Scientists have known for some time that when global warming occurs, the oceans will be the site of the most profound response.

The reason is simply that they are able to retain vastly more heat than the atmosphere. “Ninety, perhaps 95 percent of the accumulated heat is in the oceans,” said Peter Gleckler, an oceanographer at the Lawrence Livermore National Laboratory. The physical reason is that water has a far greater heat “capacity” than air, requiring more energy to raise its temperature — something that is apparent to anyone who has ever tried to boil it on a stove.

Gleckler is the lead author of a new study in the journal Nature Climate Change finding that, in the past two decades, ocean heat content has been rising rapidly and that, much more than before, heat is also mixing into the deeper layers of the ocean, rather than remaining near the surface.

“As the upper oceans have been warming over time, more and more of this heat is finding its way down into the deeper ocean, and our results indicate that the fractional amount of heat that is trapped in the deeper ocean is increasing as well,” Gleckler said.

“We find that the heat uptake of the global oceans has doubled since about 1997, compared to what took place prior to that over the industrial era. And that was a surprising result to us,” he added.

The research was conducted with scientists from the National Oceanic and Atmospheric Administration and Pennsylvania State University.

We tend to think of global warming as an overall upward trend in air temperature — but that’s simply the most immediate way in which we experience it. From a scientific perspective, it is perhaps best understood as an energy imbalance between the Earth and space, with less heat escaping and more being retained within the planet’s system.

In this sense, the new study represents a strong confirmation of this overall energy-balance shift. If large volumes of heat are trapped on Earth because of greenhouse gases in the atmosphere, then inevitably, the majority of that heat must be stored in the oceans, simply because of their greater ability to retain such energy.

“The heat capacity of the Earth’s entire atmosphere is equaled by the top 3.5 meters [11 feet] of the ocean,” explains a fact sheet released by Lawrence Livermore National Laboratory to accompany the new study.

“The overall global ocean heat uptake is a result, we know, of the increasing greenhouse gases,” Gleckler said.

Conducting the latest work required stitching together multiple data sources, including measurements from the historic Challenger expedition of the 1870s — a landmark moment for oceanography — and modern readings from Argo floats. Nearly 4,000 of these instruments are spread across the global ocean, providing temperature measurements as deep as 2,000 meters.

Scientists divide the ocean into layers, with the top portion — the one that is warming and, therefore, affects humans the most — extending from the surface down to 700 meters. The middle section spans 700 meters to 2000 meters deep, and the deepest ocean is below that.

The research suggests that two-thirds of heat accumulation has occurred in the upper layer so far, with the remaining third in the lower layers. But it also finds that the percentage stored deeper in the ocean has been increasing recently.

The consequences of upper-ocean warming are well documented. From the bleaching of corals to the potential for more-intense hurricanes, a warmer surface has profound consequences for anything living in the oceans (this is where most sea life is) but also on land. Heating the ocean also raises sea levels, because warm water expands.

The consequences of warming the middle and deepest layers are less clear and less immediate to those of us living at the surface, but they are also sure to be significant. The new study provided a global overview of increasing ocean warming, rather than any specific prediction of regional consequences. But warming the deep ocean could lead to changes in its circulation, Gleckler said. One key factor driving the oceans’ global overturning circulation is the density of water, which is in turn affected by its temperature.

Although no clear connection can be proved, it is also worth noting that scientists think the ice sheet of West Antarctica is melting because a layer of warmer middle-ocean water is now reaching the deep sub-sea base of the sheet. The cause of this phenomenon remains poorly understood, but it implicates a warmer middle layer of the ocean.

On the bright side, Gleckler notes, you can argue that increasing heat burial in the deep ocean is, in some ways, good news for humans. If the heat wasn’t at depth, then more of it would be in the surface layer and the surface of the globe would be even warmer, and feeling greater effects, than it already is.

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