An international team of researchers compared climate models with improved observations that show sea levels rose by 1.5 millimeters per year during the period, equating to an approximately 2 1/2-inch increase in ocean levels during the 42-year period.

Scientists at the Lawrence Livermore National Laboratory, which participated in the study, said the ocean warming and thermal expansion rates are 50 percent larger than previous estimates for the upper 2,295 feet of oceans, and greater than that for the upper 985 feet.

"This is just the tip of the iceberg, so to speak," said LLNL climate scientist Peter Gleckler. "Our ability to quantify structural uncertainties in observationally based estimates is critically important. This study represents important progress."

The team also included researchers from the Center for Australian Weather and Climate Research and the Antarctic Climate and Ecosystems Cooperative Research Center.

The findings appear in the journal Nature.