Melting icebergs from the North Pacific Ocean can weaken a massive ocean current system in the Atlantic, according to a groundbreaking study from the University of California, Davis. The discovery completely upends a long-standing climate framework and warns that modern climate models may be looking in the wrong ocean to predict future current collapses.

The Atlantic Meridional Overturning Circulation (AMOC) acts as the planet’s engine, transporting warm, salty water from the tropics to the North Atlantic to regulate global temperatures. For decades, scientists believed that during Earth’s last ice age, icebergs breaking off into the North Atlantic itself were the primary triggers that slowed this “conveyor belt,” which drastically cooled the Northern Hemisphere into icy periods known as Heinrich stadials.

However, the new research, published in Nature Communications, reveals these North Atlantic melting events actually occurred after the AMOC had already begun to weaken.

“So, the iceberg discharge events in the North Atlantic could not have driven the initial AMOC weakening,” said lead author Chijun Sun, an assistant professor at UC Davis.

Using advanced supercomputer simulations to recreate Earth’s climate from 19,000 years ago, Sun’s team found that the slowing of the AMOC was instead likely caused by iceberg activity in the northeastern Pacific.

The simulations showed that massive “iceberg discharge events” – or calving – in the northeastern Pacific released enormous volumes of freshwater. This freshwater travelled across the globe to the North Atlantic’s deepwater formation zones. Because freshwater is less dense than saltwater, it diluted the Atlantic’s salty waters, preventing them from sinking and effectively stalling the AMOC engine.

The resulting subsurface warming in the Atlantic then triggered a secondary chain reaction, causing North Atlantic ice sheets to melt and further weaken the system.

“What is novel and surprising here is that North Pacific meltwater could independently drive AMOC weakening,” Sun noted.

Scientists warn that the AMOC is currently weakening due to human-driven climate change, with some predicting a total collapse by the end of the 21st century.

While previous monitoring focused primarily on North Atlantic ice melt near Greenland, this research shows how sensitive the AMOC is to freshwater discharge anywhere in the global ocean. With modern Alaskan and Canadian Pacific glaciers melting at historic rates, the study underlines the urgency of global climate action on emissions.

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