Top layers of Northern hemisphere water cooled by about 0.3C; the south saw roughly the same degree of warming.
Writing in the journal Nature, the team suggests that air pollution cannot be responsible for the changes, as has been suggested for mid-century cooling.
They do not suggest a cause, and it not clear what could link all the oceans.
However, events called Great Salinity Anomalies have been recorded in the last few decades in the North Atlantic ocean - including one around 1970.
The 1970s global temperature record shows a period where the Earth's surface cooled in the Northern hemisphere, while the Southern hemisphere saw warming.
Writing in the journal Nature, the team suggests that air pollution cannot be responsible for the changes, as has been suggested for mid-century cooling.
They do not suggest a cause, and it not clear what could link all the oceans.
However, events called Great Salinity Anomalies have been recorded in the last few decades in the North Atlantic ocean - including one around 1970.
The 1970s global temperature record shows a period where the Earth's surface cooled in the Northern hemisphere, while the Southern hemisphere saw warming.
