Kilometers beneath the ocean's surface, immense currents redistribute water and heat between the ocean basins. By examining the geological record to determine how these currents have changed since the dinosaurs' days, scientists have shed light on one of the big outstanding climate questions: will higher temperatures at the surface change deep-sea flows?
Vast amounts of research have been done on how increasing greenhouse gasses are changing atmospheric behavior, and there has been plenty of work done on the consequences for the upper layers of the ocean. The depths, however, are much harder to study, and we lack baseline research on how things were operating even a few decades ago.
Dr Adriana Dutkiewicz and Professor Dietmar Müller of the University of Sydney have looked much further back to fill in the gap. In Geology, Dutkiewicz and Müller show that great deep-sea currents have sped up during warm eras, suggesting this is something we can expect to see again.
"So far, the ocean has absorbed a quarter of anthropogenic CO2 and over 90 percent of the associated excess heat," Dutkiewicz said in a statement.
However, we know relatively little about how the consequences at depth. "The satellite data typically used to inform ocean models only cover a few decades, leading to a poor understanding of longer-term ocean variability,” Dutkiewicz added.
Dutkiewicz and Müller looked at the accumulation of plankton seashells at 293 sites on submerged continental shelves and in the deep ocean over the last 66 million years. They focused on hiatuses that occur when currents are strong enough to sweep away material that would otherwise build up.
With the sites widely dispersed, simultaneous hiatuses at many locations indicate speeding up of the deep currents, rather than more local factors. Some of the observations could be tied to well-known events, such as the widening of the Drake Passage and the space between Tasmania and Antarctica, which allowed the formation of the Antarctic Circumpolar Current a little over 30 million years ago.
Most immediately relevant, however, is that deep ocean hiatuses have decreased over the last 13 million years, as the Earth entered a long-term cooling phase. This indicates abyssal currents have slowed down over that time. Nevertheless, Dutkiewicz and Müller were able to detect spikes during this phase indicating more rapid current movement, coinciding with known warm periods.
The more recent data is also more reliable because most sites are included in this period, while only a few drill holes go back to the earliest part of the study.
The paper postulates the increase in currents during warm periods is caused by stronger winds blowing over surface waters at this time, and by 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 last two to three decades of global warming, supporting our results", Müller said.
With an expectation that ocean circulation will gain strength, climatologists will be able to sharpen their predictions for how higher global temperatures will affect local climates. Other studies have revealed warmer, better mixed oceans are generally more productive and better at storing carbon. Consequently, the speeding up Dutkiewicz and Müller predict could provide considerable benefits, although whether they will come fast enough to make much difference for us in unclear. "These processes so complex, combining the physics and chemistry of the oceans even the most complex ocean atmosphere models can't capture everything, which is why we are looking further back in time," Müller told IFLScience.