Seeding the oceans with iron to combat global warming might be ineffective as well as risky, it seems.
It's long been suggested that adding iron particles to sea water would increase the amount of phytoplankton - free-floating, single-celled plants that form the base of the marine food chain.
As phytoplankton take up carbon dioxide to grow, geoengineers have suggested that seeding key regions of the ocean with iron could be one way of offsetting increasing atmospheric CO2 concentrations.
But a Canada-US team led by University of Victoria oceanographer Dr Roberta Hamme says that data from the 2008 eruption of the Kasatochi volcano in the Aleutian Islands indicates that this might not be as effective as hoped.
The volcano spewed a quarter of a cubic kilometer of iron-laden ash over a 1,000-kilometer swath of the North Pacific. This created what Hamme calls "an ocean productivity event of unprecedented magnitude" - the largest phytoplankton bloom detected in the region since ocean surface measurements by satellite began in 1997.
But although the volcanic ash fueled such a massive phytoplankton bloom, it resulted in only a modest uptake of atmospheric CO2, says Hamme. It's believed to have caused an uptake of just 0.01 petagrams of carbon - literally, a drop in the ocean compared with the estimated 6.5 petagrams released every year by the consumption of fossil fuels.
In other words, even a massive seeding effort would have little effect on atmospheric carbon dioxide.
"The event acts as an example of the necessary scale that purposeful iron fertilizations would need to be to have an impact on global atmospheric CO2 levels," says Hamme