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Updated 17 November 2004
Consequences (title)
Consequences Vol. 5, No. 2, 1999
                                                     
 

 

 

 

 

 

 

 

Article Overviews

El Niño and The Science of Climate Prediction

Questions

Have El Niños always been around? How long have they been recognized, or understood? What causes these recurrent changes in the climate system, and why do they come and go so irregularly? Are they all alike? Have they become more frequent, longer lasting, or systematically more severe in recent years? Might they be affected by global greenhouse warming? How accurately and how far in advance can they be predicted? Why are we able to predict El Niños many months before they occur when we don’t seem able to forecast the daily weather more than a few days in advance?

An Assessment

El Niños are fundamentally a warming of the surface waters of the equatorial eastern Pacific Ocean, that induces major changes in atmospheric circulation, and hence the weather, over a large fraction of the Earth. They come and go very irregularly — roughly three or four years apart, on average — and for a year or so dominate the course of the weather in many of the most heavily populated regions of the world. In the last quarter century they have been more frequent, less regular, and somewhat more severe than in the preceding 100 years. These changes could conceivably be linked to systematic global warming, although the connection is not yet certain. Truly revolutionary advances have been made in the last fifteen years in understanding the causes and mechanisms that drive El Niños (and the more general Southern Oscillation phenomenon), in large part through an intensive internationally-supported tropical ocean observing program. Sophisticated computer models that replicate the physical interactions between the ocean and the air above it can now reproduce most of the features of El Niños and La Niñas. These powerful analytical tools, coupled with a new and more extensive tropical ocean observing system, now make it possible, for the first time, to predict the occurrence of these highly important events several seasons in advance.

Consequences

Our newfound ability to foretell the occurrence of El Niños and La Niñas is significant for several reasons. It is the first time that scientists have been able to deliver accurate predictions, a season or more in advance, of seasonal and interannual changes in regional and global climate. While these long-term climatic predictions are necessarily cast in terms of probability of occurrence, we have never had this clear a picture, so far in advance, of what is coming down the road. It is also significant in that the potential benefits to individuals and societies and nations are immediate and wide reaching. There are institutions already in place and at work, like the IRI, that are dedicated to the practical application of these new capabilities. Finally, it demonstrates what organized science can accomplish that single investigators, acting on their own, cannot. While individual contributions were undeniably important, it was Big Science, in the form of the international Tropical Oceans/Global Atmosphere (TOGA) program, that mobilized cutting-edge oceanographers and atmospheric scientists in the Northern and Southern Hemispheres; held them to a well-defined task for ten years; and when its work was done, turned itself off.

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The Great El Niño of 1997-98: Impacts on Precipitation and Temperature

Questions

How is the weather affected, where, and for how long, when an El Niño or a La Niña comes to town? What continents and countries feel these effects? Are the impacts on local temperature and precipitation sufficiently great to make a difference in the lives of ordinary people? How many El Niños have come our way in the past century or so, or does anyone know? How often do they occur? In what ways was the most recent of these events different from those that had happened before?

An Assessment

The climate of the Earth, or of any region or locality, is continually changing, due to a number of different and often competing causes that act at different scales of time, from seasons to years to eons. For much of the Earth, and almost certainly for our own country, the most significant and now best known of the interannual or year-to-year changes are the distinctive, seasons-long shifts in patterns of temperature and precipitation that are identified with El Niños and La Niñas. They are initiated by systematic changes in the surface temperature of the tropical Pacific Ocean that perturb the global atmospheric circulation. The resulting changes affect the weather over much of the Earth, in a wide band of latitude, about 45° on either side of the equator, that includes India; Southern Asia; the East Indies and Australia; the islands of Oceania; North, Central and South America; and parts of Africa. They are traceable to changes in the surface temperature of the equatorial Pacific Ocean, occur irregularly but on average about twenty-five times in each century, and differ in duration and intensity. The most recent El Niño was unusually severe and lasted uncommonly long, from 1997 through much of 1998. In the course of its long stay the 1997-98 El Niño was the direct cause of droughts and floods and temperature extremes that affected, in one way or another, most of the human population of the Earth.

Consequences

The unusual El Niño of 1997-98 came at a time, fortuitously, when practical prediction schemes for these events were just becoming available. As a result, steps were taken in anticipation in many affected countries that would not have been possible in earlier years. The experience was also beneficial in illuminating the limitations of current models and forecast schemes, for in some areas — notably Australia and India — some of the weather extremes that were anticipated did not materialize. In the course of the last ten years we have been reminded again and again that El Niños are not all born equal, nor look alike. Forecasts will improve in the future, but we need to learn how to make the most of probabilistic projections. There is every reason to expect that with more experience and greater understanding, the benefits of long-term climate projection will be felt in almost every sector of our lives, and by almost everyone.

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The Application of Climate Information

Questions

How do we make the most of climate forecasts that are made a year or so in advance? To be most useful, how far ahead should such forecasts be issued, and for how small or large a geographic area? How can we learn to use the forecast information wisely? What have we learned from past experience? How much of the task of collecting data, preparing and disseminating a forecast, and issuing and following up on a forecast would rightfully fall on the federal or state government? Is there an appropriate role for private industry in this process, as is the case today for forecasts of the day-to-day weather?

An Assessment

It is now possible to make relatively reliable forecasts of seasonal or annual climate, based on what we now know of the El Niño phenomenon. We also have considerable experience in constructing and refining the analytical models that make these forecasts possible. We have most to learn, at the moment, about how to issue long-term forecasts in a manner that is most helpful to the public and private sectors, for the issues involved are more diffuse and less tangible than the physical workings of the oceans and the atmosphere. Decision makers and society itself have very little experience in responding, responsibly, to environmental changes that are projected to arrive many months or a year in the future. What is more, long term projections are and probably always will be issued in terms of probabilities, for we can never predict the future climate with absolute certainty. Significant benefits can be realized only over a long sequence of trials, on the part of those who use the forecasts as well as those who create and issue them.

Consequences

Though not the case ten years ago, it is now within our reach to foretell the times and probable intensity of coming El Niños and La Niñas, up to year or so in advance. Based on past experience, we can anticipate in every decade two or three of these climatic disruptions, which in the past have arrived at our doorstep unannounced, to wreak meteorological havoc, in one way or another, in many parts of the world. We know how to deal with warnings, issued a few hours or a few days in advance, of tornadoes and hurricanes and floods, but we have far less experience in dealing with less specific changes in the weather and climate when they are announced so far ahead. We need to develop strategies for making the most of probabilistic forecasts, and to learn how to respond responsibly to what will happen not tomorrow or next week but next season or next year. The pay-off, both economically and in terms of human welfare, could be tremendous.

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