Massive currents transport water and heat between ocean basins kilometers beneath the ocean’s surface. Scientists have thrown light on one of the key unanswered climate problems by looking at the geological record to see how these currents have altered since the dinosaurs lived: would rising surface temperatures affect deep-sea flows? There has been a lot of research into how rising greenhouse gas levels are changing atmospheric behavior, and there has also been a lot of research into the ramifications for the ocean’s upper layers. The depths, on the other hand, are far more difficult to investigate, and we don’t have any baseline information on how things worked even a few decades ago.
Professor Dietmar Müller and Dr Adriana Dutkiewicz of the University of Sydney have gone considerably further back to fill in the gaps. Dutkiewicz and Müller show in Geology that huge deep-sea currents have accelerated during warm epochs, implying that this is something we may expect to see again. “To date, the ocean has absorbed a quarter of human CO2 and over 90% of the related extra heat,” according to Dutkiewicz.
However, we know very little about the long-term repercussions. “The satellite data normally used to feed ocean models only covers a few decades,” Dutkiewicz continued, “leading to a poor knowledge of longer-term ocean variability.” Over the previous 66 million years, Dutkiewicz and Müller studied the buildup of plankton seashells at 293 sites on the continental shelves and in the deep ocean. They concentrated on hiatuses, which occur when powerful currents sweep away material that would otherwise accumulate.
Simultaneous hiatuses at multiple places imply a speeding up of the deep currents, rather than more local reasons, due to the dispersed nature of the sites. Some of the findings could be linked to well-known occurrences, such as the enlargement of the Drake Passage and the space between Tasmania and Antarctica, which allowed the Antarctic Circumpolar Current to emerge a little over 30 million years ago. The fact that deep ocean hiatuses have diminished during the last 13 million years, as the Earth has entered a long-term cooling phase, is the most immediately significant finding.
This suggests that abyssal currents have slowed over time. Nonetheless, during this phase, Dutkiewicz and Müller were able to detect spikes indicating faster current movement, which coincided with known warm times. Because most sites are included in this time, and only a few drill holes date back to the beginning of the study, the more recent data is also more credible. The increased currents during warm periods, according to the article, are caused by stronger winds blowing over surface waters at this time, as well as less ocean stratification.
“Independent studies utilizing satellite data imply that large-scale ocean circulation and ocean eddies have become more powerful during the previous two to three decades of global warming, corroborating our findings,” Müller said. Climate scientists will be able to refine their projections for how increased global temperatures would affect local climates as ocean circulation strengthens.
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