El Niño Found To Be 124,000 Years Old

August 19, 1999

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By William J. Cromie
Gazette Staff

Daniel Schrag shows one of the corals he analyzes to determine when drought and wet weather occurred in the past. Photo by Kris Snibbe

El Niño, the enfant terrible of weather, was causing droughts and floods as long ago as 124,000 years, Harvard researchers have discovered.

Records preserved in corals from Indonesia reveal that El Niño was throwing tantrums even before the last ice age began, when the climate apparently was like it has been for most of this century.

"No question about it; there were El Niños that long ago," says Daniel Schrag, associate professor of geochemistry. "The finding suggests that El Niños are much more stable than we ever thought."

Evidence of droughts and unusually wet weather, chemically etched into ancient reefs, show changes every three to seven years, a pattern that Schrag finds "remarkably similar" to those of El Niños from 1856 to 1976.

After that, a sudden change occurred. Since 1976, the pattern looks completely different, with El Niño events appearing faster and stronger.

"In 1982-83, we experienced the most severe El Niño of this century," Schrag notes. "According to previous records, you wouldn’t expect another El Niño that powerful for 100 years." Yet 15 years later, in 1997-98, an even larger one disrupted weather from Peru to Pomona, Bangladesh to Baltimore.

At the same time, global temperatures have begun to rise sharply. Nineteen ninety seven was the warmest year in this century — until 1998, which was even warmer. Sixteen of the twenty hottest years on record have occurred since 1976, including all the years since 1990.

"This altered pattern does not prove that the increased frequency and severity of El Niños are related to global warming, but by showing that the recent behavior is anomalous, it surely provides evidence for a connection between the two," Schrag says. "If I owned an insurance company, I’d be extremely concerned."

Coral Tells the Story
This year, heat waves and droughts threaten health and crops in the United States, while heavy rains and floods savage Asia. The blame cannot be placed on El Niño, but it does seem to be related to global warming.

El Niño is born when a huge mass of warm water moves from southeast Asia to the west coast of South America. Easterly trade winds normally blow ocean water in the opposite direction, but when these tropical winds slacken, the piled-up water sloshes back towards South America. The displaced warmth it carries with it changes the circulation of the tropical Pacific as well as air flow over a large part of Earth. The stronger the El Niño, the greater the effect.

Commercial fishermen can’t fill their nets off Peru and Ecuador; floods disturb South America’s coastal deserts. More rain than usual falls on the U.S. Pacific coast and in the southeastern States; winters are warmer in Canada and New England; weather is hotter and drier in the Midwest; fewer hurricanes drench the Atlantic coast. In southeast Asia, where it normally rains heavily in January, the skies turn blue and cloudless.

The last change shows up clearly in fossil corals that lived 124,000 years ago near the present location of Sulawesi Island, Indonesia. These plantlike marine animals secrete external skeletons of limestone (calcium carbonate) to protect their small, soft bodies. The skeletons, which grow into massive reefs, incorporate two different varieties (isotopes) of oxygen — oxygen 16 and oxygen 18. When conditions are dry, oxygen 16 evaporates more quickly from the water, so the heavier oxygen 18 dominates in the coral skeleton. It wet weather, oxygen 16 prevails.

Corals build their skeletons in annual rings, like tree rings. These rings can be dated, and their oxygen ratios reveal how rainfall varied from year to year. Schrag found such a record, which covers period of 65 years some 124,000 years ago.

He and postdoctoral researcher Konrad Hughen compared this record to that of changes produced by El Niños from 1856 to 1998. Despite the time gap, which includes an ice age about 60,000 years long, the records appear strikingly similar. You can’t tell one from the other until 1976.

An Ocean-Size SeeSaw
In searching for a reason why the El Niño has changed, Schrag again turned to corals. He and postdoctoral researcher Tom Guilderson studied the rocky animals that grew during the latter part of this century in the Galapagos Islands, located on the equator about 600 miles off the coast of Ecuador.

During normal winters, cold water wells up from the depths of the eastern Pacific, and this upwelling leaves its mark on the corals. During El Niños, the flood of warm water from the west cuts off the upwelling and that, too, is reflected in the chemical makeup of the corals.

This record reveals a sudden shift in 1976. After that year, the suffocating blanket of warm water seems to have gotten deeper.

You can think of the warm waters sloshing back and forth across the Pacific as a natural seesaw. The seesaw base consists of a boundary that separates warmer, lighter surface waters from colder heavier, deeper waters. Normally this border, known as a "thermocline," dips about 600 feet deep on the Asian side but only 150 feet down on the American side.

In 1976, Schrag concludes, the see-sawing thermocline dropped an additional 30 feet off South America. That doesn’t sound like much, but the tremendous volume of water it represents carries enough heat energy to produce a major change in winds and water over much of the world. The water warms the air, increasing its water content and changing its circulation, which in turn alter rain and wind patterns over millions of square miles.

"These findings apparently explain why El Niños have been stronger and more frequent," Schrag comments. "But they don’t tell us what caused the thermocline change and why it was so abrupt. We’re working hard to find answers."

Another mystery involves the possible tie to global warming. If the water and wind changes since 1976 represent a natural cycle, you’d expect them to soon switch back to conditions like those 124,000 years ago and in the 120 years before 1976. "But the longer these unusual conditions exist, the stronger is the case that they’re tied to global warming," Schrag believes.

Schrag, Hughen, and colleagues will report on their work with ancient corals in an upcoming issue of Geophysical Research Letters.