NASA's Jet Propulsion Laboratory in California just released a decade-long study which evaluated ocean temperatures at extreme depths, and the parsing of these results is incredibly interesting.
The first paragraph from the October 6th release is illustrative:
The cold waters of Earth’s deep ocean have not warmed measurably since 2005, according to a new NASA study, leaving unsolved the mystery of why global warming appears to have slowed in recent years.
If one might recall, scientists who claim that carbon dioxide drives global warming are unable to explain a 17-year hiatus in rising global temperatures, all while carbon dioxide levels continue to rise. Obviously, if carbon dioxide drives global temperatures, this is literally impossible: No correlation, no causation.
The press release continues:
Scientists at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, analyzed satellite and direct ocean temperature data from 2005 to 2013 and found the ocean abyss below 1.24 miles (1,995 meters) has not warmed measurably. Study coauthor Josh Willis of JPL said these findings do not throw suspicion on climate change itself.
"The sea level is still rising," Willis noted. "We're just trying to understand the nitty-gritty details."
The problem that they are trying to solve is why global temperatures have not risen, despite the UN's models predicting that such temperatures would increase proportionately to carbon dioxide levels. One hypothesis floated was that the oceans were absorbing the excess thermal energy. This is what the NASA release is addressing:
In the 21st century, greenhouse gases have continued to accumulate in the atmosphere, just as they did in the 20th century, but global average surface air temperatures have stopped rising in tandem with the gases. The temperature of the top half of the world's oceans — above the 1.24-mile mark — is still climbing, but not fast enough to account for the stalled air temperatures.
Many processes on land, air and sea have been invoked to explain what is happening to the "missing" heat. One of the most prominent ideas is that the bottom half of the ocean is taking up the slack, but supporting evidence is slim. This latest study is the first to test the idea using satellite observations, as well as direct temperature measurements of the upper ocean. Scientists have been taking the temperature of the top half of the ocean directly since 2005, using a network of 3,000 floating temperature probes called the Argo array.
"The deep parts of the ocean are harder to measure," said JPL's William Llovel, lead author of the study published Sunday in the journal Nature Climate Change. "The combination of satellite and direct temperature data gives us a glimpse of how much sea level rise is due to deep warming. The answer is — not much."
In summary, NASA reports that deep ocean water temperatures neither explain the increase in ocean surface temperatures, nor why global temperatures appear to have paused in recent years.