Oceans' heat-buffering ability may be weakening

For decades, the earth’s oceans have soaked up more than nine-tenths of the atmosphere’s excess heat trapped by greenhouse gas emissions. By stowing that extra energy in their depths, oceans have spared the planet from feeling the full effects of humanity’s carbon overindulgence.
But as those gases build in the air, an energy overload is rising below the waves. A raft of recent research finds that the ocean has been heating faster and deeper than scientists had previously thought. And there are new signs that the oceans might be starting to release some of that pent-up thermal to significant global temperature increases in the coming years.
The ocean has been heating at a rate of around 0.5 to 1 watt of energy per square meter over the past decade, amassing more than 2 X 1023 joules of energy — the equivalent of roughly five Hiroshima bombs exploding every second — since 1990. Vast and slow to change temperature, the oceans have a huge capacity to sequester heat, especially the deep ocean, which is playing an increasingly large uptake and storage role.

That is a major reason the planet’s surface temperatures have risen less than expected in the past dozen or so years, given the large greenhouse gas hike during the same period, said Kevin Trenberth, senior scientist with the National Center for Atmospheric Research. The phenomenon, which some call the “hiatus,” has challenged scientists to explain its cause. But new studies indicate that the forces behind the supposed hiatus are natural— and temporary — ocean processes that may already be changing course.
Pacific trade winds, for instance, which have been unusually strong for the past two decades thanks to a 20- to 30-year cycle called the Interdecadal Pacific Oscillation, have been pumping atmospheric heat down into the western Pacific. The winds are powered up by the cycle’s current negative, or cool, phase. But scientists say that when the cycle eventually swings back to its positive, warm phase, which history suggests could occur within a decade, the winds will wind down, the pumping will let up, and buried heat will rise back into the atmosphere.
“There’s a hint this might already be starting to happen,” said Matthew England, an ocean sciences professor at the University of New South Wales in Sydney, Australia. Without the winds’ cooling action, atmospheric temperatures could surge as they did in the 1980s and 1990s, the last time the oscillation was positive. During the next positive phase, “it’s very much likely that [warming] will be as fast or even faster,” he said, “because those greenhouse gases are now more elevated.”