Southeast Asian megadrought dating back 5,000 years discovered in Laos cave

In a Q&A, Penn archaeologist Joyce White discusses the partnership with paleoclimatologists that led to the finding, plus possible implications of such a dramatic climate change for societies at that time.

A group of archaeologists and excavators standing and sitting at the entrance of a cave.
Penn archaeologist Joyce White (center) has been working in Laos since 2001 with teams like the one shown here. Discovering evidence of a 1,000-year drought in a Laos cave was unexpected, she says, but does answer some questions about the Middle Holocene, a period she’d previously described as the “missing millennia.” (Pre-pandemic image: Courtesy of Joyce White)

Southeast Asia typically evokes rich and wet tropical forests. So, the discovery of a drought more than 1,000 years long beginning about 5,000 years ago was an unexpected outcome from research started by the Penn Museum’s Joyce White nearly two decades ago. She and colleagues from the University of California, Irvine; William Paterson University; the University of Quebec; and more published these findings in the journal Nature Communications

Evidence for the megadrought came from Laos’ Luang Prabang Province, where White has worked since 2001. A Henry Luce Foundation grant enabled the research program to expand starting in 2008, and a paleoclimate team that included William Paterson’s Michael Griffiths and Kathleen Johnson of UCI, co-lead authors on the latest paper, joined in 2010. Some of their work included collecting stalagmite samples from the Tham Doun Mai cave along the Ou River. 

Much like tree rings, stalagmites have rings that contain datable signs of changing climate. As rainwater drips through cracks in a cave’s roof, it interacts with a mineral called calcite to form stalactites on the cave’s ceiling. As that water-mineral mixture drips from the stalactite, stalagmites form on the floor below, building over time, layer by layer. 

“From those rings, we can interpret the occurrence of various climate events,” says White, who directs the Penn Museum’s Middle Mekong Archaeological Project and is an adjunct professor in Penn’s Department of Anthropology. “In this case, two of the stalagmites stopped growing for several hundred years, then started to grow again.” Chemical analyses confirmed that a prolonged drought lasting more than 1,000 years caused the cessation. 

When combined with climate modeling, the cave evidence seems connected to changes in vegetation and dust in northern Africa that happened around the same time—right around when the Sahara transitioned from forest to desert. The modeling also showed how such changes in northern Africa could affect rainfall across Southeast Asia. Penn Today talked with White about what the discovery means, plus the work that led to it. 

A massive rock structure with an opening at the base where people are walking.
Rock shelters in Laos near the Tham Doun Mai cave where researchers found evidence of the 1,000-year megadrought. (Pre-pandemic image courtesy of Joyce White)

What’s the main finding of this research?

There was this absolutely huge drought that lasted for more than 1,000 years that occurred in the Middle Holocene. That’s amazing in and of itself and wasn’t really anticipated by other research. This is outstanding evidence for the type of climate change that must have affected societies, what plants were available, what animals were available. All of biotic life had to adjust to this very different climate. From an archaeological point of view, this really is a game changer in how we try to understand and reconstruct this period.

When you refer to the Middle Holocene, what do you mean?

The Holocene in general is commonly considered to begin about 11,000 years ago, and the Middle Holocene is from about 6,000 to 4,000 years ago.

Before this finding, what did we know about the Holocene?

We understood pretty well what was going on in the Early Holocene, essentially hunting and gathering. We also knew that the Late Holocene was an agrarian period. The link between the two was still a mystery, mysterious partly because there is a Middle Holocene gap in the archaeological record in interior Southeast Asia, what I’d been calling the missing millennia.

There’s a mountain range between Vietnam and the Mekong Valley, where Laos is. On the Vietnam side, there are many Middle Holocene sites, but I wanted to find those on the west side, on the Laos side in the Mekong Valley. Archaeology is very much the tortoise and not the hare; you can’t necessarily go into a region and know you’re going to find evidence for whatever you’re hypothesizing. You record whatever you find, and that takes energy and time. We knew the Middle Holocene had to be there somewhere. I figured we just didn’t quite understand the landscape yet. This was before we knew about this drought.

How did this archaeological work in Laos begin?

Like many countries in Southeast Asia, Laos was not accessible to research until the ’90s. However, Thailand has been an area of archaeological study since the 1960s, and Penn was one of a handful of pioneering universities that undertook fieldwork there. The site we’re most famous for is Ban Chiang, now a UNESCO World Heritage Site, and research related to that site is one of my main research endeavors.

In the late 1990s, the director of the Penn Museum urged me to set up a project in Laos. In those days, that wasn’t an easy thing to do. When I got there, I was assigned a counterpart. We rented a truck and drove around first near Vientiane, the capital, followed by a brief trip to Luang Prabang, a former royal capital. In about two and a half days in Luang Prabang, I saw evidence of 10,000 years of human occupation, which is not an everyday occurrence for an archaeologist. It was mind-blowing.

During that initial trip, you’ve said that you noticed Luang Prabang was located at the intersection of the Seuang, Khan, and Ou rivers, where they meet and flow into the Mekong. How did that guide your next steps?

I decided I wanted to do a regional survey that looked at all three rivers, not just one, because you could pick the wrong one. We would use mobile GIS, which was cutting edge at that time, and have three separate teams exploring each river independently. Then we’d collate the data. I took another trip to get the Lao government to agree to my plan, and it took a year or two to raise money.

In 2005, with grants from the National Science Foundation and the National Geographic Society, we conducted the first formal survey of the Middle Mekong Archaeological Project. Everything was joint teams; I wanted 50-50 Lao, non-Lao teams. In about three weeks, we found nearly 60 sites, which demonstrated that this was an archaeologically rich area. We found evidence of the Stone Age, ceramics of a wide variety, the kind of thing you can find on the surface of sites and in caves.

We started test excavations of cave sites beginning in 2007. The research being published today is from 2010, the first season the paleoclimatologists joined us. They looked at many other sites, but that one on the Ou River and in the Tham Doun Mai cave was the outstanding one.

How did the team unearth the megadrought?

When rainwater from stalactites drips, stalagmites form beneath. Based on their growth and chemistry, the layers can be dated. For two of the stalactites, the dripping stopped, and preliminary data show it was for 1,000 to 2,000 years. That indicates that it wasn’t just a dry spell. It was massive.

This type of complete change in climate has to have an impact on the biotic life, but we don’t really understand that in detail yet. That being said, I think this is going to change the conversation about that whole period across Eurasia and certainly Southeast Asia. The fact that there are profound climatic phenomena at a continental scale in the Holocene timeframe is quite new in scholarly conversations among archaeologists. This kind of research, when you combine archaeology, paleoclimatology, and modeling, will more effectively bring out this type of finding.

What’s next for your work?

With COVID, who knows when we can start fieldwork again. We didn’t finish our survey on the Ou River so I would like to do that. But to flesh out the human part of the story, we need to look at aspects of our excavated evidence, including shells we had collected from the four tested sites, which were different ranges of species. Once you know what the shell is adapted to, you can get human-scaled evidence for change of subsistence and environment. We made great headway this past January and we have other animal remains to study, too. You can get some nice tight data that inform much more on the human dimension in relationship to the massive climate shifts. 

Funding for this research came from the Henry Luce Foundation, Swedish Research Council, National Geographic Society, and National Science Foundation (grants 1405472, 1603056, 1404932, and 1602947). 

Joyce White is a consulting scholar and principal investigator of the Middle Mekong Archaeological Project at the University of Pennsylvania Museum of Archaeology and Anthropology, director of the Ban Chiang Project also at the Penn Museum, executive director of the Institute for Southeast Asian Archaeology, and an adjunct professor in the Department of Anthropology in the School of Arts & Sciences at the University of Pennsylvania.