Oceans continue to warm, especially the deeps

Estimated ocean heat content by depth.

Balmaseda, Trenberth, and Källén, courtesy of the American Geophysical Union

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.)