"If you want to know if the globe has warmed, you want to look at the upper ocean," says John Lyman, a NOAA/University of Hawaii oceanographer. That's because the oceans have a very large heat capacity (about 1000 times that of the atmosphere) and take up about 80 to 90 percent of any excess heat from, say, excess greenhouse gases in the atmosphere. (The rest of the energy goes toward melting ice and warming the land and air.)
Lyman led a team of oceanographers in an analysis of the heat stored in the upper 2,000 feet of the seas. They found that since 1993 the oceans have sucked up about 0.64 Watts per square meter of energy per year. Added up over 16 years, that's equivalent to the energy found in 2 billion Hiroshima-size bombs---or the amount of energy used if every person on the planet left 500 lightbulbs (100-Watt) on continuously since 1993.
In the study, which appears in tomorrow's issue of Nature, Lyman and his team analyzed data from multiple sources to see if a warming signal could be found. Ocean temperature data is messy. The main source for many years has been from XBTs--expendable bathythermographs--that were designed in the 1960s not for gathering climate data but for the Navy to measure the thermocline (the depth in the ocean where temperature quickly changes) for using sonar. That data, however imperfect, was later drafted into use by oceanographers in ocean temperature studies. More recently, they've been relying on a network of 3000+ Argo floats around the world that were designed specifically for measuring temperature and salinity in the upper ocean.
The data is also messy because the oceans don't behave the same from year to year or basin to basin. "Heat shifts around in the ocean," says Gregory Johnson, an oceanographer at NOAA's Pacific Marine Environment Laboratory. If you look at only a specific place or too short a time, you won't notice any warming.
Combining the data from several sources and all over the globe, however, reveals a warming trend that is six times larger than any uncertainty in the data. "This is clearly a human-caused warming signal," says Josh Willis, an oceanographer at NASA's Jet Propulsion Laboratory.
Willis calls the oceans "the bellwethers of how we're changing the global climate," but warmer seas are more than a warning sign: they have serious consequences. About one-third to one-half of sea level rise can be attributed to the thermal expansion of ocean water. Warmer oceans also lead to faster melting of glaciers and even more sea level rise. And some ecologists warn that warmer water could have an effect on the phtyoplankton and zooplankton at the base of the oceanic food chain, with unknown consequences for the fish and other organisms we eat.