The South Pacific ocean current, El Ni￱o, and its associated wind movements, forms the most potent source of year by year climate change in the world. Now an international team of scientists have charted its activities over 130,000 years and raised questions about its link to global warming. Their findings are published today Friday 26 January in the journal SCIENCE*.

Strong El Ni￱o activity in the late twentieth century and increasing consensus about the reality of global warming has encouraged speculation that the two phenomena might be related. This led an international team from Scotland, USA and Australia to explore the variability of the El Ni￱o Southern Oscillation (ENSO) over geological time. The study funded by the Natural Environment Research Council included scientists from Edinburgh University, The Scottish Universities Environmental Research Centre and the Dunstaffnage Marine Laboratory.

This team has reconstructed and analysed the climate of the tropical Pacific over some 130,000 years - a full glacial- interglacial cycle.- concluding that ENSO is not a new phenomenon but has been a continual feature of Earth's climate over a period in which the world's climate has changed from warm (interglacial) to cool (glacial) conditions.

The team were able to reconstruct the weather in Papua New Guinea by the chemical analysis of corals. Corals are particularly useful climatic indicators because they contain annual banding, not unlike tree-rings, which allow researchers to isolate samples that grew over only a few weeks and thereby build up climate records with extremely fine resolution.

Particularly important is the atomic make-up of the oxygen in the coral skeleton. Oxygen is mostly composed of atoms which weigh 16 atomic mass units (amu - where 1 amu is roughly the weight of atoms in the lightest gas hydrogen). However, a few oxygen atoms, roughly 2 in every 1000, weigh in at 18 amu. Critically, the number of O-18 atoms in sea water changes with temperature and corals can be used to record that sea water signal. Hence, by measuring the abundance of the different oxygen atoms in modern living corals and comparing with those in long dead fossil corals, the team were able to build up a picture of how climate has varied over the period.

The climate records indicate that El Ni￱o was active throughout the last 130,000 years and, far from being a modern consequence of global warming, appears to be a pervasive feature of the world's climate. However, one tantalising result is that the strength of the El Ni￱o signal appears to have increased lately to the extent that ENSO was more pronounced during the twentieth century than at any other time in the past 130,000 years. This begs the question whether further global warming might result in a yet stronger ENSO. The new results support the suggestion that ENSO strength is sensitive to changes in global climate conditions, and, therefore, it is reasonable to speculate that ENSO strength may change in some way in the future.

However, the researchers are quick to point out that we do not yet know which specific aspects of the cool glacial climate were responsible for the reduced strength of ENSO at that time. Only when we know this, through further research of this type and integration of these results with coupled climate models, will we be in a position to speculate with some confidence on the likely implications of future greenhouse warming on ENSO dynamics.

The international research team includes Dr Alexander Tudhope and Dr Colin Chilcott from the University of Edinburgh and Dr Robert Ellam of the University of Glasgow who is a member of the Scottish Universities Environmental Research Centre at East Kilbride.

* "Variability in the El Ni￱o Southern Oscillation through a glacial-interglacial cycle" by A.Tudhope et al, SCIENCE, 26.1.01

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For further information contact: Dr Rob Ellam, of the University of Glasgow at the Scottish Universities Environmental Research Centre: 01355 223332 x 130 Or the University of Glasgow Press Office: 0141 330 3535.

First published: 25 January 2001