Changes in ocean heat content are thought to be the best way to detect and measure a planetary energy imbalance, since the vast majority (about 93%) of the extra heat goes into the ocean, and because its huge heat capacity -- about 1,000 times that of the atmosphere -- means heat changes there are much less fickle than in the atmosphere. As oceanographer Greg Johnson of NOAA puts it, "global warming is ocean warming."
OHC for the top half of the ocean (0-2000 meters), measured by the Argo bouys in the last 10+ years, is now clearly accelerating. The year-over-year change for the 0-700 meter region is 1.1 W/m2, and 1.5 W/m2 for the 0-2000 meter region.
For the 0-2000 meter region, a quadratic fit to the data is better than a linear fit, with an acceleration of 0.09 ± 0.03 (W/m2)/yr (statistical error, no autocorrelation):
Here, ZJ = 1 zettajoule = 1021 Joules.
A quadratic fit to these data keeps getting better and better relative to a linear fit: