Geoengineering includes a variety of large-scale efforts to reverse global climate change. One geoengineering concept involves releasing large amounts of iron-containing
fertilizers into oceans. The increased concentration of iron in ocean water would
theoretically increase the rate of photosynthesis carried out by phytoplankton. Why are
such ideas controversial?
A. Photosynthetic algae would increase, not decrease, atmospheric carbon
dioxide.

C. Scientists cannot agree whether carbon dioxide in the atmosphere relates to
global climate change.
D. Scientists cannot predict what other unintended effects large concentrations
of iron may have.