Published in Nature, the study indicates that about 20,000 years ago, the Gulf Stream was stronger than it is today due to more intense winds across the subtropical North Atlantic. This suggests that if climate change results in weaker subtropical winds in the future, as early research indicates, the Gulf Stream could also weaken. This reduction could limit the amount of tropical heat reaching Europe, cooling the continent and raising sea levels in North America, though the extent of these effects remains uncertain.
The Gulf Stream, a surface current flowing from the east coast of the US across the Atlantic to Europe, transports warm tropical water, which releases heat into the atmosphere, warming Europe.
During the last ice age, stronger winds led to a more robust and deeper Gulf Stream, even though the overall global temperature was much colder. Lead author Dr. Jack Wharton (UCL Geography) explained, "We found that during the last ice age, the Gulf Stream was much stronger because of stronger winds across the subtropical North Atlantic. As a result, the Gulf Stream was still moving lots of heat northwards, despite the rest of the planet being far colder. Our work also highlights the Gulf Stream's potential sensitivity to future changes in wind patterns. For example, if in the future winds are weaker, as shown in a recent study using climate models, it could mean a weaker Gulf Stream and a cooler Europe."
The Gulf Stream is a crucial part of the Atlantic Meridional Overturning Circulation (AMOC), which is driven by deep water formation in the subpolar North Atlantic, where cooling causes surface waters to become dense and sink, as well as by winds. There are concerns that climate change could weaken the AMOC, as melting glacial water from Greenland might disrupt deepwater formation, hindering the transport of warm tropical water to Europe and thus cooling the continent.
A combined effect of weakening winds and reduced deep water formation could substantially weaken the Gulf Stream. If the AMOC were to collapse-a scenario deemed unlikely but possible-European temperatures could drop by 10 to 15 degrees Celsius, severely impacting agriculture and weather patterns, and a decrease in the wind-driven part of the Gulf Stream would exacerbate this.
Professor Mark Maslin (UCL Geography) added, "It's not always recognised how much ocean currents are responsible for transferring heat around the planet and shaping our climate. Paradoxically, the warming of the climate could cool down much of Europe by disrupting the AMOC. Our new research adds to this understanding, and shows that the weakening of the winds which drive the Gulf Stream could reduce the circulation of heat, further affecting the continent."
The study challenges the conventional conveyor belt metaphor for the AMOC, suggesting instead a series of interconnected loops. Professor David Thornalley (UCL Geography) elaborated, "Rather than the established conveyor belt metaphor, perhaps it is better to think of the AMOC as a series of interconnected loops. There is the subtropical loop-that the Gulf Stream is part of-and a subpolar loop, which carries heat further northwards into the Arctic. During the last ice age, our findings show that the subtropical loop was stronger than it is today, whereas the subpolar loop is thought to have been weaker. Therefore, when investigating anthropogenic climate change and the AMOC, we need to consider how these different parts may change and what climate impacts each is associated with."
To determine the prehistoric strength of the Gulf Stream, the researchers examined fossil remains of foraminifera-microorganisms that live on the ocean floor-using sediment cores collected off the coast of North Carolina and Florida, in collaboration with researchers from Woods Hole Oceanographic Institute in Massachusetts.
The analysis revealed that during the last ice age, the isotopic signatures of foraminifera indicated a Gulf Stream that was twice as deep and fast as it is today.
Research Report:Deeper and Stronger North Atlantic Gyre During the Last Glacial Maximum
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