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Ocean currents emerge as climate change hot-spots

As they warm, they shift

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A global study that assesses the temperature change in ocean currents has made two findings – one surprising, the other less so. The unsurprising outcome is that as the Earth’s temperature rises, so does the temps in a collection of major ocean currents; the surprise is that those currents are warming faster than the globe as a whole.

According to the study, published this week in Nature Climate Change, a pattern of warming in the ocean’s long-distance currents has now been identified near Australia, Japan, Africa, and North America.

Moreover, the warming is also sending the currents “polewards”, meaning that species migrations already observed in Australia (in which many species are moving southwards at as much as a degree per year) are almost certain to happen on a global scale.

The study, "Enhanced warming over the global subtropical western boundary currents", aims to identify whether, and to what degree, changes in ocean currents may occur due to anthropogenic greenhouse gas forcing.

The currents are important because on their thousands-of-kilometer journeys redistributing heat from equatorial regions to the mid-latitudes, they also release both heat and moisture into the atmosphere.

The currents could be thought of as heat pumps on a global scale, collecting heat nearer to the equator, and dropping it off – along with both moisture and the energy to, among other things, drive storms – further away.

According to the study, the currents are hot-spots in the climate-change picture, warming considerably faster than the average warming of the ocean. Over all the data sets, which comprise the Gulf Stream, Kuroshio Current, East Australian Current, Brazil Current, and Aguhlas Current, the average warming over a century has been around 1.2°C, while the global mean rise in temperatures is 0.62°C for the same period.

One of the paper’s authors, CSIRO’s Dr. Wenju Cai, told the ABC’s The World Today that the change is significant because it’s an observation on a global scale. He told the program that while warming of the East Australian Current had already been documented, this study demonstrates that it’s not just happening “in an isolated part of the Tasman Sea.”

Because the change is global, and because the various currents have warmed pretty much in step, the US National Oceanic and Atmospheric Administration’s Michael McPhaden said the synchronized change in ocean circulation is “most likely [caused by] anthropogenic greenhouse gas forcing."

The scale of the change, at two to three times global temperature rises, is one thing that surprised McPhaden, because “we did not think there was such a tight connection between ocean circulation on these small scales near the western boundaries.”

To try and account for natural climate variability, the researchers present a second analysis in which the change in the currents is presented in two periods: from 1900 to 1949, and from 1949 to 2008.

The picture is far less uniform, the study finds, in the first 50 years, with some currents (the Kuroshio, Gulf Stream, and Anguhlas) rising noticeably, while the East Australian and Brazil currents did not. This, they say, might reflect the lower impact of greenhouse-gas forcing in the earlier period, such that some fluctuations were smaller than natural variability. ®

A hat-tip to The World Today, which first carried the story in Australia, and thanks to Dr. Wenju Cai for providing a copy of the paper so that The Register was able to expand upon the original report. ®

Update: Since this story went live, Dr Cai has provided further information via e-mail to The Register regarding the source data used in the study.

Before regular ocean temperature monitoring was put in place, wind speeds – which are fed by energy from the ocean – are the key proxy for ocean temperature. Hence, regarding data towards the 1900 start date of the projection, Dr Cai wrote, “Much of the early changes are inferred from winds, for which we have more observations.”

While “reconstructed data certainly have limitations,” he wrote, winds “are a major driver of climate change.”

Importantly, he noted that the outputs of this study agree with predictions from other sources: “Climate models project a synchronized warming under various emission scenarios”.

This suggests a secondary, but still important, result of this study: it serves to help validate other climate models.

Data comes from a variety of sources, Dr Cai said: “Since 1950, we have started to [get] good data, though the coverage is sparse, and the amount increases dramatically since 1979 (the satellite era).

“Since 2003, we have built a network of 3,000 floats around the globe, which measure SST [sea surface temperature] every 10 days.

“However, for over 60 years, CSIRO has continuous monthly measurements of ocean temperature of Maria Island in Tasmania, which show a warming rate of up to 3°C.” ®

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