Climate change is making the Antarctic blue—and that might be good news for the rest of the planet. Seafloor communities are flourishing beneath newly ice-free Antarctic seas, and as they grow, they are turning into carbon sinks, gobbling up carbon that might otherwise end up in the atmosphere.
“The big deal about this study is not really that Antarctica has a carbon sink—anyone who walks out into a forest sees a carbon sink,” said author David Barnes, of the British Antarctic Survey, who published his work in the September 21 issue of Current Biology. “Yet most carbon sinks don’t get bigger as climate change gets more severe. This one gets bigger.”
The mechanism Barnes found is one of the first significant examples of a negative feedback loop related to climate change, a phenomena normally associated with positive feedback loops. Positive feedback loops are snowball effects. For example, less ice on the poles means these regions reflect less sunlight, leading to more heat in the atmosphere, which in turn melts more ice. In a negative feedback loop, the opposite is true. As sea ice vanishes, life on the seafloor grows more rapidly and removes more carbon from the ocean surface, which helps prevent ice from melting further.
Sea ice, Barnes explained, acts like a heavy blanket for polar waters, blocking wind and sunlight. Without ice, the waters become more turbulent and well-mixed: rich nutrients from the bottom rise to the surface and feed phytoplankton, which are suddenly exposed to longer periods of sunlight than before. These phytoplankton produce carbon waste that sinks to the seafloor and is stored by organisms below. These benthic animals also store carbon that is naturally absorbed from the atmosphere when waves mix carbon dioxide into ocean water.
The depth of the Antarctic shelf also protects carbon from being re-suspended by activity like icebergs scraping the seafloor. The result is an explosion of life that Barnes describes as “absolutely fantastic.”
“On land, it’s a virtual desert—yet stick your head underwater with a mask on and there’s this world of life,” Barnes said, recalling sea spiders the size of dinner plates and sponges large enough to sit in. “Some of the animals are so bizarre, you couldn’t think them up in a science fiction film. And Antarctica is just full of them.”
Barnes’ research drew on seafloor animals collected over nearly 20 years. These samples, as well as photographs showing the relative abundance of animals, allowed Barnes to calculate the amount of carbon that was taken up by new communities appearing where sea ice was vanishing. The drawdown may be significant enough, particularly if it is occurring at both poles, to slow atmospheric warming.
“If [Barnes’] calculations are correct, and a significant amount of carbon…is sequestered in seafloor bryozoans, then this will help contribute to the oceans serving as a greater sink for carbon dioxide than first thought,” said James McClintock, an Antarctic benthic ecologist not involved in the study. “The caution here is that Barnes makes his more global statements with the caveat that other marine seafloor life will need to sequester carbon too, and this is unknown—thus the jury is still out.”
In February 2016, Barnes will try to answer why some Antarctic seas experience higher productivity than others, taking him back to the South Orkney Islands. In the coming years he also hopes to investigate seafloor growth in the Arctic, where sea ice loss has been dramatically higher than in the Antarctic. However, the differing geography of the poles means that melting sea ice may not mean the same thing in both places.
“There is an important difference: the Arctic is basically ocean surrounded by land, while the Antarctic is land surrounded by ocean. So in the Arctic, a lot of sea ice is over deep water,” said Ken Halanych, who studies Antarctic marine invertebrates. Deep waters are less productive, with fewer available nutrients. However, Halanych said, carbon drawdown could be occurring along the shallower coasts of northern Canada and Russia.
“This is taking up a significant part of the carbon budget—it’s not huge, it’s not going to stop [climate change], but it will make a little bit of a dent in it.”