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December 25, 2004
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Heat Islands - It’s no secret that cities are hotter than their surroundings. But scientists have found that some cities are so hot thet they’re actually changing their own weather. (9/7/00)

One Hot Year - A two year La Niña may finally be coming to an end, but it’s still going to be a hot summer. (8/10/00)


QuickTime movie of coral drilling by Robert Dunbar

Corals and Short-term Tropical Climate Variability

National Drought Mitigation Page

Frequently Asked Questions about El Niño and La Niña

Climate Prediction Center

How Has the Earths Climate Changed? (QuickTime movie)

Coral Paleoclimatology

Paleoclimatology Home Page

Many Coral Reefs Nearly Dead

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image: NASA

As we’ve all seen all too well in recent years, El Niño and its flipside, La Niña, cause extreme effects on weather across our planet. These climate cycles begin in the tropical Pacific Ocean and bring everything from severe storms to devastating droughts around the globe.

But while meteorologists and climate researchers have studied the phenomena for some fifty years, El Niño has been causing havoc for far longer. Now researchers have found a way to study the El Niños and La Niñas of the past, with the help of coral reefs. And a new study raises the question of whether the weather extremes are likely to continue, and even get worse.

Drilling into El Niño’s past

What are El Niño and La Niña?

El Niño and La Niña are oscillations in sea surface temperature (SST) of the Pacific Ocean along the equator. The temperature changes are caused by the relaxation of the easterly trade winds and the development of an anomalous westerly wind. These variations in SST, up to 4C in some years, account for wind and rainfall changes that lead to flooding and drought worldwide. The entire El Niño Southern Oscillation (ENSO) usually lasts three to seven years. The ENSO can vary greatly; some cycles cause great havoc and misery, while others pass by almost unnoticed.

The two parts of an ENSO, which scientists are still attempting to ascertain the cause of, are:

  1. El Niño, (the Christ Child); the warm part of the cycle. El Niño usually begins around Christmas, which accounts for its name.
  2. La Niña (the girl child); the cold fluctuation of the cycle. Strong trade winds and cool water are La Niñas features.

(from NOAA)

The National Oceanic and Atmospheric Administration, and other agencies have used temperature buoys to monitor the changes in sea surface temperature (SST) which characterize the El Niño/La Niña cycle. But to accurately predict El Niño’s behavior, scientists need to know more about its past, so they turned to a marine organism that lives for hundreds of years in the oceanthe coral.

A team of researchers from the University of Colorado reports in todays issue of the journal Nature that they have measured El Niño cycles back some 155 years by analyzing samples from coral reefs in the birthplace of El Niño.

Julie Cole, now at the University of Arizona, and colleagues took samples from coral beds in the equatorial Pacific Ocean, where El Niños develop. "We can get a lot of information on past climate from coral skeletons. They are much longer records than we can get from instrumental data," Cole says. Cole likens coral skeletons to tree rings in their ability to give scientists a picture of the past.

Coral skeletons are ideal because the tiny animals use the elements in the seawater to build their skeletons, including a rare form of oxygen called Oxygen-18. "When the water is warm and when the rainfall is heavy, the corals tend to take in less Oxygen-18, and more of a very common form, Oxygen-16," says Cole.

By studying the proportions of Oxygen-18 in the skeletons, the researchers discovered that El Niño cycles have changed over the past two centuries. "What we learned from this study is that on long timescales such as the last century and a half, there have been changes in the frequency and duration of El Niño events unlike anything we’ve seen in the most recent decades," says Cole. "In particular, in the mid to late 19th Century what we found was that El Niño events tended to occur less frequently and they lasted longer, so for example in a typical decade we might have, instead of two El Niños and two La Niñas, we might see just one El Niño and one La Niña. But they would last much longerinstead of lasting one or two years, they would last for five years or more."

X-ray of coral with "growth" layers indicating ocean weather patterns.

And, as the tropical Pacific region has grown warmer and wetter in recent decades as part of the global warming phenomena, the El Niño-La Niña cycle has sped up, says Cole.

Will that speedier cycle cause more intense weather extremes as the world warms? "This study doesn’t give us a firm basis for predicting that directly," says Cole. "For example, we can’t say we know El Ninos will be more frequent as the world warms; we can’t say we know we’re going to go to a slower pattern. What we do know is that as background climate changes, the period of El Nino will also change."

The costs of El Niño and La Niña

Cole points out that El Niños sister, La Niña, often leads to prolonged droughts in the Southwest United States. "In 1988, there was a drought that ended up costing something on the order of $40 billion. That was a one-year drought," states Cole. Cole notes that their study, which was funded by the National Science Foundation, has already linked one prolonged dry spell from the past to La Niñathe drought of 1859 to 1865, which devastated the Southwest and Great Plains.

Prolonged La Niña conditions also spawn more frequent and stronger hurricanes. El Niño can cause extreme floods, heat waves, snowstorms, higher heating bills in winter, higher food prices and other weather-related miseries worldwide. Scientists hope that, if they can unmask the mystery of how El Niño occurs, they can understand and predict coming cycles more accurately, which will save lives and livelihoods all over the world.

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