The oceans got hotter than we thought, but the heat stayed shallow

A CTD (conductivity, temperature, depth profiler) is lowered into the water. This is the standard tool for oceanographers making measurements from a ship.

Andrew Meijers/BAS

Of the energy added to the climate system by rising concentrations of greenhouse gases, more than 90 percent has gone into the ocean. The monitoring of ocean temperatures has improved drastically over the last decade with the deployment of a vast fleet of Argo floats that drift around being our eyes and thermometers. Even so, they don’t yet cover depths greater than 2,000 meters, and their presence today doesn’t make up for their absence in decades past.

Fortunately, time travel with gadgets from the future isn’t the only way to improve our knowledge of what’s gone on in the deeps. Ocean warming also manifests itself in another way—as rising sea level. Seawater expands ever so slightly with increasing temperature. And given how absolutely massive the world ocean is, “ever so slightly” adds up. In fact, thermal expansion and melting ice have made roughly equal contributions to sea level rise so far.

Going deep

There’s been a lot of interest in recent years in quantifying the warming of the deep ocean, but not much is currently known about what's going on below 2,000 meters. In a new study published in Nature Climate Change, a group led by William Llovel at NASA’s Jet Propulsion Laboratory combines sea level rise measurements with Argo data to look for the effect of warming in the deeps.