Kilometers below the sea surface, powerful currents distribute water and heat between the ocean basins. By examining the geological record to see how these currents have changed since the days of the dinosaurs, scientists have shed light on one of the big unanswered climate questions: Will higher surface temperatures alter deep-sea currents?
Much research has been done on how increasing greenhouse gases are changing atmospheric behavior and much work has been done on the consequences for the upper layers of the ocean. However, the depths are much more difficult to probe, and we lack basic research into how things worked just a few decades ago.
dr Adriana Dutkiewicz and Professor Dietmar Müller from the University of Sydney have looked much further back to close the gap. In geology, Dutkiewicz and Müller show that large deep-sea currents have accelerated during warm periods, suggesting that we can expect to see this again.
“So far, the ocean has absorbed a quarter of all anthropogenic CO2 and over 90 percent of the associated excess heat,” Dutkiewicz said in a statement.
However, we know relatively little about what the consequences are at depth. “The satellite data normally used to inform ocean models cover only a few decades, leading to a poor understanding of longer-term ocean variability,” Dutkiewicz added.
Dutkiewicz and Müller studied the aggregation of plankton shells at 293 sites on submerged continental shelves and in the deep sea over the past 66 million years. They focused on gaps that occur when currents are strong enough to wash away material that would otherwise accumulate.
Because sites are widely dispersed, simultaneous disruptions in many locations suggest acceleration of deep currents rather than more local factors. Some of the observations could be linked to well-known events, such as the widening of the Drake Passage and the space between Tasmania and Antarctica, which allowed for the formation of the Antarctic Circumpolar Current just over 30 million years ago.
Most directly relevant, however, is that deep-sea hiatus have decreased over the past 13 million years as Earth entered a long-term cooling phase. This suggests that the abyssal currents have slowed during this period. Nevertheless, during this phase, Dutkiewicz and Müller were able to identify peaks that indicate faster current movement and coincide with known warm periods.
The more recent data is also more reliable as most sites are included in this period while only a few drill holes date back to the earliest part of the study.
The paper posits that the increase in currents during warm periods is caused by stronger winds blowing over surface waters at the time and reduced ocean stratification.
“Fast-forward to today, independent studies using satellite data suggest that large-scale ocean circulation and ocean eddies have become more intense over the past two to three decades of global warming, supporting our findings,” Mueller said.
With the expectation that ocean circulation will gain strength, climatologists will be able to sharpen their predictions of how higher global temperatures will affect local climates.