Variability in the El Nin ˜o– Southern Oscillation Through a Glacial-Interglacial Cycle Alexander W. Tudhope, 1,2 * Colin P. Chilcott, 1 Malcolm T. McCulloch, 3 Edward R. Cook, 4 John Chappell, 3 Robert M. Ellam, 5 David W. Lea, 6 Janice M. Lough, 2 Graham B. Shimmield 7 The El Nin ˜o–Southern Oscillation (ENSO) is the most potent source of inter- annual climate variability. Uncertainty surrounding the impact of greenhouse warming on ENSO strength and frequency has stimulated efforts to develop a better understanding of the sensitivity of ENSO to climate change. Here we use annually banded corals from Papua New Guinea to show that ENSO has existed for the past 130,000 years, operating even during “glacial” times of substan- tially reduced regional and global temperature and changed solar forcing. However, we also find that during the 20th century ENSO has been strong compared with ENSO of previous cool (glacial) and warm (interglacial) times. The observed pattern of change in amplitude may be due to the combined effects of ENSO dampening during cool glacial conditions and ENSO forcing by precessional orbital variations. Coupled ocean-atmosphere interactions root- ed in the tropical Pacific Ocean play a crucial role in modulating global climate on interan- nual (1–4 ), decadal (5–7 ), and, arguably, gla- cial-interglacial (10 5 year) (8, 9) time scales. Best known in this context is the ENSO (10), which has a variable period ranging from 2.5 to 7 years, but usually focused in the 3- to 5-year band. ENSO has gained notoriety over the past two decades due to the unusually strong El Nin ˜o (“warm”) events of 1982/83 and 1997/98, both of which had widespread ecological, social, and economic impacts. Despite recent advances in our understanding of the physics behind the ENSO phenomenon (11), key aspects of the system remain poorly understood. These include the nature and or- igin of variability in the strength and frequen- cy of ENSO and the sensitivity of the ENSO system to changes in climatic boundary con- ditions. This variability, forced and unforced, is crucial to determining the predictability and global impacts of ENSO, now and in a greenhouse-warmed future (12–15). One of the major obstacles to progress in this field has been the lack of instrumental and high- resolution proxy records of sufficient length to reveal the range of natural variability in ENSO and its response to global climate change. Here we use geochemical analyses of an- nually banded massive Porites corals from Papua New Guinea to investigate variability in ENSO at intervals through the last glacial- interglacial cycle of Earth history. As they grow, reef-building corals record climatic in- formation in the chemistry of their aragonitic skeletons. Retrospective analysis of cores collected from large living coral colonies has been shown to be a particularly powerful tool for reconstructing sea surface temperature (SST) and salinity (rainfall) variations asso- ciated with ENSO (16, 17 ). Due to the pres- ence of annual bands in skeletal composition and structure, rapid growth rates (8 to 20 mm/year), and longevity of coral colonies, these coral climate-proxy records can have a temporal resolution of about a month over several hundred years. The same species of massive corals are also well preserved in some late Quaternary exposed reef sequenc- es, from which they may be sampled and accurately dated by U-series techniques. This suite of attributes, allowing annually resolved records to be constructed, makes tropical cor- als possibly unique in their potential to reveal the nature and evolution of ENSO dynamics on 10 3 to 10 5 year time scales. The record of modern ENSO in corals from Papua New Guinea. The north coast of Papua New Guinea lies in the heart of the western equatorial Pacific Warm Pool, an area of exceptionally warm surface ocean, strong atmospheric convection, and high rainfall that plays a pivotal role in ENSO dynamics (Fig. 1). The climate of the region is fundamentally linked to ENSO variability, with relative drought and slightly reduced SSTs characterizing the El Nin ˜ o phase. These climatic variations are recorded in the skele- tal geochemistry of corals living along the coast (18). In this study, we use the oxygen isotopic ( 18 O) composition of the coralline aragonite to reconstruct past ENSO activity. This tracer responds to water temperature through a tem- perature-dependent fractionation [– 0.2‰  18 O/°C (19, 20)], as well as to changes in rainfall, due to the influence of isotopically light (more negative  18 O) rainfall on surface ocean composition. Because SST and rainfall are intimately linked in tropical areas of strong convective rainfall such as the western equatorial Pacific, the two factors combine to produce an enhanced climatic signal in coral skeletal  18 O. That is, the wet and warm conditions during the La Nin ˜a phase of the Southern Oscillation result in deposition of isotopically light coral skeleton, whereas the dry and cool conditions during the El Nin ˜o phase result in isotopically heavy skeletal oxygen (Fig. 2A) (18). The approach taken in this study is to 1 Department of Geology & Geophysics, Edinburgh University, Edinburgh, EH9 3JW, UK. 2 Australian Insti- tute of Marine Science, Townsville, Queensland 4810, Australia. 3 Research School of Earth Sciences, Austra- lian National University, Canberra, ACT 0200, Austra- lia. 4 Tree-Ring Laboratory, Lamont-Doherty Earth Ob- servatory, New York 10964, USA. 5 Scottish Universi- ties Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK. 6 Department of Geological Sciences and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA. 7 Dun- staffnage Marine Laboratory, Oban, Argyll, PA34 4AD, UK. *To whom correspondence should be addressed. E- mail: sandy.tudhope@ed.ac.uk Fig. 1. Location of study sites. R ESEARCH A RTICLE www.sciencemag.org SCIENCE VOL 291 23 FEBRUARY 2001 1511