Contributed by Professor Chuan-Chou Shen, Department of Geosciences
An Asia–Europe international team, led by Professor Chuan-Chou Shen of the Department of Geosciences, National Taiwan University (NTU), has uncovered the mechanism behind the most dramatic warming and fastest sea-level rise event of the past million years. Published on June 16, 2026, in Nature Communications (1) and chosen as one of the journal’s Featured Articles, the findings show that extended weakening of the Atlantic Meridional Overturning Circulation (AMOC) resulted in the accumulation of large amounts of heat within the ocean interior. When the circulation revived, this stored heat was quickly spread again to high latitudes, causing extraordinary ice-sheet melting and the fastest sea-level rise of the last million years.
Over the past million years, the climate of Earth has been characterized by repeated ~100,000-year glacial–interglacial cycles, each consisting of a long 90,000-year ice age followed by a relatively rapid deglaciation, called a glacial termination. During the fourth glacial termination (Termination IV) about 340,000 years ago, the Earth underwent one of the most dramatic warming episodes and the most rapid sea level rise of the last million years. Sea level rose as fast as five meters per century, far exceeding the present-day global average of about 3-4 mm per year. Previous studies have suggested the event was driven by a combination of orbital forcing, rising atmospheric CO2 and ice-sheet melting, but the precise sequence of processes causing this extraordinary climate transition has remained uncertain because existing marine records did not provide sufficiently precise chronological constraints.
Since 2012, this team has carried out field expeditions to Bàsura Cave in northern Italy. Researchers collected flowstone cores under permission from the Italian government and applied high-precision uranium–thorium dating techniques developed at NTU’s High-Precision Mass Spectrometry and Environmental Change Laboratory (HISPEC) to construct a robust, high-resolution chronological framework of European hydroclimate. This independently dated framework was subsequently integrated with North Atlantic marine sediment records, including sea-surface temperature, pollen, oxygen isotopes and elemental proxies, to reconstruct the timing of hydroclimate change, AMOC variability, ocean warming and rapid sea-level rise during Termination IV.
The study shows that the ocean is not just a passive heat reservoir but an active regulator of the ice-sheet stability through changes in ocean circulation and heat redistribution. The research team built a high-resolution chronological framework for the last five glacial terminations over approximately 440,000 years. They found that during the fourth glacial termination (Termination IV, T-IV), the AMOC was weak for about 13,000 years, the longest duration among the last five terminations. The sustained influx of fresh water increased the stratification in the North Atlantic, reducing deep convection and air-sea heat exchange, and leading to large heat accumulation in the deep ocean and subsurface layers. As soon as the AMOC recovered, the extra heat transported northwards and vertical mixing in the ocean rapidly redistributed this stored heat to high latitudes, accelerating basal melting of ice shelves and destabilization of ice sheets. These processes, in combination with high atmospheric CO2 concentrations and increased Northern Hemisphere summer insolation, ultimately resulted in the extremely rapid ice loss and sea-level rise during the late stage of Termination IV.
Professor Shen notes that the climate 340,000 years ago was different to today and should not be considered as a direct analogue for future climate change. But the study notes that ice-sheet responses to warming can be strongly affected by changes in ocean circulation and the storage and redistribution of heat in the ocean interior. Future changes in the AMOC are a major uncertainty in projecting sea-level rise as the Greenland and Antarctic ice sheets continue to lose mass. If heat stored in the ocean interior is rapidly funneled to ice shelves under certain conditions, future sea-level rise may not be a gradual, linear process but may occur through much faster threshold-like changes. These results provide key insights into the stability of future ice sheets, risk of sea-level rise, and climate adaption strategies for coastal regions.
This study was completed through collaboration among more than 15 long-term partner institutions and laboratories across Asia and Europe. The principal geochemical analyses and manuscript preparation were primarily carried out by Dr. Hsun-Ming Hu, a former Ph.D. student under Prof. Shen. The research was supported by the NSTC Excellence Research Program, the Higher Education Sprout Project of the Ministry of Education, the Advanced Center for Earth Sciences, and the NTU Core Consortiums Program.
(1) Hu, H.-M.*, Marino G.*, Goñi M. F. S., Rohling E., Rodrigues T., Pérez-Mejías C., Ren Q., Jiang X., Michel V., Valensi P., Starnini E., Zunino M., Salonen J. S., Hsieh C.-J., Tan L., Chaigneau B., Chevalier M., and Shen C.-C.* (2026) Protracted ocean circulation slowdown drove exceptional ice-sheet melting during ice age termination IV. Nature Communications 17, 5675. https://www.nature.com/articles/s41467-026-73733-6
Figure 1. Distant view of Bàsura Cave in northern Italy.
Figure 2. An internal chamber in Bàsura Cave.
Figure 3. Professor Chuan-Chou Shen of the Department of Geosciences, operating a drilling machine to extract limestone core samples in Bàsura Cave, northern Italy.