Scientists examined data from 293 sites spanning 66 million years, looking for gaps in sedimentary layers known as hiatuses to determine the varying strength of ocean currents over millennia and how this relates to temperature changes.
This provided a wealth of additional data beyond the 30 years or so of satellite imagery that researchers have traditionally used to examine how ocean current activity changes as the global temperature rises.
"The satellite data typically used to inform ocean models only cover a few decades, resulting in a poor understanding of longer-term ocean variability," says Adriana Dutkiewicz of Australia's University of Sydney.
"This compelled us to examine the deep-sea geological record in order to decipher these changes."
The researchers discovered that as the Earth cooled over the last 13 million years, the gaps in the sediment record became less frequent. This indicates that current speed in the deepest parts of the ocean has slowed overall.
Deep ocean circulation appeared to be much busier during the 'hothouse climate' period preceding the 13-million-year cooling-off era. Global temperatures would have been 3-4°C (5.4-7.2°F) higher at this time than they are now.
You don't have to live on the seafloor to be affected by ocean current fluctuations: these deep whirlpools have an impact on everything from major weather patterns to the distribution of marine life.
"A break in sedimentation indicates vigorous deep-sea currents, whereas continuous sediment accumulation indicates calmer conditions," says University of Sydney geophysicist Dietmar Müller. "By combining these data with ocean basin reconstructions, geologists were able to track where and when these sediment breaks occurred."
When it comes to modelling how global warming will affect the oceans in the future, the more we know about the past, the better our predictions will be. The ocean has already absorbed a significant amount of excess carbon and heat.
Previous research has shown that during periods of climate change, the oceans can trap more carbon, primarily through plankton that use dissolved carbon to build their shells and then drift down to the ocean floor after death, trapping the absorbed carbon.
What is also clear is that as Earth's temperatures continue to rise, there will likely be more activity in the depths of the oceans. Future research will be required to determine how this will affect the balance of life and the environment.
"Fast forward to today, independent satellite data studies show that large-scale ocean circulation and ocean eddies have become more intense over the last two to three decades of global warming, supporting our findings," says Muller.
