a 1000-year megadrought starting about 5000 years ago. FBXX/ISTOCK.COM
By Charles ChoiAug. 24, 2020 , 2:05 PM
A megadrought that lasted more than 1000 years may have plagued Southeast Asia 5000 years ago, setting up dramatic shifts in regional civilizations, suggests a new study of cave rocks in northern Laos. The researchers believe the drought began when the drying of the distant Sahara Desert disrupted monsoon rains and triggered droughts throughout the rest of Asia and Africa.
For years, archaeologists studying mainland Southeast Asia—an area encompassing modern-day Cambodia, Laos, Myanmar, Malaysia, Thailand, and Vietnam—have been puzzled by what they call “the missing millennia,” a period from roughly 6000 to 4000 years ago with little evidence of human settlements. University of Pennsylvania archaeologist Joyce White, a co-author on the new paper, says she and others long thought this was because researchers hadn’t yet pinpointed where people of the era lived. Now, she believes the settlements could be missing because a megadrought devastated their populations and drove them to find water elsewhere.
To re-create the climate of that time, White and her colleagues investigated stalagmites in Tham Doun Mai, a cave in northern Laos. Stalagmites are tapering pillars of rock that rise from the floors of caves; they slowly grow taller as mineral-rich water drips from cave ceilings—often after rainfall. By analyzing the content of the slowly deposited rock, researchers can gauge not only the age of the rock, but also how wet it was at the time.
Scientists first radioisotope dated sections of three stalagmites from 9500 to 700 years ago. They next examined oxygen isotopes in the rocks to see how rainfall might have varied over those times. When rain falls, drops bearing heavy oxygen-18 isotopes land before those holding lighter oxygen-16 isotopes. Frequent downpours let loose both isotopes, but arid places that see only spotty showers tend to be depleted in light oxygen. By looking for stalagmite layers that were enriched in oxygen-18, the researchers could identify times when the climate was dry.
Paleoclimatologist Michael Griffiths collects a sample of calcite growth that precipitated onto a glass plate left in Tham Doun Mai Cave in Laos for 2 years. KATHLEEN JOHNSON
The researchers found that rainfall in the cave was relatively steady for more than 4000 years before abruptly decreasing between roughly 5100 to 3500 years ago. That suggests the region may have experienced a prolonged, heretofore unrecognized drought that lasted more than 1 millennium, the researchers report this month in Nature Communications.
If so, it may have been part of a larger series of megadroughts that hit Africa and Asia between 5000 and 4000 years ago, says study co-author Kathleen Johnson, a paleoclimatologist at the University of California, Irvine. During this time, civilizations across western Asia and the Middle East went through major upheavals, such as the collapse of the Akkadian Empire of Mesopotamia and the abandonment of cities in the Indus Valley. The climate shift, which some have dubbed the “4.2-kiloyear event,” is the basis for the Meghalayan, a controversial new geological age. It coincided with—and may have resulted from—the end of the Green Sahara, when once-verdant north Africa became a desert.
To determine whether African desertification could be linked to the Southeast Asian megadrought, the researchers simulated the ancient climate, incorporating interactions among the oceans, the atmosphere, dust, and vegetation. They found that the drying of the Sahara might have increased airborne dust, pushing the Pacific Ocean into a prolonged El Niño–like cycle that disrupted mainland Southeast Asia’s summer monsoon rains. This in turn could have triggered a megadrought over large swaths of Southeast Asia and flooding across East Asia. This was, in essence, “a redistribution of moisture across Asia,” says Michael Griffiths, a paleoclimatologist at William Paterson University and lead author on the study.
Raymond Bradley, a paleoclimatologist at the University of Massachusetts, Amherst, says the new study suggests the 4.2-kiloyear event—which many consider an abrupt climate shift—may have been part of a larger trend that began roughly 800 years earlier. He hopes the new study will spur researchers to review well-dated records from other regions across Asia to see where and when similar climatic shifts occurred. “Only then can we try to figure out why such changes occurred and how they might or might not be related to societal changes.”
To that end, Griffiths and his team are planning to explore caves in Vietnam and Thailand to get a better look at the period. And their answers may also inform modern-day climate projections, he says. “Perhaps studying the past can help illuminate our current situation in new ways.”
doi:10.1126/science.abe4757
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