Collaborative Research: Central tropical Pacific climate variability over the Last Millennium

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2). The project will assemble radiocarbon-dated segments of ancient coral recovered from islands in the central equatorial Pacific to study climate conditions during the last 1000 years. It will focus on two important time periods, the Medieval Climate Anomaly (850-1300 ago) and the Little Ice Age (1400-1850) to look at what factors cause El Nino conditions. Researchers will reconstruct monthly-resolution records of seawater temperature and salinity conditions by targeting growth bands in coral skeletons. This project supports research training educational opportunities for several undergraduate and graduate students. It also supports a research collaboration among two early career scientists and others who have a strong track record of participation in K-16 education and public outreach with an emphasis on broadening diversity and inclusion of underrepresented communities. These efforts will continue with guest lectures at a nearby minority-serving institution and by offering summer research opportunities for local high school students. Improving the accuracy of future climate projections requires a more complete understanding of internal vs. externally forced changes in tropical Pacific climate on a broad range of timescales. Coral-based paleoclimate records have dramatically improved our understanding of interannual climate variability in the tropical Pacific, however, similar insights into centennial-scale variability have thus far remained unattainable due to (i) diagenesis, which can significantly bias coral reconstructions, and (ii) colony-to- colony offsets in coral proxies, which introduce large uncertainties in estimates of mean climate change from single corals. Informed by two decades of work at Kiritimati Island (2degN, 157degW), this project will generate ~50 new well-dated and multi-proxy records of climate variability across the last millennium. Using a novel approach that layers paired coral oxygen isotope and Sr/Ca measurements, with a new paleothermometer, Sr-U, this reconstruction will provide the first set of robust, quantitative, and independent estimates of central tropical Pacific surface temperature (SST) and hydroclimate trends across the Medieval Climate Anomaly (MCA; 900-1200CE) and Little Ice Age (LIA; 1500-1800CE). The fidelity of these fossil coral records will be further ensured using rigorous screening for diagenesis and a microscale analyses to extract reliable climate information from altered corals. Comparisons of these new reconstructions with transient climate simulations will provide much-needed context for present-day trends and allow us to investigate the tropical Pacific’s long-term response to external forcings and climate feedbacks. Broader impacts to this project include a dramatic improvement to our understanding of natural climate variability in the tropical Pacific, allowing for the better quantification of regional anthropogenic climate trends, and the improvement of future climate projections by providing more accurate benchmarks for climate models. As climate change continues to dominate social consciousness, the PI will actively engage in public outreach efforts to communicate the results of the paleoclimate research as well as more general information about local/regional impacts of future climate change. It supports research training and mentoring of several graduate students and summer research experiences for high school students from a nearby school district which has a >90% underrepresented minority student population. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.