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The surprising reason why Arctic warming could be worse than previously thought

In this July 10, 2008, photo, ice floes float in Baffin Bay above the Arctic Circle, as seen from the Canadian Coast Guard icebreaker Louis S. St-Laurent. . (AP Photo/the Canadian Press, Jonathan Hayward)
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For the most part, we humans can live our lives entirely untroubled by the doings of phytoplankton.

These tiny, largely single-celled marine organisms are, basically, little photosynthesis machines — they contain chlorophyll and process sunlight to create energy. It might sound relatively dull — but according to new research, this simple lifestyle could have major climate change ramifications.

The new study, just out in the Proceedings of the National Academy of Sciences, finds that as the Arctic Ocean warms and loses its sea ice cover, populations of phytoplankton will boom. This will, in turn, further amplify warming in a region that’s already heating up twice as fast as the rest of the planet.

The reason is that phytoplankton, when they are more numerous, lead to a warmer upper layer of the ocean, because more radiation from the sun is trapped in that layer. Phytoplankton are thus changing the “physical property of the ocean,” says lead study author Jong-Yeon Park of the Max Planck Institute for Meteorology in Hamburg, Germany.

“If there is more phytoplankton in the ocean, then more shortwave radiation, radiant heating appears in the upper ocean, and less sunlight can penetrate into the deeper ocean,” Park says. “So, basically, it can change the vertical distribution of ocean heating.”

In effect, this finding adds additional oomph to arguably the most famous “feedback” process in climate science, known as the “ice-albedo feedback” (albedo refers to the reflectivity of a surface). In this widely accepted feedback, global warming leads to more melting of ice over the Arctic Ocean — which in turn exposes more of the ocean surface to the atmosphere. But because the ocean surface is darker than ice, it traps more heat, and thus warms up still further.

Now, it looks like these tiny marine organisms are further amplifying the warming ocean side of things.

The research was performed by using a climate model to simulate a one percent per year increase in atmospheric carbon dioxide levels, until a full doubling of total carbon dioxide occurred. The model simulated not only the general circulation of the atmosphere but also the response of ecosystems; in one case, the model included the phytoplankton feedback, while in the other it did not.

It was already known that phytoplankton affect climate by taking in carbon dioxide and giving off oxygen. That could actually be good a good thing, as it means less carbon dioxide in the air. But this new climatic role, centered not on chemistry but on how the density of Arctic phytoplankton in the upper layer of water leads sunlight to be trapped there, cuts in very much the other direction.

The study also notes that observational evidence suggests that in the Arctic, a “pattern of increased annual mean chlorophyll” in waters is “closely linked with that of surface warming and ice melting trends.” All seem to be increasing in sync.

How much does it matter? The study finds that this feedback could amplify warming in the Arctic by 20 percent. For a doubling of atmospheric carbon dioxide levels, says Park, that means a warming of 3.8 degrees Celsius, rather than 3.1 degrees Celsius.

The consequences could be faster sea ice decline and more areas of open water in the Arctic, further enhancing — that’s right — phytoplankton growth.