Marine microbes may fuel ocean warming

Plankton bloom off SW Ireland: A rich source of carbon dioxide. Image: By NASA Goddard Space Flight Center [public domain], via Wikimedia Commons

Warmer air means warmer seas, and marine microbes in warmer seas could mean yet warmer air. The climate cycle could get increasingly vicious.

LONDON, 6 May, 2019 − US scientists say marine microbes are the cause of yet another potentially positive feedback that could accelerate global warming.

As the oceans warm, marine microbial life might start to pump yet more carbon dioxide into the air. This process would of course increase the greenhouse gas levels still further and warm the oceans to increasing temperatures.

The finding is a reminder that the atmosphere, oceans, ice caps, rocks, algae, bacteria and forests are all intricate parts of the planetary climate machinery, and researchers still have a long way to go before they understand all the working parts in detail. But it is also a reminder that every small rise in planetary average temperatures in some way feeds back into this complex system.

The new study, based on analysis of data gathered during a research cruise in 2013 from Peru to Tahiti, is published in the Proceedings of the National Academy of Sciences.

“Warming will cause faster recycling of carbon in many areas, and that means less carbon will reach the deep ocean and get stored”

The shipboard scientists looked in depth at processes in highly productive waters off the South American coasts, and at the more or less barren waters south of the equator that cycle in a set of currents known as the South Pacific Gyre.

They did so to estimate the fate of tiny green plants – plankton – as they flourished in the ocean surface, and then perished and sank to the depths.

In the great and far-from-complete reckoning of the planet’s carbon budget – from atmosphere to plants to animals and back to the air, or to the rocks – climate scientists think that the oceans absorb around one fourth of all the extra carbon dioxide that humans burn as fossil fuels to power economic growth.

Plankton produce about 40 to 50 billion tonnes of organic carbon as they flourish, and then perish. Microbes set to work and begin the process of decay, recycling the carbon into the atmosphere. But somewhere between 8bn and 10bn tonnes of green tissue sink below 100 metres, into waters increasingly starved of oxygen, and decay stops.

Long sojourn

Once the dead plankton reach the ocean bottom, they could be there for centuries. More heat, however, could alter the balance of recycling and long-term storage.

“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,” said Robert Anderson, of Columbia University’s Lamont-Doherty Earth Observatory, and one of the authors.

The fear is that as the oceans warm, the oxygen-low zones will increase and expand. That could suggest more long-term carbon burial. But as the surface waters warm, the microbial activity could accelerate, and release even more carbon into the atmosphere. In which case, the world would warm more swiftly.

Research like this is necessarily inconclusive: marine biologists have a lot more to do before they get a convincing answer to a global puzzle. Climate scientists started worrying about oxygen depletion in the oceans years ago, but they have been more bothered by evidence that in a warmer world microbial scavengers and recyclers work ever harder, and not just on land.

Positive feedbacks

As the polar ice retreats, there are more emissions of potent greenhouse gases from the tundra. And as high latitude ice and snow retreats, the levels of radiation back into space are reduced, while deep blue sea and brown rock absorb ever higher doses of sunlight.

All these are instances of positive feedback: planetary responses that seem overall to make climate change more likely, and climate extremes more hazardous. And the increasing evidence of oxygen depletion in the oceans provides no comfort: as the seas warm, less oxygen is available for the ocean’s animals: including of course the huge hauls of fish on which millions depend for income and nourishment.

As the scientists say, in the opaque language of a research journal: “Our findings imply that climate warming will result in reduced ocean carbon storage due to expanding oligotrophic gyres, but opposing effects on ocean carbon storage from expanding suboxic waters will require modelling and future work to disentangle.”

In other words, there is more research to be done. − Climate News Network