One of the big questions surrounding climate change is how it will affect the Gulf Stream. Based on how it fared during the last ice age, it seems increasingly likely that it will weaken as the world warms, potentially shaking up the climate of Europe and beyond.
The Gulf Stream is part of the North Atlantic Gyre, a clockwise-moving highway of ocean currents driven by wind patterns that plays a role in maintaining the climate of the U.S. East Coast and parts of Europe.
In a new study, scientists from University College London (UCL) concluded that the North Atlantic Gyre was unexpectedly stronger during the last ice age than it is today, due to stronger winds in the subtropical North Atlantic Ocean.
The fossilized remains of microorganisms called foraminifera in the ocean sediment suggest that about 20,000 years ago, the Gulf Stream was about 1 kilometer (0.6 mi) deeper than it is today, reaching depths of up to 2.5 kilometers (1.5 mi). They also imply that it was powerful, due to stronger winds and denser waters.
“We found that the Gulf Stream was much stronger during the last ice age because of stronger winds across the subtropical North Atlantic. As a result, the Gulf Stream still moved a lot of heat north, even though the rest of the planet was much colder,” Dr Jack Wharton, lead author from the UCL Department of Geography, said in a statement.
This has implications for the future, the researchers say, because it shows how sensitive the Gulf Stream is to changes in wind patterns, such as those we expect as the climate crisis worsens.
“If winds weaken in the future, as suggested by a recent study using climate models, that could mean a weaker Gulf Stream and a cooler Europe,” Dr Wharton added.
The Gulf Stream is also a cog in a broader and more complex system, the Atlantic Meridional Overturning Circulation (AMOC), which consists of the northward flow of warm water in the upper Atlantic Ocean and the southward flow of cold, salty, deep water.
“Rather than the established metaphor of a conveyor belt, it might be better to think of the AMOC as a series of interconnected loops. There’s the subtropical loop, which includes the Gulf Stream, and a subpolar loop, which transports heat further north to the Arctic,” explained study co-author Professor David Thornalley from UCL Geography.
“During the last ice age, our findings show that the subtropical loop was stronger than it is today, while the subpolar loop was likely weaker. Therefore, when examining human-induced climate change and the AMOC, we need to consider how these different parts may be changing and what climate impacts each part is associated with,” Thornalley added.
Scientists have previously expressed concern that climate change is already weakening the AMOC and that this worrying trend will continue in the coming decades. Melting Arctic glacier water could disrupt the formation of deep water, preventing warm tropical water from reaching Europe and potentially cooling the continent.
If the AMOC were to collapse completely – an unlikely but possible scenario under severe climate change – it could cool parts of Europe by 10 to 15°C (18 to 27°F).
“It is not always appreciated how much ocean currents are responsible for transferring heat around the planet and shaping our climate. Paradoxically, climate warming could be cooling much of Europe by disrupting the AMOC. Our new research adds to this understanding, showing that weakening the winds that drive the Gulf Stream could reduce the circulation of heat, further affecting the continent,” noted co-author Professor Mark Maslin, also from UCL Geography.
The new research is published in the journal Nature.