The problem is lack of detailed information, according to Tim Boyer of the US National Oceanic and Atmospheric Administration (NOAA) at a press conference at the American Geophysical Union (AGU) Fall Meeting in San Francisco in December 2016.
Data on ocean temperature at various depths currently come from instruments lowered from ships, from 4000 Argo floating instruments that sink to 2000 m and return to the surface at regular intervals, or even from Antarctic seals equipped with sensors. But all of these methods leave large areas of the ocean uncovered or only sporadically monitored.
At the same press conference, NASA’s Robert Tyler presented a hypothetical method to measure oceanic heat content through what he called magnetic remote sensing. Currently under test by Tyler and his NASA colleague Terence Sabaka, the technique is based on the fact that Earth’s magnetic field lines are not static; they fluctuate and can be measured from the ground and by satellites.
One source of these fluctuations is ocean flow. The tides are "rocking back and forth, and they tend to drag the magnetic field lines with them," Tyler said. "The reason for that is that the ocean has salts in it, and it’s a pretty good conductor. The higher the conductivity of the ocean, the better the dragging, and the higher the temperature of the water, the better the conductivity."
By monitoring fluctuations in the magnetic field lines, we might be able to use them as a thermometer to learn about temperature changes in the ocean, Tyler said, adding that the concept was only in the testing stage. He was encouraged that mathematical models of magnetic fluctuations based on the diurnal ocean tides conform well to actual observations of those fluctuations. Since magnetic fluctuations depend on the electrical conductance of the ocean, "we can attempt to numerically invert the magnetic observations to gain ocean conductance," he told reporters.
There is a strong linear relationship between conductance and heat content, "which means if we have conductance, we can convert that into heat content," Tyler said. How effectively, then, can we currently infer conductance from remote magnetic observations? The good news, Tyler said, is that the modeling experiment provided an important proof-of-concept, based on theoretically generated tidal magnetic fields. The "other" news is that conductance inverted from observed magnetic fields is currently too limited in accuracy, either by noise in the observations or imperfect modeling, or both, to be reliable.
Tyler is encouraged by the potential contribution of three new European Space Agency (ESA) Swarm satellites, which are intensively studying Earth’s magnetic fluctuations in unprecedented resolution. They will provide data for years to come, he concluded, providing "an exciting new opportunity" to refine our understanding of the ocean’s conductance and thereby unlock the secrets of its hidden heat.