Every day, thousands of robotic floats bob up and down, tracking temperatures in the world's oceans, which sop up an estimated 90% of the heat from global warming. In the course of a decade, the international Argo array has provided one of the steadiest signatures of the effect of greenhouse gas emissions. But Argo has its limits. The floats go no deeper than 2000 meters, warded off by the crushing pressures at greater depths.
Now, the array is going deeper, where hidden reservoirs of heat may lurk. On 7 September, billionaire Microsoft Co-Founder Paul Allen announced a $4 million partnership with the U.S. government that would be used to purchase 33 Deep Argo floats, capable of descending 6000 meters and reaching 99% of the ocean's volume. The National Oceanic and Atmospheric Administration (NOAA), which pays for U.S. contributions to Argo, is calling it the first "formal public-private partnership for sustained ocean observation."
In a time of tight budgets, cautious federal agencies might shy away from unproven technology such as Deep Argo, says Bob Weller, a physical oceanographer at the Woods Hole Oceanographic Institution in Massachusetts, who is leading a National Academy of Sciences, Engineering, and Medicine panel investigating the future of ocean observation. That's where billionaires can step in. "It's exciting to see philanthropies bring support to innovative new sampling methods," he says.
NOAA itself already supports 28 Deep Argo floats, which are being tested in the southwest Pacific and eastern Indian oceans. But Allen's batch, which were designed by the Scripps Institution of Oceanography at the University of California, San Diego, will be the first to meet the full standards set by Argo's scientific board. Whereas shallow Argo floats are made out of metal tubes, Deep Argo floats are the shape and size of an exercise ball, with a pressure-resistant glass sphere at their core. Like their shallow peers, the floats change their buoyancy by pumping oil into or out of an attached bladder. Carrying sensors to measure temperature, salinity, and depth, the floats will descend almost to the sea floor and drift. Every 15 days, they will rise to the surface to transmit data via satellite before diving again, says Gregory Johnson, an oceanographer at NOAA's Pacific Marine Environmental Laboratory in Seattle, Washington, who will lead the project.
After test runs, 25 of the floats will be deployed several years from now in deep international waters off the coast of Brazil using Allen's private ship, the R/V Petrel. There, investigators hope to find missing heat from global warming. They want to build on what Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado, says is Argo's greatest contribution to science so far: an accurate gauge of how humans are warming the planet.
Although scientists can track rising temperatures in the atmosphere, the vast majority of the excess heat trapped by greenhouse gases goes into the ocean. Because the ocean's temperature changes slowly and steadily, Argo data provide a better gauge of global warming than atmospheric records, which are prone to natural variations, such as the short-lived slowdown or "hiatus" in the warming rate last decade that caused public and scientific confusion. "The hiatus doesn't occur in ocean heat content," Trenberth says. "So global warming is clearly evident."
Much of this heat stays in the upper 2000 meters of the oceans. But there are signs it is reaching deeper. Every decade or so since the 1980s, for example, ships have sampled the basin off the coast of Brazil where the 25 deep floats will be deployed. There, nearly 6000 meters down, a river of frigid water slowly churns north from Antarctica. Each time researchers have looked at the basin's bottom waters, they've been warmer, Johnson says. This deep layer also appears to be growing thinner. Johnson doesn't know whether the changes are due to the warming of Antarctic source waters, or a lessening of the flow that brings the cold water north. (Johnson favors the latter hypothesis.) But sampling has been so infrequent that it's impossible to say whether these changes are trends tied to global warming or just part of the current's natural variability. Deep Argo should make that distinction.
Although the ocean currents are slow to change, they have immense momentum. Even if humanity immediately ceases its emissions of greenhouse gases, the oceans will take thousands of years to settle into equilibrium again. "I always say that the deep oceans are the flywheel of the climate system," Johnson says. "Or, if you like, they're the freight train."
To keep watch on the depths, oceanographers hope to build many more deep floats. The deep floats currently cost about three times as much as the typical Argo probes, which run $20,000; Allen's financing is intended to get the program past its "valley of death" by spurring commercial interest in manufacturing the probes. If the Brazil array proves reliable, Johnson hopes the United States and its partners will consider a full deployment of 1200 floats.
The Allen deal comes as uncertainty lingers about Argo's sustainability. With lifetimes of 4 years, the floats must be constantly replenished. Cruises to deploy them in remote regions are tough to come by. Many researchers are worried that the U.S. contributions to the program—it pays for half of Argo's floats—could come under scrutiny by the administration of President Donald Trump because the floats track climate change. And there is a potential competitor: Bio-Argo, a plan to equip floats with additional sensors that could monitor biogeochemical measures of the ocean such as chlorophyll. Dean Roemmich, an oceanographer at Scripps who led Argo's original design team, is concerned. "Any of these things can turn around and bite us," he says.