More Frequent and Severe El Niños Expected

By William J. Cromie

Gazette Staff

Ocean temperatures in the equatorial Pacific increased suddenly in 1976, leading to the two most severe El Niños this century, Harvard University researchers have found. The warming portends more frequent and destructive storms, floods, and droughts in the United States, they believe.

"Since 1976, El Niños have become more common, longer, and more intense," says Daniel Schrag, associate professor of geochemistry. "We have made new observations that may explain why this happened."

Schrag thinks the change may be related to global warming. Earth's temperature has been rising since 1976, particularly in the 1990s, which have seen the hottest years on record. At the same time, the two largest known El Niños took place in 1982-83 and 1997-98.

El Niño is part of a natural climate cycle wherein a huge pool of warm water moves across the Pacific from southeast Asia to the coast of South America every three to seven years. During normal years, trade winds blow warm water westward and pile it up near Asia; when the winds slacken, the warm mass of water flows back east. This ocean flood usually arrives during the Christmas season, and so has been named El Niño for "Christ child."

Beside upsetting weather in the United States, the tremendous load of warm water off Ecuador and Peru kills fish by the billions and brings devastating torrents of rain to arid coastal lands. Monsoon-like rains that normally nourish rice and other crops in southeast Asia dry up, causing economic hardship and political instability.

"You can think of El Niño as an ocean-wide see-saw, or slow sloshing movement," Schrag explains. "When water off South America is warmer than normal, it heats the air above it, changing the pattern of winds and storms. The effects are strongest in the tropical Pacific, but can be detected worldwide."

These effects include more rain on the Pacific coast and in the southeastern U.S.; warmer winters in Canada and the northern U.S., drier, hotter weather in the Midwest, and fewer hurricanes along the Atlantic Coast. The stronger the El Niño, the greater the effect. The 1997-98 cycle was the strongest this century, causing extensive storms and floods in California and unexpected tornadoes in Florida, as well as contributing to scorching droughts in Texas and Oklahoma.

Climate Shifts

Schrag and Harvard post-doctoral researcher Thomas Guilderson discovered the 1976 warming as part of an ongoing program that uses corals to investigate past patterns of ocean circulation and climate. Growth of these plantlike sea animals, which encase themselves in rocklike coverings, is influenced by temperature, and their hard external skeletons become banded like tree rings. In Schrag's laboratory, at the Department of Earth and Planetary Sciences, measurements of the skeletons reveal information about the history of the environment in which the corals grew.

Schrag and Guilderson discovered key variations in the chemistry of corals growing in the Galapagos Islands, located on the equator about 600 miles west of Ecuador. Specifically, coral skeletons that formed after 1976 contain increased amounts of carbon-14, a radioactive form of carbon. Nuclear bomb tests in the 1950s and 1960s peppered the air with the stuff and it had since fallen into the sea. The scientific sleuths found that its abundance in Galapagos corals increased suddenly in 1976 at times when warm water from the west upwells to the surface. These upwellings are not part of El Niño floodings

Other studies show that global temperatures have been rising during this time. The first seven months of 1998, for example, have witnessed the warmest temperatures of this century.

"We believe the increases in carbon-14 and ocean temperature during the upwelling seasons after 1976 signal a change in the vertical structure of the eastern part of the equatorial Pacific Ocean," says Schrag. "This change occurred at the same time as what some oceanographers refer to as the "1976 Pacific climate shift," when El Niño became more frequent and intense."

Hundreds of feet below the surface of all oceans lies a boundary called the thermocline. It separates the warmer upper part of the sea, which is continually mixed by winds, from colder, quieter, deeper ocean. In the equatorial Pacific, the thermocline is tilted. During normal years, it extends to a depth of about 600 feet on the Asian side, but only to about 150 feet on the South American side. This tilt is caused by the trade winds pushing warm water westward. During an El Niño, when the warm pool move east, the thermocline becomes less steep.

Schrag and Guilderson conclude that in 1976

the thermocline off South America see-sawed lower than in the past by a small amount, probably less than 50 feet. It doesn't sound like much, but the additional mass of warm water changes ocean temperatures and air circulation, and likely explains the changes in El Niño.

Forecasting the Future

"Our work apparently explains why El Niños are getting stronger and coming more often," Schrag comments, "but not what caused the change in the thermocline depth. There are several theories; at this point, however, they're all too speculative to favor one over the others."

The bigger question is whether the change is temporary or the result of global warming. "I can't answer that," admits Schrag. "However, it would be surprising to me if El Niño wasn't affected in some way by the rise in temperatures globally. Specifically, it seems likely that the unusual pattern of El Niño we have seen for the past 22 years is related to global warming. We cannot prove that, but if I owned an insurance company, I'd be very concerned."

The corals studied by Guilderson and Schrag died after the 1982-83 El Niño, the second worst of the century. Schrag now has found both newer and older corals, and plans to investigate the El Niño-climate connection in both directions.

Going forward, Schrag hopes to obtain more clues about the cause of the thermocline shift. Going backward, he will search for evidence of what El Niños were like in warmer climates of the past and, therefore, what they may be like in warmer times to come.

Other studies have found that Earth about 5,000-6,000 years ago was slightly warmer than today. "Figuring out what El Niño was doing then is important for understanding what it will do in 2050 when we expect Earth to be warmer," Schrag explains. "In this case, the past may be a natural analogue for the future.

"We are also studying fossil corals from Indonesia to try to reconstruct the history of droughts brought on by El Niños," Schrag continues. The tree-ring-like history of growth enables researchers to determine years that were abnormally dry.

Schrag believes governments should decrease the effects of global warming by reducing the burning of oil, natural gas, and other fuels that load the air with carbon dioxide and other so-called greenhouse gases. These gases prevent heat from escaping into space, thereby gradually warming our planet.

"Skeptics -- those who think the warming is only temporary, or who have economic reasons for not wanting to take action -- get lots of media exposure," Schrag says. "As a result, the public has the impression that there is a large amount of uncertainty about climate warming. But most climate scientists believe the risks are real and large, regardless of whatever uncertainty remains.

"Changes in El Niño may be one of the most dramatic impacts of global warming," Schrag believes. "Scientific understanding of El Niños is good enough that we can predict them six months to a year in advance. That will soften the impact somewhat as it can give us time to prepare the way people prepare for a cold winter in New England. If El Niños do become even more frequent and intense, and I think they will, the world will have some difficult adapting to do."