Results of a large-scale experiment in the Southern Ocean that surrounds Antarctica reported in
The Southern Ocean Iron Release Experiment (SOIREE), an international collaboration, was conducted last year. Researchers released over eight and a half thousand kilograms of an iron compound into the Southern Ocean within an area 8 kilometres across and monitored its effect on the growth of phytoplankton, microscopic marine plants commonly called the 'grass of the sea'.
Why and how should iron affect marine life? In much of the oceans, the growth of phytoplankton is limited by how much nitrogen, phosphorus and silicon are available. But in the equatorial Pacific Ocean and the Southern Ocean, there is much more of them than the phytoplankton need. But plankton aren't more abundant there because something else curbs their growth.
Iron, another essential nutrient, is the limiting factor. The phytoplankton exhaust the iron supply before they use up the other elements.
This is significant not just for the ecology of the oceans, but for the global environment. All plants grow by soaking up carbon dioxide from the air during photosynthesis. Phytoplankton account for nearly half of the total photosynthesis on the planet, and so make a big difference to the amount of carbon dioxide in the atmosphere.
Because carbon dioxide is a greenhouse gas, this means that phytoplankton affect the global climate. So, in the past, changes in the amount of iron available to phytoplankton might have altered their uptake of carbon dioxide sufficiently to promote changes from ice-age to present-day conditions. Iron is delivered to the seas as wind-blown mineral dust, which becomes more or less abundant as conditions on land change -- as ice sheets recede, for instance.
So, in theory, if the oceans were 'fertilized' by addition of massive amounts of iron, the resulting increase in phytoplankton growth could remove large amounts of carbon dioxide from the air and ease the human effects on climate owing to fossil-fuel burning.
But would it work? In 1993, a team of scientists conducted an experiment similar to SOIREE in the equatorial Pacific Ocean
Yet there is no guarantee that the different oceans of the world will behave in the same way, since their patterns of circulation and mixtures of species and nutrients are different. This is why the SOIREE team wanted to test the hypothesis in the Southern Ocean.
Philip Boyd of the University of Otago in New Zealand and his colleagues now verify that iron fertilization works in these waters too
A third team -- Andrew Watson of the University of East Anglia, UK, and co-workers -- have used the SOIREE findings to investigate the role of iron limitation during climate switches to and from ice ages
They estimate that the changes in supply of iron-rich dust at the end of ice ages over the past 400,000 years (as deduced from measurements of dust in ancient Antarctic ice) account for about half of the increase in the amount of carbon dioxide in the atmosphere at these times.
Yet the temptation to see these findings as support for the idea of offsetting the greenhouse effect with iron fertilization should be resisted. Says Sallie Chisholm of the Massachusetts Institute of Technology, "The oceans are a complex system, and it is impossible to predict the long-term consequences of commercial ocean fertilization."
Past experience teaches us that attempts to tweak the environment for a desirable result can have unexpected consequences, sometimes the reverse of what we want. "Large-scale mobilization of fertilizer… to the sea is likely to cause more problems than it solves," Chisholm warns.