Reference
Gouretski, V. and Koltermann, K.P. 2007. How much is the ocean really warming? Geophysical Research Letters 34: 10.1029/2006GL027834.
Background
A global-scale study of the World Ocean by Levitus et al. (2005) suggested that between 1957 and 1997 the heat content of the upper 3-km layer rose by 14.5 x 1022J. However, to quote Gouretski and Koltermann, "the study [did] not take into account possible temperature biases associated with differing instrumentation," which - considering the great importance of the topic - is what they thus set out to do.
What was done
Noting that the large database employed by Levitus et al. was derived from five main types of instruments - Mechanical (MBT) and Expandable (XBT) bathythermographs, hydrographic bottles (Nansen and Rosette sample bottles), Conductivity-Temperature-Depth (CTD) instruments, and profiling floats - they analyzed differences (temperature offsets) among them and applied their findings to temporal trends in the degree of each instrument-type's usage over the period in question.
What was learned
The two researchers determined that the XBT data comprised the largest proportion of the data, and that "with XBT temperatures being positively biased by 0.2-0.4°C on average," this bias resulted in "a significant World Ocean warming artifact when time periods before and after introduction of XBT [were] compared." More specifically, they determined that when using the bias-correction techniques they developed, the ocean heat content increase since the 1950s was reduced by a factor of 0.62; and they say that "such corrections if applied would correspondingly reduce the estimate of the ocean warming in Levitus et al. (2005) calculations."
What it means
On the basis of Gouretski and Koltermann's careful and meticulous work, it would appear that the significant warming of the World Ocean over the last half of the 20th century, which has been enthusiastically hailed by climate alarmists the world over as evidence of accelerating global warming, may well be way over-blown.
Reference
Levitus, S., Antonov, J. and Boyer, T. 2005. Warming of the world ocean, 1955-2003. Geophysical Research Letters 32: 10.1029/2004GL021592.