To study climate systems from a time before scientific instruments, aka paleoclimate, scientists rely on simulations of earth system models and proxy-based reconstructions. Proxies for climate data include tree ring, ice core, coral, and lake core data. Both model simulations and proxies have their drawbacks. Model simulations have known biases and proxy records are geographically limited. To mitigate these limitations, paleoclimate data assimilation puts information from both models and proxies together to create improved reconstructions of past climates. This combination method may not always be the best approach, however, as researchers from the University of Washington show in their recent Journal of Climate study, the appropriate reconstruction method depends on what you want to study.
Dr. Sanchez, Dr. Hakim, and Dr. Saenger, funded in part by CPO’s Climate Observation and Monitoring (COM) program, used coral-only proxy data from a network of 125 coral archives to reconstruct tropical climate variability going back to the year 1800. They then compared the behavior and patterns of the reconstructed tropical Pacific between those generated by (1) coral archives and paleoclimate model simulations and (2) coral archives and other types of proxies. The first comparison allowed the researchers to understand how model biases impact reconstruction while the second comparison looks at the influence of proxy data that can come from regions a long distance from the tropical Pacific. The climate model simulations each came from one of two projects, the CMIP5/PMIP3 past1000 experiments or the CESM Last Millennium Ensemble, which are focused on simulating past climate metrics. Notably, the coral-only method results in more skillful temperature reconstructions, especially for the 20th century, and shows the tropical Pacific behaving opposite from other proxy-model reconstructions following large volcanic eruptions in the early 19th century. Their study focuses on understanding what drives this difference.
Studying volcanic eruptions can be extremely informative for climate researchers as eruptions provide researchers an opportunity to observe how the climate reacts to external stimuli, or forcing. Observation data on large volcanic eruptions is sparse, however, in part because scientific instruments were not present when many large eruptions occurred. This work by Sanchez, Hakim, and Saeger suggests that pan-Pacific cooling after large eruptions is a real feature of the climate system, despite few models being able to simulate such a response. At the same time, many models simulate a strong El-Nino-like response following large eruptions, but this may not be realistic. Their work highlights both the pros and cons of paleoclimate data assimilation, which can examine how realistic and robust proxy interpretations are but is also vulnerable to model biases which strongly influence how proxy information is processed during data assimilation.
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Photo credit: NOAA/NMFS/PIFSC/CRED, Oceanography Team