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Timing couldn’t have been worse for the group of colonists who came ashore on Roanoke Island in 1587, attempting to establish the first permanent English settlement in the New World. Along with the usual hardships of starting a new society on the edge of the wilderness, the colonists were confronted with the region’s worst drought in 700 years, which caused mass starvation and aggravated tense relations with Native Americans. By 1590, the ill-fated settlers had vanished with little trace. Roanoke’s collapse in the face of harsh climate puts it in distinguished company, a researcher reports in the journal Science.

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Recent concerns about global climate change, especially the impact of human activity on these changes, have sent scientists scrambling to discover how current climate trends fit into the larger picture of climate change throughout Earth’s history. With a growing pile of paleoclimate data at their disposal, researchers are also taking a closer look at how complex societies responded to past climate change.

Understanding how these cultures adapted to events like persistent droughts could provide valuable perspective on how modern societies might respond to future climate change, says Peter deMenocal, a researcher at the Lamont-Doherty Earth Observatory of Columbia University.

The long view
Modern climate data, recorded by instrument, allow us to study climate phenomena that vary on a time scale of several years, such as El Niòo, deMenocal notes in Friday’s issue of Science. But that record is far too short to capture large-scale events like the drought that wiped out Roanoke — events that we can confirm through other detailed climate records like tree rings or lake sediments.

These climate proxies, among others, indicate that the past 12,000 years of Earth’s climate have been a bumpy ride, punctuated by widespread cooling events and droughts that persisted for decades and often centuries. These events are associated with changes in ocean circulation, solar radiation and volcanism. Many of them occur on thousand-year cycles, indicating that solar variability and interactions between the ocean and atmosphere are especially important factors influencing these climate changes.

A growing store of detailed and well-dated paleoclimate records has contributed to “a transformation in the field that allows scientists to evaluate cultural records within the context of climate,” deMenocal says. Researchers are using this new perspective to examine how past societies managed to cope — or failed to cope — with prolonged and severe climate events, events without equal in modern times.

Collapse and abandonment
This new outlook suggests, for instance, that the Akkadian Empire in Mesopotamia may have been one of the earliest complex societies felled by severe drought. The Akkadian Empire was established around 2300 B.C., linking rain-fed agricultural fields in northern Mesopotamia with irrigation agriculture in the south. The empire stretched from the present-day Persian Gulf into Turkey before it abruptly collapsed less than 200 years later.

By 2170 B.C., archaeological records document a mass exodus from the north, with settlements abandoned and refugees pouring into southern Mesopotamia. Excavations at one of these settlements, Tell Leilan, show that the collapse is marked by a thick layer of windblown dust without any artifacts. Three hundred years later, smaller and more nomadic groups finally ventured north again.

Image: Excavated Akkadian dwelling
An excavated sample of residential occupation within the lower town of Tell Leilan in northeast Syria during the final days of the Akkadian Empire.
Researchers found the same telltale signature of drought in a deep-sea sediment core drilled from the Gulf of Oman. The core documents a dramatic 300-year period of windblown dust that could be dated to roughly the same period as the Akkadian collapse. Chemical traces in the dust allowed the scientists to pinpoint its origins in Mesopotamia.

The severe climate change that may have helped topple the Akkadians probably had its roots in the far-off North Atlantic Ocean. Atlantic deep-sea cores suggest that the start of the Mesopotamian drought coincides with widespread cooling in the northern part of the ocean, which could have affected rainfall that typically fed Mesopotamia’s rivers. Modern records show that water supply for Mesopotamia can be cut in half when North Atlantic sea surface temperatures are unusually cool, deMenocal notes.

Several thousand years later, on the other side of the world, the Tiwanaku culture in South America faced a similar climate challenge. Between 300 B.C. and A.D. 1100, the Tiwanaku built an urban complex that probably supported nearly half a million people. They sustained this dense settlement through raised field agriculture, a technique that improved drainage and recycled nutrients in the poor tropical soil.

The raised fields of the Tiwanaku may be a good example of a complex society already responding to the challenges of a marginal environment, and therefore vulnerable to sudden climate change, says deMenocal. Around the year 1100, the cities and fields were abruptly abandoned.

Once again, the paleoclimate record may contain clues to the Tiwanaku collapse.

The Quelccaya ice core, drilled just 125 miles from Lake Titicaca, contains an annual record of precipitation for the region. This ice core record shows close overlap between the time of the Tiwanaku abandonment and the start of an increasingly dry spell. Sediment cores from Lake Titicaca itself also chronicle the event, showing a 33-foot drop in the lake level at the time. The drought persisted for several centuries, during which the Tiwanaku went into a slow decline.

‘We have to relearn history'
The fate of the Akkadians and the Tiwanaku begs the question: If a centuries-long drought descended today, how would we respond?

One of the most destructive droughts in recent memory is the 1930s American Dust Bowl, which lasted only six years. In this short time, it managed to become “one of the most devastating and well-documented agricultural, economic and social disasters in the history of the United States,” according to deMenocal.

Researchers need to gather more data on past climates before this information can be used to accurately predict and design strategies for future large and persistent climate changes, says deMenocal, but acknowledging the historic impact of these severe events is an important step toward that goal.

“Right now, we address climate change on the basis of a painfully short climate record, just the past 150 years or so that instruments have recorded,” deMenocal says. “We now have to relearn history on a long-term scale.”

© 2013 American Association for the Advancement of Science

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