Yearly global ocean heat content compared to the 1958-65 average (dashed line at zero) for the past four decades for different layers of the ocean: from the surface to depths of 300 meters (grey) and 700 meters (blue), and total depth down to 2,000 meters (purple). Surface waters warmed more slowly (line is nearly flat since the mid-2000s) than deeper waters (steep increase). Since the core of the Argo fleet can only dive down to 2,000 meters, the amount of heat going into the deep ocean is unknown. Image adapted from Figure 1 of Balmaseda et al., 2013 (pdf).