John Lyman (left) holding an XBT and Gregory Johnson (right) holding an Argo Float. Both are used to measure ocean heat content.
The ocean has warmed significantly over the past decade and a half, a new study based on different sources of ocean warming data suggests.
The new study, detailed in the May 20 issue of the journal Nature, revealed that the top 2,300 feet (700 meters) of the world's oceans warmed 0.64 watts per square meter from 1993 to 2008. That's equal to adding the energy from 100 million atomic bombs to the ocean each year during the 16-year period, said John Lyman of the University of Hawaii.
Water takes longer to heat up and cool down than does the air or land, so ocean warming is considered to be a better indicator of global warming than measurements of global atmospheric temperatures at the Earth's surface.
But until now, scientists were not sure how the heat energy in the upper ocean had changed in recent decades or what ocean warming meant for the Earth's energy balance.
Lyman and colleagues combined different ocean monitoring groups' data sets, taking into account different sources of bias and uncertainty — due to researchers using different instruments, the lack of instrument coverage in the ocean, and different ways of analyzing data used among research groups — and put forth a warming rate estimate for the upper ocean that it is more useful in climate models.
Ocean heat content is a useful measurement for studying the Earth's warming, because the upper ocean acts as a giant heat sink and absorbs 90 percent of the heat energy that is added to the Earth's atmosphere from the warming caused by the accumulation of greenhouse gases, Lyman said.
While the ocean warmed from 1993 to 2008, the data suggest that warming has stalled since 2003. However, researchers are not certain what pattern ocean warming will follow, and the stall could be attributed to natural variability in the data, Lyman told LiveScience.
"The key thing for studying climate is what the long-term trends are doing," said climate scientist Gavin Schmidt of NASA's Goddard Institute for Space Studies, who was not involved in the study. "However you cut it, those long-term trends are still there."
Despite the variability, the long-term increase in ocean heat content is real and cannot be dismissed as an artifact of measurement error, said climate scientist Michael Mann of Penn State. Climate models based on increases in man-made greenhouse gases predict an increase in ocean warming that is similar to the new model's estimate.
"The study is a sobering reminder that human-caused climate change is very real, and by many measures actually proceeding faster than the models have projected," Mann said.