Ocean Response to Prolonged Common Era Surface Climate Trends

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Earth’s surface has generally warmed over the past 100 years, but the ocean has slowed this warming by taking up heat that otherwise would have remained in the atmosphere. Eventually, changes in the temperature of the atmosphere will affect the entire ocean, including the deep ocean. But we currently do not have enough observations to know whether the deep ocean can continue to act as a brake on surface warming. This project seeks to understand how the ocean responds to prolonged surface changes by using observations and models of the past. The team will generate and collect evidence from sea floor sediments related to ocean temperature changes of the last 2,000 years and interpret their causes with an ocean circulation model that includes the deep ocean. Climate science will be advanced by better estimates of how heat stored in the deep ocean affects surface climate. This project will support education. It will include a graduate student and community outreach through several local organizations. The results of the project will also benefit society by providing information about the influence of the ocean that is needed to improve climate projections of 21st century and beyond.Technical Description: The last 2,000 years is marked by long-term trends in surface climate, such as a 600-year cooling trend between the Medieval Climate Anomaly and the Little Ice Age. The key question is, how does the ocean respond to prolonged surface temperature trends? This question is difficult to answer because there are few observations of regional surface and deep ocean temperature before the Industrial Revolution. The team will use published and new observations from marine sediment that make it possible to investigate the deep ocean response to surface climate changes during the last 2,000 years. They will work on sites near North Atlantic Deep Water formation, and downstream in the deep Atlantic, Indian, and Pacific Oceans. The focus is to provide constraints on centennial-scale regional temperature variability. This will be achieved with observations of benthic d18O, surface temperature, and bottom water temperature data. The Common Era evolution of global ocean temperature will be reconstructed by assimilating temperature data into an ocean circulation model. The project will provide an estimate of changes in deep ocean heat content relative to the upper ocean. It will also estimate polar amplification and the difference in the timing of climate shifts between hemispheres. These results will be used to test whether heat loss in the deep ocean can offset global heat gain in the upper ocean. They will also be used to evaluate heat uptake estimates from coupled climate models, and to suggest whether it is possible for a remnant influence of the Little Ice Age to still be influencing the ocean today.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.