古代の海洋減速が将来の気候混乱を警告する(Ancient ocean slowdown warns of future climate chaos)

2024-06-13 カリフォルニア大学リバーサイド校(UCR)

＜関連情報＞

• https://news.ucr.edu/articles/2024/06/13/ancient-ocean-slowdown-warns-future-climate-chaos
• https://www.pnas.org/doi/10.1073/pnas.2311980121

前期始新世の温室効果における海洋循環の温暖化に対する感度 Sensitivity of ocean circulation to warming during the Early Eocene greenhouse

Sandra Kirtland , Andy Ridgwell, Allison L. Keller, +5, and Richard D. Norris
Proceedings of the National Academy of Sciences  Published:June 3, 2024
DOI:https://doi.org/10.1073/pnas.2311980121

Significance

Numerous transient, greenhouse-gas-fueled warming events called “hyperthermals” occurred during the Early Eocene Climate Optimum (~53.26 to 49.14 Ma). The extent of warming and carbon cycle perturbation across hyperthermals are constrained primarily by benthic foraminiferal stable isotope records. Here, we show spatial patterns in the magnitude of the negative carbon isotope excursions that characterize these events. Using an intermediate complexity Earth system model, we demonstrate that this pattern indicates transient weakening of the large-scale ocean overturning circulation and enhanced deep ocean aging gradients coincident with hyperthermal warming. Our results show that globally distributed paleoceanographic records that capture gradients in deep water mass properties are critical to accurately constrain carbon forcing across past transient climatic events.

Abstract

Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.