During the last ice age, the Atlantic Ocean's powerful current system remained active and continued to transport warm, salty water from the tropics to the North Atlantic despite extensive ice cover across much of the Northern Hemisphere, finds new research led by UCL scientists.
The findings, published in Nature, show that despite the Earth being in an ice age, part of the ocean's interior - known as North Atlantic Deep Water (NADW) - was only about 1.8°C colder than today, far from the near-freezing conditions previously assumed. Additionally, the NADW occupied a similar depth range as today, extending from roughly 1 to 4 kilometres below the surface.
This challenges the prevailing view that at the peak of the last ice age - the Last Glacial Maximum (LGM) - Atlantic circulation was weaker, and NADW was colder and confined to shallower depths. The researchers' findings also more closely agree with climate model projections for these glacial conditions, supporting the models' ability to accurately forecast future ocean circulation.
Lead author Dr Jack Wharton (UCL Geography) said: "We were amazed to find that the deep Atlantic stayed relatively warm and salty during one of Earth's coldest periods. Taken together, our data tell us the ocean's circulation system kept running even under extreme conditions, which is crucial for understanding how our climate engine works. The same climate models that correctly predicted this past behaviour also warn that these currents are vulnerable to weakening as the planet warms - and that could have dramatic consequences for future climate."
Taking the ancient ocean's temperature
To reconstruct deep Atlantic conditions during the Last Glacial Maximum, around 19,000 to 23,000 years ago, researchers analysed tiny fossil shells preserved in mud on the ocean floor. These microfossils, known as foraminifera, record the temperature and salinity of the seawater in which they lived. The team studied mud collected from sites off the coasts of the Bahamas, Bermuda, South Carolina and Iceland, from depths between 1.5 and 5 kilometres below the surface.
By analysing chemical signals locked inside these fossil shells, the team estimated deep-ocean temperature and salinity at the time the organisms were alive. These waters also carried a distinctive chemical fingerprint linking them to surface waters originating in the subtropics and Nordic Seas, indicating that large-scale heat transport through the ocean continued during this period.
Co-author Professor David Thornalley (UCL Geography) said: "The microfossils recovered from the ocean floor show that deep waters in the North Atlantic were far from freezing during the last ice age. By examining locations across the North Atlantic, we can show that warm, salty surface waters continued to sink and form North Atlantic Deep Water that reached similar depths to today."
Ocean currents and climate forecasts
The warmer ice age ocean temperatures indicated by these microfossils reflect what climate models have previously predicted, strengthening their credibility. However, it also lends credence to another prediction of these models - that climate change will cause the currents to weaken in the future, significantly cooling Europe and North Africa and disrupting weather patterns.
The ocean currents running throughout the Atlantic Ocean - known collectively as the Atlantic Meridional Overturning Circulation (AMOC) - play a critical role in regulating Earth's climate. The AMOC acts like a conveyor belt, transporting heat northward from the tropics and helping to keep Europe temperate. As surface waters cool in the North Atlantic, they sink and return southwards through the deep ocean as North Atlantic Deep Water.
Climate models predict that as the North Atlantic surface ocean warms, these waters become less dense and less able to sink to form deep waters, reducing the strength of the AMOC. Without this transport mechanism, heat from the tropics won't reach Europe and North Africa, dramatically cooling their climates.
Co-author Professor Mark Maslin (UCL Geography) said: "This research helps us better understand the mechanisms that drive ocean circulation and improves our ability to predict future climate change. Many of our best climate models indicate that Atlantic circulation is likely to weaken under the type of warming we're likely to face in the coming decades-it would have a tremendous, destabilising impact on the climate of Europe and North Africa."
Estimates are that if the AMOC were to shut down, average annual temperatures in the UK could drop by as much as 7°C by the end of the century, with winters as much as 15°C colder, which could bring frozen sea ice to the shores of Scotland. Arable land across the UK and continental Europe would be significantly reduced, and it would disrupt the rainy season monsoons in Africa.
This research was supported by the Natural Environment Research Council (NERC), the Leverhulme Trust, the European Union's Horizon Europe research and innovation programme, and the National Science Foundation (NSF), with collaboration from Utrecht University, the University of Colorado Boulder, and Woods Hole Oceanographic Institution.
