The upper ocean warmed considerably over the past decade and a half, according to a new study that attempts to make sense of conflicting analyses of the amount of heat stored in the world's seas.
Between 1993 and 2008, the study finds, the upper 700 meters of the oceans absorbed about 0.6 watts per square meter of energy. That is roughly equivalent to the power of 2 billion copies of the atomic bomb the United States dropped on Hiroshima, Japan, during World War II, said lead author John Lyman, an oceanographer at the University of Hawaii's Joint Institute for Marine and Atmospheric Research.
"Ocean heat content is a very good indicator for how the entire planet is warming," he said, because the seas serve as a massive planetary heat sink. "A percentage of the incoming radiation [from the sun] is trapped on our planet by greenhouse gases, and it turns out that about 80 to 90 percent of that heat is trapped in the ocean."
Lyman's research team, whose work was published yesterday by the journal Nature, found a warming trend six times larger than the uncertainty inherent in the ocean heat data they analyzed.
Several research teams have attempted to determine how quickly the ocean is warming. But while they agree the seas have warmed over the last 15 years, the year-to-year estimates of the groups don't agree.
Towed Navy temperature tool is replaced
The new research identifies several sources of uncertainty and bias, including that inherent in expendable bathythermographs, or XBTs, once the main tool for measuring the heat of the ocean.
From the 1960s until the widespread deployment of a much more accurate measurement tool in 2004 -- the Argo float -- XBT probes "were the dominant tool we had for looking at how the water was," said Josh Willis, an author of the study and an oceanographer at NASA's Jet Propulsion Laboratory.
Designed for the Navy, XBTs were intended to measure temperature at various depths from a moving ship. The data they collected aren't as accurate as scientists would like for climate analyses. But over time, Willis said, they inadvertently became the main source of data for understanding how warm the oceans were in past decades.
"What our study shows is because they weren't really designed to be this accurate, there's still a large amount of error in the data," he said. "Efforts are ongoing to remove and account for these errors. But despite the errors, the global warming signal is larger."
Still some puzzles to be solved
These days, scientists collect the majority of ocean heat measurements using more than 3,200 Argo floats now deployed in the world's oceans. The probes drift below the ocean surface at prescribed depths for days at a time, collecting data on their journey back to the surface, where they beam their observations to waiting satellites. Then the little floats sink back into the ocean to begin again, repeating the process thousands of times during their three- to five-year lifespans.
Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research, said in an interview that the new paper finds a warming signal that is "reasonably consistent with expectations from other indications of global warming."
But Trenberth said it does not address a key question surrounding the ocean heat measurements. Since about 2003, the rate of upper ocean warming appears to have slowed to a crawl, and scientists aren't sure why.
Measurements of the solar radiation at the top of the atmosphere suggest the upper ocean should be warming faster than it has in recent years. Trenberth suspects the ocean may be warming at depths Argo floats and other instruments aren't reaching.
"This discrepancy suggests that further problems may be hidden within the ocean observations and their processing," he wrote in an essay accompanying the new Nature paper.