Ensemble Estimation of Central Tropical Pacific Climate over the last 6,000 Years

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Significant uncertainty surrounds future climate change impacts in many regions of the world, particularly regarding potential changes in tropical rainfall levels, and their extremes. Much of the uncertainty stems from poor constraints on future trends in the El Nino-Southern Oscillation (ENSO), a natural climate cycle that affects temperature and rainfall patterns around the globe, with its largest effects in the tropics. The instrumental record of ENSO is too short to isolate statistically robust trends, so this study seeks to provide hundreds of years worth of additional observations of past ENSO variability in an effort to: 1) quantify the natural baseline for this globally-relevant climate cycle, and 2) assess whether recent changes fall outside the range of natural variability. Using a suite of well-dated and high-resolution coral records from the central tropical Pacific, the study will extend the record of ENSO and longer-term climate patterns in the heart of the ENSO region. As such, it will inform future projections of tropical Pacific climate and constrain regional rainfall trends throughout the tropics and beyond. Tropical Pacific climate trends are difficult to isolate from the short instrumental record of climate, owing to significant tropical climate variability on interannual to decadal timescales. Recent paleoclimate records hint that the El Nino-Southern Oscillation (ENSO) may be intensifying in response to anthropogenic climate forcing, implying that tropical rainfall extremes may increase in coming decades. Such records also show that natural ENSO variability is much larger than previously thought, requiring a larger number of paleo-reconstruction to define natural ENSO variability and distinguish it from possible anthropogenic signals. The last 6,000 years is an particularly important baseline for probing past ENSO variability, as global climate models show that ENSO may be sensitive to a range of natural climate forcings over this period. This project involves generation of a highly-replicated reconstruction of both mean climate and climate variability in the tropical Pacific over the last 6,000 years using hundreds of fossil coral records from Christmas Island (2ºN, 157ºW), which lies in the heart of the El Niño-Southern Oscillation (ENSO) region. The research team, which includes a graduate student, will compare the new reconstructions to numerical climate model simulations of tropical Pacific paleoclimate variability, probing the relationships between climate forcing, mean climate state, and ENSO variability.