Oceans warmed in recent decades

But measurements show slowdown in upper-ocean heat gain since 2003

Earth’s upper ocean warmed substantially between 1993 and 2008, a new analysis reveals. The trend signals growing heat storage in oceans, researchers say, a result of human-caused warming.

CIRCUITS OVERBOARD Oceanographic data, including some collected by robotic probes like this Argo float, reveal that the uppermost 700 meters of the world’s ocean warmed substantially (about 0.18 degrees Celsius, on average) between 1993 and 2008. Sea Education Association

The new study, reported in the May 20 Nature, combined oceanographic data gathered worldwide between 1993 and 2008, the time period with the most data available. During that period, the upper 700 meters of the world’s ocean warmed on average by about 0.18 degrees Celsius, says John M. Lyman, a physical oceanographer with the National Oceanographic and Atmospheric Administration in Seattle.

“But that little bit of temperature increase represents a lot of heat,” Lyman says. In fact, Lyman and his colleagues estimate that the total heat added to the oceans during that 15-year period is equivalent to the energy that would be released by exploding about 2 billion Hiroshima-scale atomic bombs, he says.

Most of that added heat comes from the greenhouse effect, which in turn stems from the heat-trapping effect of gases like carbon dioxide, Lyman says. Because water has a vastly higher capacity to absorb heat than air, between 80 and 90 percent of the heat trapped by the greenhouse effect eventually ends up in the ocean.

“Oceans are the bellwethers of how much we’re changing the global climate,” says Josh Willis, an oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and a coauthor of the new study.

The amount of heat absorbed by the ocean varied from year to year, the researchers note. The rate of ocean warming was slow from 1993 to 1998, then increased sharply between 1998 and 2003. But from 2003 to 2008 water temperatures again held fairly steady.

Data analyzed by the team were collected using a variety of instruments over the years: some by oceanographers using simple probes dropped over the side of a ship and others by free-floating, automated sensors, Lyman says.

The stable period between 2003 and 2008 roughly coincides with the large-scale deployment of a set of robotic probes called Argo, which drift with ocean currents and measure temperature and salinity in the uppermost 2,000 meters of ocean (SN: 1/8/05, p. 30). Although it’s possible the leveling off of upper-ocean temperature measurements after 2003 is related to the change in instrumentation, it’s more likely a temporary lull in heat storage due to variations in natural processes like ocean circulation, says Lyman.

Other data suggest that some heat may have been transferred deeper than the current set of measurements, partially by diffusion from surface layers and partially due to ocean currents carrying warm water downward, says Gregory Johnson, a study coauthor and NOAA oceanographer in Seattle. This could help explain why surface waters haven’t warmed much in recent years.

The lack of warming between 1993 and 1998 may stem from a different natural phenomenon, says Kevin Trenberth, a climatologist at the National Center for Atmospheric Research in Boulder, Colo. In 1991, he notes, Earth’s climate was substantially chilled by the eruption of Mount Pinatubo in the Philippines, an event that cooled the oceans and stopped sea level rise for several years (SN: 11/5/05, p. 294).

Regardless of the source of year-to-year variations in the warming rate, says Trenberth, the slowdown in warming near the ocean’s surface since 2003 is inconsistent with satellite measurements that suggest Earth’s heat absorption increased during that time frame. Despite these discrepancies, he writes in a commentary in Nature, “as the relevant analytical methods mature, ocean heat content is likely to become a key indicator of climate change.”

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