Warming Oceans May Pump Out More Greenhouse Gas Than They Absorb

Bioluminiscent phytoplankton.

(CN) – Until recently scientists thought the world’s oceans were our ally in the fight against greenhouse gas emissions, but a new study published Monday in the scientific journal Proceedings of the National Academy of Sciences suggests otherwise.

Oceans normally act as a powerful brake on the greenhouse effect by soaking up about a quarter of the carbon dioxide that humans pump into the air each year. Photosynthetic plankton in the water incorporate carbon into their bodies and as the plankton die, they sink, taking the carbon with them.

Scientists describe the sinking plankton as organic rain falling down to the deep ocean where it will be insulated from the atmosphere for centuries or more. Eventually, the ocean gives back the carbon dioxide to the atmosphere through the respiration of aerobic bacteria that eat the raining plankton breathing much like humans by exchanging oxygen for CO2 and releasing it back into the air.

Unfortunately, in many regions climate change is causing the oceans to warm. This in turn speeds up CO2 regeneration as the bacteria feed on plankton at shallower depths, leaving shorter distances and less time before the carbon travels back up and out to the air.

“The results are telling us that warming will cause faster recycling of carbon in many areas, and that means less carbon will reach the deep ocean and get stored there,” said Robert Anderson, study coauthor and oceanographer at Columbia University’s Lamont-Doherty Earth Observatory.

Scientists say plankton produce about 40 to 50 billion tons of solid organic carbon each year. Depending on the region and conditions, they estimate that about 8 to 10 billion tons manage to sink out of the surface ocean into greater depths, past about 100 meters, without getting eaten by bacteria. Prior to this study scientists had a poor understanding of the depths at which CO2 is breathed out by bacteria and its rate of return to the atmosphere.

For this study researchers used data from a 2013 research cruise from Peru to Tahiti. They looked at two distinct regions: the nutrient-rich, highly productive waters off South America, and the largely infertile waters that circle slowly in the central ocean below the equator in a set of currents known as the South Pacific Gyre.

Scientists pumped large amounts of seawater at different depths and sifted through it to isolate particles of organic carbon and isotopes of the element thorium. Combined, the elements enabled researchers to calculate the amount of carbon sinking through each depth that they sampled. This procedure yielded far more data than past study methods for capturing and measuring particles.

In the nutrient rich fertile South American zone, oxygen gets used up quickly near the surface, as bacteria and other organisms gobble up organic matter. At a depth of about 150 meters, oxygen content reaches near zero, halting aerobic activity. Once organic material reaches this layer, called the oxygen minimum zone, it can sink untouched to the deeper ocean.

The oxygen minimum zone acts as a protective cap over any organic matter that sinks past it. Oxygen levels do pick up again and aerobic bacteria can go back to work in the far deep but any CO2 produced that far down will take centuries to get back into the air via upwelling currents.

Many scientists thought much of the organic matter produced near the surface made it through the oxygen minimum zone, meaning that most CO2 regeneration would take place in the deep ocean taking longer to be released. Now researchers’ measurements suggest that only about 15 % makes it this far while the rest is converted back to CO2 above the minimum zone.

“People did not think that much regeneration was taking place in the shallower zone,” Lamont-Doherty graduate student and lead author, Frank Pavia said. “The fact that it’s happening at all shows that the model totally doesn’t work in the way we thought it did.”

This is important because researchers project that as the oceans warm, oxygen minimum zones will both spread horizontally over wider areas, and vertically, toward the surface. Based on this new study, the release of CO2 above these spreading zones will also increase and counteract any greater amounts of trapping of organic matter below.

Pavia says that more research will be needed to determine whether near surface regeneration or the cap provided by the oxygen minimum zones will win out. His team’s discovery indicates that the spread of these zones may not be as beneficial as previously thought for carbon storage. At least carbon capping had been a silver lining to the already harmful zones that tend to kill off marine life in what are now important fishing areas.

In the South Pacific Gyre, the results were less ambiguous.

As in areas of oxygen minimum zones, the new study methods also reversed previous research predictions but with more definitive results. Previous research suggested less biologic activity due to a lack of nutrients, with whatever organic matter that formed on the surface sinking to the cold deeps to remain for centuries. Instead, the new study showed far more regeneration near the warmer surface than previous study estimates.

As the ocean warms, just like the oxygen minimum zones, the South Pacific Gyre and similar current systems in other parts of the oceans are projected to grow. The gyres will divide these regions into stratified layer cakes of warmer waters on top and colder waters below.

The warm waters on top will allow more CO2 regeneration, with more of it going back into the air over wider regions with shallower water and shorter distances to pass up and through. There are four other major gyres besides South Pacific Gyre: the north Pacific, the south and north Atlantic, and the Indian Ocean.

Anderson explained that the gyres are of even greater concern because there is no counterbalancing effect like below the nearer-shore oxygen minimum zones.

“The story with the gyres is that over wide areas of the ocean, carbon storage is going to get less efficient,” Anderson said.  

The study authors point out that the processes they focused on are only part of the overall ocean carbon cycle. Physical and chemical reactions independent of biology are responsible for much of the exchange of carbon between atmosphere and oceans, and these processes could interact with the biology in complex and unpredictable ways.

“This gives us information that we didn’t have before, that we can plug into future models to make better estimates,” Pavia said.

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