Atlantic Ocean currents stayed strong during the last ice age, study finds

Powerful Atlantic Ocean currents continued to transport heat around the planet during the last ice age, despite vast ice sheets covering much of the Northern Hemisphere, according to new research led by scientists at UCL Geography.

The study, published in Nature, reveals that a key component of the Atlantic’s circulation system - North Atlantic Deep Water - remained surprisingly warm and occupied similar depths to today, challenging long-held assumptions about ice age oceans.

Researchers found that deep Atlantic waters were only around 1.8°C colder than modern conditions during the Last Glacial Maximum, around 19,000–23,000 years ago. Previous theories suggested these waters were near freezing and that ocean circulation was much weaker.

The findings indicate that the Atlantic Meridional Overturning Circulation (AMOC) - a system of currents that transports warm, salty water northwards and helps regulate Europe’s climate - remained active even during one of the coldest periods in Earth’s history.

Lead author Dr Jack Wharton said the results highlight the resilience of the ocean’s “climate engine” under extreme conditions. “Our data show that deep Atlantic circulation kept going during the ice age,” he said. “That helps explain how heat continued to move around the planet, even when much of the surface was frozen.”

To reconstruct ancient ocean conditions, the team analysed microscopic fossil shells preserved in deep-sea sediments from across the North Atlantic, including sites near the Bahamas, Bermuda, Iceland and the eastern United States. These fossils record the temperature and salinity of the seawater in which they formed, allowing scientists to trace the origins and behaviour of deep ocean waters.

Professor David Thornalley explained that the chemical signatures preserved in the fossils show warm, salty surface waters continued to sink and form deep water, much as they do today.

The findings also lend confidence to climate models, which have long suggested that Atlantic circulation remained relatively strong during the ice age. However, the research also reinforces warnings from those same models that modern climate change could weaken the AMOC in the future, with serious consequences for Europe, North Africa and global weather patterns.

Professor Mark Maslin said the study improves understanding of how ocean circulation responds to climate change - knowledge that is critical for predicting future climate risks.

The research was supported by the Natural Environment Research Council, the Leverhulme Trust, the European Union’s Horizon Europe programme and the US National Science Foundation, with international collaborators from Europe and the United States.