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Coral loss linked to El Nino


The World Today Archive - Friday, 27 October , 2000  00:00:00

Reporter: John Stewart

COMPERE: Coral lies at the heart of a breakthrough made by American scientists in coming to an understanding of one of the world's most complex weather patterns, the El Nino phenomenon. They've found that coral in the Pacific can be used to measure El Nino cycles over the past 150 years.

In Australia many farmers are familiar with the El Nino cycles and the La Nina effect, the other one, the reverse, which has been linked to drought and its opposite.

Julia Cole from the University of Colorado has been telling John Stewart that coral is forcing scientists to rethink their theories about El Nina, which now appears to be far less predictable than it was ever first thought.

JOHN STEWART: Why coral, why did you choose to study coral in the first place?

JULIA COLE: Well, our interest is in the history of El Nina variability and we know that corals can live for several centuries and they grow skeletons that often have annual bands. So we can use those annual bands to construct a time history of coral growth. So corals were a good choice for us because they have this combination of long life and an accurate calendar within the samples.

JOHN STEWART: And what do you actually measure, is it the size of the coral or a certain part of it that indicates the temperature changes in the ocean?

COLE: No, what we're measuring is the chemistry of the coral skeleton and in particular we measure the relative abundance of a rare form or isotope of oxygen, that's oxygen 18. It turns out that when corals produce their skeletons during warm and rainy periods they tend to take up relatively less of this rare isotope of oxygen, and during cooler drier periods they tend to take up relatively more of the rare form.

JOHN STEWART: What have you been able to learn about the history of El Nino and the patterns that it works through?

COLE: One of the things that we found is that in the mid to late nineteenth century we see a pattern of El Nino that is very different from the late twentieth century pattern. In the late twentieth century pattern typically we see El Ninos that come and go every few years, they may last for a year or two. So in a typical decade you might see a couple of El Nino events and a couple of La Nina events, which is the opposite of El Nino.

In the mid to late nineteenth century we see a very different pattern, we see El Nino and La Nina events that last much longer, they may last for up to five or six years, and you may see only one El Nino in a decade and one La Nina in a decade. So the whole variation of El Nino is slowed down and the individual events are much longer lived.

JOHN STEWART: What can Australian farmers expect in the next few decades in terms of the type of cycles that El Nino is working through?

COLE: Getting a simple prediction from our study is not possible. One of the things that we see is that we tend more rapid El Ninos now than we did in the past, but we don't see a simple relationship between, say, a warming climate and the frequency of El Ninos. It's not like you're putting a pot on the stove to boil, it's more complicated than that.

JOHN STEWART: So the coral allows scientists to understand what's going on behind the scenes, the mechanisms behind El Nino and La Nina rather than being a direct indicator or predictor.

COLE: I guess what I'm saying is that we don't know as much about El Nino as we thought we did. We used to think it had a very predictable time scale and what we're seeing is that it doesn't have such a constant time scale, the time scale of El Nino can vary quite a lot. So in a way it's a heads up that we don't have as much certainty about that as we might have thought.