LONDON (Reuters) - The top layer of the world’s ocean has warmed steadily since 1993, a strong sign of global warming and a key driver of sea level rise, according to a study by an international team of scientists.

“The ocean is the biggest reservoir for heat in the climate system, so as the planet warms, we’re finding that 80 to 90 percent of the increased heat ends up in the ocean,” said Josh Willis, an oceanographer at the U.S. National Aeronautics and Space Administration (NASA).

Scientists from NASA and National Oceanic and Atmospheric Administration (NOAA), Britain’s Met Office, the University of Hamburg in Germany and the Meteorological Research Institute in Japan analyzed different estimates of heat content in the upper ocean from 1993 to 2008 to assess the size and certainty of growing heat storage in the ocean.

They estimated that the heat content of the ocean has increased over the last 16 years and the energy stored is now enough to light nearly 500 100-watt light bulbs for every person on the planet.

Warmer oceans cause sea levels to rise as seawater expands as it heats up, accounting for about one-third to one-half of global sea level rise, scientists say.