The Atlantic's Powerful Current System Persisted Through the Last Ice Age
University College London
A groundbreaking study led by scientists at University College London (UCL) has revealed that the Atlantic Ocean's mighty current system remained active during the last ice age, transporting warm, salty water from the tropics to the North Atlantic despite the extensive ice cover across much of the Northern Hemisphere. The research, published in Nature, challenges previous assumptions and highlights the ocean's resilience during one of Earth's coldest periods.
The findings indicate that the North Atlantic Deep Water (NADW) was only about 1.8°C colder than present-day temperatures, far from the near-freezing conditions previously thought. This deep-ocean current maintained its depth range, extending from approximately 1 to 4 kilometers below the surface, during the Last Glacial Maximum (LGM).
This discovery contradicts the prevailing theory that Atlantic circulation weakened at the LGM, with NADW being colder and confined to shallower depths. The researchers' findings align more closely with climate model projections, bolstering the models' accuracy in forecasting future ocean circulation.
Dr. Jack Wharton, the lead author from UCL Geography, expressed amazement at the deep Atlantic's relative warmth and salinity during the ice age. He emphasized the significance of the ocean's circulation system, which continued to function under extreme conditions, providing crucial insights into the climate engine's inner workings. Wharton also noted that the same climate models predicting past behavior also warn of potential consequences as the planet warms, underscoring the vulnerability of these currents.
Reconstructing the ancient ocean's temperature
To reconstruct deep Atlantic conditions during the LGM, around 19,000 to 23,000 years ago, the researchers analyzed tiny fossil shells, known as foraminifera, preserved in mud on the ocean floor. These microfossils provide valuable records of the temperature and salinity of the seawater in which they lived. The team studied mud samples from various sites off the coasts of the Bahamas, Bermuda, South Carolina, and Iceland, at depths ranging from 1.5 to 5 kilometers below the surface.
By examining chemical signals within these fossil shells, the researchers estimated the deep-ocean temperature and salinity during the organisms' lifetimes. They also identified a distinct chemical signature linking these waters to surface waters originating in the subtropics and Nordic Seas, indicating that large-scale heat transport through the ocean persisted during this period.
Professor David Thornalley, a co-author from UCL Geography, emphasized the microfossils' revelation that deep waters in the North Atlantic were far from freezing during the last ice age. By examining multiple locations across the North Atlantic, the team demonstrated that warm, salty surface waters continued to sink and form NADW, reaching similar depths as today.
Ocean currents and climate forecasts
The warmer ocean temperatures during the ice age, as indicated by the microfossils, align with climate model predictions, enhancing their credibility. This finding also supports another model prediction: that climate change will cause the currents to weaken in the future, significantly cooling Europe and North Africa and disrupting weather patterns.
The Atlantic Meridional Overturning Circulation (AMOC), a critical component of the ocean's circulation system, plays a vital role in regulating Earth's climate. It acts as a heat transporter, moving warm water northward from the tropics and contributing to Europe's temperate climate. As surface waters cool in the North Atlantic, they sink and return southwards as NADW.
Climate models predict that as the North Atlantic surface ocean warms, these waters will become less dense and less capable of sinking to form deep waters, leading to a reduction in AMOC strength. Without this heat transport mechanism, Europe and North Africa will experience dramatic cooling, significantly impacting their climates.
Professor Mark Maslin from UCL Geography highlighted the research's contribution to our understanding of ocean circulation mechanisms and its implications for predicting future climate change. He warned that many climate models indicate a likely weakening of Atlantic circulation under the warming expected in the coming decades, which could have a destabilizing effect on the climate of Europe and North Africa.
The potential consequences of AMOC shutdown are alarming. Estimates suggest that average annual temperatures in the UK could drop by up to 7°C by the end of the century, with winters as much as 15°C colder, potentially bringing frozen sea ice to Scottish shores. Arable land across the UK and continental Europe would diminish, and it would disrupt the rainy season monsoons in Africa.
This research was supported by various funding agencies, including the Natural Environment Research Council (NERC), the Leverhulme Trust, the European Union's Horizon Europe research and innovation program, and the National Science Foundation (NSF), with collaboration from Utrecht University, the University of Colorado Boulder, and Woods Hole Oceanographic Institution.
