At the end of the 20th century, climate scientists noticed what they thought at first was an anomaly: a slowdown in the pace of global warming in the lower atmosphere. Today it is a recognized trend that has lasted more than 15 years. Perplexed, oceanographers are on a hunt to find where this missing heat has gone.
In the latest report out of Nature Geoscience this week, University of Miami physical oceanographer Sang-Ki Lee and colleagues may have found some of this missing heat: The Pacific Ocean is keeping its cool by sending heat over to the Indian Ocean. This heat redistribution, the researchers say, could play a role in regulating the rate of global warming.
Oceans: A Complex Buffer
Why the global warming hiatus has happened and how long it will last are mysteries. However, scientists do know that the ocean has recently helped to buffer what was otherwise an accelerated surface warming, one that has not yet stopped. Warming in the upper atmosphere continues to show that the planet is undergoing a radiation imbalance.
However, rather than showing any signs of storing heat, as is the case in the Atlantic Ocean, the Pacific Ocean has actually cooled over the past decade.
“When I noticed from the hydrographic data that the Pacific Ocean heat content has been decreasing since 2003 or so, I was very surprised and puzzled,” Lee told Eos. “And when I found a large heat increase in the Indian Ocean, I was almost convinced that there was something wrong with the hydrographic data.”
How Does Heat Escape to the Indian Ocean?
Lee ran a computer model simulation and found that he could explain the difference if a massive amount of heat from the Pacific flowed through Indonesia’s archipelago into the Indian Ocean. However, how best to move the heat?
Warm water, like warm air, rises—or, rather, stays at the surface when nothing else is disturbing it. This is why, in a lake, the upper layer is warmer than the bottom layer.
To get warm surface water from the Pacific to the Indian Ocean requires wind—and not just any wind. The trade winds need to be strong enough to push water from the eastern Pacific all the way across the ocean basin to the west, where it piles up and creates a region of above-average sea surface height.
Warm surface water can then flow like a river down around the Indonesian archipelago to the Indian Ocean. A difference in height of less than a dozen centimeters is enough to get the heat moving.
The Role of La Niña
Often, the trade winds over the Pacific come up against westerly winds from over the Indian Ocean. During El Niño events, these westerly winds are stronger than the trade winds, and the two will converge in the middle of the Pacific. However, during La Niña events, the westerly winds are extremely weak, and the result is a lower than average sea surface height in the Indian Ocean.
When Lee and his colleagues looked at the temperature observations between the Pacific and Indian Oceans, warming episodes in the latter matched the pattern of the more frequent La Niña events that have occurred over the years.
“We were all very excited to find a good match between the model simulation and the direct measurements,” Lee said. The observations show that “ocean heat transport plays a vital role in redistributing the global energy imbalance,” he added.
Can the Indian Ocean House All of this Hidden Heat?
Lee admits his work is looking at only part of the puzzle—that of the surface heat transport to the Indian Ocean.
Last year, other oceanographers put the focus on deeper water layers in the Atlantic Ocean, dismissing the Indian Ocean’s role during this hiatus period. Ka Kit Tung of the University of Washington told Eos that in the journal Science “we reported…that the Indian Ocean is the only ocean that warmed at the surface and in its upper layers, but it is a minor player in terms of the heat uptake that is needed.”
“The upper 700 meters of the heat content increase in the Indian Ocean, and, for that matter, in the global ocean, is not enough to explain the near-zero trends in the surface temperature and upper 200 meters’ ocean heat content. The key is the heat storage between 200 and 1500 meters of the oceans, which is about 70 zettajoules during the hiatus period,” Tung explained. For comparison, the global energy consumption per year is 0.5 zettajoule.
The paper by Lee and his team is “a useful contribution concerning the recent cause of the warming in the upper 700 meters of the Indian Ocean,” Tung added.
“It is not a budget calculation,” he explained. “There is a difference between finding some warming in the Indian Ocean and justifying the proposition that the amount of heat storage explains what is needed to account for the global hiatus. [We] not only calculated the heat storage in the Indian Ocean in the upper 700 meters but…calculated it down to 1500 meters and showed that it was not enough.”
An Important Piece of the Puzzle
Despite heat budget complexities, some oceanographers find the new results intriguing.
“The report finds that the interbasin heat transport carried by ocean currents may hold the key to deciphering the unsolved missing heat,” said Lisan Yu of the Woods Hole Oceanographic Institution.
“Although the study did not explain what drives La Niña conditions in the tropical Pacific, it identifies a potentially important pathway of heat redistribution and provides valuable insight [into] the role of ocean currents in the global warming hiatus,” she explained. “The study adds further evidence that the oceans are key to explaining the climate anomalies.”
For Jérôme Vialard of the Laboratoire d’Océanographie et du Climat: Expérimentations et Approches Numériques (LOCEAN) in Paris, France, the search for the missing heat is the first step in addressing a broader issue. “The key question is now to understand if this heat will soon be released back to the atmosphere,” he said. In such a case, “the ‘hiatus’ of the last 10 years will be compensated by accelerated surface global warming over the next decade.”
—Christina Reed, Freelance Writer
Citation: Reed, C. (2015), Tracking the missing heat from the global warming hiatus, Eos, 96, doi:10.1029/2015EO029947. Published on 21 May 2015.