None of us should care about the chances. Let's study it!
At the very least, look at the opportunity cost. You can have climate change as it's moving now. Or you can try your best at political solutions (good luck stopping China&India's economic growth and associated pollution). Or you can try some geo-engineering.
Rabbits have completely transformed the Australian continent, one so dramatic you can see the effects from space. (http://www.sciencephoto.com/media/181383/view) Cane toads have a smaller but measurable effect since they utterly ruin entire ecosystems.
If you think it's impossible for a single organism to have an effect on climate you're mistaken. Wolves, once reintroduced to Yellowstone, have changed the entire park, altering water flow on a massive scale: https://www.youtube.com/watch?v=ysa5OBhXz-Q
"Our results contribute to a growing body of evidence showing that changes in growth of woody deciduous plants following the reintroduction of wolves cannot be explained by the trophic cascade model alone"
It's the right side, but the so-called effects purported to be visible in this photo are not from the rabbits - the left side of the picture is densely agricultural, while the right side is not.
First of all, I think you are missing my point. These kinds of experiments (i.e. the introduction of variables to "improve" an environment) have a long and sordid history. The Cane Toad is a cautionary tale: it was supposed to eat pests, but instead it took over large swaths of Australia and caused devastation.
What this person is trying to do is seed plankton. Plankton, as anyone who grew up near a coast can attest, commit mass suicide as they over-bloom. This is called the red tide and it caused me to spend a lot of evenings after beach days itchy with watery eyes. It also kills enormous numbers of fish, whales, jellyfish, etc.
EDIT: I can't reply, but I saw your comment that "unlike the cane toad, iron can't reproduce." Unfortunately, you're in error. Iron, like a lot of metals and toxins, can bioaccumulate in the food chain, pass from prey to predator and from gestating mother to fetus, and persist in ways that cause all kinds of havoc [1]
Unlike the cane toad though, iron can't reproduce. It seems to me entirely possible to study the effects of iron fertilization in a controlled manner, and indeed to tune how it's done for maximum benefit and minimum risk. It seems like that's what George is advocating.
At the very least, look at the opportunity cost. You can have climate change as it's moving now. Or you can try your best at political solutions (good luck stopping China&India's economic growth and associated pollution). Or you can try some geo-engineering.