Are Ocean Currents Speeding Up ... Or Are They Slowing Down? Nobody Knows

The latest research contradicts previous studies which suggested that global warming will weaken ocean circulation, especially in tropical waters.

“Global warming is speeding up Earth’s massive ocean currents,” said one headline. “Global ocean circulation is accelerating from the surface to the abyss,” said another.” But this is another of those climate stories in which the top line is not backed up by the qualifications raised by oceanic researchers when looking at the results of this fascinating paper. Published in Science Advances it suggests that for almost 25 years, ocean currents have been rapidly speeding up, partly due to global warming, according to a new study.

It contradicts previous studies that suggested that global warming will weaken ocean circulation, especially in tropical waters. This new study suggests the acceleration in ocean currents will be especially strong in tropical waters!

A key point is that there is no sustained direct measurement of the ocean’s currents, so it has to be inferred using other means. When this is done the numerous gaps in the data are filled in with results from computer models and anyone can see the caution this method should raise. Based on observations and models, study authors claim that from 1990 to 2013, the energy of the world’s currents increased by some 15% per decade. The researchers put this down to strengthening winds driving ocean currents. Ocean winds have increased over the past 30 years. The increase is about 2% per decade and is itself part of a longer-term trend.

The main evidence for this change comes from six years of Argo data whose floating and diving buoys have been operating since 2005 and have produced the most coherent database on ocean parameters we have. They do not directly measure ocean currents, but a good inference can be obtained from their movements and indications where winds are piling up regions of ocean.

Hu Shijian of the Chinese Academy of Science’s Institute of Oceanography is the lead author of the study. He points out that that this new paper is different from previous studies that looked for an ocean circulation increase. Indeed, given varying regional responses to global warming it has not been possible to deduce how and whether global ocean circulation has been altered. “So far observations haven’t shown a trend,” Shijian said. So, he set about the reanalysis route to see if he could find one.

A review article in Science noted that as yet natural fluctuations cannot be ruled out and that it will take another decade at least to see if the trend is real and possibly associated with global warming. Quoted in Science, Susan Wijffels, of the Woods Hole Oceanographic Institution said, “It’s going to stimulate a lot of other work.”

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