"We are seeing the global ocean store more heat than it gives off," said John Lyman, an oceanographer at NOAA's Joint Institute for Marine and Atmospheric Research.

An international team of scientists analysed nine different estimates of heat content in the upper ocean from 1993 to 2008.

They combined these estimates to assess the size and certainty of growing heat storage in the ocean.

"The ocean is the biggest reservoir for heat in the climate system," said Josh Willis, an oceanographer at NASA's Jet Propulsion Laboratory.

"So as the planet warms, we're finding that 80 to 90 per cent of the increased heat ends up in the ocean."

Measurements by an array of autonomous free-floating ocean floats called Argo and earlier devices called expendable bathythermographs (XBTs) demonstrate that ocean heat content has increased over the last 16 years.

But the team notes that there are still uncertainties and some biases.

"The XBT data give us vital information about past changes in the ocean, but they are not as accurate as the more recent Argo data," said Gregory Johnson, an oceanographer at NOAA's Pacific Marine Environmental Laboratory.

"However, our analysis of these data gives us confidence that on average, the ocean has warmed over the past decade and a half, signalling a climate imbalance."