Every few months a number goes around about how much the ocean has warmed, and it gets quoted like somebody rowed out and took a reading. Somebody did, sort of. There are about 4,000 robot floats drifting out there right now, sinking on a timer, measuring on the way back up and phoning the numbers in by satellite.<\/p>\n
The catch is, almost all of them stop at 6,500 feet. The ocean averages around 12,000 feet deep and runs past 20,000 in the trenches, so the fleet we lean on to tell us what the ocean’s doing has been skipping most of the ocean.<\/p>\n
Two French floats went down to 19,700 feet in January to start closing that gap.<\/p>\n
They’re called Argo Deep-6000, they were built by Ifremer<\/a>, which is France’s state ocean research institute, and each one weighs 88 pounds (40 kg). That’s double what a conventional Argo float weighs. Both went into the water on January 11, three days’ sail north of the Antilles, directly above an ocean trench.<\/p>\n They went in off a sailboat, chartered from a company called OCEOPS that runs science expeditions under sail. Ifremer could’ve booked deck space on a gray research vessel and didn’t, which I like more than I can really justify.<\/p>\n By February 2, when Ifremer published the results, each float had dropped to 6,000 meters and come back five times. Here’s what a cycle looks like: the float sinks, drifts at depth, goes down to profile depth, measures salinity, temperature, oxygen and pressure on the climb, then surfaces to transmit before it starts over.<\/p>\n Now here’s what I can’t tell you. Ifremer put out that five-cycle milestone and hasn’t published an English-language update since, and I couldn’t find public float IDs for the two prototypes to look them up on the Argo fleet dashboard myself. Five cycles each as of early February is the confirmed number. Whether they’re both still cycling today, I don’t know. They’re built to last seven years.<\/p>\n One more thing about those cycles. Ifremer says these floats run on ten-day cycles like the rest of the fleet, but January 11 to February 2 is 22 days, and five ten-day cycles don’t fit into 22 days. My guess is the trial dives ran back to back without the long drift in the middle, which isn’t unusual when you’re commissioning something. Ifremer doesn’t spell it out either way.<\/p>\n Pressure. Seawater adds roughly one atmosphere for every 33 feet you go down, so a float sitting at 19,700 feet is under about 600 atmospheres, which works out to somewhere around 8,700 pounds on every square inch of it. Then it’s got to survive that hundreds of times and still have enough battery left to pump itself back up to the surface afterward.<\/p>\n The usual answer to that is titanium. Ifremer went with a composite hull instead, and that’s where the extra 44 pounds went. Xavier Andr\u00e9, who heads profiler technology development at Ifremer, said the main challenge was building something that could withstand “the pressures of the deep sea” while running seven years on its own energy and keeping the cost under control.<\/p>\n That last bit matters more than you’d think, because Ifremer wants 30 of these, and a float you can only afford to build twice is a science project.<\/p>\n Euro-Argo<\/a>, the European end of the program, calls these the only European cylindrical floats able to work at 6,000 meters. France is now the third country to design a float rated that deep, after the US and China.<\/p>\n Ifremer’s release says 1,250 of the 4,000 floats can measure below 2,000 meters. I’d read that as the plan rather than a headcount. Argo’s own Deep Argo mission page<\/a> gives 1,228 floats as the target for a standing deep array, which is close enough to the same number that I’d want it clarified, and the published counts of what’s actually in the water are a lot smaller. A 2023 paper in Frontiers in Marine Science<\/a> mapped 197 active Deep Argo floats, and Euro-Argo said in March that Europe’s deep array stood at 53.<\/p>\n The rest of it’s clearer. The US operates more than 100 floats rated to 6,000 meters and China runs about 30, according to Ifremer. France’s own fleet is 270 floats working between 2,000 and 4,000 meters, and the 30 deep ones are due by 2028.<\/p>\n About 10% of the ocean’s total warming is happening below 2,000 meters, and Ifremer says the strongest signal sits below 4,000 meters in the Southern Ocean, spreading north into the Pacific, Atlantic and Indian.<\/p>\n That matters for a fairly dull physical reason. Warm water takes up more room than cold water, so a good share of sea level rise comes from the ocean swelling as it heats, on top of whatever the ice adds. Measure the top 6,500 feet and you’re estimating the rest of the column.<\/p>\n You can buy ocean profiles other ways, and some of them are absurdly cheap. A University of Delaware team’s been bolting sensors onto mako sharks and getting temperature profiles for $296 apiece<\/a>. What a float does that a shark won’t is turn up on schedule for years at a depth nothing else visits, and that’s the same thinking behind the 8.5-foot rover crawling the seafloor off California on two watts<\/a>.<\/p>\n Production goes to nke Instrumentation in Hennebont, the Brittany firm that already builds Ifremer’s 4,000-meter float, so this is a manufacturing job now. Ifremer says the 30 deep floats are due by 2028, mostly for the North Atlantic, funded through the PIANO project and the Equipex+ Argo-2030 project.<\/p>\n","protected":false},"excerpt":{"rendered":" Every few months a number goes around about how much the ocean has warmed, and it gets quoted like somebody … <\/p>\nSo why is 19,700 feet so hard?<\/h2>\n
Most of the fleet stops less than a third of the way down<\/h2>\n
What’s down there that they want?<\/h2>\n