In the 15 years ending in 2008, the oceans grew hotter with energy equal to 2 billion Hiroshima-size atomic bombs, a recent report estimates.

"All the impacts you find from global warming—this shows you it is really happening," said John M. Lyman, an oceanographer with the Joint Institute for Marine and Atmospheric Research, operated by the University of Hawaii at Manoa and the National Oceanic and Atmospheric Administration. "This confirms they would occur."

The Hiroshima bomb was the equivalent of 13,000 tons of TNT.

By another measure, the additional energy stored in the ocean is enough to power nearly 500 100-watt light bulbs per each of the planet’s 6.7 billion people continuously for 16 years, said Lyman, one of the researchers involved in the report.

"You can think of the ocean as a bellwether for global warming," he said in a telephone interview from NOAA’s Pacific Marine Environmental Laboratory in Seattle. "The ocean traps 80 to 90 percent of heat on the planet from greenhouse gases."

He added, "Water has much larger heat capacity than air, so the top 3 meters (about 10 feet) of the ocean can produce the same amount of heat as the atmosphere."

The warming ocean is responsible for about one-third to one-half of global sea level rise because seawater expands and takes up more space as it heats up, the scientists said.

They analyzed nine different estimates of heat content in the upper ocean from 1993 to 2008. Their findings were reported in an article titled "Robust Warming of the Global Upper Ocean" in a recent issue of the journal Nature.

"We used every data source available since 1993 to estimate the heat content," Lyman said, citing studies by groups from Germany, the United Kingdom, the United States and Japan.

"We tried to quantify what the uncertainty was (in the estimates)," he said. "I was surprised a teeny bit that when you follow different curves, when we put them together, the uncertainty was small." Combining heat estimates from the surface to a depth of about 6,000 feet, the team found "strong multiyear warming" throughout the world’s oceans.

The scientists looked at measurements by a global array of about 3,276 autonomous ocean floats called Argo deployed in 1994 and earlier devices called expendable bathythermographs or XBTs that were dropped from ships to obtain temperature data.

Lyman said the XBTs provide data for the top 2,000 feet of the ocean and the Argo instruments go to 6,000 feet. "Below 6,000 feet is not measured very often," he said.

Oceanographer Gregory Johnson, also at the Seattle laboratory, said analysis of XBT data about past changes and more recent Argo data "gives us confidence that on average, the ocean has warmed over the past decade and a half, signaling a climate imbalance."

The Argo floats represent an international effort to continuously monitor the upper-ocean temperature, salinity and velocity.

The data, sent via satellite to scientists, greatly reduces uncertainties in estimates of ocean heat content over the past several years, Lyman’s group said.

Increased ocean acidity because of rising levels of atmospheric carbon dioxide dissolved in seawater poses a threat to coral reefs and certain plankton and other marine organisms, UH oceanographers have reported.

Chemical oceanographer Richard Zeebe, among those studying the warming oceans, has warned, "Drastic emission cuts at some point in the future might be an option in terms of climate change, but it could be too late for coral reefs and other marine organisms."

Lyman’s team includes scientists from the NASA Jet Propulsion Laboratory in Pasadena, Calif.; the Met Office Hadley Centre in the United Kingdom; University of Hamburg, Germany; the Meteorological Research Institute in Japan; and NOAA.