It’s time we pay more attention to past episodes of ‘great warming.’

With some climate scientists and media outlets increasingly attributing severe weather events in part to global warming, a big challenge going forward is about proportionality. In other words, to what extent can we connect hurricanes, heat waves, and droughts to greenhouse gases? Scientists are working to clarify any linkages, but it’s unlikely there will be clear-cut answers for at least a few years.

The odd thing about this endeavor and the larger severe weather/global warming debate is that we have yet to appreciate what science has already learned about climate change in the distant past, specifically (tree ring) evidence of devastating, prolonged droughts.


“The dry spells of a thousand years ago spanned not years, but generations, anthropologist Brian Fagan writes in his 2008 book, The Great Warming: Climate Change and the Rise and Fall of Civilizations. “The medieval droughts in California’s Sierra Nevada lasted decades, far longer than those of modern times.” Fagan’s book examines other nasty ancient droughts and how they laid waste to some of the world’s great empires, from the Maya in Central America to the Khmer in Cambodia.

But just as the contemporary question of drought and its global warming signal is a complicated one, so too is the question of drought and its impact on prehistoric societies.

For example, new research published in PNAS, as reported by Live Science, postulates that the Khmer’s ancient city of Angkor “might have collapsed” while battling an extended drought.

Researchers have been trying to puzzle-out this story for years. Live Science provides some context:

Suggested causes for the fall of the Khmer Empire in the late 14th to early 15th centuries have included war and land overexploitation. However, recent evidence suggests that prolonged droughts might have been linked to the decline of Angkor — for instance, tree rings from Vietnam suggest the region experienced long spans of drought interspersed with unusually heavy rainfall.

In fact, previous studies (which Live Science might have mentioned and linked to in its dispatch) have found that extended droughts and other climate forces played a major role in Angkor’s collapse. Columbia University’s Brendan Buckley, for instance, led a study published in PNAS in 2010, which declared:

The Angkor droughts were of a duration and severity that would have impacted the sprawling city’s water supply and agricultural productivity, while high-magnitude monsoon years damaged its water control infrastructure.

Science writer Ed Yong, in a post at his Discover magazine blog, provides a detailed perspective on the Buckley study and on the larger question of what ultimately led to Angkor’s demise. As Yong noted, “By the time the droughts kicked in, the city was already weakened by social, economic and political strife.”

What’s interesting about Ankgor’s story is how similar it may be to other famous “collapse” stories in prehistory, like the Maya and Anasazi, which Jared Diamond chronicled in his best-selling book, Collapse: How societies choose to fail or succeed. Additionally, in an article for Science several years ago, I explored the complex mix of social and environmental factors that led to the unraveling of prehistoric southwestern societies, such as the Fremont and Anasazi, a millennium ago.

The general theme that runs through all these cases, according to archaeologists, is this: In boom times, when the environment was good, cultures flourished and populations expanded; when the climate went bad, hard times set in, along with societal conflict. But when ecosystems got stretched beyond their capacity, as a result of multiple stresses, including a series of extended droughts, the cultures eventually broke under the strain. For example, that Science article cited one study published in Quaternary Science Reviews, whose authors wrote that, “In some sense, the 13th century drought may have simply ‘finished off’ some cultures that were already in decline.”

But as many scholars will also attest, these were cultures that had withstood previous ferocious episodes of climate instability and had built a good bit of resilience into their systems and institutions. Why would one bad drought knock them out when previous ones didn’t? Kevin Anchukaitis, a Columbia University paleoclimatologist and co-author with Buckley on the 2010 PNAS Angkor paper, offered some insight into this question in a recent e-mail exchange:

So, were the crops of the Maya or the [Angkor] Khmer withering in the field during megadroughts? Perhaps, probably. But I tend to subscribe to the theory — elegantly described by University of Illinois anthropologist Lisa Lucero — that it wasn’t drought per se that tipped these civilizations into collapse, but rather the failure of “elites” and the infrastructure that was part of their power, effectiveness, and control. There may have been direct consequences of drought — crop failure, famine, malnutrition — that played a role, but before those aspects would have become lethal in and of themselves, there likely would have been a crisis of confidence and elite control. Add to that enemies at the gates — like the Siamese of Ayutthaya for the Khmer at Angkor — and it may have become too much for complex societies to withstand.

Lessons as We Anticipate Projected Droughts?

While such a nuanced picture that emerges from accumulated scholarship is fascinating, are there lessons for us here in the 21st century, as we stare down the barrel of projected, long-lasting droughts that are likely to be worsened by the build-up of greenhouse gases? After all, writes Fagan in his book, the U.S. has “experienced droughts, but none of them have endured like that descended on the Sierra Nevada a millennium ago.”

Referring to the same history that Fagan documents in The Great Warming, Anchukaitis, again in an e-mail, wrote:

We know the climate system is capable of producing longer and more severe drought than we’ve seen in the instrumental record even without the influence of anthropogenic climate change, and we’ve constructed massive infrastructure and have large sedentary populations in hydrologically-marginal environments. But at least in the [U.S.] Southwest (and, I think, southeast Asia) perhaps more so than the direct consequences of drought will be the institutional and infrastructural consequences: People aren’t going to die of thirst or famine this time in the modern and future southwestern United States, but you have to wonder what the social and political consequences of a Puebloan-style [Anasazi] Great Drought would be today for the infrastructure and institutions that make it possible to sustain large populations in the region.

Fagan, in his book, discusses this possibility and says that, in terms of water, “if the lengthy droughts of a millennium ago were to return, much of the western United States is living on borrowed time.” He also suggests that the contemporary public debate, with its singular focus on climate change, extreme weather, and sea level rise, may be shortsighted: “The melting of ice caps and the increased danger of flooding are no trivial matter.” But the experience of the civilizations he surveys in his book is that “the silent and oft-ignored killer is drought, even during a period of mild warming.”

Lest one think Fagan has misread the dynamics of the public discourse, consider what Kate Galbraith wrote last year, when a severe dry spell was starting to bake Texas: “The drought has become a touchstone for climate scientists and journalists in the debate over global warming.”

But there is also evidence that local Texas authorities and populations most affected by the persistent drought are managing it well, and that this story of resilience, as Albuquerque Journal science reporter John Fleck suggests, is not getting enough attention.

Still, Columbia University’s Anchukaitis cautions: “What I think we don’t know yet is what combination of drought characteristics and societal response to them would strain that resilience. To what extent are we immune from the type of drought-civilization synergies that brought about the demise of past complex civilizations?”

That’s a good question, one deserving a lot more serious consideration.

Keith Kloor

Keith Kloor is a New York City-based freelance journalist who writes often about the environment and climate change.