Since the late 1990s, global warming has stabilized, even as greenhouse gases have risen. That defies simple models that say the temperature should keep going up. Many scientists think the so-called “hiatus” is taking place in part because much of the heat trapped in the atmosphere by greenhouse gases is being soaked up and stored by the oceans–at least for now. The Pacific is believed to play an especially powerful role, with winds in its eastern regions sweeping heat into its depths, like dirt getting swept under the rug. The problem is, scientists checking under the rug by measuring subsurface temperatures have not necessarily found the predicted increases in heat. This has come to be known as the riddle of the “missing heat.” A team of oceanographers now says they know where it went: It has been exported from the Pacific to the Indian Ocean. Their study, out this week in the journal Nature Geoscience, finds that this movement may account for more than 70 percent of all heat absorbed by the entire upper world ocean in the past decade.
The team, led by Sang-Ki Lee of the National Oceanic and Atmospheric Administration, modeled existing data assembled by other researchers to reach their conclusion. According to this data, the top 700 meters of the Pacific have cooled, not warmed, in recent years. At the same time, westward-bound winds over the Pacific have strengthened, pushing great amounts of subsurface water toward the straits through the island nation of Indonesia, which continually feed Pacific waters to the Indian Ocean. It is part of a vast conveyor belt that carries subsurface currents around the globe. (The conveyor continues around the horn of Africa and into the Atlantic, then heads north; deep water eventually surfaces in the north Atlantic, helping warm that region.)
In line with the increases in Pacific winds, measurements by coauthor Arnold Gordon of Lamont-Doherty Earth Observatory show that flow through the main Indonesian channel, the Makassar Strait, has increased since the early 2000s. Calculations show that the increased flow could account for enough water movement to carry the heat that should be in the Pacific. And this lines up with data showing that the upper layers of the Indian Ocean have been warming in rough correspondence with the missing heat since 2003. The conclusion: the heat is just getting pushed from one ocean to the other. “Everyone’s looking for the missing heat but the data was already there,” said Gordon. “All we did was connect the dots.”
Oceanographer Matthew England of Australia’s University of New South Wales, who was not involved in the research, told the journal Nature that the study clarifies the oceans’ role in recent temperature trends. “It’s resolving a question that has a lot of people stumped,” he said.
Others are not so sure. While the team analyzed only the top 700 meters of ocean, other studies suggest that a lot of heat may have sunk below that level–maybe a third or more of recent total ocean warming. However, measurements at those depths are less comprehensive. Kevin Trenberth, a climate scientist at the U.S. National Center for Atmospheric Research, says also that the datasets the researchers used in the shallower depths may not be accurate; other measurements show the upper Pacific in fact has been slightly warming, not cooling.
Despite the disagreements, most oceanographers who study this subject concur on two things. First: due to human-influenced warming, heat is collecting somewhere in the oceans, even if not everyone agrees on the details. Second: Some time. somewhere, that absorbed heat will have to re-emerge into the atmosphere, whether it is years, decades or centuries from now. This could cause a spike in global warming that would be all too obvious on the surface.
For more detail, see news coverage of the paper: