We know that global warming affects ocean currents in a variety of ways, but questions remain about exactly how this relationship works. A new study attempts to answer some of those questions.
Scientists pored over data stretching back 66 million years over 293 sites, looking at gaps in sedimentary layers, known as hiatuses, to figure out the varying strength of ocean currents across the millennia and how this relates to temperature changes.
Doing so provided a wealth of extra data beyond the 30 years or so of satellite imagery that researchers traditionally used to look at how ocean current activity changes as the temperature of the globe keeps on rising.
"The satellite data typically used to inform ocean models only cover a few decades, leading to a poor understanding of longer-term ocean variability," says sedimentologist Adriana Dutkiewicz from the University of Sydney in Australia.
"This prompted us to look at the deep-sea geological record to decipher these changes."
The team found that over the last 13 million years, as the Earth has gradually cooled down, the hiatuses in the sediment record became less frequent. That suggests current speed in the deepest parts of the ocean has slowed down overall.
In comparison, in the 'hothouse climate' period that preceded the 13-million-year cooling-off era, deep ocean circulation appeared to be much busier. At this time global temperatures would have been 3-4°C (5.4-7.2°F) warmer than they are today.
You don't have to live on the sea bed to be affected by fluctuations in ocean currents: These deep whirlings have an influence on everything, from major weather patterns to the distribution of marine life.
"A break in sedimentation indicates vigorous deep-sea currents, while continuous sediment accumulation indicates calmer conditions," says geophysicist Dietmar Müller from the University of Sydney.
"Combining these data with reconstructions of ocean basins has allowed geologists to track where and when these sediment breaks occurred."
The more we know about the past, the better our predictions will be when it comes to modeling how global warming will change the oceans in the future. The ocean has already soaked up a huge amount of excess carbon and heat.
Previous studies indicate that in periods of climate warming, oceans can trap more carbon, primarily via plankton using dissolved carbon to build their shells, and then drifting down to the ocean floor after death, trapping the soaked up carbon.
What's also now clear is that there's likely to be more activity down in the depths of the oceans as temperatures on Earth continue to rise. Future research will be required to assess exactly how that's going to impact the balance of life and atmosphere.
"Fast-forward to today, independent studies using satellite data suggest that large-scale ocean circulation and ocean eddies have become more intense over the last two to three decades of global warming, supporting our results," says Müller.
The research has been published in Geology.