Stronger winds during the last ice age strengthened the Gulf Stream, signalling the risks of future cooling of Europe and rising sea levels from climate-induced wind changes.

Researchers at UCL found that the Gulf Stream was stronger during the last ice age, around 20,000 years ago, due to stronger winds in the subtropical North Atlantic. Their recently published study Naturesuggests that future reductions in these winds due to climate change could weaken the Gulf Stream. Such a change would reduce the flow of tropical heat to Europe, potentially cooling the continent and raising sea levels in North America.

Historical information about the dynamics of the Gulf Stream

The Gulf Stream is a surface current that flows up the east coast of the United States and then crosses the Atlantic to Europe, carrying with it warm tropical water. This water gives off heat to the atmosphere, warming Europe.

The researchers found that during the last ice age, when ice sheets covered much of the northern hemisphere, stronger winds in the region led to a stronger, deeper Gulf Stream. But despite the stronger Gulf Stream, the planet as a whole was much colder than it is today.

The role of winds in the dynamics of the Gulf Stream

“We found that during the last ice age the Gulf Stream was much stronger due to stronger winds in the subtropical North Atlantic. As a result, the Gulf Stream was still carrying a lot of heat northwards, even though the rest of the planet was much colder. Our study also highlights the potential sensitivity of the Gulf Stream to future wind changes. For example, if winds are weaker in the future, as shown in a recent study using climate models, this could mean a weaker Gulf Stream and a cooler Europe,” said lead author Dr Jack Wharton (UCL Geography).

The Gulf Stream is also part of the larger Atlantic Meridional Overturning Circulation (AMOC), which is driven by deep-sea formations and winds in the subpolar North Atlantic, where cooling causes surface waters to condense and sink. Scientists have previously expressed concerns about how climate change could weaken the AMOC, as glacial meltwater flowing off Greenland could disrupt deep-sea formations, preventing warm tropical water from reaching Europe and thus cooling the continent.

Possible consequences of AMOC failure

The combined effects of weaker winds and reduced deepwater formation could significantly weaken the Gulf Stream. If the AMOC were to deteriorate (an unlikely but possible future scenario), temperatures in Europe would fall by 10–15 degrees Celsius, damaging continental agriculture and weather conditions, and the reduction in the windward part of the Gulf Stream would make this worse.

Co-author Professor Mark Maslin (UCL Geography) said: “It is not always appreciated how much ocean currents are responsible for transporting heat around the planet and shaping our climate. Paradoxically, a warming climate could cool much of Europe due to AMOC disturbances. Our new study adds to this understanding and shows that weakening of the winds driving the Gulf Stream could further impact the continent by reducing heat circulation.”

AMOC complexity and climate interaction

Although the AMOC and its component currents, including the Gulf Stream, are sometimes referred to as a giant conveyor belt, this study highlights the complexity of the system, with each part of the current capable of unique responses to climate change.

Co-author Professor David Thornally (UCL Geography) said: “Instead of the established metaphor of an assembly line, it is perhaps better to think of the AMOC as a series of interconnected cycles. There is a subtropical cycle, of which the Gulf Stream is a part, and a subpolar cycle, which transports heat further north towards the Arctic. During the last ice age, our findings suggest that the subtropical cycle was stronger than today, while the subpolar cycle is thought to have been weaker. So when investigating anthropogenic climate change and the AMOC, we need to think about how these different parts might be changing and what climate impacts each is associated with.”

Research methods and results

To assess the strength of the prehistoric Gulf Stream, the researchers analyzed fossil remains of foraminifera (microorganisms that live on the ocean floor) taken from sediment cores found off the coast of North Carolina and Florida in collaboration with Woods researchers at the Hall Institution of Oceanography in Massachusetts.

The researchers found that foraminifera from layers of the last ice age in sediment cores taken from various parts of the Gulf Stream had isotope signatures (the ratio of oxygen-18 to oxygen-16, controlled by a combination of temperature and salinity) that suggested the Gulf Stream was twice as deep and flowing twice as fast as it is today.