A new Scripps Institution of Oceanography, UC San Diego-led study confirms a hypothesis that a retreat in sea ice could increase the Arctic Ocean’s ability to absorb carbon dioxide from the atmosphere.
Sea ice in the Arctic has decreased rapidly over the past three decades. In the summer of 2007, 50 percent less ice was observed compared to the summer of 1980. This dramatic reduction in ice cover has altered the Arctic ecosystem. The study used model simulations to find that between 1996 and 2007, carbon dioxide taken up by the Arctic Ocean increased by an average of 1.4 megatons per year.
The increased carbon uptake mostly occurs in the summer, as ice melts and sunlight can reach the ocean’s surface, creating a habitat for phytoplankton. These marine plants develop through photosynthesis, absorbing carbon dioxide from the atmosphere as they grow. As ice cover recedes, the Arctic Ocean can support larger phytoplankton populations, and carbon absorption increases. When these organisms die, some of their carbon material sinks to the bottom of the ocean, creating a long-term sink, or reservoir, of carbon. Oceanographers call this process of carbon uptake and storage the “biological pump.”
Using a combination of models integrating data on temperature, salinity, ocean currents, sea ice, nutrients and carbon transport, study lead author Manfredi Manizza and his coauthors calculated that between 1996 and 2007, the Arctic Ocean stored about 58 megatons of carbon per year, and the model results suggest that increased biological activity caused carbon uptake to grow by about 1.4 megatons of carbon each year.
These numbers weren’t surprising, as scientists already knew the Arctic Ocean ice shelves to be a large carbon sink. But the authors observed another, more surprising effect of the average 0.04° C (0.072°F) warming in the Arctic over this time period. They found that the greatest warming actually resulted in the least carbon uptake.
“I was expecting a greater carbon uptake going from 2006 to 2007 given that the reduction in sea-ice was more severe in 2007 than 2005,” said Manizza, a project scientist in Scripps’ Geosciences Research Division, “[but] the model showed the opposite.”
Instead, the results showed that a process that scientists call the “solubility pump” had a significant impact on carbon uptake.
Carbon dioxide, like all gases, becomes less soluble in water as the water’s temperature increases. As the ocean in some parts of the Arctic became warmer, CO2 began escaping from the ocean into the atmosphere in a process called de-gassing.
“This shows that the current hypothesis that less sea-ice area directly corresponds to an increase in ocean carbon uptake in the Arctic Ocean is not always correct,” Manizza said.
The study appears in the American Geophysical Union journal Global Biogeochemical Cycles.
The findings suggest that the future of carbon cycling in the Arctic will create a balance between the dual effects of warming: less ice cover allowing more photosynthetic activity, and warmer waters being less able to absorb CO2. But the researchers aren’t certain how the balance between these processes will change in the future, and what it will mean for global climate.
“We’re just beginning to scratch the surface,” Manizza said. “A large uncertainty is the change of the biological pump in response to climate warming.”
Many Arctic scientists believe that nutrient concentrations in the Arctic Ocean will become depleted in the future due to less sea-ice and more nutrient utilization. This would cause a decline in phytoplankton populations, a weaker biological pump, and less ocean carbon uptake.
Studies of phytoplankton in other parts of the Arctic have shown that temperature changes can affect communities of these microorganisms, resulting in changes to carbon storage. While Manizza’s work suggests that carbon cycling in the Arctic is already changing, the many uncertainties in the responses of physical, chemical, and biological processes to warming make it hard to reliably predict the future of the Arctic carbon sink.
Gathering more data from this region is crucial, Manizza said, to “both inform us about the change in the polar area, and make our models highly reliable for policymaking decisions.”
This research was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration and contributes to a project sponsored by NASA’s Modeling Analysis and Prediction program.
– Mallory Pickett is a master’s student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography, UC San Diego