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September 2004

Clues to an ancient lakebed

An NCAR scientist finds signs of an abrupt climate shift more than 4,000 years ago. Her research may yield insights into future climate change.

Carrie Morrill.

When Carrie Morrill collected sediment from an ancient lakebed in Tibet, she wasn’t expecting any major surprises. She thought the sediment record would show a relatively stable climate existed during the mid-Holocene period, about 4,000 to 6,000 years ago.

So it was with some surprise that Carrie detected signs of a sharply shifting climate — a finding that may have ramifications for abrupt climate change in the not-too-distant future.

“For a while, people thought the Holocene was just a flat line, climatically,” says the CGD scientist. “Now we know that abrupt climate changes happened. Understanding what caused them can be key for predicting what may be in store for us.”

At the time she collected the sediment, Carrie was getting her doctorate in geology from the University of Arizona. Her research involved reconstructing past climate through a painstaking analysis of two different isotopes of oxygen. (She jokes that her work involved three weeks in Tibet and four years in a lab.)

The sediment yielded clues to the past because precipitation from the Indian Ocean that fell on the lake contained a different mix of oxygen isotopes than did precipitation that resulted from local evaporation. By analyzing the isotopes in the sediment, Carrie detected that, about 4,700 years ago, the local climate changed sharply over the course of about a century. Monsoons from the Indian Ocean stopped delivering as much precipitation.

“I wasn’t expecting the climate to change so abruptly,” she said. “I had expected it to be more cyclical.”

Looking for records. Paleoclimatologists, such as Carrie Morrill, try to reconstruct past climate from a very limited set of natural records. The world map, above, shows the types and locations of records that Carrie has used to piece together climate in the mid-Holocene period. The bar graph shows when abrupt climate changes occurred, according to the records. (Courtesy Carrie Morrill.)

At NCAR, Carrie is probing deeper into the picture. By pulling together data from a range of natural records, including ice cores, marine and lakebed sediments, pollen, and windblown silt, she has found that the events in Tibet appear to be part of a series of dramatic climate shifts between 4,000 and 4,800 years ago that affected much of the Northern Hemisphere, and possibly the Southern Hemisphere as well. The paleoclimate records indicate the midlatitudes became colder and the tropics dried significantly. The shifts in each region appear to have occurred within a century, but Carrie needs more records to determine whether they took place around the world simultaneously or at different times during the 800-year span.

Positive feedback cycles accentuated some of these changes. As the African monsoon weakened, for example, soil and vegetation in the region released less water vapor into the air, providing less fuel for the storms.

During or shortly after the climate shifts, several ancient civilizations collapsed. They include the Akkadian society in Mesopotamia and the Indus civilization in India and Pakistan. Carrie speculates that climate change may have been a factor.

“Was the collapse of these civilizations caused by an abrupt climate change?” she asks. “Maybe they had built up big cities, and then there wasn’t enough water to sustain them as the climate became drier.”

Paleoclimatologists aren’t sure why the climate shifted so dramatically. Some point to changes in oceanic circulation; others to variations in Earth’s orbit. Carrie says more data are needed, especially from the tropics and the Southern Hemisphere, to flesh out what happened to global climate during the mid-Holocene.

Growing concerns

Carrie’s research comes amid increasing scientific work on abrupt climate change. Although global climate typically changes over thousands or even millions of years, natural records indicate it can also shift dramatically in a few decades or centuries. Many scientists point to a period known as the Younger Dryas, about 11,500 years ago, when melting ice sheets affected ocean circulation patterns. Regional — and possibly global — temperatures dropped by several degrees within a few decades, and the climate did not recover for more than 1,000 years.

Some worry that current emissions of carbon dioxide and other greenhouse gases, in addition to warming the planet, may have unforeseen and sudden impacts on our climate. Carrie says abrupt climate change is characterized by the climate reacting in an unexpected way to an external event, or a “forcing,” such as a change in solar energy output.

“Climate changes in a direction determined by the forcing, but it’s not linear,” she explains. “There are multiple equilibria in the climate system and the climate jumps from one to another. When a threshold is crossed, the climate shifts.”

Carrie plans to continue her research into the mid-Holocene, assembling more complete natural records to create a better picture of global climate during that time. What fascinates her about the mid-Holocene is its climate was similar to our own. For that reason, understanding the abrupt changes that occurred then may shed insight into our own changing climate.

“It’s relevant to what can happen in the future,” she says.

• David Hosansky

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