John Martin of the Moss Landing Marine
Laboratory proposed (Nature 345, issue 3/4 in 1990) that excess
carbon dioxide could be removed from the atmosphere by using iron
fertilizer to stimulate the growth of algae in the polar oceans.
The idea comes from the fact that iron is the limiting nutrient
for biological growth in the polar oceans, which have abundant
levels of phosphate, nitrate and silicate (see Robin S. Keir,
Nature 349, 198 [1991]). This was shown (Nature 348, 188 [1990])
to be hugely expensive, potentially dangerous, and minimally
effective. More recent discussion points out
advantages and disadvantages.
A news release in October 1996 by the National Science Foundation reported
preliminary results with an experiment off the Galapagos Islands in which
phytoplankton were fertilized with about 450 kg of iron. The result was
that the growth of phytoplankton stimulated by this fertilization consumed
an additional 2.3 million kg of carbon dioxide at the atmosphere/ocean
interface.
Below are Nimbus-7 scanner data for phytoplankton concentration from November 1978 to
June 1986 showing the abundance of biological activity in polar oceans.