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Sudden Ice Age Warming Linked to El Niño

John Roach
for National Geographic News
March 17, 2004
 
During the last ice age, temperatures in the North Atlantic region rose sharply and quickly about every 1,500 years. The result: massive discharges of melted ice into the ocean which disrupted weather patterns around the world.

The trigger behind these abrupt climate cycles—called Dansgaard-Oeschger events—is the subject of a long-running debate that bears on the ability of scientists to predict climate.


Some researchers believe the climate events were triggered by changes in the North Atlantic's dominant ocean-circulation pattern, whereas other researchers believe the events are associated with the El Niño/Southern Oscillation (ENSO) phenomenon in the tropical Pacific (see sidebar for an explanation of El Niño and ENSO).

Reporting in tomorrow's issue of the science journal Nature, an international team of scientists suggests millennial-scale dry periods associated with ENSO triggered the surges of melted ice into the North Atlantic.

"We've not got conclusive evidence, but we have the strongest evidence yet that long-term changes in El Niño are driving these changes in the North Atlantic," said Chris Turney, a paleoecologist at Queen's University Belfast in the United Kingdom.

Turney and Peter Kershaw, a paleoecologist at Monash University in Victoria, Australia, were the lead authors of the study. The study is based on data collected as part of ongoing research into Australia's historical climate. The research is supported by the National Geographic Society's Committee for Research and Exploration.

Richard Alley is a geoscientist at Pennsylvania State University in University Park and expert on abrupt climate changes such as Dansgaard-Oeschger. He said that, though he disagrees with the paper's conclusion, the authors' work is to be commended.

"The paper is interesting, the data careful, the hypotheses provocative and [they] undoubtedly will stimulate much further discussion," he said.

Alley comes down on the other side of the debate, suggesting that long-term changes in the global climate are driven by changes in the dominant ocean pattern in the North Atlantic, called the thermohaline circulation (see sidebar for an explanation of the thermohaline circulation).

El Niño Record

Distinguishing whether long-term changes in the North Atlantic or the tropical Pacific drive the global climate could allow scientists to better forecast major and potentially damaging shifts in weather patterns.

"ENSO telling the thermohaline circulation what to do in the North Atlantic is very different from the North Atlantic thermohaline circulation telling the monsoon what to do and in turn influencing ENSO," Alley said.

To determine whether El Niño influences weather in the North Atlantic, researchers must first separate the signal of El Niño from the rest of the world's weather.

The task is not easy, as global weather patterns are, well, global. For example, during the Northern Hemisphere's winter, weather in the North Atlantic sweeps across Europe and may spark summer monsoons in Asia.

Turney and his team have found an ancient volcanic crater in northern Queensland, Australia, that is sufficiently removed from the influence of the monsoon and thus experiences weather driven only by the tropical Pacific and is sensitive to changes in the strength of El Niño.

The top 36 feet (11 meters) of the crater, called Lynch's Crater, are covered in peat. "What we've got are various things in the sediment we can measure to get a handle on environmental change when the peat itself was laid down," Turney said.

For example, peat normally forms in wet, swampy conditions, but when drier conditions prevail, such as during El Niño, the top part of the peat dries out. When the peat dries, it becomes vulnerable to bacteria, which cause the peat to decompose.

By examining the level of decomposition in the layers of peat and associated factors such as the ratio of sedge (which is indicative of wet, non-El Niño, weather) to grass (which is indicative of dry, El Niño, weather), the researchers were able to establish a climate record.

Glacial Surge

The Lynch's Crater climate record shows dry periods that last for about 1,500 years, indicating frequent El Niño events that Turney and colleagues say are correlated to the Dansgaard-Oeschger events in the North Atlantic.

The researchers hypothesize that during the last ice age these 1,500-year periods of frequent El Niño activity caused more evaporation in the Pacific and warming across northern North America. This in turn either melted the ice so that it slipped into the sea or caused more snow to fall and created a surge of glacial ice towards the sea.

Either way, the influx of melted ice disrupted the North Atlantic thermohaline circulation, which works like a giant conveyor belt transporting warm, tropical waters from South America up to Greenland, where it gets colder, saltier, and denser. This dense water sinks and drifts along the ocean bottom back to South America.

"If you can upset that by putting a whole lot of fresh water [in the North Atlantic], it forms a cap and doesn't allow salt water to sink, affecting climate around the world," Turney said.

According to Alley, the hypothesis is interesting "but far from proven."

He said that changes in methane—swamp gas from tropical wetlands—are recorded in bubbles in ice cores and occurred at the same time as Dansgaard-Oeschger events in the North Atlantic. If El Niño was the trigger, the methane would be expected to occur much earlier than the Dansgaard-Oeschger events because it takes a while for ice to melt.

Alley also said that the timing for ice sheets to surge in response to increased snow fall or melting is likely to be a bit random, making "it unlikely that one would see a direct, tight correlation in time between a tropical forcing and ice-sheet surging in the far north."

Turney, who anticipates this paper sparking debate, will continue to refine and extend Australia's historical climate record, in hopes of better preparing the world for future climate changes.

For more on the climate and the environment, scroll down for related stories and links.
 

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