Nearly a quarter century ago, Greg Johnson was a freshly minted PhD in oceanography puttering north in the South Pacific Ocean. About every 35 miles, the boat he was on stopped and scientists dropped an instrument overboard to measure temperature and salinity at regular intervals all the way to the seafloor.
This process was repeated in a crisscross pattern throughout the world's oceans over the course of the 1990s. "At the end of it, we had kind of a blurred snapshot of the state of the ocean in that decade all the way from the surface to the bottom from coast to coast," he explained to NBC News.
The following decade, scientists re-sampled some transects for the sake of comparison. Taken together, the measurements collected during the World Ocean Circulation Experiment are the best — and for most of the oceans only — data available on temperature and salinity 1.4 miles below the surface.
Today, Johnson is spearheading a project to deploy a global array of robotic floats that will probe to a depth of 3.75 miles, allowing scientists to continuously monitor changing temperature and salinity in the entire ocean except for the deepest trenches.
The data would help fix bridge a major gap in knowledge about the deep, dark abyss and its linkage to climate change, according to project scientists and other ocean experts.
For example, data from the ocean-crossing ships indicated deep-ocean warming, especially in the Southern Hemisphere. "But we don't know whether that was a one-time thing, whether that was a decade of warming and now it is going to be followed by a decade of cooling, or if it is a trend or part of an oscillation," said Johnson, now with the National Oceanic and Atmospheric Administration's Pacific Marine Laboratory in Seattle. "We don’t know what it is."
The envisioned robotic array known as Deep Argo is an extension of a 3,579-member-strong army of floats already deployed throughout the world's oceans that regularly beam out data on temperature and salinity to a depth of 1.4 miles.
To date, that army of floats has returned an "awesome magnitude more data" than previously available, revealing "that the ocean is just full of systems which are analogous to the atmosphere's weather," John Marshall, an oceanographer and director of the climate modeling initiative at the Massachusetts Institute of Technology, told NBC News.
Deep Argo, he said, would further clarify that picture and provide a trove of data on just where and how much heat is stored in the ocean. Scientists have reported that about 93 percent of the extra energy trapped in the Earth's climate system by rising greenhouse gas concentrations is stored as heat in the world's oceans.
More of that warming after 1970 than previously estimated appears concentrated in the upper ocean, according to a recent study in Nature Climate Change, while a companion study found that "no significant global ocean warming below 2000" meters had occurred over 2005-2013.
Johnson co-authored a perspective on the research, also published in Nature Climate Change, that noted the finding’s “uncertainties are large” and could be resolved with the Deep Argo network.
"In part of the ocean, that heat is kept up near the surface and in other places the heat is carried deep into the interior of the ocean," Marshall said. "For example, in the Southern Ocean, we think there are pathways from the surface right down to the bottom of the ocean, so the patterns and the timing of global warming are being affected by the ocean taking up heat."
In fact, storage of heat in the deep ocean may help explain what's known as the hiatus – a slowdown in the pace of atmospheric warming over the past 15 years. As it stands now, the paucity of deep ocean observations limits scientists' ability to shore up the theory, he added.
"I think personally a lot of the excitement in Deep Argo has to do (with) seeing the ocean circulation and ocean circulation changes that are responsible for getting these property changes into the deep oceans," Dean Roemmich, an oceanographer heading up Deep Argo efforts at the Scripps Institution of Oceanography in La Jolla, Calif., told NBC News.
The network of floats would also help scientists better understand how much ocean warming and freshwater runoff from melting ice sheets contribute to sea level rise and thus model how high and how quickly seas will rise in the coming decades. In addition, the real-world observations will help validate and calibrate models scientists use to forecast future climate change, according to Johnson.
This June, scientists with Scripps Institution of Oceanography involved with the Deep Argo initiative deployed two prototype floats east of New Zealand, a deep ocean basin that is known to have warmed over the past two decades.
Those floats have already completed about 40 profiles each to a depth of 3.75 miles. The group will retrieve and check the equipment in a year and deploy about a dozen more of them in the basin as an initial foray into the development of a global Deep Argo array, Roemmich explained. Ultimately, he and other researchers envision a Deep Argo network of about 500-1,000 floats globally.
Johnson dreams even bigger, perhaps as many as 1,200 floats, which each carrying a price tag of $40,000 to $50,000.
"My idea is that we need to measure accurately locally to be able to resolve on decadal timescales temperature changes that we have seen with the (World Ocean Circulation Experiment) repeat sections," he said. "That is something at present that we have to average over large basins to get statistically significant results. With Deep Argo, we will be able to hopefully look at things locally."