When discussing global warming, the public eye is mostly directed to global average surface air temperatures, but that’s just one slice of the climate pie. If you haven’t noticed, the ocean is awfully big, and it holds a great deal more heat energy than the atmosphere. In fact, about 90 percent of the energy that’s been added to the climate system by human activities has gone into the ocean.

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.

The resulting ocean heat content data shows some interesting features. Drops in ocean heat content coincide with large volcanic eruptions, which pump sunlight-reflecting aerosol particles into the atmosphere for a time. But there’s also a small drop after 1998—a year known for the incredibly strong El Niño that pushed global surface temperature to a (then) high point.

That may seem odd at first blush, but it makes sense if you’re not fixated on the atmosphere. An El Niño involves above-average sea surface temperatures in the eastern Pacific, and some of that energy is transferred to the atmosphere, which becomes warmer as a result. All that warm water means evaporation, and evaporation means cooling of the ocean, just like sweat cools your skin.

After this period, ocean heat content continued to rise sharply—especially in the deep ocean. The paper states that “recent warming rates of the waters below 700 [meters] appear to be unprecedented” in the record. Supporting some earlier estimates, the data shows about 30 percent of ocean warming after 1998 taking place more than 700 meters down.

While it’s certainly useful to note the extent to which heat energy is accumulating in the climate system, it’s more interesting to ask why the deep ocean has taken such a large share of it recently. It seems to relate to changes in ocean circulation. A 2011 study indicated that La Niñas and a circulation pattern called the Pacific Decadal Oscillation could cause lulls in surface warming while energy is stuffed into the deep ocean. That may be exactly what we’ve experienced over the past decade. When those conditions change, we’ll see the effects in higher surface temperatures.

Geophysical Research Letters, 2013. DOI: 10.1002/grl.50382 (About DOIs).