The Heat Is Online

ENSO Intensity Linked To Warming

Coral reef exposes worst El Ninos ever are now

Reuters News Service, Jan. 28, 2001

WASHINGTON - An ancient coral reef in Papua New Guinea has let scientists check up on the history of El Nino and suggest the weather pattern, blamed for droughts, floods and storms, has never been stronger than it is now.


Other experts said their findings show that El Nino, which can disrupt weather around the world, is probably unpredictable and affected by subtle changes in climate - bad news for people trying to forecast El Nino influences.

The international team of scientists was able to track El Nino going back 130,000 years by reading annual bands in the fossilized coral much like scientists read the annual bands in sawn-off tree trunks.

Coral reef terraces in the Huon Peninsula of Papua New Guinea lie in the north coast of the island in an area particularly affected by the El Nino Southern Oscillation (ENSO), a pattern of weather and currents named after the Christ child by Pacific fishermen who noticed it around Christmas time.

Corals are small animals that build themselves a hard skeleton and live in colonies that eventually form reefs. As they slowly grow, they build into their skeletons a record of the current sea surface temperature and salinity.

Geological forces are pushing the ancient coral upwards to where it can be seen and sampled.

"This situation makes it possible to sample corals which grew during periods when climatic boundary conditions were substantially different to those today," the researchers wrote in their paper, published on Friday in the journal Science.

They traced oxygen isotopes in the coral. Isotopes, or chemical variants, can be detected using special equipment.

"The samples indicated that El Nino was never more intense than the events of the last hundred years," David Lea, a professor of geological sciences at the University of California Santa Barbara who helped write the study, said in a statement.

"Over the last 100 years we have very accurate records of El Nino, with 1982-83 and 1997-98 being the largest events on record."

Lea said global warming may be to blame but this is not certain from his team's study.

"Of course, everyone wants to know if the intensity of these large events is somehow related to global warming. Our data suggest that the behavior of the tropical Pacific over the last 100 years is atypical, but it does not pinpoint which factors modulate El Nino," he said.

But Julia Cole of the University of Arizona, who studies El Nino and ancient climate patterns, said the study fits in with other studies that show background climate change - whether caused by people or other forces - can affect El Nino.

"Their data show really clearly that in the early part of the 20th century, before you had global warming, you still had very strong El Ninos," Cole said in a telephone interview.

"From their data alone you cannot blame warming. But ... what that suggests is that background climate change in the future, whatever the cause, is also likely to have an influence on El Nino. We can't assume that El Nino will keep on ticking along in the same way that it has."

And that is a frightening thought for weather experts and government officials who would like to be able to anticipate and prepare for weather chaos wrought by El Ninos.

Centuries ago El Nino occurred every two to 15 years, but recent research shows the pattern has become more frequent. It is blamed for causing much of the $89 billion in weather-related damage in 1998.

REUTERS NEWS SERVICE

Ancient Coral May Hold Hint of Worsening Weather Cycle
By Andrew C. Revkin , The New York Times, Jan 26, 2001

Reading chemical clues etched in coral up to 130,000 years old, geologists say it appears that El Niño and La Niña, the pulses of warm and cool Pacific waters that can lead to Indonesian droughts and Idaho blizzards, have almost never before reached the sustained intensity seen in the past century.

The research also shows that the swings in Pacific temperatures tend to increase in warmer times — like now — but weakened by as much as 50 percent during the protracted cold of the last ice age.

The scientists say their findings provide a hint, but only a hint, that additional global warming could further intensify the cycle, the El Niño- Southern Oscillation, or ENSO, and thus bring more bouts of destructive weather. The cycle is one of the dominant forces shaping year-to-year storm and precipitation patterns.

The researchers, led by Dr. Alexander W. Tudhope of the University of Edinburgh, describe their work in today's issue of the journal Science.

"Their results provide strong support for the idea that ENSO may be more responsive to global change than previously thought," said Dr. Julia Cole, an expert in ocean-climate links at the University of Arizona, in a separate paper in Science that evaluates the research.

In a telephone interview, Dr. Tudhope emphasized that there was no way to tell yet whether the recent period of sharp Pacific temperature swings was related to warming from rising levels of heat-trapping greenhouse gases like carbon dioxide. He pointed out that the coral record indicates that the recent period of intense swings started before 1900, long before the peak of emissions of heat-trapping gases from human activities.

But he said further warming from rising levels of these gases could further stir El Niño and La Niña, and thus global weather patterns.

"We're already at the outward bounds of natural variability," Dr. Tudhope said, explaining that the global average temperature is now as warm as it has been for tens of thousands of years. "From here, we're going into uncharted territory really."

He also emphasized that the climate chronology drawn from the corals was very fragmentary, representing only about a dozen "windows on climate," each covering several decades to a century, spread over the last 130,000 years.

But other geologists and climate experts said the work appeared to provide the broadest view yet of short-term flutters in tropical weather patterns, with measurable shifts in the chemistry of the coral skeleton reflecting the subtle changes in the environment.

"This is exactly the kind of information we need," said Dr. S. George Philander, a professor of geo sciences at Princeton. "We know the Earth was colder 20,000 years ago or warmer 120,000 years ago, but we have no information about how high- frequency oscillations changed in the past."
The ENSO cycle is important because of the great reach and consequence of the destructive weather it leads to.

El Niño, the giant patch of unusually warm water, can persist for several years, disrupting fisheries in the eastern Pacific, producing sustained rough weather along the West Coast of the United States and causing harsh droughts in places like Indonesia, which several years ago battled enormous wildfires resulting from El Niño.

La Niña, the cold variant, typically drives a relentless freight train of bad winter storms that hit the Pacific Northwest and then rumble east across the northern states, while hot, dry summertime spells settle in the West.

A La Niña event is under way and is expected to fade in a few months, federal weather officials say. A new cycle is inevitable, but its timing and strength are largely unpredictable.

Dr. David W. Lea, an author of the study and a geology professor at the University of California at Santa Barbara, said the coral data should help improve computer models used to try to predict future ENSO cycles and global climate.

To divine the cycles of the past, the researchers zeroed in on fossil reefs and living ones in eastern Papua New Guinea, where the climate is strongly influenced by the Pacific temperature shifts.

Corals record changes in local rainfall and temperature as subtle variations in the ratio of two isotopes of oxygen incorporated in their calcium carbonate skeletons. Cores are drilled through boulder-size corals and successive thin slices reveal the change in oxygen composition — and climate — back in time, Dr. Tudhope said.

A challenge for climate sleuths has been to find a place holding a series of corals dating back into and beyond the last ice age, when sea levels were more than 300 feet lower than they are now because so much water was locked up in glacial ice.

Normally, such fossils would be deep underwater, but the geologically active part of New Guinea where the team collected its samples has been rising from the sea at a fast pace, exposing ancient reefs that would otherwise be nearly impossible to find.

To collect a record of the last 100 or so years, the scientists donned scuba gear and drilled cores from living boulder corals. It was those samples that showed the most sustained period of sharp temperature shifts of all — 120 years long.

There were two previous peaks of ENSO activity that were as strong, but each lasted only a few years, according to the study.

The researchers speculate that the recent sustained ENSO period may be the result of two influences superimposed on each other: today's warm climate and a variation in Earth's orbit that tends to weaken westerly trade winds in the Pacific.

When the winds diminish, the warm pool of El Niño waters has an easier time spreading eastward and changing weather patterns, the researchers said.

For the moment that remains a theory, Dr. Tudhope said. The best way to answer such questions, and to get an inkling of coming cycles, he said, is to go back and try to fill in the blanks that still dominate the calendar of climates past.




Science, Jan. 26, 2001
Variability in the El Niño Southern Oscillation Through a Glacial-interglacial Cycle
Alexander W. Tudhope 1*, Colin P. Chilcott 2, Malcolm T. McCulloch 3, Edward R. Cook 4, John Chappell 3, Robert M. Ellam 5, David W. Lea 6, Janice M. Lough 7, Graham B. Shimmield 8
1 Department of Geology & Geophysics, Edinburgh University, Edinburgh, EH9 3JW, UK.; Australian Institute of Marine Science, Townsville, Queensland 4810, Australia.
2 Department of Geology & Geophysics, Edinburgh University, Edinburgh, EH9 3JW, UK.
3 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.
4 Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, New York 10964, USA.
5 Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK.
6 Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA.
7 Australian Institute of Marine Science, Townsville, Queensland 4810, Australia.
8 Dunstaffnage Marine Laboratory, Oban, Argyll, PA34 4AD, UK.
* To whom correspondence should be addressed. E-mail: sandy.tudhope@ed.ac.uk.

The El Niño Southern Oscillation (ENSO) is the most potent source of interannual climate variability. Uncertainty surrounding the impact of greenhouse warming on ENSO strength and frequency has stimulated efforts to develop a better understanding of the sensitivity of ENSO to climate change. Here we use annually-banded corals from Papua New Guinea to show that ENSO has existed for the past 130,000 years, operating even during 'glacial' times of substantially reduced regional and global temperature and changed solar forcing. However, we also find that during the 20th century ENSO has been strong compared to ENSO of previous cool (glacial) and warm (interglacial) times. The observed pattern of change in amplitude may be due to the combined effects of ENSO dampening during cool 'glacial' conditions and ENSO forcing by precessional orbital variations.

Science 10.1126/science.1057969
Copyright © 2001 by The American Association for the Advancement of Science.