What coral could have told scientists ages ago

El Niño is a climate event with a past, as Andrew Goudie explains.

In spring 1997, the American people discovered El Niño, and in the ensuing months there developed what some claimed to be "the climate event of the century". Changes in water temperatures in the Pacific Ocean caused enormous climate disruption on the west coast of the United States and in the southern states. As storm after storm caused flooding and landslides, California became a more unpleasant place in which to live.

In the Yankee heartland of the northeast, however, the unusually mild winter had many benefits, and the impact on the US as a whole may have been positive. Scientists claimed a great deal of credit for predicting the event and its impacts, but questions were asked about whether its intensity could have been exacerbated by global warming.

However, although the El Niño of 1997-98 attracted more attention from society, scientists and politicians than any previous climatic event, the phenomenon itself has been known for a long time. Strong sea water warming on the west coast of south America has been recognised as a periodic phenomenon by generations of fisherman in Ecuador and Peru. In strong El Niño warming years, their fish catches would dwindle and heavy floods and rainfall would occur in the normally dry Peruvian desert.

Moreover, it was in the 1920s that Sir Gilbert Walker, director-general of observatories in India in the era before computers, used the services of teams of Indian clerks to process copious amounts of statistical data. He established that oscillations in one area could have consequences ("teleconnections") at widely spaced localities. He recognised, for example, as a general principle that, when air pressure is high in the Pacific Ocean, it tends to be low in the Indian Ocean from Africa to Australia.

This has important climatic implications. Not only has the El Niño phenomenon been known for a long time, it is a phenomenon that has a long history. Notwithstanding claims from the United States that 1997-98 was the strongest El Niño in history, historical records show that stronger and longer El Niño events have occurred in the past. This is true of some Indian droughts, including those of 1596, 1629-33, 1685-88 and 1788-1793.

This archival story is recounted in the first chapter by the editors. Archives are, however, not the sole source of historical information on El Niño. A more novel repository of information is provided by massive corals. These typically grow 1-2cm annually, and trace elements, isotopes and organic compounds included in their skeletons are affected by climatic factors. These can be sampled and analysed at intervals corresponding to less than a month of coral growth. In other words, they furnish a high-resolution record that extends over hundreds of years. Michael Gagan and John Chappell describe how annual flooding of tropical rivers can be identified from ultraviolet fluorescent bands in long cores drilled from massive Porites corals growing in the inshore waters of the Great Barrier Reef. Malcolm McCulloch and colleagues describe the use of trace-element ratios in corals to provide information on past sea surface temperatures and hence potentially identify El Niño events. Such studies form the first half of this book.

Most of the second half discusses the question of El Niño impacts. One of these impacts is on health, for weather anomalies can result in natural disasters, affect human nutrition and lead to changes in disease vectors. Excessively wet years, for example, can cause dangerous floods, overload sewerage systems, lead to a spread of cholera, or flush out mosquito breeding sites. Another impact is famine. The El Niño Southern Oscillation-related drought in Irian Jaya, Indonesia, in 1997-98 not only caused hardship because of water shortages, it was also associated with severe frosts that eliminated many garden vegetables (especially tubers) and trees. There were also severe bush fires in Irian Jaya and in Kalimantan and Sumatra. These fires burnt 9.75 million hectares of forest and released between 250 and 1,000 million tons of carbon into the air. They were the cause of severe air pollution over large parts of Southeast Asia.

Richard Grove and Chappell have chosen a fascinating theme, and some of the studies they have brought together are major contributions. However, it is far from being the definitive work on El Niño , partly because of its limited geographical scope and partly because some of the chapters are rather perfunctory. It brings together a somewhat eclectic and variable group of papers that is strong in two prime areas - an overview of the history of El Niño and the impacts of the 1997-98 event in Southeast Asia.

Andrew Goudie is professor of geography, University of Oxford.