Whether the unknowns will get us is.. unknown. We are going to learn about whole-planet ecology in the next century whether we intervene or not. Pretending that we know less than we really do is kind of a trademark regressive-right tactic, so you'll forgive me when I say it's quite disengenuous and harmful to compare coal combustion to iron fertilization.

The negative externalities of coal combustion have been known in broad strokes at both the macro and micro levels for centuries. The benefits of fertilization have been known for much longer.