Unfortunately, it’s hard to monitor that. There are a multitude of measuring stations for surface air temperatures, but our presence in the ocean is limited. With the advent of the Argo array—a fleet of autonomous, drifting floats that measure ocean temperatures—in the early 2000s, our data improved drastically. Still, the uncertainty has historically been greater for deeper waters.
In 2010, researchers identified an imbalance in our global energy arithmetic. If we measure the energy that's being trapped by increasing greenhouse gases, some of it seems to disappear—there wasn’t enough warming in the atmosphere or shallow ocean to account for all that extra energy— and there's been a deficit since 2004. (Though a later study suggested the mismatch might be within the margin of error for the temperature estimates.)
Some expected the “missing energy” would be found in deeper waters, but we didn’t have the data to demonstrate that. Meanwhile, the rapid atmospheric warming trend of the 1990s, boosted by strong El Niños, slowed in the La Niña-ridden 2000s, prompting some to posit that global warming was over and the scientists could all go home.
A new paper published in Geophysical Research Letters compiles the available measurements of the ocean’s heat content, including information on the deep ocean. The study finds that those deep waters have absorbed a surprising amount of heat—and they are doing so at an increasing rate over the last decade.
The researchers—Magdalena Balmaseda and Erland Källén of the European Centre for Medium Range Weather Forecasts (ECMWF) and Kevin Trenberth of the US National Center for Atmospheric Research—assembled the available data from 1958 to 2009 using a reanalysis model. These models are used to reconstruct global conditions from available measurements. In this case, the reanalysis model focused on the ocean, though atmospheric conditions were also included to complete the picture.