The Gulf Stream brings warm surface water northwards from the tropics to high latitudes, where it cools, sinks and flows southwards at depth. Changes in this Atlantic 'meridional overturning circulation' (MOC) would have profound implications for climate.
Credit: National Oceanography Centre
SYDNEY: The Atlantic Ocean current, which may be affected by future climate change, today takes heat north to Europe but 10,000 years ago it was weaker and flowed in the opposite direction.
"[The opposite flow in the Atlantic Ocean] explains the presence of huge ice sheets in Europe and North America during that cold climatic period," said César Negre, an environmental scientist at the Autonomous University of Barcelona in Spain, and co-author of the letter in the British journal Nature.
"The ocean currents transport heat, and different configurations of Atlantic circulation have had an impact on its redistribution over the climate system."
Warming up Europe
The current is the deep-water section of the 'meridional overturning circulation' (MOC), which is made up of warm surface currents, such as the Gulf stream, that cool as they go. The currents then sink, flowing south along the ocean floor, ventilating the deep ocean and affecting the storage of carbon away from the atmosphere.
Until now, "there has been a lot of debate in the last few years in the scientific community about the configuration of the Atlantic circulation during the last Ice Age," said Negre.
In this study, the researchers took ocean cores from deep in the Southeast Atlantic, and found that gradient concentrations of two natural radioactive isotopes - Protactinium-231 and Thorium-230 - were the opposite 10,000 years ago from what they are today, showing that the current flowed in reverse.
A constantly changing circulation
While surface currents can be driven by a number of things, in deep ocean the driving force is differences in density - and the density is controlled by salinity and temperature.
"It has previously been suggested that increased seawater salinity in the Southern Ocean in combination with ocean surface cooling during the Last Glacial Maximum potentially favoured a reversed deepwater abyssal flow," the researchers wrote in their paper.
In 2005, scientists at the U.K. National Oceanography Centre discovered that the surface current of the Atlantic circulation was slowing down, and climate change models suggest that a slowing Atlantic circulation could create a positive feedback cycle that would further drive climate change.
A constantly changing circulation
"To determine the likelihood and consequences of such a change, we need to understand the sensitivity of the ocean circulation to changes in [environmental input]," said Steve Rintoul at CSIRO in Hobart, Tasmania, Australia, who was not connected to the study.
"This study is very important because it helps to reconcile earlier, conflicting studies" said Rintoul, however "[we] still have a lot to learn about how the ocean circulation worked during glacial times".
