An ‘unfathomable amount’ of rising heat in the oceans — study

Gayathri Vaidyanathan, E&E reporter

The global oceans, like a giant sponge, have sopped up 90 percent of the excess heat trapped in the planet since the Industrial Revolution.

Almost half of that heat uptake, equaling “an unfathomable amount of heat” — 330 zeta joules, with 21 zeros — has occurred since 1997, said Peter Gleckler, a climate scientist at the Lawrence Livermore National Laboratory. The finding was published this week in Nature Climate Change.

The heat is mostly in the upper reaches of the ocean, and 35 percent has trickled down to depths greater than 700 meters, the study finds.

“What is happening in the deeper ocean is more of a mystery; the heat is trapped there for a very long time, and it could have important implications for circulation and marine ecology that we don’t understand,” Gleckler said.

The oceans are among the least observed ecosystems on Earth, and the changes unfolding in the deepest reaches due to human activity have been largely unobserved and could have dramatic consequences.

The earliest attempt to quantify the ocean’s heat content was in 1872, when the HMS Challenger undertook a global voyage and sailors used thermometers to measure sea temperature. The latest, and most comprehensive, attempts began in 2004, when scientists with the Argo program deployed automated floats that dove to 700 meters to measure ocean heat content. In 2014, “Deep Argo” floats began diving down to 2,000 meters (ClimateWire, Oct. 7, 2014).

Scientists with the Global Ocean Ship-based Hydrographic Investigations Program, or GO-SHIP, have been tracking heat content down to the ocean bed at an average depth of 4,500 meters.

Gleckler and his colleagues used all available data sets to compile a record of ocean heat content over 150 years.

“As you might imagine, there are many times and places where we don’t have measurements,” he said.

They filled in gaps using data simulated using climate models and then studied the changes in ocean heat content following the Industrial Revolution. The models suggested that half of the observed ocean warming has occurred in the past two decades.

“The oceans are the largest heat storage reservoir in the climate system,” said Paul Durack, a climate scientist at the Lawrence Livermore National Laboratory and a co-author of the study. The magnitude of change in ocean warming that scientists are seeing is actually very small compared to the overheat capacity of the global oceans, he said. But the warming is expected to increase in the future.

“As changes become larger and larger, they will actually feed back on changing the way the ocean operates,” he said.

This underscores the importance of maintaining global ocean observing networks such as Argo and GO-SHIP, Durack said.

“You can’t take this for granted, because when it is gone, it is gone,” he said. “It is very hard to re-establish these networks if they do start to decay.”