Off the coast of Nova Scotia, buried in ocean-floor mud, lies evidence of a climate tipping point that happened 12,900 years ago—and might happen again. UCL researchers have discovered the first geological proof that during the Younger Dryas, an abrupt global cold snap, the Gulf Stream ocean current shifted hundreds of miles to the north, transforming eastern Canada's marine ecosystems in ways that modern climate models have long predicted but never before confirmed in the rock record.
The discovery matters because it shows us a tangible example of what happens when one of Earth's most powerful heat-conveying systems destabilizes. The Gulf Stream transports warm tropical waters northward and is central to regulating climate across the entire North Atlantic. When it moves, everything changes. Scientists are watching closely because our warming oceans could trigger similar shifts in the coming decades, and now we have a blueprint, written in ancient sediment, of what that cascade of change looks like.
Lead author Fangjingcheng Zhu, who conducted the research at UCL and is now at the University of Southampton, explained the interconnectedness of these systems: "Our research shows how the many different components of the Atlantic circulation system are all interconnected and can be abruptly altered during climate change." The team reconstructed 12,900-year-old ocean conditions by analyzing microfossils called foraminifera preserved in mud cores extracted from the seafloor off Canada's East Coast. These tiny fossil shells record the temperature and salinity of ancient seawater; by analyzing the chemical signals locked inside them, researchers estimated what the ocean was like when these organisms were alive.
During the Younger Dryas—a sudden cold snap that lasted slightly more than 1,000 years—global mean temperatures plummeted about 0.6 degrees Celsius in less than a century. But the Northern Hemisphere experienced far steeper drops, with annual mean temperatures in some regions falling by as much as 10 degrees Celsius. The culprit was a weakening of the Atlantic Meridional Overturning Circulation, the vast system of currents that acts like a conveyor belt, moving heat from the tropics northward. When AMOC weakened, the Gulf Stream shifted northward, bringing it closer to Nova Scotia's coast.
The sediment cores reveal something counterintuitive: this northward shift initially warmed the waters around Nova Scotia by 4 to 5 degrees Celsius. Yet this warming was only temporary. Within the first few hundred years of the Younger Dryas, cold freshwater from expanding sea ice farther north eventually covered the warm Gulf Stream waters, insulating the current from the atmosphere above and negating its warming effect. Meanwhile, across the Atlantic, reduced heat transport to the far north caused sea ice to expand off the UK and continental Europe, dropping mean annual temperatures by up to 6 degrees Celsius.
What makes this discovery significant is its empirical weight. Northward shifts of the Gulf Stream have been predicted by climate models for years, but this is the first direct geological evidence of such a shift during a major abrupt climate event. The research provides advanced warning of the sequence of shifts that Atlantic currents are expected to undergo as climate change progresses. We now have not just theory, but proof from Earth's own geological archive, showing that these systems can reorganize rapidly—and that the effects cascade unpredictably across regions. As the climate continues warming, that precedent demands our attention.