It’s not often that disappearing Arctic ice is presented as good news for the planet. Yet new research suggests that as the northern polar cap melts, it could lift the lid off a new carbon sink capable of soaking up carbon dioxide.
The findings, from two separate research groups, raise the possibility - albeit a remote one - of weakening the greenhouse effect. The researchers say the process of carbon sequestration is already underway. Even so, the new carbon sink is unlikely to make a significant dent in the huge amounts of CO2 pumped into the atmosphere by industrial activities.
Kevin Arrigo and colleagues at Stanford University studied satellite data collected between 1998 and 2007 to see how sea surface temperatures and the quantities of sea ice and phytoplankton had changed during that time.
Phytoplankton produce chlorophyll to obtain energy from the sun and assimilate CO2, and so increased phytoplankton productivity would remove more carbon from the atmosphere.
“We found that as sea ice diminishes, annual productivity goes up,” says Arrigo. Satellite remote sensing measures the amount of chlorophyll in surface waters, and so provides an estimate of ocean productivity.
Catch some rays
From one year to the next, the phytoplankton grew more in areas where the ice had disappeared: less ice meant more open water for longer, allowing the plankton to soak up more energy from the Sun. In some areas, production was boosted more than three-fold.
Ken Denman of the Canadian Centre for Climate Modelling and Analysis warns that the Arctic sink as it stands is likely to have only a very small impact on human emissions. Typically between half and a quarter of the carbon soaked up by phytoplankton ends up stored at the bottom of the ocean.
If the Arctic became completely ice-free and phytoplankton productivity levels were maintained, Arrigo and his colleagues calculated that the new carbon sink could in theory absorb an extra 160 million tonnes of carbon each year. “Given the current rate of human emissions, that would only account for 0.7% of total annual emissions,” says Arrigo.
“When you look carefully at the amounts involved, they just are not significant relative to the massive amounts of CO2 that we are and will be putting into the atmosphere,” agrees Denman.
A bigger sink?
For the sink to have a larger effect, productivity would have to rise further, something Arrigo says is uncertain. “The Arctic has relatively low nutrients in surface waters, so once they are all used up production will not increase any further,” he told New Scientist.
To sustain the growing phytoplankton, more nutrients would need to be brought to the surface waters, for instance from the silt on the seabed.
Some studies have suggested that strong winds and more storms are mixing up Arctic waters and could eventually bring more nutrients to the surface. The findings, however, are debated.
What is clear is that the impacts of the melting Arctic ice are complex and will continue to unfold as temperatures rise. Less ice cover may mean more phytoplankton, but it also means a darker sea surface, which will reflect less solar energy back out into space.
What’s more, the entire Arctic food chain could be affected, from top to bottom. “Food supplies for lower trophic levels may indeed be greater, but the loss of sea ice could precipitate profound ecological shifts,” says Arrigo. Most likely, fish that live in open water would be favoured over the predators that rely on ice - ringed seals and polar bears for example - that dominate today.
“It is clear that careful monitoring of climate and ecosystem changes in the Arctic is necessary to determine the longer-term implications of substantial losses of Arctic sea ice,” conclude the team.
Journal references: Arrigo et al.: Geophysical Research Letters, DOI: 10.1029/2008GL035028 (in press) and 10.1029/2008GL034791 (in press).