Collected using XBTs, this was the dominant source of ocean temperature data up until 2003 or 2004, when its measurements were superseded
by those done by the Argo system.
A problem with the data
According to Jet Propulsion Laboratory climatologist Josh Willis:
"There is still a good deal of uncertainty in observational estimates of ocean heat content during the 1990s and into the early part of the 2000s. This is because of known biases in the XBT data set, which are the dominant source of ocean temperature data up until 2003 or 2004. Numerous authors have attempted to correct these biases, but substantial difference remain in the “corrected” data. As a result, the period from 1993 to 2003 still has uncertainties that are probably larger than the natural or anthropogenic signals in ocean heat content that happen over a period of 1 to 3 years.
However, the decadal trend of 10 to 15 years seems to be large enough to see despite the uncertainties. Because Argo begins to become the dominant source of temperature data in about 2004, the period from 2000 to 2005 is especially worriesome because of the transition from an XBT-dominated estimate of ocean heat content."