Harrison, D.E. and Carson, M. 2007. Is the world ocean warming? Upper-ocean temperature trends: 1950-2000. Journal of Physical Oceanography 37: 174-187.
What was done
The authors sorted individual temperature observations in the World Ocean Database 2001 into 1° x 1° and 2° x 2° bins, after which - working only with bins having at least five observations per decade for four of the five decades since 1950 - they calculated 51-year temperature trends for depths of 100, 300 and 500 meters, as well as sequential 20-year trends, i.e., 1950-1970, 1955-1975, 1960-1980 ... 1980-2000, for the same depths. Then, based on the results that were statistically significant at the 90% confidence level, they determined their implications.
What was learned
Harrison and Carson report that the upper ocean "is replete with variability in space and time, and multi-decadal variability is quite energetic almost everywhere." In fact, they found that 95% of the 2° x 2° regions they studied "had both warming and cooling trends over sequential 20-year periods," and that "the 51-year trends are determined in a number of regions by large trends over 20- to 25-year sub-periods."
What it means
The two researchers say their results suggest that "trends based on records of one or two decades in length are unlikely to represent accurately longer-term trends," and, therefore, that "it is unwise to attempt to infer long-term trends based on data from only one or two decades." In addition, they note that "the magnitude of the 20-year trend variability is great enough to call into question how well even the statistically significant 51-year trends ... represent longer-term trends." This situation suggests to us that we really don't know what may be happening, in the mean, with respect to the temperature trajectory of the global ocean; and this would appear to be Harrison and Carson's conclusion as well.